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Drugs Acting On Autonomic Nervous System Introduction

The human nervous system consists of the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). CNS is composed of the brain (located in the cranial cavity) and the spinal cord (located in the vertebral cavity), which serve as the main control centers. for all body activities.

PNS is composed of nerves derived from the brain and spinal cord (12 pairs of cranial nerves and 31 pairs of spinal nerves) which serve as a linkage between the CNS and the body.

PNS can be subdivided into sensory (afferent) nerves and motor (efferent) nerves. Sensory nerves send nerve impulses from the body to the CNS to effector organs.

Motor nerves are divided into the Somatic Nervous System (SNS) which regulates the voluntary contraction of the skeletal muscles and the Autonomic Nervous System (ANS) which regulates the involuntary control of smooth, cardiac muscles and glands.

The Autonomic Nervous System (ANS) can be divided into sympathetic and parasympathetic branches where in general sympathetic nerves stimulate activities of the effect or organs (except digestive organs) and parasympathetic nerves inhibit activities of the effect or organs (except digestive organs).

The Autonomic Nervous System (ANS) controls a variety of involuntary and reflexive regulatory responses, e.g. the beating of the heart, expansion or contraction of blood vessels or pupils, etc.

It is responsible for both the “fight or flight” response that represents the body’s physiological response to crisis or stress and for the less crisis-driven functions of resting, repairing, digesting, and reproductive activities. Many of the drugs used to treat common conditions of the heart, circulation, and blood pressure by altering the functioning of the ANS.

Read and Learn More Medicinal Chemistry III Notes

There are three main components of ANS, viz., Sympathetic Nervous System (SNS), Parasympathetic Nervous System (PNS), and Enteric Nervous System (ENS).

ENS is intrinsic to the digestive system; it carries the key functions in support of systemic neurologic and immunologic well-being and is highly responsible for both physical and emotional stimuli. Comparison chart between Sympathetic Nervous System (SNS) and Parasympathetic Nervous System (PNS):

Adrenergic Drugs

The sympathetic system activates and prepares the body for vigorous muscular activity, stress, and emergencies. Adrenergic drugs are chemical agents that exert their principal pharmacological and therapeutic effects by either enhancing or reducing the activity of the various components of the sympathetic division of the Autonomic Nervous System.

Adrenergic drugs act by stimulation of the peripheral nerve endings of the sympathetic or adrenergic nerves, the action being exerted on the effector cells supplied by postganglionic endings.

As these drugs stimulate the adrenergic nerves directly by mimicking the action of norepinephrine or indirectly by stimulating the release of norepinephrine, they are also called adrenomimetic or sympathomimetic agents.

A sympathomimetic drug plays a vital role in various life-threatening disorders, like acute attacks of bronchial asthma, cardiac arrest, shock and allergic reactions. As most of adrenergic agents contain an intact or partially substituted amino group, they are also called as sympathomimetic amines.

Adrenergic Neurotransmitters

Adrenergic neurons release nor-epinephrine (nor-adrenaline) as the primary neurotransmitter. These neurons are found in the Central Nervous System and also in the Sympathetic Nervous System, where they serve as links between the ganglia and the effector organs.

The adrenergic neurons and receptors located either pre-synaptically on the neuron or post-synaptically on the effector organ, are the sites of action of the adrenergic drugs.

Catecholamines

The catecholamines are so named because they contain a catechol group and an amine group. A catechol group is simply a benzene ring with hydroxyl groups on two adjacent carbons. The amine component of the catecholamines is ethylamine.

Because of their chemistry, all catecholamines have three properties in common: (1) They cannot be used orally. (2) They have a long duration of action. (3) They cannot cross the blood-brain barrier.

Catechol is highly susceptible to oxidation in the presence of oxygen to produce an ortho- quinone-like compound, which undergoes further reaction to give a mixture of colored products. Hence solution of catechol amines is stabilized by the addition of antioxidants (reducing agents) such as ascorbic acid or sodium bisulfite.

Synthesis, Storage, and Release of Catecholamine

Catecholamine biosynthesis takes place in adrenergic and dopaminergic neurons in the CNS, in sympathetic neurons in the ANS, and in the adrenal medulla. Epinephrine and norepinephrine each possess chiral carbon atoms, and thus exist an enantiomeric pair of isomers.

The enantiomer with R-configuration is biosynthesized by the body and possesses biological activity. Neurotransmission in adrenergic neurons is a process that involves five steps: synthesis, storage, release, and receptor binding of norepinephrine, followed by removal of the neurotransmitter from the synaptic gap.

• Synthesis Of Nor-Epinephrine (Nor-Adrenaline): Hydroxylation of tyrosine is rate rate-limiting step.
• Uptake Into Storage Vesicles: Dopamine enters a vesicle and is converted to norepinephrine. Norepinephrine is protected from degradation in the vesicle.
• Release Of Neurotransmitter: Influx of the calcium causes fusion of the vesicle with the cell membrane in a process known as exocytosis.
• Binding To Receptor: The post synaptic receptor is activated by the binding of neurotransmitters.
• Removal Of Norepinephrine: Released norepinephrine is rapidly taken into the neuron.
• Metabolism: Norepinephrine is methylated by COMT and oxidized by MAO.

All five enzymes that are involved in the pathway of the biosynthesis of norepinephrine are synthesized within the cell bodies of the adrenergic neurons and transported to their nerve terminals. Phenylalanine-hydroxylase also called phenylalanine-4-monooxygenase coverts phenylalanine to tyrosine.

Conversion of tyrosine to L-B-(3,4-dihydroxy phenyl)-α- alanine (DOPA) is a rate-limiting step catalyzed by the enzyme, tyrosine-hydroxylase. Decarboxylation of DOPA to dopamine takes place by the enzyme aromatic amino-acid decarboxylase. Dopamine formation takes place in the cytoplasm.

In synaptic vesicles, dopamine is hydroxylated stereospecifically by the enzyme dopamine B-monooxygenase (dopamine-ß- hydroxylase) to give noradrenaline (norepinephrine). Norepinephrine undergoes methylation in the presence of the enzyme phenylethanolamine-N-methyl transferase to give adrenaline (epinephrine).

The norepinephrine formed is stored in vesicles as its adenosine triphosphate complex until the depolarization of neuron initiates the process of vesicle fusion with the plasma membrane and extrusion of norepinephrine into the synaptic cleft. Synthesis of norepinephrine and epinephrine also takes place in adrenal medulla.

The release of neurotransmitters occurs when an action potential opens voltage-sensitive calcium channels and increases intracellular calcium. Influx of calcium triggers the fusion of the vesicle with the surface membrane. This causes the release of neurotransmitter (norepinephrine) from the synaptic vesicle through exocytosis.

It then binds to the adrenoreceptor on the effector cells and produces effects by various mechanisms. The excess norepinephrine that are not bound to the postsynaptic receptor, binds to presynaptic receptors to decrease its own release. It then diffuses out, either by metabolism by COMT or is taken back up by the presynaptic neuron.

Uptake And Metabolism Of Catecholamines

The actions of norepinephrine at adrenergic receptors are terminated by a combination of processes, such as

• Uptake into the neuron and into extraneuronal tissues,
• Diffusion away from the synapse and uptake at non-neuronal sites, and
• Metabolism.

The two principal enzymes involved in catecholamine metabolism are namely monoamine oxidase (MAO) and catechol-o-methyl transferase (COMT).

Both of these enzymes are distributed widely throughout the body, with high concentrations found in the liver and kidney. Catecholamines that are administered orally become inactivated before they can reach to the systemic circulation.

Hence, catecholamines are ineffective if given by mouth. Both enzymes are very active and quickly destroy catecholamines, therefore, three catecholamines viz., norepinephrine, dopamine, and dobutamine are effective only if administered by continuous infusion (by IV route in an emergency), whereas other parenteral routes like subcutaneous and intramuscular will not yield adequate blood levels.

Because of the hydroxyl groups on the catechol portion of the catecholamines, they cannot cross the blood-brain barrier; therefore catecholamines have minimal effects on the CNS.

Norepinephrine (NE) is deaminated oxidatively by MAO to give 3,4-dihydroxy- phenylglycoaldehyde which is then reduced by aldehyde reductase to 3,4-dihydroxy- phenylethylene glycol. This glycol metabolite is primarily released into the circulation, where it undergoes methylation by the COMT to give 3-methoxy-4-hydroxyphenylethylene glycol.

3,4-dihydroxyphenylglycolaldehyde undergoes dehydrogenation in the presence of aldehyde dehydrogenase to give 3,4-dihydroxy-mandelic acid, which is then by the action of COMT convert to 3-methoxy-4-hydroxy mandelic acid. This metabolite is commonly referred to as vanillylmandelic acid. It is the end product of several pathways of norepinephrine metabolism.

Non-Catecholamines

The non-catecholamines have ethylamine in their structure but do not contain the catechol moiety that characterizes the catecholamines. The non-catecholamines differ from the catecholamines in three important respects:

• As they do not have a catechol group, non-catecholamines are not substrates for COMT and are metabolized slowly by MAO. As a result, the half-lives of non-catecholamines are much longer than those of catecholamines.
• As they do not undergo rapid degradation by MAO and COMT, non-catecholamines can be administered orally, whereas catecholamines cannot.
• Non-catecholamines are considerably less polar than catecholamines, and hence are more able to cross the blood-brain barrier.

Adrenergic Receptors

Catecholamines bind to specific receptors known as adrenergic receptors. There are two basic types of receptors: a-adrenergic receptors and B-adrenergic receptors. a-adrenergic receptors have subtypes such as a and α2, whereas ẞ-adrenergic receptors have subtypes such as B1 B2, and B3.

The action of neurotransmitters adrenergic drugs or sympathomimetic drugs upon these receptors is an important means of alternate ANS functions for the therapy of different disease states.

Receptor Distribution:

• α-Adrenergic Receptors: They are found in the smooth muscles of the iris, arteries, arterioles, and veins.
• a-Adrenergic Receptors: They mediate the inhibition of adrenergic neurotransmitter release.
• B1-Adrenergic Receptors: They are found in the myocardium where their stimulation increases the force and rate of myocardial contraction.
• B2-Adrenergic Receptors: They are found in bronchial and vascular smooth muscles where their stimulation causes smooth muscle dilation or relaxation.
• B3-Adrenergic Receptors: These receptors are expressed on fat cells and their stimulation causes lipolysis.

Norepinephrine activates primarily a-receptors and epinephrine activates primarily B-receptors, although it may also activate a-receptors. Stimulation of a-receptors is associated with constriction of small blood vessels in the bronchial mucosa and relaxation of smooth muscles of the intestinal tract. B-receptor activation relaxes bronchial smooth muscles which cause the bronchi of the lungs to dilate.

In addition, B-receptor stimulatory effects cause an increase in the rate and force of heart contractions. As a result, an increased amount of blood leaves the heart and is diverted from non-active organs to areas that actively participate in the body’s reaction to stress such as skeletal muscles, brain, and liver.

Fight Or Flight Response: The changes experienced by the body during emergencies have been referred to as the fight or flight response. These reactions are triggered both by direct sympathetic activation of the effector organs and by stimulation of the adrenal medulla to release epinephrine and lesser amounts of norepinephrine. These hormones enter the bloodstream and promote responses in effector organs that contain adrenergic receptors.

Effects of stimulation of the sympathetic division:

• To increase heart rate and blood pressure.
• To mobilize energy stores of the body.
• To increase blood flow to skeletal muscles and the heart while diverting flow from the skin and internal organs.
• Dilation of the pupils and the bronchioles.
• Affects gastrointestinal motility and the function of the bladder and sexual organs.

Selected Tissue Responses to Stimulation of Adrenoceptor Subtypes

Classification Of Sympathomimetic Agents

Sympathomimetic agents are subdivided into three classes: direct-acting, indirect-acting and dual (mixed) acting sympathomimetic agents.

Direct-Acting Sympathomimetics

Direct-acting agents elicit a sympathomimetic response by interacting directly with adrenergic receptors. They bind to and activate α1, α2, B1, and B2 receptors.

Naturally occurring molecules that bind to these receptors include,

Norepinephrine: A neurotransmitter which binds to a1, a2, and ẞ1 receptors. It is non-selective sympathomimetic.

Epinephrine: A hormone produced and secreted from the adrenal medulla that binds α1, α2, B1, and B2 receptors. It is non-selective sympathomimetic.

Dopamine: A neurotransmitter that binds to D1 or D2 receptors as well as α1, α2, and ẞ1 receptors.

a-receptor stimulation:

α-agonists act by G-protein activation of the enzyme phospholipase C, resulting eventually in the release of calcium, thereby increasing the intracellular calcium concentration. 2-agonists act by inhibiting adenylyl cyclase, the enzyme that catalyzes the conversion of adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP). This leads to decreased intracellular cAMP levels.

Alpha receptors are further subdivided into α1A, 1B, αic and α1D as well as α2A, 2B and

B-receptor stimulation:

B-agonists are selective to a subtype (B1, B2, and ẞ3) or nonselective- stimulate adenylyl cyclase, causing increased intracellular cAMP levels.

Dopamine-receptor stimulation:

Dopamine receptor agonists can act on D1 or D2 receptors. D1-receptors activate adenylyl cyclase and increase intracellular cAMP mainly in neurons and vascular smooth muscle. D2- receptors reduce intracellular cAMP and are found in the brain and as pre-synaptic receptors. Receptor Drug

Indirect Acting Sympathomimetics

Indirect-acting sympathomimetic agents produce effects, primarily by causing the release of NE (e.g., amphetamine) from adrenergic nerve terminals and preventing its re-uptake (e.g., cocaine and tricyclic antidepressants). The NE that is released by the indirect-acting agent activates the receptors to produce the response.

Mixed-acting Sympathomimetics

Mixed-acting drugs employ both mechanisms i.e., direct and indirect (e.g., ephedrine, metaraminol). They bind directly with adrenergic receptors and cause the release of NE. Ephedrine is used to treat nasal congestion.

Mechanisms For Activation Of Adrenergic Receptor

Drugs can activate adrenergic receptors by four basic mechanisms:

• Direct Receptor Binding: Direct interaction with receptors is the most common mechanism by which drugs activate peripheral adrenergic receptors. The direct-acting receptor stimulants produce their effects by binding to adrenergic receptors and mimicking the actions of natural transmitters (norepinephrine, epinephrine, and dopamine).
• Promotion of Norepinephrine Release: By acting on terminals of sympathetic nerves to cause norepinephrine release, drugs can bring about activation of adrenergic receptors.
  Example: Amphetamines and ephedrine.
• Inhibition of Norepinephrine Reuptake: Recall that reuptake of norepinephrine into terminals of sympathetic nerves is the major mechanism by which adrenergic transmission is terminated. By blocking norepinephrine reuptake, drugs can cause norepinephrine to accumulate within the synaptic gap, and can thereby increase receptor activation.
  Example: Cocaine and tricyclic antidepressants (imipramine).
• Inhibition of Norepinephrine Inactivation: Some of the norepinephrine in terminals of adrenergic neurons is subject to inactivation by monoamine oxidase (MAO). Hence, drugs that inhibit MAO can increase the amount of NE available for release, and can thereby enhance receptor activation.

Direct Acting Sympathomimetics

Structure-Activity Relationship:

• The sympathomimetic drugs may be divided into catechol and non-catechol amines.
• All catecholamines possess the catechol nucleus (o-dihydroxybenzene). Catechol-amines have only a brief duration of action, and are ineffective orally, due to degradation by COMT.

Non-catecholamines consist of a benzene ring and an ethylamine side chain (i.e. B-phenylethylamine). Substitution on the meta and para positions of the aromatic ring and, on a and B-positions of the ethylamine side chain influences not only the mechanism of sympathomimetic action but also the receptor selectivity of the drug.

Substitution on the aromatic nucleus, specifically -OH groups at the 3 (meta) and 4 (para) positions of the ring and a B-OH group are required for maximal a and B activity.

Examples: Adrenaline And Noradrenaline.

B-hydroxyl (B-OH) Group:

• Due to chirality, they exhibit high stereo selectivity in producing their agonistic effect. The more potent enantiomer i.e. R-configuration is 100 times more potent than S-configuration.
• All direct-acting B-phenylethylamine-derived agonists have the arrangement in space of the catechol group, the amino group, and the B-OH group in the fashion resembling that of R-norepinephrine stereo selectively acts as three critical pharmacophoric groups which bind with three complementary binding areas on the receptor and called as Easson-Stedman Hypothesis (Three point attachment theory).

Amino (-NH2) Group:

• The presence of the amino group in phenylethylamines is important for direct agonist activity.
• The amino group should be separated from the aromatic ring by two carbon atoms for optimal activity.
• Both primary and secondary amines are found among the potent direct-acting agonists, but tertiary or quaternary amines tend to be poor direct agonists.
• Substitution of the bulkier group at the nitrogen decreases a-receptor agonist activity and increases B-receptor activity. Thus, norepinephrine is an effective ẞ-receptor agonist as well as a potent a-receptor agonist, while epinephrine is a potent a, ẞ1 and B2-receptor agonist. Isoproterenol is a potent and ẞ2-receptor agonist, but has little affinity for a-receptors.

Example: Isoproterenol

Large substituents on the amino group also protect the amino group front undergoing oxidative deamination by MAO as well as increase ẞ2 selectivity.

Examples: N-tert-butyl norepinephrine (Colterol)

α-Carbon Atom:

• Substitution of CH3 or C2Hs at a-carbon atom reduces direct receptor agonist activity at both a and ẞ receptors.
• a-alkyl group increases the duration of action by making the compound resistant to metabolic deamination by MAO. It also increases oral effectiveness and greater CNS activity.
• Another effect of a-substitution is the introduction of a chiral center i.e. a-methyl norepinephrine, is the erythro isomer that possesses significant activity at a-receptor.

Catechol Moiety:

• Catechol moiety is important for maximal agonist activity. It can be replaced with other substituted phenyl moieties to provide selective adrenergic agonist activity. This approach can be used in the design of selective ẞ2-receptor agonists.
• Replacement of the meta-OH of the catechol structure with a hydroxymethyl group shows B2 selectivity.

Examples: Albuterol (Salbutamol)

Hydroxyl (-OH) groups at the 3 and 5 positions of the aromatic ring (Resorcinol ring), in compounds with large amino substituents, show ẞ2 selective activity. As the resorcinol ring is not a substrate for COMT, it tends to have better absorption and longer duration of action.

Examples: Metaproterenol, Terbutaline

Modification at the catechol ring can also bring about a-selectivity at receptors. Removal of the p-OH group from epinephrine gives phenylephrine, which, in contrast to epinephrine, is selective for the a-adrenergic receptor with B-activity almost entirely absent.

Examples: Phenylephrine

Optical isomerism is conferred by substitution on either of the ethyl carbon atoms.

Laevorotatory: Substitution at the B-carbon atom produces naturally occurring epinephrine and norepinephrine, both of which are over 10 times as potent as their isomers.

Dextrorotatory: Substitution at the a-carbon atom generally confers greater potency in CNS stimulation, e.g. d-amphetamine.

• Agents lacking -OH substitution, especially the compounds with 3-OH, are resistant to COMT and have a longer duration of action and oral effectiveness.
• Lacking of both, aromatic -OH groups and the B-OH on the ethyl chain produce almost all of their effects by NE release, thus their effects are mainly on a and ẞ1.

Examples: Ephedrine

Substitution on the B-C atom generally decreases central stimulant action, due to the lower lipid solubility of these agents. However, this also greatly enhances both a and ẞ potency. Thus, ephedrine is less potent than methamphetamine as a CNS stimulant but is more potent vasoconstrictor and bronchodilator.

Absence of the benzene ring reduces the CNS stimulant action, without reducing peripheral effects. e.g. Cyclopentamine, and many of these agents are used as nasal decongestants.

Endogenous Catecholamines

Adrenaline (Epinephrine)

• Adrenaline is chemically 1-(3,4-dihydroxyphenyl)-2-methylaminoethanol and available as adrenaline acid tartarate.
• Adrenaline is a potent stimulator of both a and ẞ receptors.
• It is light sensitive and easily oxidized on exposure to air because of the catechol ring system and turns to brown colour.
• It is ineffective by the oral route because of poor absorption and rapid metabolism by MAO and COMT.
• Adrenaline is rapidly inactivated in the body, despite its stability in blood.
• It shows prominent actions on the heart and vascular smooth muscle.
• Adrenaline is one of the most potent vasopressors known, given by IV route, it evokes a characteristic rise in blood pressure.
• Increase in blood pressure is due to direct myocardial stimulation (positive inotropic effect), an increased heart rate (positive chronotropic effect) and peripheral vasoconstriction.

Adrenaline Uses:

• As it activates ẞ1 receptors, epinephrine is used to overcome AV heart block and restore cardiac function in patients experiencing cardiac arrest.
• Activation of ẞ2 receptors in the lung promotes bronchodilation, which can be useful in patients with asthma.
• Used to prevent bleeding during surgery or in the case of inner organ bleeding.
• It can cause. α-mediated vasoconstriction; it is administered in combination with local anaesthetics which leads to longer lasting effect of local anaesthetics in smaller doses.
• Used in the treatment of open-angle glaucoma, where it apparently reduces intraocular pressure increasing the rate of outflow of aqueous humour from its anterior chamber of the eye.
• Used to produce mydriasis during ophthalmologic procedures.
• It activates a combination of a and ẞ receptors, epinephrine used as treatment of choice for anaphylactic shock.

Noradrenaline (Norepinephrine):

• Noradrenaline is chemically 1-(3,4-dihydroxyphenyl) amino ethanol, available as acid tartarate salt.
• Both adrenaline and noradrenaline are approximately equipotent at cardiac ẞ1 receptors. Noradrenaline is a potent agonist for a-receptors but has little action on B2 receptors.
• Noradrenaline should be protected from air and light as it darkens on exposure to air and light.
• Like the other. endogenous catecholamines, it is a substrate for both MAO and COMT and thus is not effective by the oral route of administration. It is given by intravenous injection.
• Levo-isomer is pharmacologically active.

Noradrenaline Uses:

• Noradrenaline is used to maintain blood pressure in acute hypotensive states resulting from surgical or non-surgical trauma, central vasomotor depression and haemorrhage.

Dopamine:

• Dopamine is chemically 3,4-dihydroxyphenylethylamine, is a central neurotransmitter. It differs from the other naturally occurring catecholamines, lacking the B-OH group on the ethylamine side chain.
• It is the metabolic precursor of noradrenaline and adrenaline..
• It is a substrate for both MAO and COMT and is thus ineffective orally.
• As it cannot cross blood brain barrier, it has minimal effects on the CNS.
• It stimulates ẞ1-receptors of heart and exerts a positive inotropic effect on the heart.
• It increases blood flow to the kidney in doses that have no chronotropic effect on the heart or that cause no increase in blood pressure.

Dopamine Uses:

• Dopamine is used in acute congestive cardiac failure with imminent renal failure, septic shock, cardiogenic shock, surgical shock and acute pancreatitis.

α-adrenergic Receptor Agonists

Phenylephrine:

• Phenylephrine is chemically 1-(3-hydroxyphenyl)-2-methylaminoethanol and available as hydrochloride salt.
• It should be stored in airtight container to protect from light because it is decomposed by light.
• Phenylephrine differs from adrenaline only by lacking the 4-OH group on the benzene ring and subsequently is resistant to COMT and has predominantly α-agonist effects.
• It gets metabolized by MAO.
• Phenylephrine is a selective α1-receptor agonist.
• Oral absorption is not reliable, and so it is given parenterally or topically as eye or nasal drops.

Phenylephrine Uses:

• Phenylephrine is potent vasoconstrictor, but is less potent than epinephrine and norepinephrine.
• It is used as a nasal decongestant, mydriatric and as a vasopressor agent.
• It is used to dilate the pupil and to treat open-angle glaucoma.
• It is used in spinal anesthesia, to prolong the anesthesia and to prevent a drop in blood pressure.
• It is used in the treatment of severe hypotension resulting from either shock or drug administration.

Methoxamine:

• Methoxamine is chemically, 2-amino-1-(2,5-dimethoxyphenyl)propan-1-ol.
• It is a selective direct acting a1-receptor agonist.
• It is less potent than phenylephrine as a vasoconstrictor that has no stimulant action on the heart.
• It is not substrate for COMT, its duration of action is significantly longer than that of norepinephrine.

Methoxamine Uses:

• It is used primarily during surgery or shock to maintain adequate arterial blood- pressure.

Imidazolines:

Structure-Activity Relationship:

• These compounds interact differently with receptors to that of B-phenylethylamine. The heterocyclic imidazoline ring is linked to a substituted aromatic moiety.
• Modification in imidazoline ring decreases the activity.
• Open ring imidazoline are highly active i.e. Guanabenz and Guanfacine.
• Aromatic substitution is flexible. Agonist activity increases on substitution of aromatic ring with halogen (CI) or small alkyl group (CH3), particularly when placed at two ortho positions.
• • Naphazoline: (2-(1-naphthylmethyl)-2-imidazoline}
  • Tetrahydrozoline: 2-(1,2,3,4-tetrahydronaphthalen-1-yl)-4,5-dihydro-1H-imidazole}
  • Xylometazoline: (2-(4-tert-butyl)2, 6-dimethylbenzyl)-2-imid ‘ine}
  • Oxymetazoline: (6-tert-butyl-3-(2-imidazoline-2-ylmethyl)-2,4-dimethylphenol}

• These drugs are agonists at both α, and a2-adrenergic receptors.
• Because of the basic nature of the imidazoline ring, these drugs essentially exist in an ionized form at physiological pH.

Imidazolines Uses:

• These agents are used for their vasoconstrictive effects as nasal decongestants and ophthalmic decongestants.
• • (e) Guanabenz: (2-[(2,6-dichlorophenyl) methylideneamino] guanidine}
  • (f) Guanfacine: (2-[(2,6-dichlorophenyl) acetamido] guanidine}

• Structurally, these two compounds can be considered “open-ring imidazolidines.
• 2,6-dichlorophenyl moiety is connected to a guanidino group by a two-atom bridge.
• In guanabenz the bridge is -CH-N- group and in guanfacine the bridge is -CH2CO-group.
• Conjugation of the guanidine moiety with the bridging moiety helps to decrease the pka, therefore because of basic group these drugs exist in their non-ionized form at physiological pH.

Imidazolines Uses:

• Guanabenz and guanfacine are used as antihypertensive drugs.

Clonidine:

• Clonidine is chemically, 2-(2,6-dichloroanilino)-2-imidazoline.
• It has selective action on α2 receptor.
• It exhibits vasoconstrictive activity by stimulation of peripheral a-adrenergic receptors.
• It enters into the CNS and stimulates 2 receptors located in regions of the brain leads to decrease in sympathetic outflow from the CNS, which in turn, decreases in peripheral vascular resistance and blood pressure.
• It produces bradycardia due to stimulation of cardiac a2-adrenergic receptor.

Clonidine Uses:

• Clonidine is useful in the treatment of hypertension.

Methyldopa:

• Methyldopa is 3-(3,4-dihydroxyphenyl)-2-methyl-L-alanine.
• Methyldopa is used only by oral administration since its zwitter ionic character limits its solubility.
• a-methyl norepinephrine is the metabolic product of the drug methyldopa. It shows selectivity towards the α1 receptor.
• a-methyl norepinephrine acts on a2 receptors in the CNS in the same manner as clonidine to decrease sympathetic outflow and to lower blood pressure.

Methyldopa Uses:

• Methyldopa is useful in the treatment of hypertension.

α and ẞ-adrenergic Receptor Agonists

Dobutamine:

• Dobutamine is chemically 3,4-dihydroxy-N-[3-(4-hydroxyphenyl)-1-methylpropyl]-b- phenylethylamine.
• Dobutamine is a synthetic catecholamine derivative.
• Dobutamine is given by intravenous infusion since it is not effective orally.
• Dobutamine gets metabolized by COMT and conjugation, but not by MAO.
• Dobutamine resembles dopamine chemically but possesses a bulky aromatic residue on the amino group i.e. 1-(methyl)-3-(4-hydroxyphenyl) propyl and the absence of a B-OH group.
• Dobutamine is a selective ẞ1 receptor agonist and has only slight indirect actions.
• Dobutamine increases cardiac output without any effect on heart rate and blood pressure.
• Dobutamine is a racemic mixture of two enantiomeric forms. The (+) isomer has potent B-agonistic actions. The (-) isomer has potent agonistic and poor ẞ-agonistic actions.
• Racemic dobutamine increases the inotropic activity of the heart to a much greater extent than chronotropic activity.
• Dobutamine does not act as an agonist at the dopaminergic receptors that mediate renal vasodilation.

Dobutamine Uses:

• Dobutamine is used in patients with heart failure associated with myocardial infarction, open heart surgery and cardiomyopathy.

B-adrenergic Receptor Agonists

Isoproterenol:

• Isoproterenol is chemically, 4-[1-hydroxy-2-[isopropylaminoethyl]-1,2-benzenediol. It acts on both ẞ1 and B2 receptors.
• As it contains an isopropyl substitution on the nitrogen atom, it has virtually no effect on a-receptors.
• Isoproterenol increases cardiac output by stimulating cardiac ẞ1 receptors and can bring about bronchodilation through stimulation of B2 receptors in the respiratory tract.
• Isoproterenol is available for use by inhalation and injection.
• Isoproterenol is metabolized by COMT.

Isoproterenol Uses:

• Isoproterenol is clinically used for the relief of bronchospasms associated with bronchial asthma.
• It produces adverse effect as cardiac stimulation which leads to use it sometimes in the treatment of heart block.

Selective ẞ2-adrenergic Receptor Agonists:

• Modification of the catechol portion of a ẞ-agonist (isoproterenol) has resulted in the development of selective ẞ2 receptor agonists.
• Further the shift of the OH group produces resistance to MAO and prolongs the duration of action.
• Increased ẞ2 agonist activity is also conferred by the substitution of increasing bulky lipophilic groups on the amino group of isoproterenol.
• Because of their relative selectivity, these agents relax the smooth muscle of the bronchi, uterus and blood vessels.
• They have far less action on the heart than Isoproterenol and other agents.

Metaproterenol:

• Metaproterenol is chemically,1-(3,5-dihydroxy phenyl)-2-isopropylaminoethanol.
• Metaproterenol is a photosensitive compound and, hence should be protected from light and air.
• Metaproterenol is less ẞ2 selective than terbutaline.
• Metaproterenol is not metabolized by either COMT or MAO, therefore more effective when given orally, and have a longer duration of action.

Terbutaline:

• Terbutaline is chemically, N-tert-butyl-N-[2-(3,5-dihydroxyphenyl)-2-hydroxy-methyl]amine or 1-(3,5-dihydroxyphenyl)-2-(tert-butyl amino) ethanol.
• It is a non-catecholamine, therefore is resistant to COMT.

Salbutamol (Albuterol):

• Salbutamol is chemically, 1-(4-hydroxy-3-hydroxymethylphenyl)-2-(tert-butylamino) ethanol.
• It is a selective ẞ2 receptor agonist whose selectivity results from the replacement of the meta-hydroxyl group of the catechol ring with a hydroxyl methyl moiety.
• It is not metabolized by either COMT or MAO.
• It is active orally and they exhibit a longer duration of action.

Salbutamol Uses:

• Metaproterenol, Terbutaline, and Salbutamol (albuterol) possess strong ẞ2 agonistic properties.
• They are used in the treatment of bronchial asthma.

Bitolterol:

• Bitolterol is a prodrug of the ẞ2 selective adrenergic agonist colterol.
• It is chemically, [4-[2-(tert-butylamino)-1-hydroxyethyl]-2-(4-methylbenzoyl)-oxyphenyl] 4-methylbenzoate.
• The presence of the two p-toluic acid esters in bitolterol makes it considerably more lipophilic than colterol.
• It is. hydrolyzed by esterases in the lung and other tissues to produce the active agent, colterol.
• It has a longer duration of action and is metabolized by COMT and conjugation after hydrolysis of the esters.

Bitolterol Uses:

• Bitolterol is administered by inhalation for bronchial asthma and reversible bronchospasm.

Selective ẞ3 adrenergic receptor agonists

Selective ẞ3 receptor agonists have been developed, but not approved for therapeutic use. B3 receptor stimulation promotes lipolysis. These agents act as anti-obesity drugs and are used for the treatment of non-insulin-dependent diabetes.

Indirect Acting Sympathomimetics

Indirect-acting sympathomimetics act by releasing endogenous NE. They enter the nerve endings by way of the active uptake process and displace NE from its storage granules. This class is comprised of non-catecholamines.

These compounds are resistant to COMT and MAO enzymes due to the lack of phenolic hydroxyl groups and the presence of a-methyl groups. These compounds pass more readily through blood blood-brain barrier because of increased lipophilicity.

Structure-Activity Relationship:

• Most of these drugs retain phenyl ethylamine skeleton.
• The presence of a B-OH group decreases, and an α-CH3 group increases the effectiveness of indirect-acting agents.
• The presence of nitrogen substituents decreases indirect activity,
• Substituent larger than the methyl group at nitrogen renders the compound virtually inactive.
• Tertiary amino group substitution is ineffective as an NE-releasing agent.

Amphetamine:

• Amphetamine is chemically, 1-phenylpropan-2-amine.
• It is one of the most potent sympathomimetic.
• It is an indirect-acting sympathomimetic amine and its action depends on the release of norepinephrine from adrenergic nerves.
• CNS stimulant effects are thought to be due to stimulation of the cortex.
• The d-isomer of amphetamine is 3-4 times more potent than the I-isomer.

Amphetamine Uses:

• Amphetamine causes increased wakefulness, elevated mood, increased initiative, self- confidence and ability to concentrate.
• It has an anorexic action and can be used in the treatment of obesity.

Hydroxyamphetamine:

• Hydroxyamphetamine is chemically, 4-(2-aminopropyl)phenol or 1-(4-hydroxy- phenyl)-propan-2-amine.
• It possesses a-receptor stimulant activity, but lacks CNS activity.
• It is a powerful vasoconstrictor.

Hydroxyamphetamine Uses:

• Hydroxyamphetamine is used as an anorexiant in the treatment of obesity.
• It is used to dilate the pupil for diagnostic eye examinations and for surgical procedures on the eye.
• It is used sometimes with cholinergic blocking drugs like atropine to produce a mydriatic effect.
• It is used in hyperkinetic syndrome in children.
• It is used in narcolepsy (sudden attack of sleep in completely inappropriate situations).

Pseudoephedrine:

• L-(+)-Pseudoephedrine is chemically, (15,25)-2-(methylamino)-1-phenylpropan-1-ol. It is the (S,S) diastereoisomer of ephedrine in which B-OH group is having S-configuration.
• It is a naturally occurring alkaloid from the Ephedra species.
• Ephedrine has a mixed mechanism of action whereas pseudoephedrine acts predominantly by an indirect mechanism.

Pseudoephedrine Uses:

• Pseudoephedrine is used as a nasal decongestant and in the treatment of cold.
• Pseudoephedrine can also be used in the treatment of hypertension.

Propylhexedrine:

• Propylhexedrine is an analogue of amphetamine in which the aromatic ring has been replaced with a cyclohexane ring.
• It is chemically, 1-cyclohexyl-2-methylaminopropane.

Propylhexedrine Uses:

• Propylhexedrine is used as nasal decongestant.
• It has local vasoconstrictor effect on nasal mucosa in the symptomatic relief of nasal congestion caused by the common cold, allergic rhinitis or sinusitis.

Sympathomimetics With Mixed Action

These drugs act both, directly with the receptor sites and partly by the release of endogeneous norepinephrine. These agents have no -OH on the aromatic ring, but do have 6-OH group.

Ephedrine:

• Ephedrine is chemically, (1R, 25)-2-(methylamino)-1-phenylpropan-1-ol.
• It occurs naturally in many plants, being the principal alkaloid obtained from various species of Ephedra.
• Ma Huang, the plant containing ephedrine, has been used in China for over 2000 years.
• It contains two asymmetric carbon atoms, thus there are four optically active forms. The erythro racemate is called as ephedrine, whereas threo racemate is known as pseudoephedrine.
• Among four compounds available, D(-) isomer is clinically most active.
• It has agonist activity at both a. and B-receptors.
• Ephedrine decomposes gradually and darkens when exposed to light.
• It is not metabolized by either MAO or COMT.

Ephedrine Uses:

• The pharmacological activity of ephedrine resembles with epinephrine. Ephedrine differs from adrenaline mainly by its
• • effectiveness after oral administration,
  • longer duration of action,
  • more pronounced central actions,
  • much lower potency.
• It produces a sharp rise in systolic, diastolic and pulse pressures, with a reflex bradycardia, similar to adrenaline, but lasting for 10 times as long.
• Ephedrine is mainly used as a bronchodilator in asthma.
• It is used to treat narcolepsy and depressive state.
• It is also used as nasal decongestant, mydriatic and in certain allergic disorders. • It is also used to treat hay fever and urticaria.

Phenylpropanolamine:

• Phenylpropanolamine is similar in structure to ephedrine except that, it possesses primary amine instead of a secondary amine.
• It is chemically, (1S 2R)-2-amino-1-phenyl propan-1-ol.
• This structural difference makes Phenylpropanolamine to slightly higher vassopressive and lower central stimulatory action than ephedrine.

Phenylpropanolamine Uses:

• Phenylpropanolamine is used as nasal decongestant with prolonged action than that of ephedrine.
• Earlier it was used as appetite suppressant and also used in the treatment of cough and cold. But because of increased risk of hemorrhagic stroke in young women, FDA recommended its removal from such medication.

Metaraminol:

• Metaraminol is chemically, 3-[(1R,25)-2-amino-1-hydroxypropyl]phenol.
• It is an isomer of phenylephrine.
• It possesses a mixed mechanism of action.
• It’s direct-acting effects mainly on a-adrenergic receptor.

Metaraminol Uses:

• Metaraminol is used for its vasopressor action for maintaining blood pressure during spinal anesthesia and hemorrhage.
• It has also been used to treat severe hypotension brought on by other traumas that induce shock.

Adrenergic Receptor Antagonists

Adrenergic receptor antagonists are also called as adrenergic blocking agents or adrenergic receptor blockers or sympatholytic agents.

α-Adrenergic Receptor Antagonists

Drugs that attach to a-adrenergic receptors and block the effect of norepinephrine and epinephrine are categorized as a- adrenergic antagonists.

These drugs consist of a number of compounds of diverse chemical structure that bear little obvious resemblance to the a-adrenergic receptor agonist. a-adrenergic receptor antagonists may be non-selective or selective for α1 receptor.

Non-selective a-adrenergic receptor antagonists block both a1 and α2 adrenergic receptors resulting in reducing blood pressure as a major cardiovascular effect.

Generally, a-adrenergic receptor antagonists are useful in the management of hypertension, prostatic hypertrophy (prostate enlargement) and heart failure.

Non-selective a-Receptor Antagonists:

Reversible a-Receptor Antagonists:

Example: Tolazoline and Phentolamine

• Tolazoline and phentolamine are imidazoline a-antagonists that also have antihypertensive activity.
• They have structural similarities to the imidazoline α-agonists, such as naphazoline and xylometazoline, but does not have the lipophilic substituents required for agonist activity.
• They are potent non-selective competitive (reversible) a-adrenergic receptor antagonists.
• They have both α1- and a2- antagonistic activity and produce tachycardia.
• Tolazoline is chemically, 2-benzyl-4,5-dihydro-1H-imidazole.
• Tolazoline is relatively weak a-antagonist, its histamine like and acetylcholine like action contributes to its vasodilatory activity.
• Tolazoline stimulates stimulation of gastric acid secretion, render its use in patients who have gastric or peptic ulcer.
• Phentolamine is chemically, 3-[N-(4,5-dihydro-1H-imidazol-2-ylmethyl)-4- methylanilino] phenol.
• Phentolamine is the more effective a-antagonist. The vasodilatory effect of phentolamine is used in the management of hypertension.

Reversible a-Receptor Antagonists Uses:

• Tolazoline is a vasodilator that is used to treat spasms of peripheral blood vessels.
• Tolazoline is used in treatment of persistent pulmonary hypertension of the newborn. Tolazoline is used to treat Raynaud’s syndrome and other conditions involving peripheral vasospasm.
• Tolazoline is also used to treat thrombophlebitis.
• Phentolamine is used to prevent or control hypertensive episodes that occur in patients with pheochromocytoma.
• Phentolamine can be used as an aid in the diagnosis of pheochromocytoma (Tumor of the adrenal medulla).
• Phentolamine is used in combination with papaverine to treat impotence.

Irreversible a-Receptor Antagonists:

Example: Phenoxybenzamine

• Phenoxybenzamine is chemically, N-benzyl-N-(2-chloroethyl)-1-phenoxypropan-2-amine.
• It is non-competitive, non-selective and irreversible a-adrenergic receptor antagonist. It is a B-haloalkylamine that alkylates a-receptors.
• It is non-selectively and irreversibly blocks the postsynaptic a-adrenergic receptor in smooth muscle, leading to a muscle relaxation and a widening of the blood vessels.
• It has small onset of action, but long duration of action which may last for 3-4 days.
• As phenoxybenzamine forms covalent bonds with the receptors is irreversible, new receptors must be synthesized before the effects can be overcome, therefore, the a-receptor blocking action is long-lasting.

Irreversible a-Receptor Antagonists Uses:

• Phenoxybenzamine is used to treat hypertension and as a peripheral vasodilator.
• It is used in alleviating the sympathetic effects of pheochromocytoma.

Selective α-receptor Antagonists:

Example: Prazosin, Terazosin, Doxazosin

• These agents contain quinazoline ring, the piperazine ring, and the acyl moiety.
• The 4-amino group on the quinazoline ring is very important for α-rcceptor affinity. Replacement of piperazine ring with other heterocyclic ring like piperidine does not lose the activity.
• The pharmacokinetic activity of these agents is significantly affected by nature of acyl group.
• These drugs produce peripheral vasodilation without an increase in heart rate or cardiac output.
• Prazosin is chemically, [4-(4-amino-6,7-dimethoxyquinazolin-2-yl)piperazin-1-yl]- (furan-2-yl)methanone.
• Terazosin is chemically, [4-(4-amino-6,7-dimethoxyquinazolin-2-yl)piperazin-1-yl]- (oxolan-2-yl)methanone.
• Doxazosin is chemically, [4-(4-amino-6,7-dimethoxyquinazolin-2-yl)piperazin-1-yl]- (2,3-dihydro-1,4-benzodioxin-3-yl)methanone.
• They are selective adrenergic α- antagonist.

Selective α-receptor Antagonists Uses:

• Prazosin is used in the treatment of heart failure; hypertension; pheochromocytoma; Raynaud’s syndrome; prostatic hypertrophy and urinary retention.
• Terazosin is used for treatment of symptoms of an enlarged prostate. It also acts to lower the blood pressure, and is therefore a drug of choice for men with hypertension and prostate enlargement.
• Doxazosin is used to treat high blood pressure and urinary retention associated with prostate enlargement.

Selective α2-receptor Antagonists:

Example: Yohimbine

• Yohimbine, an indole alkaloid is isolated from Pausinystalia yohimbe bark and Rauwolfia roots.
• It is an α2-antagonist with greater selectivity for a2- than for α1-adrenoceptors.
• It is also used as a serotonin antagonist.
• It has actions both in the CNS and in the periphery, inducing hypertension and increase in heart rate.

Selective α2-receptor Antagonists Uses:

• Yohimbine is used to treat male erectile impotence and postural hypotension.
• It increases heart rate and blood pressure as a result of its blockade of a2-receptors in the CNS.
• It is also used in research to induce anxiety.

Other α-receptor antagonists:

Dihydroergotamine:

• Dihydroergotamine is an ergot derivative with agonistic activity for alpha- adrenergic, serotonergic, and dopaminergic receptors.
• It acts by stimulating the 5-HT1D receptor subtype and causes constriction of cerebral blood vessels.
• It is administered as a nasal spray or injection. Nausea is a common side effect.

Dihydroergotamine Uses:

• Dihydroergotamine is used to treat migraine headaches and cluster headaches.

Methysergide:

• Methysergide is an ergot derivative.
• It is chemically, (6aR,9R)-N-[(2S)-1-hydroxybutan-2-yl]-4,7-dimethyl-6,6a,8,9- tetrahydro-indolo[4,3-fg]quinoline-9-carboxamide.
• It antagonizes the effects of serotonin in blood vessels and gastrointestinal smooth muscle.

Methysergide Uses:

• Methysergide is used prophylactically in migraine and other vascular headaches.

B-Adrenergic Receptor Antagonists

B-adrenergic receptor antagonists are drugs that attach to B-adrenergic receptors and block the effect of agonists (e.g. Norepinephrine and epinephrine) at those B-receptor sites. B-antagonists are classified as non-selective and cardioselective.

Non-selective ẞ- antagonist blocks both B1 and B2-receptors; whereas cardio-selective ẞ-antagonist preferentially blocks B1-receptors and decreases heart rate and myocardial contraction. ẞ2-receptor antagonistic effects include inhibition of vascular dilation and inhibition of bronchodilation.

B-adrenergic receptor antagonists are useful in the management of hypertension, arrhythmia, ischemic heart disease, chronic open-angle glaucoma, migraine, and thyrotoxicosis. It is also useful in the management of chronic but not acute heart failure.

The common side effects of B-adrenergic receptor antagonists are bradycardia, bronchospasm, fatigue, sleep disturbance, impotence in men, and attenuated responses to hypoglycemia. Non-selective ẞ-antagonists are generally avoided in patients with asthma, as it may lead to progressive heart block, bronchoconstriction, and heart failure.

Sudden discontinuation of ẞ-antagonist may result in “rebound” increases in heart rate and blood pressure, which may lead to myocardial ischemia, infarction, or sudden death in susceptible individuals, therefore for termination of ẞ-antagonist therapy gradual reduction of the dose is recommended.

Structure-Activity Relationships:

• Structural modification by replacing the aromatic hydroxyl groups of Isoproterenol by chlorines leads to the formation of dichloro isoproterenol, which is a ẞ-antagonist that blocks the effects of sympathomimetic amines on bronchodilation, uterine relaxation, and heart stimulation.

• Dichloroisoproterenol is a partial agonist, it has no clinical utility, therefore it is not used as a clinically useful drug.
• Pronethalol, 3,4-dichloro substitutes with a carbon bridge to form a naphthyl-ethanolamine derivative which is a clinically useful drug. It has less intrinsic sympathomimetic activity than dichloro isoproterenol, but it was withdrawn from clinical testing because of tumor induction in animal tests.

• Introduction of an oxymethylene bridge, (-OCH2-) between the aromatic ring and the ethylamino side chain and changing the side chain from C2 to C position of the naphthyl group of pronethalol (arylethanolamines) increases the B-blocker activity, i.e. propranolol (an aryloxypropanolamine).

• Oxymethylene bridge (-OCH2-) is responsible for the antagonistic properties of the molecules which depend on the nature of the aromatic ring and its substituents.
• Lengthening the side chain would prevent the appropriate binding of the required functional groups to the same receptor site.
• The nature of the aromatic ring is also a determinant in the ẞ1-selectivity of the antagonists.
• Substitution of sufficient size on the para and absence at the meta position of an aromatic ring is a common structural feature of cardio-selective antagonist (B) i.e., practolol.

• Bulky aliphatic groups, such as the tert-butyl and isopropyl groups are essential for B-receptor antagonist activity.
• Secondary amine is essential for optimal activity. N, N-disubstituted compounds are inactive.
• B-blocking agent exhibits its effect based on stereoselectivity.
• The -OH-bearing C-atom possesses S-configuration for optimal activity to ẞ-receptor. The enantiomer with the R-configuration is typically 100 times less potent.
• Although most of the B-antagonistic activity resides in one enantiomer, propranolol, and most other B-blockers are used clinically as racemic mixtures. The only exceptions are levobunolol, timolol, and penbutolol with which the S-enantiomer is used.

Non-Selective ẞ-Blockers:

Example: Propranolol

• Propranolol is chemically, 1-naphthalen-1-yloxy-3-(propan-2-ylamino)propan-2-ol.
• Non-Selective ẞ-Blockers is a non-selective ẞ-adrenergic receptor antagonist, which blocks both B1 and B2 receptors.
• Non-Selective ẞ-Blockers act by blocking the B-receptor in the heart. It slows down the heart rate, reduces the force of contraction, and reduces cardiac output.
• Its antihypertensive action is due to its ability to reduce cardiac output and suppress renin release from the kidney.
• Non-selective ẞ-Blockers is contraindicated in the presence of conditions such as asthma and bronchitis.
• Its receptor blocking action can be reversed with a sufficient quantity of B-agonist.
• Non-selective ẞ-Blockers is well absorbed after oral administration, but undergo extensive first-pass metabolism before it reach the systemic circulation.

Non-Selective ẞ-Blockers Uses:

• Propranolol is used in the treatment of hypertension, cardiac arrhythmias, angina pectoris, myocardial infarction, hypertrophic cardiomyopathy, pheochromocytoma, and migraine prophylaxis.
• Non-Selective ẞ-Blockers have high lipophilicity and the ability to penetrate the CNS and found to be useful in treating diseases of the CNS, such as anxiety.
• Non-Selective ẞ-Blockers is under investigation for the treatment of schizophrenia, alcohol withdrawal syndrome, and aggressive behavior.

Other Non-selective ẞ-blockers:

Several other nonselective ẞ-blockers, such as pindolol, nadolol, penbutolol, carteolol, timolol, levobunolol, sotalol, and metipranolol are found to be clinically useful agents.

These drugs are chemically,

• Pindolol: 1-(1H-indol-4-yloxy)-3-(propan-2-ylamino)propan-2-ol.
• Nadolol: (2R,3S)-5-(3-(tert-butylamino)-2-hydroxypropoxy]-1,2,3,4-tetrahydro naphthalene-2,3-diol.
• Penbutolol: (25)-1-(tert-butylamino)-3-(2-cyclopentylphenoxy)propan-2-ol.
• Carteolol: 5-[3-(tert-butylamino)-2-hydroxypropoxy]-3,4-dihydro-1H-quinolin-2-one.
• Timolol: (25)-1-(tert-butylamino)-3-[(4-morpholin-4-yl-1,2,5-thiadiazol-3-yl)oxy] propan-2-ol.
• Levobunolol: 5-[(25)-3-(tert-butylamino)-2-hydroxypropoxy]-3,4-dihydro-2H-naphthalen-1-one.
• Sotalol: N-[4-[1-hydroxy-2-(propan-2-ylamino)ethyl]phenyl]methanesulfonamide.
• Metipranolol: [4-[2-hydroxy-3-(propan-2-ylamino)propoxy]-2,3,6-trimethylphenyl] acetate.

Other Non-selective ẞ-blockers Uses:

• Pindolol, nadolol, penbutolol, carteolol and timolol are used to treat hypertension.
• Nadolol is found to be useful in the long-term management of angina pectoris.
• Timolol is used in the prophylaxis of migraine headaches and in myocardial infarction.
• In addition to its ẞ-adrenergic blocking activity, sotalol also blocks the inward K* current which delays cardiac repolarization and leads to be a useful agent in the treatment of ventricular arrhythmias and atrial fibrillation.
• Carteolol, timolol, levobunolol and metipranolol are used topically to treat open-angle glaucoma. This agent lowers intraocular pressure by decreasing the amount of aqueous humour fluid produced in the eye by the ciliary body.

B1-Selective Blockers:

Cardioselective ẞ-antagonist drugs have a greater affinity for the ẞ1-receptors of the heart than for B2-receptors in other tissues. These agents are useful in the treatment of cardiovascular diseases, such as hypertension. These drugs do not possess ẞ2-receptor antagonistic properties and, therefore can be used safely in patients who have bronchitis or bronchial asthma.

Because of the absence of vascular ẞ2-receptor antagonistic activity, these agents reduce the increase in peripheral resistance that sometimes occurs after the administration of non-selective ẞ-antagonists. These drugs show its ẞi-receptors blocking cardio selectivity at relatively low doses only. At normal therapeutic doses, most of the selective activity is lost.

These drugs are chemically,

• Acebutolol: N-[3-acetyl-4-[2-hydroxy-3-(propan-2-ylamino) propoxy]phenyl]butanamide. Atenolol: 2-[4-[2-hydroxy-3-(propan-2-ylamino)propoxy]phenyl]acetamide.
• Betaxolol: 1-[4-[2-(cyclopropylmethoxy)ethyl]phenoxy]-3-(propan-2-ylamino) propan-2-ol.
• Bisoprolol: 1-(propan-2-ylamino)-3-[4-(2-propan-2-yloxyethoxymethyl)phenoxy] propan-2-ol. Esmolol: Methyl 3-[4-[2-hydroxy-3-(propan-2-ylamino)propoxy]phenyl]propanoate.
• Metoprolol: 1-[4-(2-methoxyethyl)phenoxy]-3-(propan-2-ylamino)propan-2-ol.

B1-Selective Blockers Uses:

• All drugs except esmolol are used in the treatment of hypertension. Acebutolol and esmolol are used for treating certain cardiac arrhythmias.
• Atenolol and metoprolol are useful in treating angina pectoris and in myocardial infarction. Atenolol has low lipid solubility and does not readily cross the blood brain barrier.
• Betaxolol is used in the treatment of glaucoma.
• Esmolol possesses a rapid onset of action and a very short duration of action. Its effects disappear within 20 to 30 minutes after the infusion is discontinued. Therefore it is used in controlling heart rate during surgery after operation.

Mixed α / ẞ-Adrenergic Receptor Antagonists

These drugs possess both ẞ- and a-receptor blocking within the same molecule. Labetalol and carvedilol are anti-hypertensives with α1, B1 and B2-blocking activity.

Labetalol:

• Labetalol is chemically, 2-hydroxy-5-(1-hydroxy-2-(4-phenylbutan-2-ylamino)ethyl] benzamide.
• It is a phenyl ethanolamine derivative that is a competitive inhibitor of both ẞ1 and B2-adrenergic receptors as well as adrenergic receptors.
• It is a more potent ẞ-antagonist than a-antagonist.
• It has two asymmetric carbon atoms, therefore it exists as a mixture of four isomers. The different isomers possess different a and ẞ antagonistic activities. The ẞ-blocking activity is seen only in the (1R, 1’R) isomer, whereas α-antagonistic activity is seen in the (15, 1’R) isomer.
• It is very well absorbed and it undergoes extensive first-pass metabolism.

Labetalol Uses:

• Labetalol is a clinically useful as an antihypertensive drug.
• Due to its a-receptor blocking effect, it produces vasodilation and because of the B-receptor blocking effect, it prevents the reflex tachycardia usually associated with vasodilation.

Carvedilol:

• Carvedilol is chemically, 1-(9H-carbazol-4-yloxy)-3-[2-(2-methoxyphenoxy)-ethyl- amino] propan-2-ol.
• It is a B-blocker that also possesses α- adrenergic receptor blocking activity.
• Its only S-enantiomer possesses the ẞ-blocking activity, whereas both enantiomers possess α1-receptor antagonistic activity.
• It also possesses antioxidant activity and an anti-proliferative effect on vascular smooth muscle cells.

Carvedilol Uses:

• It is used in the treatment of hypertension and congestive cardiac failure.
• It has neuro-protective effect and the ability to provide major cardiovascular protection.

Synthesis

Phenylephrine

Salbutamol

Tolazoline

Propranolol

Multiple Choice Questions

Question 1. Sympathetic stimulation is mediated by:

1. release of norepinephrine from nerve terminals
2. activation of adrenoreceptors on postsynaptic sites
3. release of epinephrine from the adrenal medulla
4. all of the above

Answer. 4. all of the above

Question 2. Clonidine hydrochloride IP is:

1. monoamine oxidase inhibitor which contains an imidazoline ring system
2. arterial and venous vasodilator which contains an imidazoline ring system
3. monoamine oxidase inhibitor which contains a pyrimidine ring system
4. monoamine oxidase inhibitor which contains a phthalazine ring system

Answer. 2. arterial and venous vasodilator which contains an imidazoline ring system

Question 3. DOPA is an important natural amino acid and a precursor in the biosynthesis of

1. Dopamine
2. Adrenaline
3. Nor-adrenaline
4. Methyldopa

Answer. 3. Nor-adrenaline

Question 4. Characteristics of epinephrine include all of the following EXCEPT:

1. It is synthesized into the adrenal medulla.
2. It is mainly synthesized into the nerve ending.
3. It is transported in the blood to target tissues.
4. It directly interacts with and activates adrenoreceptors.

Answer. 2. It is mainly synthesized into the nerve ending.

Question 5. Chemically a-methyldopa is:

1. 2-methyl-3-hydroxy-2-tyrosine
2. 3-methyl-2-hydroxy-2-tyrosine
3. 6-methyl-4-hydroxy-2-tyrosine
4. 3-hydroxy-a-methyl-2-tyrosine

Answer. 4. 3-hydroxy-a-methyl-2-tyrosine

Question 6. Which of the following sympathomimetics acts indirectly?

1. Epinephrine
2. Norepinephrine
3. Ephedrine
4. Methoxamine

Answer. 3. Ephedrine

Question 7. The raw material for the synthesis of Propranolol is

1. a-Naphthylamine
2. B-Naphthol
3. α-Naphthol
4. a-Naphtholdehyde

Answer. 3. α-Naphthol

Question 8. Indirect action includes all of the following properties EXCEPT:

1. Displacement of stored catecholamines from the adrenergic nerve ending.
2. Inhibition of reuptake of catecholamines already released.
3. Interaction with adrenoreceptors.
4. Inhibition of the metabolism of endogenous catecholamines from peripheral adrenergic neurons.

Answer. 3. Interaction with adrenoreceptors.

Question 9. Tyrosine is a p-hydroxy derivative of phenylalanine. It loses carbon dioxide on heating with formation of:

1. Dopamine
2. 2-p-Hydroxypropionic acid
3. 2-p-Hydroxy benzoic acid
4. 2-p-Hydroxyphenylethylamine

Answer. 4. 2-p-Hydroxyphenylethylamine

Question 10. Catecholamine includes the following EXCEPT:

1. Ephedrine
2. Epinephrine
3. Isoproterenol
4. Norepinephrine

Answer. 1. Ephedrine

Question 11. Identify the biologically active isomer of adrenaline:

1. R (+) enatiomer
2. R (-) enatiomer
3. S (+) enatiomer
4. S (-) enatiomer

Answer. 2. R (-) enatiomer

Question 12. Epinephrine decreases intracellular cAMP levels by acting on:

1. a receptor
2. a2 receptor
3. B1 receptor
4. B2 receptor

Answer. 2. a2 receptor

Question 13. IUPAC name of terbutaline is:

1. 5-[2-[(1,1-dimethyl ethyl)amino]-1-hydroxy ethyl]-1,3-benzenediol
2. 5-[2-[(2,2-dimethyl ethyl)amino]-1-hydroxy ethyl]-1,3-benzenediol
3. 5-[3-[(1,1-dimethyl ethyl)amino]-1-hydroxy ethyl]-1,3-benzenediol
4. None of above

Answer. 1. 5-[2-[(1,1-dimethyl ethyl)amino]-1-hydroxy ethyl]-1,3-benzenediol

Question 14. Direct effects on the heart are determined largely by:

1. α1 receptor
2. a2 receptor
3. B1 receptor
4. B2 receptor

Answer. 3. B1 receptor

Question 15. Which isomer of the Propranolol is more active?

1. Dextro isomer
2. Levo isomer
3. Meso isomer
4. Racemic isomer

Answer. 1. Dextro isomer

Question 16. Which of the following effects is related to direct ẞ1-adrenoreceptor stimulation?

1. Bronchodilation
2. Vasodilation
3. Tachycardia
4. Bradycardia

Answer. 3. Tachycardia

Question 17. The aromatic nucleus present in Propranolol is

1. Benzene
2. Naphthalene
3. Anthracene
4. Pyridine

Answer. 2. Naphthalene

Question 18. The most appropriate starting material for propranolol is

1. a-naphthol, epichlorohydrin, and isopropyl amine
2. B-naphthol, epichlorohydrin, and isopropyl amine
3. a-naphthol, epichlorhydrine and butyl amine
4. a-naphthol, epichlorohydrin and dimethyl amine

Answer. 1. a-naphthol, epichlorohydrin and isopropyl amine

Question 19. Distribution of a-adrenoreceptor subtypes is associated with all of the following tissues except those of:

1. Heart
2. Blood vessels
3. Prostate
4. Pupillary dilator muscle

Answer. 1. Heart

Question 20. B-adrenoreceptor subtypes are contained in all of the following tissues EXCEPT:

1. Bronchial muscles
2. Heart
3. Pupillary dilator muscle
4. Fat cells

Answer. 3. Pupillary dilator muscle

Question 21. In which of the following tissues both a and ẞ1 adrenergic stimulation produce the same effect?

1. Blood vessels
2. Intestine
3. Uterus
4. Bronchial muscles

Answer. 1. Blood vessels

Question 22. The effects of sympathomimetics on blood pressure are associated with their effects on:

1. The heart rate
2. The peripheral resistance
3. The cardiac output
4. All of the above

Answer. 4. All of the above

Question 23. A relatively pure a-agonist causes all of the following effects EXCEPT:

1. Increase in peripheral arterial resistance
2. Increase in venous return
3. Has no effect on blood vessels
4. Reflex bradycardia

Answer. 3. Has no effect on blood vessels

Question 24. A non-selective ẞ-receptor agonist causes all of the following effects EXCEPT:

1. Increase in cardiac output
2. Increase in peripheral arterial resistance
3. Decrease in peripheral arterial resistance
4. Decrease in mean pressure

Answer. 2. Increase in peripheral arterial resistance

Question 25. Which of the following statements is not correct?

1. a-agonists cause miosis.
2. a-agonists cause mydriasis.
3. B-antagonists decrease the production of aqueous humor.
4. a-agonists increase the outflow of aqueous humor from the eye.

Answer. 1. a-agonists cause miosis.

Question 26. A bronchial smooth muscle contains:

1. a1-receptor
2. α2-receptor
3. B1-receptor
4. B2-receptor

Answer. 4. B2-receptor

Question 27. All of the following agents are ẞ-receptor agonists EXCEPT:

1. Epinephrine
2. Isoproterenol
3. oxymetazoline
4. Dobutamine

Answer. 3. oxymetazoline

Question 28. Which of the following drugs causes bronchodilation without significant cardiac stimulation?

1. Isoproterenol
2. Terbutaline
3. Xylometazoline
4. Clonidine

Answer. 2. Terbutaline

Question 29. B1-receptor stimulation includes all of the following effects EXCEPT:

1. Increase in heart contractility
2. Bronchodilation
3. Tachycardia
4. Increase in conduction velocity in the atrioventricular node

Answer. 2. Bronchodilation

Question 30. B2-receptor stimulation includes all of the following effects EXCEPT:

1. Stimulation of renin secretion
2. Fall of potassium concentration in plasma
3. Relaxation of bladder, uterus
4. Tachycardia

Answer. 1. Stimulation of renin secretion

Question 31. Hyperglycemia induced by epinephrine can be due to:

1. Gluconeogenesis (B2)
2. Inhibition of insulin secretion (α2)
3. Stimulation of glycogenolysis (B2)
4. All of the above

Answer. 4. All of the above

Question 32. Which of the following effects is associated with ẞ3-receptor stimulation?

1. Lipolysis
2. Decrease in platelet aggregation
3. Bronchodilation
4. Tachycardia

Answer. 1. Lipolysis

Question 33. Which of the following statements is not correct?

1. Epinephrine acts on both a- and ẞ-receptors.
2. Norepinephrine has a predominantly ẞ-action.
3. Phenylephrine has a predominantly a-action.
4. Isoproterenol has a predominantly ẞ-action.

Answer. 2. Norepinephrine has a predominantly ẞ-action.

Question 34. Indicate the drug, which is a direct-acting both a- and B-receptor agonist:

1. Norepinephrine
2. Methoxamine
3. Isoproterenol
4. Ephedrine

Answer. 1. Norepinephrine

Question 35. Which of the following agents is an a2-selective agonist?

1. Norepinephrine
2. Clonidine
3. Ritodrine
4. Ephedrine

Answer. 2. Clonidine

Question 36. Which of the following agents is a non-selective ẞ-receptor agonist?

1. Norepinephrine
2. Terbutaline
3. Isoproterenol
4. Dobutamine

Answer. 3. Isoproterenol

Question 37. Indicate the ẞ1-selective agonist:

1. Isoproterenol
2. Dobutamine
3. Metaproterenol
4. Epinephrine

Answer. 2. Dobutamine

Question 38. Which of the following sympathomimetics is a ẞ2-selective agonist?

1. Terbutaline
2. Xylometazoline
3. Isoproterenol
4. Dobutamine

Answer. 1. Terbutaline

Question 39. Indicate the indirect-acting sympathomimetic agent:

1. Epinephrine
2. Phenylephrine
3. Ephedrine
4. Isoproterenol

Answer. 3. Ephedrine

Question 40. Epinephrine produces all of the following effects EXCEPT:

1. Decrease in oxygen consumption
2. Bronchodilation
3. Hyperglycemia
4. Mydriasis

Answer. 1. Decrease in oxygen consumption

Question 41. Epinephrine is used in the treatment of all of the following disorders EXCEPT:

1. Bronchospasm
2. Anaphylactic shock
3. Cardiac arrhythmias
4. Open-angle glaucoma

Answer. 4. Open-angle glaucoma

Question 42. Which of the following direct-acting drugs is a relatively pure a-agonist, an effective mydriatic and decongestant and can be used to raise blood pressure?

1. Epinephrine
2. Norepinephrine
3. Phenylephrine
4. Ephedrine

Answer. 3. Phenylephrine

Question 43. Isoproterenol is:

1. Both an a- and B-receptor agonist
2. B1-selective agonist
3. B2-selective agonist
4. Non-selective ẞ-receptor agonist

Answer. 4. Non-selective ẞ-receptor agonist

Question 44. Ephedrine causes:

1. Miosis
2. Bronchodilation
3. Hypotension
4. Bradycardia

Answer. 2. Bronchodilation

Question 45. Which of the following sympathomimetics is preferable for the emergency therapy of cardiogenic shock?

1. Ephedrine
2. Dobutamine
3. Isoproterenol
4. Methoxamine

Answer. 2. Dobutamine

Question 46. Which of the following sympathomimetics is related to short-acting topical decongestant agents?

1. Albuterol
2. Phenylephrine
3. Terbutaline
4. Norepinephrine

Answer. 3. Terbutaline

Question 47. Indicate the long-acting topical decongestant agent:

1. Epinephrine
2. Norepinephrine
3. Isoproterenol
4. Xylometazoline

Answer. 4. Xylometazoline

Question 48. Which of the following topical decongestant agents is an a1-selective agonist?

1. Phenylephrine
2. Clonidine
3. Ephedrine
4. Epinephrine

Answer. 1. Phenylephrine

Question 49. Indicate the agent of choice in the emergency therapy of anaphylactic shock:

1. Methoxamine
2. Terbutaline
3. Phenylephrine
4. Epinephrine

Answer. 4. Epinephrine

Question 50. Which of the following sympathomimetics is an effective mydriatic?

1. Salmeterol
2. Phenylephrine
3. Dobutamine
4. Norepinephrine

Answer. 2. Phenylephrine

Question 51. Sympathetic stimulation is mediated by.

1. Release of norepinephrine from nerve terminals
2. Activation of adrenoreceptors on postsynaptic sites
3. Release of epinephrine from the adrenal medulla
4. All of the above

Answer. 4. All of the above

Question 52. Which of the following sympathomimetics acts indirectly?

1. Epinephrine
2. Norepinephrine
3. Ephedrine
4. Methoxamine

Answer. 3. Ephedrine

Question 53. Catecholamine includes following EXCEPT:

1. Ephedrine
2. Epinephrine
3. Isoprenaline
4. Norepinephrine

Answer. 1. Ephedrine

Question 54. Which of the following agents is an a1-selective agonist?

1. Norepinephrine
2. Methoxamine
3. Ritodrine
4. Ephedrine

Answer. 2. Methoxamine

Question 55. All of the following agents are B-receptor agonists EXCEPT:

1. Epinephrine
2. Isoproterenol
3. Methoxamine
4. Dobutamine

Answer. 3. Methoxamine

Question 56. Indicate the direct-acting sympathomimetic, which is an a12 and B1 receptor agonist:

1. Isoproterenol
2. Ephedrine
3. Dobutamine
4. Norepinephrine

Answer. 4. Norepinephrine

Question 57. Indicate the indirect-acting sympathomimetic agent:

1. Epinephrine
2. Phenylephrine
3. Ephedrine
4. Isoproterenol

Answer. 3. Ephedrine

Question 58. Which of the following agents is an a1, a2, B1, and B2 receptor agonist?

1. Methoxamine
2. Albuterol
3. Epinephrine
4. Norepinephrine

Answer. 3. Epinephrine

Question 59. Which of the following sympathomimetics is a B2-selective agonist?

1. Terbutaline
2. Xylometazoline
3. Isoproterenol
4. Dobutamine

Answer. 1. Terbutaline

Question 60. Isoproterenol is:

1. Both a- and ẞ-receptor agonist
2. B1-selective agonist
3. B2-selective agonist
4. Nonselective ẞ-receptor agonist

Answer. 4. Nonselective ẞ-receptor agonist

Question 61. Ephedrine causes:

1. Miosis
2. Bronchodilation
3. Hypotension
4. Bradycardia

Answer. 2. Bronchodilation

Question 62. Indicate the long-acting topical decongestant agents:

1. Epinephrine
2. Norepinephrine
3. Phenylephrine
4. Xylometazoline

Answer. 4. Xylometazoline

Question 63. Indicate the agent of choice in the emergency therapy of anaphylactic shock:

1. Methoxamine
2. Terbutaline
3. Norepinephrine
4. Epinephrine

Answer. 4. Epinephrine

Question 64. Which of the following sympathomimetics is an effective mydriatic?

1. Salmeterol
2. Phenylephrine
3. Dobutamine
4. Norepinephrine

Answer. 2. Phenylephrine

Question 65. Which of the following drugs is a nonselective a-receptor antagonist?

1. Prazosin
2. Phentolamine
3. Metoprolol
4. Reserpine

Answer. 2. Phentolamine

Question 66. Indicate the α-selective antagonist:

1. Phentolamine
2. Dihydroergotamine
3. Prazosin
4. Labetalol

Answer. 3. Prazosin

Question 67. Which of the following drugs is a non-selective ẞ-receptor antagonist?

1. Metoprolol
2. Atenolol
3. Propranolol
4. Acebutolol

Answer. 3. Propranolol

Question 68. Indicate the ẞ1-selective antagonist:

1. Propranolol
2. Metoprolol
3. Carvedilol
4. Sotalol

Answer. 2. Metoprolol

Question 69. Which of the following drugs is a reversible non-selective a, ẞ-antagonist?

1. Labetalol
2. Phentolamine
3. Metoprolol
4. Propranolol

Answer. 1. Labetalol

Question 70. The principal mechanism of action of adrenoreceptor antagonists is:

1. Reversible or irreversible interaction with adrenoreceptors
2. Depletion of the storage of catecholamines
3. Blockade of the amine reuptake pumps
4. Non-selective MAO inhibition

Answer. 1. Reversible or irreversible interaction with adrenoreceptors

Question 71. Non-selective a-receptor antagonists are most useful in the treatment of:

1. Asthma
2. Cardiac arrhythmias
3. Pheochromocytoma
4. Chronic hypertension

Answer. 3. Pheochromocytoma

Question 72. Which of the following drugs is useful in the treatment of pheochromocytoma?

1. Phenylephrine
2. Propranolol
3. Phentolamine
4. Epinephrine

Answer. 3. Phentolamine

Question 73. Indicate adrenoreceptor antagonist agents, which are used for the management of pheochromocytoma:

1. Selective ẞ2-receptor antagonists
2. Non-selective ẞ-receptor antagonists
3. Indirect-acting adrenoreceptor antagonist drugs
4. a-receptor antagonists

Answer. 4. a-receptor antagonists

Question 74. Propranolol is used in the treatment all of the following diseases EXCEPT:

1. Cardiovascular diseases
2. Hyperthyroidism
3. Migraine headache
4. Bronchial asthma

Answer. 4. Bronchial asthma

Drugs Acting On Autonomic Nervous System Introduction

The central nervous system (CNS) is composed of the brain and spinal cord. These neurons cannot be regenerated if damaged. The peripheral nervous system (PNS) is made up of peripheral nerves that connect the CNS to the rest of the body. These neurons can be regenerated if damaged. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves. There are 31 pairs of spinal nerves and 12 pairs of cranial nerves.

Functional Importance:

• Sensory: Gathers information about changes occurring within and around the body; sensory receptors, at the ends of peripheral nerves, send signals to CNS.
• Integrative: Information is “brought together,” interpreted, to create sensations, create thoughts, add, to memory, make decisions, etc.
• Motor: Sending of signals from CNS to muscles and/or glands to elicit a response.

Peripheral Nervous System

Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent. Spinal nerves serve both functions and are called mixed nerves.

The motor (efferent) portion of the nervous system is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Both autonomic and enteric nervous systems function involuntarily.

• The somatic nervous system (mediate voluntary activities).
• The autonomic nervous system (mediate involuntary activities).
• The enteric nervous system (functions to control the gastrointestinal system).

The autonomic nervous system is largely independent (activities are not under direct conscious control). The cranial and spinal nerves connect CNS to heart, stomach, intestines and glands. It controls unconscious activities.

Read and Learn More Medicinal Chemistry III Notes

It is concerned primarily with visceral functions such as cardiac output, blood flow to various organs and digestion. The ANS is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state.

The somatic division is largely concerned with consciously controlled functions such as movement, respiration, and posture. The cranial and spinal nerves connect CNS to skin and skeletal muscles. It controls conscious activities.

Autonomic Nervous System (ANS):

The ANS has two major portions: The sympathetic (thoracolumbar) division and the parasympathetic (craniosacral) division. Both divisions originate in nuclei within the CNS and give rise to preganglionic efferent fibers that exit from the brain stem or spinal cord and terminate in motor ganglia.

• The sympathetic preganglionic fibers leave the CNS through the thoracic and lumbar spinal nerves.
• The parasympathetic preganglionic fibers leave the CNS through the cranial nerves (especially the 3rd, 7th, 9th and 10th) and the 3rd and 4th sacral spinal roots.

The sympathetic system leads to decrease in digestion, pupil size, urinary output and at the same time increases heart rate, bronchiole dilation, blood glucose, blood to skeletal muscle more of “fight or flight” response.

The parasympathetic system leads to decrease in heart rate, bronchiole dilation, blood glucose, blood to skeletal muscle and whilst, increase in digestion, pupil size, urinary output; more of “rest and digest” response.

Cholinergic Drugs

Nerve impulses elicit responses in smooth, cardiac, and skeletal muscles, exocrine glands, and postsynaptic neurons by liberating specific chemical neurotransmitters. By using drugs that mimic or block the actions of chemical transmitters, we can selectively modify many autonomic functions. These functions involve a variety of effector tissues, including cardiac muscle, smooth muscle, vascular endothelium, exocrine glands and presynaptic nerve terminals. Autonomic drugs are useful in many clinical conditions.

Cholinergic Neurotransmitters:

Cholinergic nerves (Parasympathetic nerves) are found in the peripheral nervous system and central nervous system (CNS) of humans. Parasympathetic nerves regulate processes connected with energy assimilation (food intake, digestion, absorption) and storage. These processes operate when the body is at rest, allowing increased bronchomotor tone and decreased cardiac activity.

Secretion of saliva and intestinal fluids promotes the digestion of foodstuffs, transport of intestinal contents is speeded up because of enhanced peristaltic activity and lowered tone of sphincter muscles.

To empty the urinary bladder (micturition), wall tension is increased by detrusor activation with a concurrent relaxation of sphincter tonus. Activation of ocular parasympathetic fibers results in narrowing of the pupil and increased curvature of the lens, enabling near objects to be brought into focus (accommodation).

Cholinergic neurons release acetylcholine (Ach) as the primary neurotransmitter, serves as mediator at terminals of all postganglionic parasympathetic fibers, in addition to fulfilling its transmitter role at ganglionic synapses within both the sympathetic and parasympathetic divisions and the motor endplates on striated muscle. However, different types of receptors are present at these synaptic junctions.

Synthesis, Storage and Release of Acetylcholine Biosynthesis:

Acetylcholine (Ach) is biosynthesized in cholinergic neuron, in which active transport mechanism is involved in picking up choline molecule from extra synaptic fluid into axoplasm. This transport depends on concentration of Na* and K+ ions. Choline is acetylated in cytoplasm by acetyl coenzyme-A (acetyl-CoA) which is biosynthesized in mitochondria present in nerve terminal. The acetylation is catalyzed by the enzyme choline acetyltransferase (biosynthesized itself in the cholinergic neurons).

Some of the choline is biosynthesized from amino acid serine, but most of choline used to form Ach is recycled following the enzymatic hydrolysis of Ach in synaptic space.

Storage:

As soon as acetylcholine is synthesized, it is actively transported into cystolic storage vesicle (5000-10000 Ach) located in presynaptic nerve ending, where it is maintained until it is released. Some Ach remains in cystol and is eventually hydrolyzed to acetate and choline. Only the stored form of Ach serves as the functional neurotransmitter.

Release:

When impulse reaches to the nerve terminal, it results in release of Ach due to depolarization of the nerve terminal by the action potential. It alters membrane permeability to Ca++, which results into opening of voltage dependant Ca** channel affording an influx of Ca** causes exocytotic release of Ach into the synaptic cleft. (4 Ca** are taken up for each molecule of ACh release).

The ruptures cystolic (synaptic) vesicle is again re-shaped to store the fresh neurotransmitter. Each synaptic vesicle contains a quantum of Ach (one Quanta is equal to 12,000-60,000 molecules of ACh). A single action potential causes the release of several hundred quanta of ACh into the synapse.

Metabolism:

At the postsynaptic effector cell membrane, ACh interacts with its receptors. Because these receptors can also be activated by the alkaloid muscarine, they are referred to as muscarinic (M) cholinoceptors. In contrast, at ganglionic and motor endplate cholinoceptors, the action of ACh is mimicked by nicotine, therefore they are also said to be nicotinic (N) cholinoceptors.

Released ACh is rapidly hydrolyzed and inactivated by a specific acetylcholine esterase (AChE), present on pre- and post-junctional membranes, or by a less specific serum choline esterase (butyryl choline esterase), a soluble enzyme present in serum and interstitial fluid. Enzymatic hydrolysis, results into formation of choline which poorly binds to acetylcholine receptors.

There are enough AChE present in synapse to hydrolyse approximately 3 x 108 molecules of Ach in 1 millisecond. Thus there is adequate enzyme activity to hydrolyse all of ACh released by one action potential.

The empty synaptic vesicles, which are returned to the axonal terminal bulb by endocytosis, are filled with ACh.

The vascular release of ACh is reported to be inhibited by excess of Mg**. The release of ACh along with the extracelluler Ca** then mobilize the intracellular Ca** from sac present on sarcoplasmic reticulum. The increase in concentration of free intracellular Ca** activates calmodulin-dependent myosin light kinase and phosphorylation of myosin takes place which leads to muscle contraction.

Cholinergic Receptors

There are two different types of cholinergic receptors i.e. nicotinic receptor and muscarinic receptor. These receptors differ in composition, location and pharmacological functioning. They have specific agonists and antagonists activity. They have been characterised as nicotinic and muscarinic on the basis of their ability to be bound by the natural alkaloids nicotine and muscarine respectively. Both the receptors have subtypes that differ in location and specificity.

Nicotinic Receptors

Nicotinic receptors are coupled directly to ion channels and when activated by Ach, mediate very rapid responses. Ion channels are responsible for the electrical excitability of nerve and muscle cells and for the sensitivity of sensory cells. The channels are pores that open or close in an all or nothing fashion on time scales ranging from 0.1 to 10 milliseconds to provide aqueous pathways through the plasma membrane that ions can transverse.

The nicotinic ACh receptor is a glycoprotein embedded into the polysynaptic membrane and consists of 5 subunits of polypeptide chains. Subunits are arranged like a rosette surrounding the Na channel. The two alpha subunits carry two ACh binding sites with negatively charged groups which combine with the cationic group of ACh and open Na* channel.

When the neurotransmitter ACh binds to the nicotinic receptor, it causes a change in the permeability of the membrane to allow passage of small cations Ca** and Na* and K+. The. physiological action is to temporarily depolarize the nerve end plate to muscular contraction at a neuromuscular junction.

Nicotinic Receptor Subtypes:

Nu-cholinoceptors (N):

• Location: Neuromuscular junction.
• Function: Depolarization of muscle end plate and contraction of skeletal muscle.
• They are blocked by succinylcholine, d-tubocurarine and decamethonium, and are stimulated by phenyl trimethyl ammonium.

NN-cholinoceptors (N2):

• Location: Autonomic ganglia.
• Function: Depolarization of post ganglonic membrane (in adrenal medulla- catecholamine release).
• They are blocked by hexamethonium and trimethaphan, but are stimulated by tetramethyl ammonium and dimethyl-4-phenylpiperazinium (DMPP).

Muscarinic Receptors

Muscarinic receptors play an important role in regulating the functions of organs innervated by the autonomic nervous system. The action of ACh on muscarinic receptors is to stimulate secretions from salivary and sweat glands, secretions and contraction of the gut and constriction of the respiratory tract. It inhibits contraction of the heart and relaxes smooth muscle of blood vessels.

Muscarinic receptors mediate their effects by activating guanosine triphosphate (GTP)- binding proteins (G-proteins). These receptors have seven protein helixes that passed through the plasma membrane, creating four extracellular domains and four intracellular domains. The extracellular domain of the receptor contains the binding site for Ach. The intracellular domain couples with G-proteins to initiate the biochemical changes that results in pharmacological action from receptor activation.

Muscarinic Receptor Subtypes:

Subtypes of muscarinic receptors are located at the CNS and peripheral nervous system. Muscarinic receptor subtypes have been defined on the basis of their affinity for agonists, antagonists and the pharmacological effects cause.

They are classified into subtypes according to their molecular structure, signal transduction, and ligand affinity in the M1, M2, M3, M4 and Ms.

• M1-receptors are present on nerve cells, exocrine glands and autonomic ganglia, where they mediate a facilitation of impulse transmission from preganglionic axon terminals to ganglion cells. In humans, these receptors seem to affect the rapid eye movement (REM), sleep, emotional responses, affective disorders including depression and modulation of stress. They are also located in intramural ganglia of stomach wall which on stimulation cause gastric secretion.
• M2-receptors are also called cardiac muscarinic receptors because they are located in the atria and conducting tissue of the heart. Their stimulation causes a decrease in the strength and rate of cardiac muscle contraction. These effects may be produced by affecting intracellular K+ and Ca** levels in heart tissue. Opening of K+ channels leads to slowing of diastolic depolarization in sinoatrial pacemaker cells and a decrease in heart rate. These receptors may also act through an inhibitory G protein (G1) to reduce adenylate cyclase activity and lower cyclic 3′,5′-adenosine monophosphate (CAMP) levels in cardiac cells. M2-receptors can also serve as autoreceptors and inhibit release of ACh’ on presynaptic terminals of postganglionic cholinergic nerves.
• M3-receptors are referred to as glandular muscarinic receptors. They are located in exocrine glands and smooth muscles and increase secretory activity such as secretions from lacrimal, salivary, bronchial, pancreatic and mucosal cells in the GIT. Stimulation of M3-receptors also results into contraction of visceral smooth muscles.
• M4-receptors, like M2-receptors, they act through G1 protein to inhibit adenylate cyclase. They also function by a direct regulatory action on K* and Ca++ ion channels. Stimulation of M4-receptors in tracheal smooth, inhibit the release of Ach in same manner as M2-receptors.
• Ms-receptors, messenger RNA (mRNA) is found in the substantia nigra. It may regulate dopamine release at terminals within the striatum.

Classification Of Para-Sympathomimetic Agents

Acetylcholine receptor stimulants and cholinesterase inhibitors:

Acetylcholine receptor stimulants comprise a large group of drugs that mimic ACh (cholinomimetic agents). Cholinoceptor stimulants are classified by their spectrum of action depending on the type of receptor-muscarinic (M) or nicotinic (N) that is activated. They are also classified by their mechanism of action, as some cholinomimetic drugs bind directly to (and activate) cholinoceptors, while others act indirectly by inhibiting the hydrolysis of endogenous ACh.

Direct-Acting Cholinergic Drugs:

• Choline ester (stimulants of M- and N-receptors):
  e.g. Acetylcholine, Carbachol.
• Cholinomimetic Alkaloids:
• • Stimulants of M-receptors
    e.g. Pilocarpine, Cevimeline, Bethanechol, Musacarine, Phalloidin

• • Stimulants of N-receptors
    e.g. Nicotine, Cytisine, Lobeline

Indirect-Acting Cholinergic Drugs (anticholinesterase (Anti-ChEs) drugs):

• Reversible drugs (carbamates class)
• • With N3+ (cross BBB)
    e.g. Alkaloids: Galantamine, Physostigmine
    e.g. Synthetic drugs: Donepezil, Rivastigmine, Tacrin

• • With N4+ (do not cross BBB)
    e.g. Demecarium, Edrophonium, Neostigmine, Pyridostigmine

• Irreversible anticholinesterase agents (organophosphates class)
• • Thiophosphate insecticides:
    e.g. Parathion, Malathion

• • Nerve paralytic gases (chemical warfare):
    e.g. Tabun, Sarin, Soman.

Structure-Activity Relationship: Choline ester (stimulants of M- and N- receptors):

Quaternary Ammonium Group (N4*):

• The quaternary ammonium group is essential for activity; at the cationic head is the major point of receptor interaction.
• The replacement of nitrogen (N) by sulfur (S), arsenic (As) or selenium (Se) leads to decrease in activity.
• The conversion of the quaternary ammonium group (N) to primary, secondary or tertiary leads to less active derivatives.
• The replacement of the methyl (CH3) substitution with higher carbon skeleton (more ‘C’-atoms) functional groups like Propyl (C3H7) or higher alkyl groups leads to inactive derivatives.
• The quaternary ammonium group should be followed by a chain of five atoms for maximal muscarinic activity; it is referred as “five-atom rule”.

Ethylene Bridge (-CH2-CH2-):

• With the increase or decrease in the carbon chain length, the activity of the derivatives is rapidly reduced.
• Substitution on the a or ẞ position with methyl (CH3) group results in decreased activity, but increased duration of action, this phenomenon is due to the reduced metabolism due to steric hindrance.
• B-substituted choline derivative exhibits longer duration of action than that of its a-substituted choline analog.
• The substitution of methyl (CH3) group on a or ẞ position leads to selectivity towards receptors.
• a-Methylcholine (methyl group substituted at a-position derivative) is less potent than Ach, but has more nicotinic activity than muscarinic activity.
• B-Methylcholine; Methacholine (methyl group substituted at B-position derivative) is less potent than ACh, but more selective to Muscarinic receptor.
• Substitution on the aor B.position with alkyl function larger than methyl group like propyl or butyl leads to derivatives with no activity.

Acyl oxy group:

• The conversion of acetyl (CH3CO) group to higher group like propionyl (C2H, CO) or butyryl (C3H,CO) functions results in decrease in activity, indicated by the five-atom rule.
• The ester derivatives of higher molecular carboxylic acids or of aromatic acids results in antagonist property.
• The acetyl (CH3CO) group/methyl ester in Ach is rapidly hydrolyzed by AchE, replacement of the same with carbomate (NH2CO) ester results in resistance to hydrolysis leading to potent derivative with increased duration of activity.
• Carbachol (carbamate ester of choline) is more stable to hydrolysis and can be administered orally.
• The replacement of the ester function with ether or ketonic function while sticking by the five-atom rule will also result in stable and potent derivatives.
• Choline ethyl ether derivative is as potent as ACh.

Direct-Acting Cholinergic Drugs

Choline Ester

Acetylcholine:

• Acetylcholine is chemically, 2-acetyloxyethyl (trimethyl)azanium.
• Acetylcholine chloride exerts a powerful stimulant effect on the parasympathetic nervous system.
• It does not have any therapeutic importance as it is rapidly hydrolyzed by cholinesterase.
• Stimulation of the vagus and the parasympathetic nervous system produces a tonic action on smooth muscle and induces a flow from the salivary and lacrimal glands. Its cardiac-depressant results in negative chronotropic effect (decrease in heart rate) and negative inotropic effect (decrease in the force of myocardial contractions).
• It also has vasodilatory action on the peripheral vascular system.
• When it gives systemically, it produces bronchial constriction as a characteristic side effect.
• A non-selective muscarinic receptor antagonist (i.e. atropine) is the most effective antagonist to the action of Ach.

Uses:

• Acetylchloline is a cardiac depressant and an effective vasodilator.
• It is used during ophthalmic surgery (cataract)..
• It is used as ex-vivo agent to study pharmacological effect of parasympathomimetic.

Carbachol:

• Carbachol is chemically, 2-carbamoyloxyethyl(trimethyl)azanium.
• It is a choline carbamate, a parasympathomimetic that stimulates both muscarinic and nicotinic receptors.
• The pharmacological activity of carbachol is similar to that of ACh.
• It is not well absorbed in the gastro-intestinal tract and does not cross the blood- brain barrier.
• It is usually administered topical ocular or through intraocular injection.
• It is not easily metabolized by cholinesterase therefore, more stable in aqueous solutions.
• It can also act indirectly by promoting release of ACh and by its weak anticho- linesterase activity.
• It has a 2 to 5 minute onset of action and its duration of action is 4 to 8 hours with topical administration and 24 hours for intraocular administration.
• Since carbachol is poorly absorbed through topical administration, benzalkonium chloride is mixed in it to promote absorption.

Uses:

• Carbachol is primarily used for treating glaucoma, or during ophthalmic surgery (cataract).
• It is administered as an ophthalmic solution, its principal effects are miosis and increased aqueous humour outflow.

Bethanechol:

• Bethanechol is chemically, 2-carbamoyloxypropyl(trimethyl)azanium.
• It is B-methylcholine carbamate and non-specific, that selectively stimulates muscarinic receptors without any effect on nicotinic receptors.
• It is not readily hydrolyzed by cholinesterase and therefore has a long duration of action.
• It has pharmacological properties similar to that of methacholine.

Uses:

• Bethanechol is given orally or subcutaneously to treat urinary retention and abdominal distention resulting from general anaesthetic, diabetic neuropathy of the bladder, or a side effect of antidepressants; or to treat gastrointestinal lack of muscular tone.
• It has potential benefit in the treatment of cerebral palsy.
• It is contraindicated in patients with asthma, coronary insufficiency, peptic ulcers, intestinal obstruction and hyperthyroidism.

Methacholine:

• Methacholine is chemically, 2-acetyloxypropyl (trimethyl)azanium.
• It is ẞ-methylcholine acetate and is a synthetic choline ester that acts as a non- selective muscarinic receptor agonist but has little effect on the nicotinic receptors.
• It is not readily hydrolyzed by cholinesterase and therefore has a long duration of action.

Uses:

• Methacholine is primarily used to diagnose bronchial hyper-reactivity (asthma and chronic obstructive pulmonary disease), known as the bronchial challenge test or methacholine challenge.
• It also leads to adverse cardiovascular effects, bradycardia and hypotension.

Cholinomimetic Alkaloids

Pilocarpine:

• Pilocarpine is an alkaloid obtained from the dried leaflets of Pilocarpus jaborandi or Pilocarpus microphyllus.
• It is chemically, (3S,4R)-3-ethyl-4-[(3-methylimidazol-4-yl)methyl]oxolan-2-one.
• It is a non-selective agonist on the muscarinic receptors.
• It works by activating cholinergic receptors of the muscarinic type which cause the trabecular meshwork to open and the aqueous humor to drain from the eye.

Uses:

• Pilocarpine is used to treat increased pressure inside the eye (glaucoma).
• As ophthalmic solution, it is used for angle closure glaucoma, ocular hypertension, and open angle glaucoma.
• By oral medication, it stimulates the secretion of saliva and sweat and use to treat dry mouth.
• Common side effects include irritation of the eye, increased tearing, headache, and blurry vision.
• Systemic injection of pilocarpine can cross blood-brain barrier, which can lead to chronic epilepsy and can be used as epilepsy inducing agent in rodents in order to study human epilepsy.

Indirect-Acting Cholinergic Drugs/Cholinesterase Inhibitors

The acetylcholine action at the synaptic junction is terminated by metabolism catalyzed by the action of AchE. The AchE enzyme hydrolyses Ach to choline and acetate. Inhibition of the enzyme leads to increased concentration of Ach and prolongs the life of the available Ach at the synaptic junction, thus producing a cholinergic agonist effect. The enzyme inhibitors are classified based on the affinity towards the enzyme and the type of interaction.

Reversible Cholinesterase Inhibitors

Physostigmine:

• Physostigmine is an alkaloid obtained from the dried ripe seed of Physostigma venenosum.
• It occurs naturally in the Calabar bean and the Manchineel tree.
• It is chemically, [(3aR,8bS)-3,4,8b-trimethyl-2,3a-dihydro-1H-pyrrolo[2,3-b]indol-7-yl] N-methylcarbamate.
• It is a highly toxic parasympathomimetic alkaloid, a reversible cholinesterase. inhibitor.
• It acts by interfering with the metabolism of acetylcholine.
• In solution it undergoes hydrolysis to form methyl carbamic acid and eseroline, neither of which inhibits Ach.

Uses:

• Physostigmine is used to treat glaucoma and delayed gastric emptying.
• As it enhances the transmission of acetylcholine signals in the brain and can cross the blood-brain barrier, physostigmine is used as an antidote to treat anticholinergic poisoning (Datura stramonium, Atropa belladonna, atropine, scopolamine, and other anticholinergic drug overdoses poisoning).

Neostigmine:

• Neostigmine is chemically, [3-(dimethylcarbamoyloxy)phenyl]-trimethylazanium.
• It has quaternary nitrogen; hence, it is polar and does not cross the blood-brain barrier, but it does cross the placenta.
• It works by blocking the action of acetylcholinesterase and therefore increases the levels of acetylcholine.

Uses:

• Neostigmine is used to treat myasthenia gravis, ogilvie syndrome (acute dilation of the colon), and urinary retention. Common side effects include nausea, increased saliva and abdominal pain.
• In myasthenia gravis there are too few acetylcholine receptors so with the acetyl cholinesterase blocked, acetylcholine can bind to the few receptors and trigger a muscular contraction.
• It is also used as an antidote to non-depolarizing neuromuscular blocking drug.

Pyridostigmine:

• Pyridostigmine is chemically, (1-methylpyridin-1-ium-3-yl) N,N-dimethylcarbamate.
• It is a quaternary carbamate inhibitor of cholinesterase that does not cross the blood-brain barrier.
• Common side effects include nausea, diarrhea, frequent urination, and abdominal pain.

Uses:

• Pyridostigmine is used to treat myasthenia gravis. (Long term neuromuscular disease that leads to varying degrees of skeletal muscle weekness).
• It is used to treat muscle weakness in people with myasthenia gravis or forms of congenital myasthenic syndrome.
• It is also used together with atropine to end the effects of neuromuscular blocking medication of the non-depolarizing type.

Edrophonium Chloride:

• Edrophonium is chemically, ethyl-(3-hydroxyphenyl)-dimethylazanium, chloride.
• It is a readily reversible acetylcholinesterase inhibitor and acts by competitively inhibiting the enzyme.
• It has a direct cholinomimetic effect on skeletal muscle, which is greater than that of most other anticholinesterase drugs.

Uses:

• Edrophonium chloride is used as a diagnostic agent to differentiate myasthenia gravis from cholinergic crisis and Lambert-Eaton. (Lambert-Eaton myasthenic syndrome is an autoimmune disease where the neuron is unable to release enough ACh for normal muscle function).
• In myasthenia gravis, the body produces autoantibodies which block, inhibit or destroy nicotinic acetylcholine receptors in the neuromuscular junction. Edrophonium reduces the muscle weakness in myasthenia gravis.
• In a cholinergic crisis, resulting from too much neuromuscular stimulation, edrophonium will make the muscle weakness worse by inducing a depolarizing block.

Tacrine Hydrochloride:

• Tacrine hydrochloride is chemically, 1,2,3,4-tetrahydro-9-aminoacridine, hydro- chloride.
• It is a centrally acting reversible anticholinesterase and indirect cholinergic agonist.

Uses:

• It was the first centrally acting cholinesterase inhibitor approved for the treatment of Alzheimer’s disease (progressive loss of brain cells that leads to memory loss and decline of other thinking skills).

Ambenonium Chloride:

• Ambenonium dichloride is chemically, (2-chlorophenyl)methyl-[2-[[2-[2-[(2-chloro- phenyl) methyl-diethylazaniumyl)ethylamino]-2-oxoacetyl]amino]ethyl]-diethylazanium; dichloride.
• It acts by suppressing the activity of AChE.
• It possesses a relatively prolonged duration of action and causes fewer side effects in the GI tract than the other anticholinesterase agents.
• Because of its quaternary ammonium structure, it is absorbed poorly from the GI tract.
• In moderate doses, the drug does not cross the blood brain barrier.
• Ambenonium chloride is not hydrolyzed by cholinesterases.

Uses:

• Ambenonium is a reversible cholinesterase inhibitor used in the management of myasthenia gravis in patients who do not respond satisfactorily to neostigmine or pyridostigmine.
• It is used to treat muscle weakness due to myasthenia gravis disease or defect of the neuromuscular junction.

Irreversible Cholinesterase Inhibitors

They covalently bond with the AchE enzyme to irreversibly deactivate them, they are referred as nervé poisons and are used as agricultural insecticides. Due to the irreversible inhibition of the enzyme, Ach accumulates at the synaptic junction leading to a high concentration of the Ach and thus to produce agonist effect. These compounds belong to the class of Organophosphorus esters. A general formula for such compounds is as follows:

• • R1 = Alkoxyl group
  • R2 = Alkyl / Alkoxyl / aryl/aryloxy/Tertiary Amine.
  • X = A good leaving group like F, CN, thiomalate, p-nitrophenyl
  • A Oxygen / Sulphur
• ‘A’ is usually oxygen or sulphur, but may also be selenium.
• When ‘A’ is other than oxygen, biological activation is required before the compound becomes effective as an inhibitor cholinesterase.
• Phosphorothionates (when ‘A’ is ‘S’) have much poorer electrophilic character and 105 folds weaker anticholinesterase activity than their oxygen analogues.
• ‘X’ is the leaving group when the molecule reacts with the enzyme.
• The ‘R’ moiety imparts lipophilicity to the molecule and contributes to its absorption through the skin.
• Inhibition of AChE by organo-phosphorous compound takes place in two steps, association of enzyme and inhibitor and the phosphorylation step.

Isofluorophate (Diisopropyl fluorophosphate):

• Diisopropyl fluorophosphates is chemically, 2-[fluoro(propan-2-yloxy) phosphoryl]- oxypropane.
• It is an organophosphorus insecticide, a parasympathomimetic drug irreversible anti- cholinesterase.
• Since it irreversibly inhibits cholinesterase, its activity lasts for days or even weeks.
• It must be handled with extreme caution, as it can be absorbed readily through intact epidermis and more so through mucous tissues.

Uses:

• Isofluorophate has been used in ophthalmology as a miotic agent in treatment of chronic glaucoma.
• It is also used as a miotic in veterinary medicine.
• It is used as an experimental agent in neuroscience because of its acetyl- cholinesterase inhibitory properties and ability to induce delayed peripheral neuropathy.

Echothiophate Iodide:

• Echothiophate iodide is chemically, 2-diethoxyphosphorylsulfanylethyl (trimethyl)- azanium; iodide.
• It is also known as Phospholine. It is a parasympathomimetic drug, an irreversible acetylcholinesterase inhibitor.
• It has long duration of action and its effects can last a week or more.

Uses:

• Echothiophate iodide is used as an ocular antihypertensive in the treatment of chronic glaucoma and in accommodative esotropia.

Parathion:

• Parathion is chemically, O,O-Diethyl O-(4-nitrophenyl) phosphorothioate.
• It is a relatively weak cholinesterase inhibitor.
• It is highly toxic to non-target organisms, including humans, so its use has been restricted.
• Absorbed parathion is rapidly metabolized to paraoxon by enzymes present in liver microsomes and insect tissue.
• Parathion is also metabolized by liver microsomes to yield p-nitrophenol and diethylphosphate; the later is inactive as an irreversible cholinesterase inhibitor.

Uses:

• Parathion is an organophosphate insecticide and acaricide.
• It is used as an agricultural insecticide.

Malathion:

• Malathion is chemically, 2-[(dimethoxyphosphinothioyl)-thio]butanedionic acid diethyl ester, or diethyl-2-dimethoxyphosphinothioyl sulfanyl butane dioate.
• It is a poor inhibitor of cholinesterases.
• It is an organophosphate insecticide which acts as an acetylcholinesterase inhibitor.

Uses:

• Malathion is used as an agricultural insecticide.

Acetylcholine Sterase Reactivators

Oxime class of compounds can provide a nucleophilic attack on the phosphorylated AchE (deactivated) and can regenerate the free enzyme. Choline-reactivating oximes are effective antidotes in organophosphorus insecticide or sulfonate poisoning, a state of acetylcholine excess because of cholinesterase inhibition.

Pralidoxime chloride:

• Pralidoxime chloride is chemically, 2-formyl-1-methylpyridinium chloride oxime.
• It is also said to be 2-pyridine aldoxime methyl chloride or 2-PAM, usually as the chloride or iodide salts, belongs to a family of compounds called oximes that bind to organophosphate-inactivated acetylcholinesterase.
• These drugs antagonize the effects of accumulated acetylcholine at the cholinergic synapses by reactivating the inhibited cholinesterase.
• Reactivation of cholinesterases is importance for the survival due to organo- phosphorus insecticide poisoning.
• It is a quaternary ammonium compound and most effective by intramuscular, subcutaneous, or intravenous administration.

Uses:

• Pralidoxime chloride or 2-PAM is used to combat poisoning by organophosphates or acetyl cholinesterase inhibitors (nerve agents) in conjunction with atropine.
• 2-PAM is the most popular oxime for this purpose.
• It may be effective against some phosphates that have a quaternary nitrogen.
• It is also an effective antagonist for some carbamates, such as neostigmine methylsulfate and pyridostigmine bromide.

Cholinergic Blocking Agents

Agents that block or antagonize the action of acetylcholine, at post ganglionic nerve ending are called as cholinergic blockers or parasympatholytic agents. They act by inhibiting muscarinic action of acetylcholine on smooth muscle contraction and including exocrine gland secretion.

Cholinergic blockers are classified into two classes, based on mechanism of action:

• Agents that inhibit, synthesis or release of Ach: These agents are not been effective clinically and hence not used.
• Agents that block Muscarinic receptor at nerve ending: These agents are also said to be parasympathetic post ganglionic muscarinic receptor blockers. These competitive reversible antagonists are large molecules, derived from muscarinic antagonist with one or more bulky groups, thus capable of binding to the receptor.

Structure-activity relationship of cholinergic blockers:

• The Pivotal carbon is of crucial importance in relation to the development of cholinergic antagonists.
• Generally, the amine function need to be a quaternary ammonium group (N4*) or it can be tertiary (N3*) that is protonated at physiological pH to form the cationic head. The quaternary ammonium group is essential for activity, as the cationic head is the major point of receptor interaction.
• The amine function is separated from the pivotal carbon by a chain / bridge, the chain may be a ester, ether or a hydrocarbon function bridge.
• The substitution ‘A’ and ‘B’ contains at least one aromatic moiety that can exhibit Van der Waals’ interaction with the receptor and the other one as cycloalkyl, aliphatic hydrocarbon or alkyl group for hydrophobic interaction with the receptor.
• The ‘X’ function can be hydroxy function or a carboxamide group or just hydrogen to exhibit hydrogen bond interaction with the receptor.

The Amine Function:

• The amine group should be cationic in nature, which is the primary point for interaction with the receptor core.
• It may be a quaternary ammonium group (N4) or it can be tertiary (N3+) that is protonated at physiological pH to form the cationic head.

The ‘X’ Functions as Hydrogen (H) / Hydroxyl group (OH):

• The hydroxyl group (OH) is not prerequisite for activity, but suitably placed alcoholic hydroxyl function enhances the activity.
• The ‘OH’ group contributes in hydrogen bonding with the amino acid residues present in the receptor.

The Chain Function:

• The chain function can be an ester/ether / amino alcohol / hydrocarbon moiety.
• The ester moiety is found to be more effective, this can be indicated as the agonist, also poses a ester function, contributing for a better binding affinity at the receptor site.
• The ether amino alcohol/ hydrocarbon moieties are also clinically effective derivatives.

The Cyclic Substituents – ‘A’ and ‘B’:

• At least one cyclic moiety as a substituent at position ‘A’ / ‘B’ is prerequisite for activity.
• The cyclic substituent may be phenyl, cyclohexyl etc., the phenyl moiety predominant as an important substitution.
• Aromatic acid ester derivatives produce decrease in anticholinergic activity, but results in compounds with potential local anaesthetic activity.

Classification Of Muscarinic Receptor Blockers

• Natural and semi-synthetic derivatives:
• • Solanaceous alkaloids and their synthetic analogs
• Synthetic derivatives:
  • Amino alcohol ester derivatives
  • Amino alcohol ether derivatives
  • Amino alcohol derivatives
  • Amino amide derivatives
  • Miscellaneous derivatives

Natural and Semi-Synthetic Derivatives

Solanaceous Alkaloids and their Synthetic Analogs:

The solanaceous alkaloids, represented by hyoscyamine, atropine and scopolamine (hyoscine) are the major class of antimuscarinic drugs. These alkaloids are found principally in Henbane (Hyoscymus niger), deadly nightshade (Atropa belladonna) and Jimson weed (Datura stramonium).

Crude drugs containing these alkaloids have been used since early times for their marked medicinal properties which depend largely on inhibition of the parasympathetic nervous system and stimulation of the higher nervous centers.

• Belladonna alkaloid (Atropine) possesses a weak local anaesthetic activity and has been used topically for its analgesic effect on hemorrhoids, certain skin infections and various itching dermatoses.
• It is also used as mydriasis.
• It increases the heart rate (by depression of the vagus nerve).
• It depresses the motility of the GIT, and acts as an antispasmodic on various smooth muscles (ureter, bladder, and biliary tract).
• It also directly stimulates the respiratory center.
• The action of scopolamine containing drugs differs from those containing hyoscyamine and atropine in having no CNS stimulation; they have significant narcotic or sedative effect.
• The use of this group of drugs is accompanied by a fairly high incidence of reactions because of individual idiosyncrasies.
• Death from over dosage usually results from respiratory failure.

Atropine Sulphate:

• Atropine occurs naturally in a number of plants of the nightshade family including deadly nightshade, Jimson weed, and mandrake.
• It is chemically, [(1R,5S)-8-methyl-8-azabicyclo[3.2.1]octan-3-yl] 3-hydroxy-2-phenyl- propanoate.
• It is the tropine ester of racemic tropic acid and is optically inactive.
• It is an enantiomeric / racemic mixture of d-hyoscyamine and l-hyoscyamine, with most of its physiological effects due to l-hyoscyamine.
• Atropine sulfate is prepared by neutralizing atropine in acetone or ether with an alcoholic solution of sulfuric acid, with care used to prevent hydrolysis.
• It is an antimuscarinic drug that works by inhibiting the parasympathetic nervous system.
• Atropine is a competitive reversible antagonist of the muscarinic acetylcholine receptor types M1, M2, M3, M4 and Ms.
• In the eye, atropine induces mydriasis by blocking contraction of the circular pupillary sphincter muscle, thereby allowing the radial iris dilator muscle to contract and dilate the pupil.

Uses:

• Atropine is used to treat pesticide poisonings and to decrease saliva production during surgery.
• As an ophthalmic preparation it is used to treat uveitis (swelling of the middle layer of the eye) and early amblyopia (vision development disorder).
• Atropine induces cycloplegia by paralyzing the ciliary muscles and helps to relieve pain associated with iridocyclitis (an inflammation of the iris), and treats ciliary block (malignant) glaucoma.
• Common side effects include a dry mouth, large pupils, urinary retention and constipation.

Hyoscyamine Sulphate:

• Hyoscyamine (daturine) is a tropane alkaloid.
• It is a secondary metabolite found in certain plants of the family Solanaceae like Atropa belladonna.
• It is chemically, [(1S,5R)-8-methyl-8-azabicyclo[3.2.1]octan-3-yl] (2S)-3-hydroxy-2- phenylpropanoate.
• It is the levorotatory isomer of atropine.

Uses:

• Hyoscyamine is used to provide symptomatic relief of spasms caused by various lower abdominal and bladder disorders including peptic ulcers, irritable bowel syndrome, diverticulitis, pancreatitis, colic, and interstitial cystitis.
• It has also been used to relieve some heart problems, control some of the symptoms of Parkinson’s disease, as well as for control of abnormal respiratory symptoms and “hyper-mucus secretions” in patients with lung disease.
• It is also useful in pain control for neuropathic pain, chronic pain and palliative care; and for those with intractable pain from treatment resistant, untreatable, and incurable diseases.
• It is used to treat disorders of the urinary tract.
• It is used to treat spasms of the bladder and serves as a urinary stimulant.
• When combined with opioids, it increases the level of analgesia obtained.

Scopolamine:

• Hyoscine is produced from plants of the family Solanaceae like Atropa belladonna.
• The name “scopolamine” is derived from one type of nightshade known as Scopolia, while the name “hyoscine” is derived from another type known as Hyoscyamusniger.
• Hyoscine is in the antimuscarinic family of medications and works by blocking some of the effects of acetylcholine within the nervous system.

Uses:

• Scopolamine (Hyoscine) is a medication used to treat motion sickness and postoperative nausea and vomiting.
• It is also used before surgery to decrease saliva.
• It is not recommended in people with glaucoma or bowel obstruction.

Homatropine:

• Homatropine is chemically, [(15,5R)-8-methyl-8-azabicyclo[3.2.1]octan-3-yl]-2-hydroxy-2-phenylacetate.
• It is available as the hydrobromide salt.
• It is an anticholinergic medication that is an antagonist at muscarinic acetylcholine receptors.
• It is less potent than atropine and has a shorter duration of action:-

Uses:

• It is used as an ophthalmic preparation as a cycloplegic (to temporarily paralyze accommodation), and as a mydriatic (to dilate the pupil).
• Homatropine is also given as an atropine substitute given to reverse the muscarinic and CNS effects associated with indirect cholinomimetic (Anti AChE) poisoning.

Ipratropium Bromide:

• Ipratropium bromide is chemically, (8-methyl-8-propan-2-yl-8-azoniabicyclo[3.2.1]- octan-3-yl) 3-hydroxy-2-phenylpropanoate;bromide.
• It is a muscarinic antagonist, a type of anticholinergic, which works by causing smooth muscles to relax.

Uses:

• Ipratropium bromide is used to treat the symptoms of chronic obstructive pulmonary disease and asthma.
• Ipratropium, sprayed into the nostrils as a nasal solution, can reduce rhinorrhea, but will not help nasal congestion.
• Combination with beta-adrenergic agonists increases the dilating effect on the bronchi.
• Common side effects include dry mouth, cough, and inflammation of the airways.

Synthetic Derivatives

The solanaceous alkaloids are found to be potent parasympatholytics, but they have the undesirable wide range of effects because of their non-specific action. For example, antispasmodic effects of the alkaloids most often result in side effects such as dryness of the mouth and fluctuations in pulse rate. To minimize such non-specific undesirable effects, the synthetic compounds are derived which possess specific cholinolytic actions.

The synthetic derivatives have tertiary amino function that is protonated in physiological pH or a quaternary ammonium group to form the cationic head, a crucial point of interaction. at the receptor cites. The tropic acid moiety is replaced by a large aromatic moiety like phenyl to provide hydrophobic interaction at the receptor cites.

Amino Alcohol Ester Derivatives:

Clidinium Bromide:

• Clidinium bromide is chemically, (1-methyl-1-azoniabicyclo[2.2.2]octan-3-yl)- 2-hydroxy-2,2-diphenylacetate;bromide.
• It is a muscarinic antagonist and finds importance in management of the symptoms of cramping and abdominal/stomach pain by decreasing stomach acid, and slowing the intestines.

Uses:

• Clidinium bromide is used in combination for the management of Peptic ulcer disease, GI motility disturbances (irritable bowel syndrome) and in acute enterocolitis.
• It is contraindicated in glaucoma.

Cyclopentolate Hydrochloride:

• Cyclopentolate hydrochloride is chemically, 2-(dimethylamino)ethyl 2-(1-hydroxy-cyclopentyl)-2-phenylacetate;hydrochloride.
• It blocks the acetylcholine receptor in the sphincter muscle of the iris and the ciliary muscle, thereby preventing contraction. This dilates the pupil, producing mydriasis, and prevents accommodation of the eye to different distances (cycloplegic).

Uses:

• Cyclopentolate hydrochloride is used as a mydriatic medication where it acts as parasympatholytic.
• It is commonly used during pediatric eye examinations.
• It is also administered as an atropine substitute to reverse muscarinic and central nervous system effects of indirect cholinomimetic (anti AchE) administration.

Dicyclomine Hydrochloride:

• Dicyclomine hydrochloride is chemically, 2-(diethylamino)ethyl. 1-cyclohexyl- cyclohexane-1-carboxylate;hydrochloride.
• It has some muscarinic receptor subtype selectivity. It binds more firmly to M1 and M3 than M2 and M4 receptors.
• It antagonizes muscarinic receptors on smooth muscle in the gastrointestinal (GI) tract, thereby preventing the actions of acetylcholine and reducing GI smooth muscle spasms.

Uses:

• Dicyclomine hydrochloride is used as an antispasmodic and in urinary incontinence.
• It has local anesthetic properties and is used in gastrointestinal, biliary, and urinary tract spasms.
• It is used to treat the symptoms of irritable bowel syndrome, specifically hypermotility.
• It is also useful in dysmenorrhea (pain during menstruation), pylorospasm (spasm of the pyloric sphincter) and biliary dysfunction.

Glycopyrrolate:

• Glycopyrrolate is chemically, (1,1-dimethylpyrrolidin-1-ium-3-yl) 2-cyclopentyl-2- hydroxy-2-phenylacetate;bromide.
• It is a synthetic quaternary ammonium amine anticholinergic agent with antispasmodic activity.
• It competitively binds to peripheral muscarinic receptors in the autonomic effector cells.
• It does not cross the blood-brain barrier and consequently has no CNS effects.

Uses:

• Glycopyrrolate is a preanesthetic medication to reduce salivary, tracheobronchial, and pharyngeal secretions as well as decreasing the acidity of gastric secretion.
• It is also used to reduce excessive saliva.
• It decreases acid secretion in the stomach and so may be used for treating stomach ulcers in combination with other medications.

Methantheline Bromide:

• Methantheline bromide is chemically, diethyl-methyl-[2-(9H-xanthene-9-carbonyloxy)ethyl]azanium;bromide.
• It is a muscarinic antagonist and finds importance in management of the symptoms of cramping and abdominal/stomach pain by decreasing stomach acid and slowing the intestines.
• It is an anticholinergic agent that also acts at the nicotinic cholinergic receptors of the sympathetic and parasympathetic systems as well as at the myoneural junction of the postganglionic cholinergic fibers.

Uses:

• Methantheline bromide is used in combination for the managment of peptic ulcer and irritable bowel syndrome.
• It is also used in treatment of gastritis, intestinal hypermotility, bladder irritability, cholinergic spasm and pancreatitis.
• It is contraindicated in glaucoma.

Propantheline Bromide:

• Propantheline bromide is chemically, methyl-di(propan-2-yl)-[2-(9H-xanthene-9- carbonyloxy)ethyl]azanium;bromide.
• It competitively antagonizes acetylcholine activity mediated by muscarinic receptors at neuroeffector sites on smooth muscle and exocrine gland cells.
• It is five times more potent than methantheline and also finds importance to control the symptoms of irritable bowel syndrome.

Uses:

• Propantheline bromide is an antimuscarinic agent used for the treatment of excessive sweating (hyperhidrosis), cramps or spasms of the stomach, intestines or bladder, and involuntary urination (enuresis).
• Propantheline relaxes the gut muscle and can relieve pain in conditions caused by spasm of the muscle in the gut.
• It relaxes the smooth muscle in the bladder and prevents the involuntary spasms that can allow leakage of urine from the bladder in the condition known as enuresis (involuntary urination in adults).
• Propantheline can also be used to treat excessive sweating because acetylcholine block also reduces secretions such as sweat and tears.

Amino Alcohol Ether Derivatives:

These cholinergic blockers were introduced as antiparkinsonian drugs due to their beneficial effect in the management of parkinsonism. These derivatives also posess antihistaminic properties since they are structurally related to H1 class of a antihistaminic drugs.

Benztropine Mesylate:

• Benztropine mesylate is chemically, 3-benzhydryloxy-8-methyl-8-azabicyclo[3.2.1] octane; methanesulfonic acid.
• It is a centrally acting anticholinergic/antihistamine agent.
• It is a selective M1 muscarinic acetylcholine receptor antagonist.
• It partially blocks cholinergic activity in the basal ganglia and has also been shown to increase the availability of dopamine by blocking its reuptake and storage in central sites, and as a result, increasing dopaminergic activity.
• It antagonizes the effect of acetylcholine, decreasing the imbalance between the neurotransmitters acetylcholine and dopamine, which may improve the symptoms of early parkinson’s disease.

Uses:

• Benztropine mesylate is a centrally active muscarinic antagonist that has been used in the symptomatic treatment of Parkinson’s disease.
• It is used to reduce extrapyramidal side effects of antipsychotic treatment.
• It improves tremor and may alleviate rigidity and bradykinesia.
• Benzatropine is also used for the treatment of dystonia, a rare disorder that causes abnormal muscle contraction, resulting in twisting postures of limbs, trunk, or face.

Orphenadrine Citrate:

• Orphenadrine citrate is chemically, phenylmethoxy]ethanamine; 2-hydroxypropane-1,2,3-tricarboxylic acid.
• It is an anticholinergic drug of the ethanolamine antihistamine class (it is closely related to diphenhydramine).
• It blocks cholinergic receptors, thereby interfering with the transmission of nerve impulses from the spinal cord to the muscles.

Uses:

• Orphenadrine citrate is used to treat muscle pain and to help with motor control in parkinson’s disease.
• It is used to relieve pain caused by muscle injuries like strains and sprains.
• Orphenadrine and other muscle relaxants are sometimes used to treat pain arising from rheumatoid arthritis.
• It also possess antihistamine property due to H1 receptor antagonist action.

Amino Alcohol Derivatives:

These cholinergic blockers were introduced in 1940s, and were established for their antispasmodic importance. They are also used as antiparkinsonian drugs due to their beneficial effect in the management of parkinsonism.

These derivatives have structural similarity with atropine derivatives possessing a bulky group and a cyclic amino function, amino propanol arrangement with three carbon chains between the hydroxy function and the amino group is the common structural features of these amino alcohol derivatives.

Biperidin Hydrochloride:

• Biperiden hydrochloride is chemically, 1-(5-bicyclo[2.2.1]hept-2-enyl)-1-phenyl-3- piperidin-1-ylpropan-1-ol;hydrochloride.
• It is a muscarinic antagonist that has effects in both the central and peripheral nervous systems.

Uses:

• Biperiden hydrochloride is used to treat Parkinson disease.
• Common side effects include blurred vision, dry mouth, sleepiness, constipation, and confusion.
• It should not be used in people with a bowel obstruction or glaucoma.
• It is used to improve acute extrapyramidal side effects, it relieves muscle rigidity, reduces abnormal sweating and salivation, improves abnormal gait (particular way of walking) and to lesser extent tremor.
• It has a strong nicotine receptor blocking effect and helpful in nicotine induced convulsions hence, biperiden has been analyzed as an alternative anticonvulsant for usage.
• It is contraindicated in all forms of epilepsy.

Procyclidine Hydrochloride:

• Procyclidine hydrochloride is chemically, (1R)-1-cyclohexyl-1-phenyl-3-pyrrolidin- 1-ylpropan-1-ol;hydrochloride.
• It is a muscarinic antagonist that crosses the blood-brain barrier.
• It acts by blocking central cholinergic receptors, and thus balancing cholinergic and dopaminergic activity in the basal ganglia.

Uses:

• Procyclidine is principally used for the treatment of drug-induced extra pyramidal disorders and in parkinsonism, akathisia (feeling of inner restlessness) and acute dystonia (sustained muscle contractions) or secondary dystonia.
• It is a second-line drug for the treatment of parkinson’s disease. It improves tremor, but not rigidity or bradykinesia.
• It also has antispasmodic effect on smooth muscle and may produce mydriasis and reduction in salivation.

Tridihexethyl Chloride:

• Tridihexethyl chloride is chemically, (3-cyclohexyl-3-hydroxy-3-phenylpropyl)- triethylazanium; chloride.
• It is an anticholinergic, antimuscarinic and antispasmodic drug.
• It may block all the three (M1, M2 and M3) types of muscarinic receptors.
• It has a pronounced antispasmodic and antisecretory effect on the gastrointestinal tract.

Uses:

• It may be used, usually in combination with other drugs, to treat acquired nystagmus (involuntary eye movement) or peptic ulcer disease.

Amino Amide Derivatives:

The amino amide derivatives are structurally similar to the amino alcohol derivatives, where the polar hydroxyl function is replaced by a polar amide function resulting in reduced metabolism, hence these derivatives have increased duration of action in comparison to the amino alcohol derivatives.

They also have structural features similarity with atropin derivatives. They possess a bulky aromatic group usually a phenyl moiety with respect to the both ‘A’ and ‘B’ substitutents on the pivotal carbon and amide function.

Isopropamide Iodide:

• Isopropamide iodide is chemically, (4-amino-4-oxo-3,3-diphenylbutyl)-methyl- di(propan-2-yl)azanium;iodide.
• It is most often provided as an iodide salt, but is also available as a bromide or chloride salt.
• It is a long-acting quaternary anticholinergic drug.

Uses:

• Isopropamide iodide is used in the treatment of peptic ulcers and other gastrointestinal disorders involving hyperacidity (gastrointestinal acidosis) and hypermotility.

Tropicamide:

• Tropicamide is chemically, N-ethyl-3-hydroxy-2-phenyl-N-(pyridin-4-ylmethyl)- propanamide.
• It is a synthetic muscarinic antagonist with having similar action to that of atropine and with an anticholinergic property.
• It binds to and blocks the muscarinic receptors (M4) in the sphincter and ciliary muscle in the eye and thus works as a mydriatic and relaxes the sphincter muscle of the Iris and paralysis of the ciliary muscle.

Uses:

• Tropicamide is an anticholinergic drug and is used to dilate the pupil and helps in examination of the eye.
• It is a diagnostic agent and is used to produce short-duration mydriasis and cycloplegia.

Miscellaneous Derivatives:

Drugs discussed in the miscellaneous derivatives are synthetic molecules without any structural resemblance with the other synthetic anticholinergic agents.

Ethopropazine Hydrochloride:

• Ethopropazine hydrochloride is chemically, N,N-diethyl-1-phenothiazin-10-ylpropan- 2-amine;hydrochloride.
• It is a phenothiazine derivative with anticholinergic activity.
• Drowsiness and dizziness are the most common side effects.
• It is contraindicated in conditions such as glaucoma because of its mydriatic effect.

Uses:

• Ethopropazine hydrochloride is used as an antiparkinsonian agent that also has antihistamine, and antiadrenergic actions.
• It is also used in the alleviation of the extrapyramidal syndrome induced by drugs.
• It is also effective on tremor, sialorrhea, (excessive salivation) and oculogyric crises (prolonged involuntary upward deviation of the eyes).

Synthesis

Carbachol

Neostigmine

Ipratropium Bromide

Dicyclomine Hydrochloride

Procyclidine Hydrochloride

Multiple Choice Questions

Question 1. Acetylcholine is not a specific neurotransmitter at:

1. Sympathetic ganglia
2. Sympathetic postganglionic nerve endings
3. Parasympathetic ganglia
4. Parasympathetic postganglionic nerve endings

Answer. 2. Sympathetic postganglionic nerve endings

Question 2. Muscarinic receptors are located in:

1. Autonomic ganglia
2. Skeletal muscle neuromuscular junctions
3. Autonomic effector cells
4. Sensory carotid sinus baroreceptor zone

Answer. 3. Autonomic effector cells

Question 3. Substitution of acetylcholine at ẞ-position with respect to the nitrogen atom:

1. Decreases the nicotinic activity
2. Increases the nicotinic activity
3. Increases the muscarinic activity
4. Decreases both nicotinic and muscarinic activity

Answer. 3. Increases the muscarinic activity

Question 4. Indicate the location of M2 cholinoreceptor type:

1. Heart
2. Glands
3. Smooth muscle
4. Endothelium

Answer. 1. Heart

Question 5. Which of the following cholinomimetics activates both muscarinic and nicotinic receptors?

1. Lobeline
2. Pilocarpine
3. Nicotine
4. Bethanechol

Answer. 4. Bethanechol

Question 6. Atropine contains

1. One asymmetric carbon
2. Three asymmetric carbons
3. Two asymmetric carbons
4. Four asymmetric carbons

Answer. 2. Three asymmetric carbons

Question 7. Indicate a cholinomimetic agent, which is related to direct-acting drugs:

1. Edrophonium
2. Physostigmine
3. Carbachol
4. Isofluorophate

Answer. 3. Carbachol

Question 8. Acetylcholine is not used in clinical practice because:

1. It is very toxic.
2. The doses required are very high.
3. It is very rapidly hydrolyzed.
4. It is very costly.

Answer. 3. It is very rapidly hydrolyzed.

Question 9. Parasympathomimetic drugs cause:

1. Bronchodilation
2. Mydriasis
3. Bradycardia
4. Constipation

Answer. 3. Bradycardia

Question 10. Which of the following direct-acting cholinomimetics is mainly muscarinic in action?

1. Bethanechol
2. Carbachol
3. Acetylcholine
4. None of the above

Answer. 1. Bethanechol

Question 11. Which of the following direct-acting cholinomimetics has the shortest duration of action?

1. Acetylcholine
2. Methacholine
3. Carbachol
4. Bethanechol

Answer. 1. Acetylcholine

Question 12. Which of the following cholinomimetics is indirect-acting?

1. Lobeline
2. Edrophonium
3. Pilocarpine
4. Carbachol

Answer. 2. Edrophonium

Question 13. Indicate a reversible cholinesterase inhibitor:

1. Isofluorophate
2. Carbachol
3. Physostigmine
4. Parathion

Answer. 3. Physostigmine

Question 14. Which of the following cholinesterase inhibitors is irreversible?

1. Physostigmine
2. Edrophonium
3. Neostigmine
4. Isofluorophate

Answer. 4. Isofluorophate

Question 15. Indicate cholinesterase activator:

1. Pralidoxime
2. Edrophonium
3. Pilocarpine
4. Isofluorophate

Answer. 1. Pralidoxime

Question 16. Which of the following cholinomimetics is commonly used in the treatment of glaucoma?

1. Pilocarpine
2. Lobeline
3. Acetylcholine
4. Neostigmine

Answer. 1. Pilocarpine

Question 17. Chronic long-term therapy of myasthenia is usually accomplished with:

1. Edrophonium
2. Neostigmine
3. Echothiophate
4. Carbachol

Answer. 2. Neostigmine

Question 18. Indicate the reversible cholinesterase inhibitor, which penetrates the blood-brain barrier:

1. Physostigmine
2. Edrophonium
3. Neostigmine
4. Piridostigmine

Answer. 1. Physostigmine

Question 19. Indicate a muscarinic receptor-blocking drug:

1. Scopolamine
2. Pipecuronium
3. Trimethaphan
4. Pilocarpine

Answer. 1. Scopolamine

Question 20. The mechanism of atropine action is:

1. Competitive ganglion blockade
2. Competitive muscarinic blockade
3. Competitive neuromuscular blockade
4. Non-competitive neuromuscular blockade

Answer. 2. Competitive muscarinic blockade

Question 21. Atropine is highly selective for:

1. M1-receptor subtype
2. M2-receptor subtype
3. M3-receptor subtype
4. All of the above

Answer. 4. All of the above

Question 22. Atropine causes:

1. Miosis, a reduction in intraocular pressure and cyclospasm
2. Mydriasis, a rise in intraocular pressure and cycloplegia
3. Miosis, a rise in intraocular pressure and cycloplegia
4. Mydriasis, a rise in intraocular pressure and cyclospasm

Answer. 2. Mydriasis, a rise in intraocular pressure and cycloplegia

Question 23. Atropine causes:

1. Bradycardia, hypotension and bronchoconstriction
2. Tachycardia, little effect on blood pressure and bronchodilation
3. Decrease in contractile strength,conduction velocity through the AV node
4. Tachycardia, hypertensive crisis and bronchodilation

Answer. 2. Tachycardia, little effect on blood pressure and bronchodilation

Question 24. Atropine is frequently used prior to administration of inhalant anesthetics to reduce:

1. Muscle tone
2. Secretions
3. Nausea and vomiting
4. All of the above

Answer. 2. Secretions

Question 25. Which of the following drugs is useful in the treatment of uterine spasms?

1. Carbachol
2. Vecuronium
3. Atropine
4. Edrophonium

Answer. 3. Atropine

Question 26. Indicate the antimuscarinic drug, which is used as a mydriatic:

1. Pilocarpine
2. Neostigmine
3. Homatropine
4. Ipratropium

Answer. 3. Homatropine

Question 27. Which of the following agents is used as an inhalation drug in asthma?

1. Atropine
2. Ipratropium
3. Lobeline
4. Homatropine

Answer. 2. Ipratropium

Question 28. Indicate an antimuscarinic drug, which is effective in the treatment of mushroom poisoning:

1. Pralidoxime
2. Homatropine
3. Pilocarpine
4. Atropine

Answer. 4. Atropine

Drugs Action On Central Nervous System Introduction

The drugs fall under this category having common effects as depression of neuronal activity in the Central Nervous System (CNS), i.e. the brain and spinal cord. In most of the cases the entire brain is intended as target organ, whereas in a few cases skeletal muscle or the spinal cord is targeted. Various classes of the drugs falling under CNS depressant are anxiolytics, sedatives and hypnotics, general anaesthetics, anticonvulsants and antipsychotics.

The Anxiolytic drugs are used is in the treatment of the anxiety disorders, which are conditions characterized by excessive or inappropriate anxiety. The major use of sedative is to calm the patient, whereas hypnotic drugs are used in the treatment of insomnias which are failures to get adequate sleep. The clinical significance of general anaesthetic agents is to produce unconsciousness and loss of perception to pain during surgical procedures.

Anticonvulsant drugs are mainly used to prevent or lessen the sudden excessive electrical activity in brain neurons that is a characteristic of the convulsions or epilepsies. Antipsychotics are used in the disorders like psychoses, most notably the schizophrenias.

Sedatives And Hypnotics Introduction

Hypnotics are used to induce sleep resembling the natural one. They are central nervous system depressants; and produce drowsiness and sound sleep. They find importance in the management of insomnia (sleeplessness) and also as a preanesthetic medication. They find similarity with sedatives.

A sedative drugs induce sedation by reducing irritability or excitement and are central nervous system depressants. Sedative drugs serve to calm or relieve anxiety, restlessness and emotional tension. Whilst, hypnotic drugs are to initiate and/or to sustain a quality sleep in the management of insomnia.

These drugs produce dose-dependent effects highlighting their anxiolytic effect and to that to produce sound sleep. Hence, they are collectively known as sedative-hypnotic drugs. Sedative-hypnotic drugs may produce dependence, and that optimal treatment is to use the lowest effective dose for the shortest therapeutic time period necessary, with gradual discontinuation. They are habit-forming and chronic use is known to disturb the human sleep pattern.

Insomnia is a sleep disorder highlighted with trouble sleeping or sleeplessness which may either no quality or quantity of sound sleep. Insomnia may be indicated with difficulty in falling asleep or staying asleep for the definite duration. Insomnia is followed by daytime sleepiness with low energy, irritability, and also a depressed mood.

Read and Learn More Medicinal Chemistry III Notes

Insomnia can be transient, short term or long term, lasting for days or weeks, or lasting for more than a month. Insomnia can occur independently or as a result of another problem like psychological stress, chronic pain, heart failure, hyperthyroidism, heartburn, restless leg syndrome, menopause and even dependence for caffeine, nicotine, and alcohol.

In addition to benzodiazepines, barbiturates, and a miscellaneous group, many drugs belonging to other pharmacological classes may possess one or more of the anxiolytic, sedative and hypnotic activities.

Mechanism Of Action Of Sedatives And Hypnotics

Gama Amino Butyric Acid (GABA) Activity Enhancers / GABA Receptor Modulators:

The GABAA receptor is one of the two ligand-gated ion channels responsible for mediating the effects of GABA, the major inhibitory neurotransmitter in the brain. The GABAergic stimulation in the brain via the activation of the GABAA also known as benzodiazepine receptors or referred as chloride channel complex, allows increased chloride conductance, thereby resulting in CNS depressant effect. The agonist drugs like barbiturates, benzodiazepines; bind to an allosteric site on the postsynaptic GABAA receptor complex that increase chloride conductance.

Classification Of Hypnotics And Sedatives

An arbitrary classification of hypnotics and sedatives is as follows:

• Benzodiazepine derivatives eg. Chlordiazepoxide, Diazepam, Oxazepam, Chlorazepate, Lorazepam, Alprazolam, Zolpidem.
• Barbiturate derivatives e.g. Barbital, Phenobarbital, Mephobarbital, Amobarbital, Butabarbital, Pentobarbital, Secobarbital.
• • Long acting (Duration of action more than 6 hours) e.g. Barbital, Phenobarbital, Mephobarbital.
  • Intermediate acting (Duration of action of 3-6 hours) e.g. Amobarbital, Butabarbital.
  • Short acting (Duration of action of less than 3 hours) e.g. Pentobarbital, Secobarbital.
• Miscellaneous:
  • Amide and Imide derivatives e.g. Glutethmide.
  • Alcohol and their carbamate derivatives e.g. Meprobomate, Ethchlorvynol.
  • Aldehyde and their derivatives e.g. Triclofos sodium, Paraldehyde.

Benzodiazepines

The benzodiazepines are lipophilic in nature and are in non-ionized form and thus well absorbed from the GI tract. The benzodiazepine derivatives with 3-hydroxyl function are polar compounds and are absorbed slowly in comparison to the other lipophilic derivatives.

Due to higher lipophilic nature, these drugs are bound to plasma proteins; but do not compete with other protein bound drugs. The benzodiazepine derivatives are effectively distributed because of its more lipophilic nature.

Structure-Activity Relationship Of Benzodiazepines:

Benzodiazepine derivatives are chemically 5-aryl-1,4-benzodiazepine with a carboxamide function in the seven-member diazepine ring moiety. The SAR study has been classified into three fragments of the 5-aryl-1,4-benzodiazepine nucleus.

Ring A-Aryl or Heteroaryl Moiety

• The primary fragment of the aryl or heteroaryl moiety as the ring A, the aromatic or heteroaromatic ring A is prerequisite for the activity. The moiety exhibits л-л stacking with the aromatic amino acid residues of the GABAA receptor.
• Heteroaromatic moiety derivatives are more potent than aromatic moiety derivatives. An electronegative substituent like -Cl, -NO2 or -CF3 function with electron withdrawing ability at position 7 is required for activity, and better electro-negativity directly correlates with higher activity..
• Any substitution at positions 6, 8, and 9 leads to loss of activity.

Ring B-1,4-benzodiazepine Moiety

• The 1,4-benzodiazepine moiety represents the ring B; the second fragment of the SAR study.
• The saturation of the double bond at positions 4 and 5 (N=C) or shifting of the same to the positions 3 and 4 results in decrease in the activity.
• Any alkyl substitution irrespective of the higher or lower function at the position 3 leads to decrease in the activity.
• Substitution with a hydroxyl (-OH) function at position 3 does not affect the activity of the molecule, but the polar function is readily converted to the glucuronide conjugate and is excreted in urine and thus is short-lived, resulting in decreased duration of action. It highlighted the importance of derivatives with short activity and helpful to induce sleep.
• The absence of the hydroxyl group at position 3 (no substitution) indicates compounds with better duration of action. These derivatives are pharmacokinetically important, derivatives lacking a hydroxyl group at position 3 are non-polar in nature, and are metabolized to the pharmacologically active 3-hydroxylated metabolite in the hepatic CYP450 system slowly. The active 3-hydroxyl metabolites have longer half- lives resulting in increased duration of action.
• The nitrogen at position 1 and the carbonyl (C=O) function at position 2 are important for activity.
• The N-alkyl side chains are tolerated, usually a lower alkyl function.
• A proton-accepting group, i.e. carbonyl (C=O) function at position 2 is required and interacts with histidine residue at the receptor core (a proton donor) in benzodiazepine binding site of GABAA receptor.
• A triazole or imidazole rings are also equally effective, since they are also capable accepting a proton from the histidine residue at the receptor core and exhibiting the H-bonding. The triazole or imidazole moiety can be fused on positions 1 and 2, and eventually lead to the increase in the activity. Derivatives like triazolam and alprazolam highlight drugs with a fused triazolo ring.
• While, Midazolam is a derivative indicated with a fused imidazole ring. Interestingly, an electron-attracting group at position 7 is not required for activity in few of these derivatives.
• These triazole or imidazole rings derivatives are short acting as they are rapidly metabolized by a-hydroxylation of the methyl substituent on the triazolo or imidazolo moiety similar to benzylic oxidation via CYP450 mixed oxidase function. The resulting active a-hydroxylated metabolites are inactivated by glucuronidation. These derivatives are also metabolized by 3-hydroxylation of the benzodiazepine ring.

Ring C-Phenyl Substituent

• A phenyl ring C at position 5 promotes activity.
• Substitution on the phenyl group at ortho (2′) or diortho (2′,6′) positions with electron-withdrawing groups results in increased activity.
• Whilst, substitution on the para (4′) results in profound decrease in activity.

Metabolism of Benzodiazepines:

The benzodiazepines are extensively metabolized via CYP450 mixed function oxidase system, majorly CYP4503A4 and CYP2C19; some of the metabolites of selective benzodiazepines are active and few also have longer half-lives indicating their increased duration of action.

The CYP3A4 inhibitors like ketoconazole or food like grapefruit juice inhibits their metabolism and can affect their duration of action and equally toxicity. The benzodiazepine derivatives have lower abuse potential and a much greater safety margin than that of the barbiturates.

Chlordiazepoxide:

• Chlordiazepoxide is chemically, 7-chloro-2-methylamino-5-phenyl-3H-1,4-benzo- diazepine-4-oxide.
• It is a sedative and hypnotic drug under the benzodiazepine class and the first benzodiazepine to be synthesized.
• It is a long-acting drug and even its metabolite is active and has a long half-life.
• It enhances GABAA receptor activity and results in an increased binding of the inhibitory neurotransmitter GABA to the GABAA receptor leading to inhibitory effects on the Central Nervous System.

Uses:

• Chlordiazepoxide is used in the management of epilepsy, anxiety, insomnia and alcohol or drug abuse withdrawal symptoms.

Diazepam:

• Diazepam is chemically, 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzo- diazepin-2-one.
• It is a sedative and hypnotic drug under the benzodiazepine class and produces a calming effect.
• It acts by increasing the effect of the neurotransmitter GABA. It enhances GABAA receptor activity leading to Central Nervous System depression.

Uses:

• Diazepam is most frequently prescribed medications in the world under the class of benzodiazepines.
• It is used in the management of epilepsy, anxiety, insomnia, muscle spasms, seizures, trouble sleeping, restless legs syndrome and alcohol or drug abuse withdrawal symptoms.

Oxazepam:

• Oxazepam is chemically, 7-Chloro-1,3-dihydro-3-hydroxy-5-phenyl-2H-1, 4-benzo- diazepin-2-one.
• Oxazepam acts on benzodiazepine receptors, resulting in increased effect of GABA to the GABAA receptor which results in inhibitory effects on the Central Nervous System. Oxazepam exists as a racemic mixture, and has no therapeutic benefit to the administration of a single enantiomer over the racemic mixture.
• It is a short acting benzodiazepine with a slow onset of action. It is an example for an active metabolite formed during the metabolism of diazepam and other benzodiazepine drugs.
• It is a 3-hydroxy benzodiazepine derivative highlighting its slow absorption and short action, and safer than other benzodiazepines in patients with impaired liver function as it does not require hepatic oxidation, and is simply metabolized by glucuronidation. So oxazepam is less likely to accumulate and cause adverse reactions.

Uses:

• Oxazepam is used in the management of anxiety, insomnia and in the control of symptoms of alcohol withdrawal syndrome.

Clorazepate:

• Clorazepate is chemically, 7-chloro-2,3-dihydro-2-oxo-5-phenyl-1H-1,4-benzo- diazepine-3-carboxylic acid.
• It acts on benzodiazepine receptors, resulting in increased effect of GABA to the GABAA receptor which results in inhibitory effects on the Central Nervous System.
• Clorazepate is an example for prodrug, the parent drug undergoes decarboxylation in the acidic environment of the GIT to form nordiazepam; a non-polar and active metabolite with a half-life of more than forty hours.
• The metabolite desmethyldiazepam is responsible for its therapeutic effects. The desmethyldiazepam is further metabolized to oxazepam which also possesses activity.

Uses:

• It is a long acting benzodiazepine medication, and finds importance as anxiolytic, anticonvulsant, sedative, hypnotic, and skeletal muscle relaxant.

Lorazepam:

• Lorazepam is chemically, 7-Chloro-5-(2-chlorophenyl)-1,3-dihydro-3-hydroxy-2H- 1,4-benzodiazepin-2-one.
• It acts on benzodiazepine receptors, resulting in increased effect of GABA to the GABAA receptor which results in inhibitory effects on the Central Nervous System.

Uses:

• Lorazepam is a benzodiazepine medication, used to treat anxiety, insomnia, seizures, and chemotherapy-induced nausea and vomiting.
• It is also finds importance as preoperative medication.

Alprazolam:

• Alprazolam is chemically, 8-chloro-1-methyl-6-phenyl-4H-[1,2,4]triazolo[4,3-a][1,4] benzodiazepine.
• It acts on benzodiazepine receptors, resulting in increased effect of GABA to the GABAA receptor which results in inhibitory effects on the Central Nervous System.

Uses:

• Alprazolam is a potent, short-acting benzodiazepine drug, and finds importance as an anxiolytic, sedative and hypnotic.
• It is commonly used for the treatment of generalized anxiety disorder or social anxiety disorder.

Zolpidem:

• Zolpidem is chemically, N,N,6-Trimethyl-2-(4-methylphenyl)imidazo[1,2-a]pyridine-3- acetamide.
• It acts on benzodiazepine receptors, resulting in increased effect of GABA to the GABAA receptor which results in inhibitory effects on the Central Nervous System.

Uses:

• Zolpidem is a non-benzodiazepine and hypnotic of the imidazopyridine class, a medication primarily used for the short term treatment of sleeping problems.

Barbiturates

The barbiturates were previously used extensively as sedative-hypnotic drugs. Now, they have been replaced by the safer benzodiazepine. Barbiturates act by binding to an allosteric recognition site on GABAA receptors that positively modulate the effect of the GABAA. They bind at different binding sites unlike benzodiazepines, and have increased duration of the GABA-gated chloride channel openings.

• The barbiturates are chemically 5,5-disubstituted-2,4,6-trioxohexahydropyrimidine or 5,5-disubstituted barbituric acids.
• The structure of barbituric acids indicates their acidic character.
• The derivatives without methyl substituents on the nitrogen exhibit a pKa’s of about
• 7.6; and with a methyl substituent they have pKa’s of about 8.4.
• The free acids have poor water solubility and good lipid solubility.
• The 2-thiobarbiturates also have greater lipophilicity, as the sulfur atom increases lipid solubility.
• Sodium salts of the barbiturates are readily prepared and are water soluble.

Structure-Activity Relationship Of Barbiturates:

• The barbituric acid molety Le. 24,6-trioxohexahydropyrimidine lacks any kind of depressant activity.
• The disubstitution at position 5 of barbituric acid, where both the hydrogens are needed to be replaced by an akyl/aryl group for CNS depression activity.
• The activity is marked due to the replacement of both the hydrogens at position 5, as even if one of the hydrogen is available at position 5, the molecule 2,4,6-trioxohexahydropyrimidine or 5-substituted-24,6-trioxohexahydropyrimidine exhibits tautomerization resulting in a highly acidic trihydroxypyrimidine close to a pka value 4. Thus the compound is largely in the anionic form at physiological pH, with little nonionic lipid-soluble nature available to cross the blood-brain barrier.
• The substitution with alkyls function of both the hydrogens at position 5 of the molecule confers activity with a rapid onset, but a decrease in duration of action.
• This is attributed to the increasing lipophilicity, as an increase in lipophilicity leads to quick or rapid onset of action, and this may be attributed to an ability to penetrate the BBB. An increase in lipophilicity also enhances the hypnotic potency. Redistribution of the molecules confers to decreased concentration of the molecule in CNS and partitioning out of the brain to other sites results in decreased duration of action.
• The short duration of action may also be attributed to its ability to penetrate liver microsomes and biotransformed inactive metabolite.
• The substitution at position 5 with an alkyl group can be of increasing carbon skeleton up to about 5 to 6 carbon moiety for optimal activity. An increase beyond 7 to 9 carbon moiety leads to decrease in depression activity and precipitation of convulsion.
• Substitution of branched cyclic or unsaturated carbon moiety leads to derivatives with short duration of action. These derivatives are oxidative metabolized to form hydroxyl function polar metabolite and eliminated rapidly via conjugation.
• Substitution of lower alkyl group like ethyl and a phenyl moiety at position 5 results in compounds with lower lipophilicity and leads to derivatives with slower onset of action, but relatively longer duration of action.
• There is an inverse correlation between the total number of carbon atoms substituted on the position 5 and the duration of action.
• • This can be better understood based on the character of the substituent taken into account.
  • The relatively polar character of a phenyl substituent results in long acting barbiturate.
  • An alkyl substituent (3-4 carbon aliphatic chain), branching of alkyls, presence of an isolated double or triple bond substituted derivatives result in compounds with intermediate to short active.
• Substitution at position 1 on nitrogens with lower alkyl group results with increased lipophilicity of the molecule leading to increased activity.
• Di-substitution on both the nitrogen at position 1 and 3 results in inactive compounds, this is attributed to the pharmacokinetic parameter, due to the inability to from sodium salt of the barbituric acid and completely water insoluble derivative.
• Therefore following derivatives are only active:
  • 5,5-disubstituted barbituric acid.
  • 1,5,5-trisubstituted barbituric acid.
  • 5,5-disubstituted thio-barbituric acid.
• While derivatives given below are inactive:
  • Non-substituted barbituric acid.
  • 5-monosubstituted barbituric acid.
  • 1,3-disubstituted barbituric acid.
  • 1,3,5,5-tetrasubstituted barbituric acid.

Synthesis of Barbiturates:

The barbiturates are 5,5-disubstituted barbituric acids. The following scheme shows how the 5.5-dialkyl compounds are synthesized.

Metabolism of Barbiturates:

The substituent at position 5 can influence the ease of oxidative metabolism by effects on bond strengths as well as by influencing partitioning. The absorption from the GI tract of the barbiturates is optimal and exhibits substantial plasma proteins binding. The metabolism pattern of the compounds with low lipophilicity is usually excreted intact in the urine, while highly lipophilic compounds are excreted after metabolism to polar metabolites.

With the increase in the lipophilicity, generally the rate of metabolism increases, except for compounds having extremely high lipophilicity like thiopental, which tend to depotize and are thus relatively unavailable for metabolism.

Metabolism generally follows an oxidative biotransformation. N-methylation decreases duration of action, by increasing the concentration of the lipid-soluble free barbituric acid. 2-thiobarbiturates have a very short duration of action because its lipophilicity is extremely high, promoting depotization.

Side-effects of Barbiturates:

The common side effects of barbiturates include headache, drowsiness, dizziness, ataxia, respiratory depression, hypersensitivity reactions, paradoxical excitement and confusion.

Long Acting Barbiturates (Duration of action > 6 hours):

Barbital:

• Barbital is chemically, 5,5-diethyl-2,4,6(1H,3H,5H)-pyrimidinetrione.
• It is also known as barbitone which is chemically 5,5-diethylbarbituric acid, the first commercially available barbiturate.
• It increases the activity of the inhibitory neurotransmitter GABA to the GABAA receptor which results in inhibitory effects on the Central Nervous System.

Uses:

• Barbital is used as a hypnotic and antiepileptic..

Phenobarbital:

• Phenobarbital is chemically, 5-ethyl-5-phenylbarbituric acid.
• It is a long acting barbiturate derivative and also known as Phenobarbitone, which is the oldest, but still used as anti-epileptic medication.
• It increases the activity of the inhibitory neurotransmitter GABA to the GABAA receptor which results in increased influx of chloride ions leading to decreased excitability.
• It also produces blockade of excitatory glutamate signaling.
• Phenobarbital is a CYP450 inducer and hence dosing of other drugs is needed to be monitored.

Uses:

• Phenobarbital is used as a sedative, hypnotic and also as an anticonvulsant for both generalized tonic-clonic and partial seizures.
• It is occasionally used to treat drug withdrawal, as a pre anaesthetic medication, Crigler-Najjar syndrome and Gilbert syndrome patients to aid in the conjugation of bilirubin.

Mephobarbital:

• Mephobarbital is chemically, 3-methyl-5-ethyl-5-phenylbarbituric acid.
• It is a long acting barbiturate derivative, as the parent drug, it undergoes metabolically N-demethylated to phenobarbital and responsible for its activity.
• It increases the activity of the inhibitory neurotransmitter GABA to the GABAA receptor which results in inhibitory effects on the Central Nervous System.

Uses:

• Mephobarbital is used as a hypnotic and as an anticonvulsant.

Intermediate Acting Barbiturates (Duration of action = 3-6 hours)

Amobarbital:

• Amobarbital is chemically, 5-ethyl-5-(3-methylbutyl)-2,4,6-(1H,3H,5H)-pyrimidine- trione.
• It is also known as amylobarbitone, which is an intermediate acting barbiturate with moderate duration of action of 4-5 hours.
• It increases the activity of the inhibitory neurotransmitter GABA to the GABAA receptor which results in increased influx of chloride ions leading to decreased excitability.

Uses:

• Amobarbital is used as sedative and hypnotic.

Butabarbital:

• Butabarbital is chemically, 5-ethyl-5-(1-methylpropyl)-2,4,6(1H,3H,SH)-pyrimidine- trione.
• It is an intermediate acting barbiturate with moderate duration of action of 3-6 hours. It increases the activity of the inhibitory neurotransmitter GABA to the GABAA receptor which results in increased influx of chloride ions leading to decreased excitability.
• It has a particularly fast onset of effects and short duration of action, has importance in management of severe insomnia and relieving anxiety before surgical procedures.

Uses:

• Butabarbital is used as sedative and hypnotic.

Short Acting Barbiturates (Duration of action < 3 hours)

Pentobarbital:

• Pentobarbital is chemically, 5-ethyl-5-(1-methylbutyl)-2,4,6(1H,3H,5H)-pyrimidine-trione.
• It is a short acting barbiturate with duration of action less than 3 hrs.
• It increases the activity of the inhibitory neurotransmitter GABA to the GABAA receptor which results in increased influx of chloride ions leading to decreased excitability.

Uses:

• Pentobarbital is used as sedative and hypnotic.
• It also finds importance as a preanesthetic medication and in control of convulsions in emergencies.
• It has an application in reducing intracranial pressure in Reye’s syndrome, traumatic brain injury and induction of coma in cerebral ischemia patients.
• It is also used as a veterinary anaesthetic agent.

Secobarbital:

• Secobarbital is chemically, 5-(1-methylbutyl)-5-(2-propenyl)-2,4,6(1H,3H,5H)-pyrimidinetrione.
• It is a short acting barbiturate derivative.
• It increases the activity of the inhibitory neurotransmitter GABA to the GABAA receptor which results in increased influx of chloride ions leading to decreased’ excitability.

Uses:

• Secobarbital is used as an anaesthetic, anticonvulsant, anxiolytic, sedative, and hypnotic drug.

Miscellaneous Sedatives and Hypnotic Derivatives

A wide range of chemical structures (e.g. imides, amides, alcohols) can produce sedation and hypnosis resembling those produced by the barbiturates. Despite this apparent structural diversity, the compounds have generally similar structural characteristics and chemical properties.

Amide and Imide Derivatives:

Glutethimide:

• Glutethimide is chemically, 2-ethyl-2-phenyl glutarimide.
• It is one of the most active non-barbiturate hypnotics that is structurally similar to the barbiturates. Glutethimide is highly lipophilic and undergoes extensive oxidative metabolism with a half-life period of approximately 10 hours.

Uses:

• It is used as a hypnotic and sedative drug.

Alcohols and their Carbamate Derivatives:

Meprobamate:

• Meprobamate is chemically, 2-methyl-2-propyl-1,3-propanediol dicarbamate.
• Meprobamate does not act through GABA system.
• It has inter-neuronal blocking properties at the level of the spinal cord, and responsible for its skeletal muscle relaxation.

Uses:

• Meprobamate is indicated as an antianxiety agent and also as a sedative and hypnotic agent.
• It is also effective against absence seizures and also a centrally acting skeletal muscle relaxant.

Ethchlorvynol:

• Ethchlorvynol is chemically, 1-chloro-3-ethyl-1-penten-4-yn-3-ol.
• It is a mild sedative and hypnotic with a quick onset and short duration of action. It is highly lipophilic and extensively metabolized to its secondary alcohol.
• It is highly habit forming and extremely physically addictive.
• A mechanism of action is similar to the benzodiazepines and barbiturates.
• It increases the activity of the inhibitory neurotransmitter GABA to the GABAA receptor which results in increased influx of chloride ions leading to decreased excitability.

Uses:

• Ethchlorvynol is used as a mild sedative and hypnotic drug.

Aldehydes and Their Derivatives

Triclofos sodium:

• Triclofos sodium is chemically, sodium-2,2,2-trichloroethyl hydrogen phosphate.
• It is a prodrug which is metabolized in the liver into the active drug trichloro ethanol.
• As trichloroethanol may cause liver damage, therefore triclofos should not be used for extended periods.
• The half-life of triclofos is long and it may cause drowsiness the next day.

Uses:

• Triclofos sodium is a sedative hypnotic drug used seldom for treating insomnia, as it caused dependence and only be used for the short term to relief of severe insomnia.

Paraldehyde:

• Paraldehyde is chemically, 2,4,6-Trimethyl-1,3,5-trioxane.
• It, also known as paracetaldehyde, is the cyclic trimer of acetaldehyde, a liquid with a strong characteristic odour and an unpleasant taste. These properties limit its use as medicine.

Uses:

• Paraldehyde is a CNS depressant and an anticonvulsant, hypnotic and sedative.
• It was also included in some cough medicines as an expectorant.
• Paraldehyde is the oldest as it was introduced into clinical practice in 1882 and one of the safest hypnotics and was given to psychiatric and geriatric patients up to the 1960s.
• As an anti-seizure, it was used in the treatment of convulsions to treat status epilepticus.
• Industrially paraldehyde is used in resin manufacture and also as a preservative.

Synthesis

Diazepam

Barbital

Multiple Choice Questions:

Question 1. Hypnotic drugs are used to treat:

1. Psychosis
2. Sleep disorders
3. Narcolepsy
4. Parkinsonian disorders

Answer. 2. Sleep disorders

Question 2. 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepine-2-one is IUPAC name of

1. Oxazepam
2. Nitrazepam
3. Diazepam
4. Chlordiazepoxide

Answer. 3. Diazepam

Question 3. The IUPAC name of glutethimide is:

1. 3-Ethyl-3-phenyl-2,6-piperdine dione
2. p-Sulphonamido chloroimido benzoic acid
3. 3-(5-Nitrofurfurylideneamino)-oxazolidin-2-one
4. 2-(2-Fluorobiphenyl-4-yl)propionic acid

Answer. 1. 3-Ethyl-3-phenyl-2,6-piperdine dione

Question 4. Which of the following chemical agents are used in the treatment of insomnia?

1. Benzodiazepines
2. Imidazopyridines
3. Barbiturates
4. All of the above

Answer. 4. All of the above

Question 5. Oxazepam is psychoneurotic has lower side effect due to:

1. Geometric hydroxylation
2. Ring oxidation
3. Conjugation of 3-OH group
4. M-demethylation

Answer. 3. Conjugation of 3-OH group

Question 6. Select a hypnotic drug, which is a benzodiazepine derivative:

1. Zolpidem
2. Chlorazepate
3. Secobarbital
4. Phenobarbitone

Answer. 2. Chlorazepate

Question 7. In benzodiazepine ring, some positions cannot be substituted to retain the depressant activity:

1. Position 6, 8 and 9
2. Position 4, 6 and 8
3. Position 3, 6 and 8
4. Position 2, 3 and 8

Answer. 1. Position 6, 8 and 9

Question 8. Introduction of -OH group at position 3 of benzodiazepine causes:

1. Increase in activity
2. Loss of activity
3. Lowering of activity
4. None of above

Answer. 3. Lowering of activity

Question 9. Tick a hypnotic agent – a barbituric acid derivative:

1. Flurazepam
2. Zaleplon
3. Thiopental
4. Triazolam

Answer. 3. Thiopental

Question 10. Substitution at 8 and 9 position in a benzodiazepine ring causes:

1. Loss in the activity
2. Increase in the activity
3. Decrease in the activity
4. None of above

Answer. 3. Decrease in the activity

Question 11. IUPAC name of Diazepam is:

1. 7-chloro-1-methyl-5-phenyl-1,4-benzodiazepine-2-one
2. 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepine-2-one
3. 7-chloro-1-methyl-5-phenyl-1,4-benzodiazepine
4. 6-chloro-1,3-dihydro-5-phenyl-2H-1,4-benzodiazepine-2-one

Answer. 2. 7-chloro-1,3-dihydro-1-methyl-5-phenyl-2H-1,4-benzodiazepine-2-one

Question 12. Select a hypnotic drug, which is an imidazopyridine derivative:

1. Pentobarbital
2. Temazepam
3. Zolpidem
4. Chloral hydrate

Answer. 3. Zolpidem

Question 13. 2-amino-5-chlorobenzophenone is the convenient starting material for the synthesis of:

1. Nitrazepam
2. Diazepam
3. Chloramphenicol
4. Trimethoprim

Answer. 2. Diazepam

Question 14. Which of the following barbiturates is an ultra-short-acting drug?

1. Secobarbital
2. Amobarbital
3. Thiopental
4. Phenobarbital

Answer. 3. Thiopental

Question 15. Indicate the mechanism of barbiturate action (at hypnotic doses):

1. Increasing the duration of the GABA-gated Cl- channel openings
2. Directly activating the chloride channels
3. Increasing the frequency of Cl-channel opening events
4. All of the above

Answer. 1. Increasing the duration of the GABA-gated Cl- channel openings

Question 16. Alprazolam apart from 1,4-benzodiazepine ring also contains:

1. Pyrole ring
2. Pyrazole ring
3. Triazole ring
4. Furan ring

Answer. 3. Triazole ring

Question 17. Which of the following agents is preferred in the treatment of insomnia?

1. Barbiturates
2. Hypnotic benzodiazepines
3. Ethanol
4. Phenothiazide

Answer. 2. Hypnotic benzodiazepines

Question 18. Barbiturates are being replaced by hypnotic benzodiazepines because of:

1. Low therapeutic index
2. Suppression in REM sleep
3. High potential of physical dependence and abuse
4. All of the above

Answer. 4. All of the above

Question 19. The onset and duration of action of barbiturate are affected by:

1. Number of carbon atoms situated on the 5-position
2. Substitution of nitrogen atom at position-1
3. Number of aromatic groups
4. None of the above

Answer. 1. Number of carbon atoms situated on the 5-position

Question 20. Indicate the main claim for an ideal hypnotic agent:

1. Rapid onset and sufficient duration of action
2. Minor effects on sleep patterns
3. Minimal “hangover” effects
4. All of the above

Answer. 4. All of the above

Question 21. Sedative action of barbiturate is due to substitution at Cs, it is due to:

1. High lipophilicity of group at Cs position
2. Electronic withdrawing effect
3. Steric effect
4. Metal chelation

Answer. 1. High lipophilicity of group at Cs position

Question 22. Which of the following hypnotic drugs is used intravenously as anesthesia?

1. Thiopental
2. Phenobarbital
3. Flurazepam
4. Zolpidem

Answer. 1. Thiopental

Antipsychotics

Introduction

The term ‘psychosis’ used to describe a type of mental health issue that seriously affects the way that a person thinks or feels and where the person can lose contact with reality due to a disturbance in the functioning of the brain.

This type of mental health problem can happen to anyone and is much more common than most people realize. Sometimes people can have a one-off episode and after that they get better and never experience psychosis again. Other people may have episodes that come and go and they can be well for long periods in between.

There are two broad classes of functional psychotic disorders i.e. schizophrenia and bipolar disorder. Schizophrenia is a chronic condition with exacerbations, but always with some background symptoms. Bipolar disorder is generally an intermittent condition with the expectation of full recovery between episodes.

Symptoms of schizophrenia are grouped into two categories:

• Positive symptoms: Hallucinations and delusions.
• Negative symptoms: Social withdrawal and lack of energy and motivation that are similar to those found in depression.

Psychosis is characterized by

• Loss of connectedness with reality.
• Feeling of very anxious or agitated.
• Have very low or high moods.
• Persons may develop false ideas or beliefs about reality (delusions).
• Persons may have false perceptions (hallucinations).
• Persons also experience flaws in the ways they think (thought disorders).
• Persons may think that people are against them and they may hear voices or sounds that aren’t real.
• Poor physical health.
• Significantly impairs work, family and social functioning.

The drugs that increase the dopaminergic activity like levodopa (a precursor), amphetamine (release of dopamine) and apomorphine (a direct dopamine receptor agonist) may aggrevate schizophrenia or produce psychosis in some patients.

Antipsychotic drugs are used mainly for the treatment of psychosis. Antipsychotic drugs diminish the underlying thought disorder associated with schizophrenia. As these agents often have a calming effect in agitated psychotic patients, they are also called as major tranquilizers and due to its lessen reactivity to emotional stimuli, with little effect on consciousness, these drugs are also known as neuroleptics.

Antipsychotic drugs are derived from several chemical groups. These drugs may be broadly classified as first-generation or typical or conventional agents (phenothiazines and older non-phenothiazines, such as haloperidol, with similar pharmacologic actions, clinical uses, and adverse effects), and second-generation or atypical agents, which are also called as newer non-phenothiazines.

Mechanism Of Action Of Antipsychotic Drugs

Many antipsychotic drugs strongly block post-synaptic D2 receptors in CNS, especially in the mesolimbic-frontal limbic system and block the action of dopamine.

The most frequent uses of these agents are in manic disorders and the schizophrenias. In the manic disorders, the agents may block dopamine, (3,4-dihydroxyphenethylamine) at limbic D2 and D3 receptors, reducing euphoria, delusional thinking and hyperactivity.

In the chronic idiopathic psychoses (schizophrenias), both conventional (typical) and newer (atypical) antipsychotics appear to act to benefit positive symptoms by blocking dopamine at D2 and D3 limbic receptors. The activity of the atypical agents against negative symptoms may be due to blocking of serotonine2A receptor (5-HT2A).

Classification Of Antipsychotic Drugs

Antipsychotic drugs are classified into following categories:

• Phenothiazines: Promazine hydrochloride, Chlorpromazine Triflupromazine, Thioridazine hydrochloride, Piperacetazine Prochlorperazine meleate, Trifluoperazine hydrochloride. hydrochloride, hydrochloride,
• Ring Analogues of Phenothiazines: Chlorprothixene, Thiothixene, Loxapine succinate, Clozapine.
• Fluoro buterophenones: Haloperidol, Droperidol, Risperidone.
• Beta amino ketones: Molindone hydrochloride.
• Benzamides: Sulpieride.

Phenothiazines

Many potentially useful phenothiazine derivatives have been synthesized and evaluated pharmacologically. Three sub-families of phenothiazine derivatives based on side chain. substitutions were found to be active against psychotic disorder.

Substitution with aliphatic derivatives (e.g., promazine, chlorpromazine and triflupromazine) and piperidine derivatives (e.g., thioridazine) are less potent, whereas substitutions with piperazine derivatives (e.g., prochlorperazine and trifluoperazine) are more potent and effective in small doses. The piperazine derivatives are more selective in their pharmacological activity.

Structure-Activity Relationship of Phenothiazines

Phenothiazines are fused tricyclic (heterocyclic) system, chemically constituted by both lipophilic and hydrophilic groups. Structural features are associated with activity. The general structure of phenothiazine antipsychotic drugs is as follows,

• Position 2 in phenothiazine nucleus was found to be best position for substitution.
• Substitution of electron withdrawing group (e.g. Chlorine) at position 2, increases the antipsychotic activity. (The importance of this substitution is formation of hydrogen bond between the hydrogen atoms of the protonated amino group of the side chain with an electron pair of an electron withdrawing group at position 2 substituent, to develop a dopamine like arrangement.)
• Substitution at the position 3 can improve activity over non-substituted compounds, but not as significantly as substitution at the 2 position.
• Substitution at positions 1 and 4 has a deleterious effect on antipsychotic activity.
• The sulfur atom at position 5 is in a position analogous with that of p-hydroxyl of dopamine leads to assign a receptor-binding function.
• A substituent at position 4 at phenothiazine nucleus might interfere with receptor binding by the sulfur atom.
• Nitrogen-containing side-chain substituent at position 10 at phenothiazine nucleus is required for antipsychotic activity;

• The ring nitrogen and side-chain nitrogen must be separated by a three carbon chain.
• Shortening or lengthening the chain results into drastically decreased antipsychotic activity. (The three-atom chain length may be necessary to bring the protonated amino nitrogen into proximity with the 2-substituent.).
• Decrease in size from a dimethylamino group to a monomethylamino group at side chain greatly decreases activity.
• Substitution on the side chain with a large group (e.g. phenyl) decreases antipsychotic activity.
• Branching with polar groups such as methyl branching on the B-position has a variable effect on antipsychotic activity.
• The side chains are either aliphatic, piperazine, or piperidine derivatives. Substitution with piperazine side chains leads to greatest potency as well as pharmacological selectivity.

Phenothiazine Derivatives:

Promazine Hydrochloride:

• Promazine hydrochloride is chemically, N,N-dimethyl-3-phenothiazin-10-yl-propan- 1-amine; hydrochloride.
• Promazine hydrochloride blocks postsynaptic dopamine receptors D1 and D2, mesolimbic receptors and decreases stimulation of psychotic effects, such as hallucinations and delusions.
• It also blocks medullary chemoreceptor trigger zone (CTZ), of vomiting center and thus acts as antiemetic.
• It also blocks alpha-adrenergic receptors and exhibits strong anticholinergic activity.

Uses:

• It is primarily used as antipsychotic agent in short-term treatment of disturbed behaviour.
• It is also used as antiemetic.

Chlorpromazine Hydrochloride:

• Chlorpromazine hydrochloride is chemically, N, N-dimethyl-3(2-chloropheno- thiazine)-10-yl)-propan-1-amine; hydrochloride.
• It exerts its antipsychotic effect by blocking postsynaptic dopamine receptors in cortical and limbic areas of the brain and thus preventing the excess of dopamine in the brain. Thereby it decreases stimulation of psychotic effects, such as hallucinations ,and delusions.
• It also blocks dopamine receptors in the chemical trigger zone (CTZ) in the brain, thereby relieving nausea and vomiting.

Uses:

• Chlorpromazine hydrochloride acts as an antipsychotic agent used to treat hallucinations and delusions.
• It is also used as antiemetic and used in the treatment of uncontrollable hiccup.
• It also has significant sedative and hypotensive properties, possibly reflecting central and peripheral noradrenergic blocking activity, respectively.

Triflupromazine Hydrochloride:

• Triflupromazine hydrochloride is chemically, N,N-dimethyl-3-[2-trifluoromethyl) phenothiazine-10-yl]propan-1-amine; hydrochloride.
• It is analogous with chlorpromazine hydrochloride, only changes of -CF3 in place of – Cl at position 2 leads to increase in antipsychotic activity.

Uses:

• Triflupromazine hydrochloride is used as antipsychotic drug and is used to treat hallucinations and delusions.
• It has lower sedative and hypotensive properties compared to chlorpromazine hydrochloride.

Thioridazine Hydrochloride:

• Thioridazine hydrochloride is chemically, 10-[2-(1-methylpiperidin-2-yl)ethyl]-2- methylsulfanylphenothiazine; hydrochloride.
• It is a piperidine sub-group of the phenothiazine block to mesolimbic postsynaptic dopamine receptor D2 and decreases dopamine activity, thereby it decreases stimulation of psychotic effects, such as hallucinations and delusions.
• It also binds to serotonin 5-HT2 receptors, resulting in decreased serotonin activity.

Uses:

• Thioridazine hydrochloride is a phenothiazine derivative used in the treatment of pshycotic disorder including schizophrenia.
• It has sedative and hypotensive activity in common with chlorpromazine and less antiemetic activity.
• Piperacetazine Hydrochloride:
• Piperacetazine hydrochloride is chemically, 1-(10-(3-[4-(2-hydroxyethyl)-1- piperidinyl]propyl)-10H-phenothiazin-2-yl)ethanone; hydrochloride.
• It is an antipsychotic prodrug, most notably used for schizophrenia.

Prochlorperazine Meleate:

• Prochlorperazine meleate is chemically, 2-chloro-10-[3-(4-methylpiperazin-1-yl)- propyl]phenothiazine, meleate.
• It is piperazine phenothiazine derivative and it blocks the postsynaptic dopamine D2-receptor in the chemoreceptor trigger zone (CTZ) of the brain and may prevent emesis.
• It also blocks a-adrenergic receptors and results in sedation, muscle relaxation, and hypotension.

Uses:

• Prochlorperazine maleate is a phenothiazine antipsychotic drug.
• It is principally used in the treatment of nausea vomiting and vertigo.
• It also has antihistaminic and anticholinergic activities.
• It is used mainly for its antiemetic effect and not for its antipsychotic effect.

Trifluoperazine Hydrochloride:

• Trifluoperazine hydrochloride is chemically,10-(3-(4-methylpiperazin-1-yl)propyl]-2- (trifluoromethyl) phenothiazine; hydrochloride.
• It blocks central dopamine receptors and is used to treat delusions and hallucinations caused by an excess of dopamine.
• It also blocks the postsynaptic dopamine D2-receptor in the chemoreceptor trigger zone (CTZ) of the brain and may prevent emesis.
• It also blocks central adrenergic receptors and leads to anxiolytic effects.

Uses:

• Trifluoperazine is piperazine phenothiazine derivative and antipsychotic agent that is no longer commonly used in clinical practice.
• It also possesses anxiolytic, and antiemetic activities.

Ring Analogues of Phenothiazines

These agents have structural resemblance to that of the phenothiazine antipsychotics. Most of the drugs also have similar clinical properties and uses as that of phenothiazines.

Thioxanthene Derivative:

The thioxanthene system differs from the phenothiazine system by replacement of the N-H moiety with a carbon atom doubly bonded to the propylidene side chain with the substituent in the 2 position. These compounds (Chlorprothixene and Thiothixene) are very similar in pharmacological activities as that of phenothiazines.

Chlorprothixene:

• Chlorprothixene is chemically, 3-(2-chlorothioxanthen-9-ylidene)-N,N-dimethyl- propan-1-amine.
• It is a tertiary amine typical antipsychotic drug of thioxanthenes class.
• It produces its antipsychotic action by blocking the 5-HT2 D1, D2 and D3 receptors.
• It also possesses histamine (H1), muscarinic and a-1 adrenergic receptors blocking action.

Uses:

• Chlorprothixene is used as first generation antipsychotic.
• It also has non-narcotic analgesic, antiemetic, sedative and cholinergic antagonist activity.

Thiothixene:

• Thiothixene is chemically, N,N-dimethyl-9-[3-(4-methylpiperazin-1-yl)propylidene] thioxanthene-2-sulfonamide.
• It is a thioxanthene derivative. It produces antipsychotic activity by blocking postsynaptic dopamine receptors in the mesolimbic system.
• It also blocks medullary chemoreceptor trigger zone leading to decreased stimulation of the vomiting center.

Uses:

• Thiothixene is used as an antipsychotic agent.
• It is also used as antiemetics.

Dibenzoxazepine Derivative:

Loxapine Succinate:

• Loxapine succinate is chemically 8-chloro-6-(4-methylpiperazin-1-yl)benzo[b] [1,4]benzoxazepine, butanedioic acid.
• It produces its action by blocking the dopamine receptors at postsynaptic receptor sites in the limbic system, cortical system and basal ganglia, thereby reducing the hallucinations and delusions.

Uses:

• Loxapine succinate is an antipsychotic agent used in schizophrenia.
• It also possesses antiemetic, sedative, anticholinergic and antiadrenergic actions.

Dibenzodiazepine Derivative:

Clozapine:

• Clozapine is chemically 3-chloro-6-(4-methylpiperazin-1-yl)-5H-benzo[b][1,4] benzo- diazepine.
• It is an important atypical antipsychotic.
• It weakly blocks D2 receptors and relieves schizophrenic symptoms such as hallucinations, delusions and dementia.
• It also has serotonin (bind with 5-HT2A/2c receptor) receptors antagonist activity.
• It has notably low production of extra pyramidal symptoms (EPS) and reduction of negative symptoms.

Uses:

• Clozapine is an important atypical antipsychotic.
• Its use is restricted because of a relatively high frequency of agranulocytosis as a severe side effect.

Fluorobutyrophenones

Many of the fluorobutyrophenones possess antipsychotic activity. The general structural features for antipsychotic activity are as follows:

Structure Activity Relationship:

• Aromatic ring with para fluoro substitution at R1 is required for optimum activity.
• Carbonyl group (–) at ‘X’ is required for optimal activity, although the other groups like C(H)OH and C(H)aryl also have good antipsychotic activity.
• Carbon chain length with 3 carbon atoms is required for optimal activity, whereas longer or shorter chain length decreases the activity.
• The aliphatic amino nitrogen incorporated into a cyclic form is required for highest activity.
• R2 must be an aromatic ring attached directly or occasionally separated by one intervening atom to the 4th position for optimal activity.
• The ‘Y’ group can vary with different groups required to assist antipsychotic activity, Example: -OH group in haloperidol.

Haloperidol:

• Haloperidol is butyrophenone, chemically it is, 4-[4-(4-chlorophenyl)-4-hydroxy- piperidin-1-yl)-1-(4-fluorophenyl)butan-1-one.
• It competitively blocks postsynaptic dopamine receptors in the mesolimbic system of the brain and so is used to treat delusion and hallucination.
• It has strong affinity for D2 and D3 receptors.
• It also blocks dopamine receptors in the chemoreceptive trigger zone (CTZ) and leads to its anti-emetic effect.

Uses:

• Haloperidol is a typical antipsychotic drug.
• It is a potent antipsychotic useful in schizophrenia and in psychoses associated with brain damage.
• It also possesses neuroleptic and antiemetic activities.

Droperidol:

• Droperidol is butyrophenone, chemically it is, 3-[1-[4-(4-fluorophenyl)-4-oxobutyl]- 3,6-dihydro-2H-pyridin-4-yl]-1H-benzimidazol-2-one.
• It has general properties similar to those of haloperidol.
• It acts by blocking dopamine D2 receptors.
• It blocks dopamine receptors in the chemoreceptor trigger zone (CTZ) and leads to its anti-emetic effect.
• It also acts on postsynaptic GABA receptors in the CNS and increases the inhibitory effect of GABA which leads to sedative and anti-anxiety activities.

Uses:

• Droperidol is used as preanaesthetic neuroleptics.
• It is used in combination with an opioid analgesic agent fentanyl preanaesthetically.
• It also possesses anti-emetic, sedative and anti-anxiety properties.

Risperidone:

• Risperidone is a benzisoxazole derivative, chemically it is, 3-[2-[4-(6-fluoro-1,2- benzoxazol-3-yl)piperidin-1-yl]ethyl]-2-methyl-6,7,8,9-tetrahydropyrido[1,2-a]- pyrimidin-4-one.
• Risperidone selectively antagonizes serotonin 5-HT2 receptor.
• It also binds with the limbic dopamine D2 receptor which leads to antipsychotic activity.

Uses:

• Risperidone acts as an atypical antipsychotic agent.
• It is reported to decrease the negative (e.g., withdrawal, apathy) as well as the positive (e.g., delusions, hallucination) symptoms of schizophrenia.

Beta Amino Ketones

Molindone Hydrochloride:

• Molindone hydrochloride is chemically 3-ethyl-2-methyl-5-(morpholin-4-ylmethyl)- 1,5,6,7-tetrahydroindol-4-one;hydrochloride.
• It exerts its effect by blocking dopamine receptors (D2 and D3) in the reticular activating and limbic systems, thereby decreasing dopamine excess in the brain. It also has moderate affinity for cholinergic and alpha-adrenergic receptors.

Uses:

• Molindone is a conventional antipsychotic used in the therapy of schizophrenia and other psychoses.
• It is useful in the treatment of the aggressive type of undersocialized conduct disorder.

Benzamides

Sulpieride:

• Sulpieride is benzamide derivative, chemically N-[(1-ethylpyrrolidin-2-yl)methyl]-2- methoxy-5-sulfamoylbenzamide.
• It is more selective and acts primarily as a dopamine D2 antagonist.

Uses:

• Sulpieride is used therapeutically as an antipsychotic.
• It is also used as antidepressant and as a digestive aid.

Synthesis

Chlorpromazine Hydrochloride

Multiple Choice Questions:

Question 1. Neuroleptics are used to treat:

1. Neurosis
2. Psychosis
3. Narcolepsy
4. Parkinsonian disorders

Answer. 2. Psychosis

Question 2. 5H-dibenz [b,f]azepine-5-carboxamide is chemical name for:

1. Imipramine
2. Carbamazepine
3. Carbapenam
4. Diazepam

Answer. 2. Carbamazepine

Question 3. Most antipsychotic drugs:

1. Strongly block postsynaptic D2 receptor
2. Stimulate postsynaptic D2 receptor
3. Block NMDA receptor
4. Stimulate 5-HT receptor

Answer. 1. Strongly block postsynaptic D2 receptor

Question 4. In phenothiazine tranquillizing agents, replacement of C-2 hydrogen by chlorine:

1. Decreases activity
2. Increases activity
3. Do not affect activity
4. Leads to penetration to CNS

Answer. 2. Increases activity

Question 5. Which of the following antipsychotic drugs is typical?

1. Clozapine
2. Quetiapine
3. Haloperidol
4. Olanzapine

Answer. 3. Haloperidol

Question 6. Chlorpromazine is derivative of 2-chlorphenothiazine which can be prepared from starting material:

1. 2-Chlorophenothiazine
2. 2- Nitrophenothiazine
3. 2-Aminophenothiazine
4. 2-Trifluoromethyl phenothiazine

Answer. 1. 2-Chlorophenothiazine

Question 7. Indicate the atypical antipsychotic drug:

1. Haloperidol
2. Clozapine
3. Thioridazine
4. Thiothixene

Answer. 2. Clozapine

Question 8. Which of the following antipsychotic drugs has high affinity for D4 and 5-HT2 receptors?

1. Clozapine
2. Fluphenazine
3. Thioridazine
4. Haloperidole

Answer. 1. Clozapine

Question 9. Choose the correct chemical name for chlorpromazine hydrochloride:

1. [3-(2-chlorophenothiazine-10-yl)propyl]diethyl amine hydrochloride
2. [2-(3-chlorophenothiazine-10-yl)propyl]dimethyl amine hydrochloride
3. [3-(2-chlorophenothiazine-10-yl)propyl]dimethyl amine hydrochloride
4. [3-(3-chlorophenothiazine-10-yl)propyl]dimethyl amine hydrochloride

Answer. 3. [3-(2-chlorophenothiazine-10-yl)propyl]dimethyl amine hydrochloride

Question 10. Indicate the antipsychotic drug, which is a phenothiazine aliphatic derivative:

1. Thiothixene
2. Risperidone
3. Chlorpromazine
4. Clozapine

Answer. 3. Chlorpromazine

Question 11. Indicate the antipsychotic drug, which is a butyrophenone derivative:

1. Droperidol
2. Thioridazine
3. Sertindole
4. Fluphenazine

Answer. 1. Droperidol

Question 12. Droperidol belongs to the class of:

1. Carbamates
2. Xanthanes
3. Butyrophenone
4. Phenothiazine

Answer. 3. Butyrophenone

Question 13. Indicate the antipsychotic drug, which is a thioxanthene derivative:

1. Haloperidol
2. Clozapine
3. Chlorpromazine
4. Thiothixene

Answer. 4. Thiothixene

Question 14. Indicate the antipsychotic agent – a dibenzodiazepine derivative:

1. Fluphenazine
2. Clozapine
3. Risperidone
4. Droperidol

Answer. 2. Clozapine

Question 15. Indicate the antipsychotic drug having significant peripheral a-adrenergic blocking activity:

1. Haloperidol
2. Chlorpromazine
3. Clozapine
4. Risperidone

Answer. 2. Chlorpromazine

Question 16. Indicate the antipsychotic drug having a muscarinic-cholinergic blocking activity:

1. Chlorpromazine
2. Clorprothixene
3. Risperidone
4. Haloperidol

Answer. 1. Chlorpromazine

Question 17. Which of the following antipsychotic agents is preferable in patients with coronary and cerebrovascular disease?

1. Chlorpromazine
2. Fluphenazine
3. Haloperidol
4. Perphenazine

Answer. 3. Haloperidol

Question 18. Haloperidol is a major tranquilizer, is belongs to the class of:

1. Carbamates
2. Propanediol
3. Butyrophenone
4. Phenothiazine

Answer. 3. Butyrophenone

Question 19. The mechanism of haloperidol antipsychotic action is:

1. Blocking D, receptors
2. Central alpha-adrenergic blocking
3. Inhibition of norepinephrine uptake mechanisms
4. All of the above

Answer. 4. All of the above

Question 20. Which of the following antipsychotic drugs has high affinity for D, and 5-HT2 receptors?

1. Droperidol
2. Clozapine
3. Thlothixene
4. Risperidone

Answer. 4. Risperidone

Anticonvulsants

Introduction

The word ‘epilepsy’ comes from the Greek word epilambanein (to seize). Epilepsy is not a contagious disease or psychological disorder; it can develop at any age. It is said to be common neuralgic disorder found in approximately 0.7% of the population of any age. Epilepsy is a group of chronic CNS disorders due to sudden and transitory seizures of abnormal motor and sensory neurons resulting into a repeated neuronal discharge.

It is a physical condition which causes burst of hyperactivity in brain characterized by chronic, recurrent, paroxysmal changes in neuralgic function. This hyperactivity produces ‘seizures’ which vary from one person to another in frequency and form. It is seen as a sudden abnormal function of the body, often with loss of consciousness, an excess of muscular activity, or sometimes loss of it, or an abnormal sensation. A seizure may last for few seconds to few minutes.

The terms epilepsy or convulsion or seizure are often used interchangeably and basically have the same meaning.

Epilepsy is said to be ‘symptomatic epilepsy’ which develops after a particular identifiable events such as asphyxia, head injury, meningitis, birth trauma or brain injury during pregnancy whereas, epilepsy is called as ‘idiopathic epilepsy’ which develops without any identifiable cause.

Classification Of Different Types Of Epilepsy (Seizure)

A brief classification of different types of epilepsy is important as it facilitates accurate diagnosis and drug selection for precise treatment against seizure disorder.

The major classification is as follows:

Generalized Seizures:

Generalized seizures essentially involve the entire brain and do not have an apparent local onset. Two major types of generalized seizures are as follows:

The generalized tonic-clonic seizure (grand mal)

It is the most common type of epilepsy in which the person often experiences an aura (feeling something around the body, fear and discomfort). It proceeds by a series of bilateral muscular jerks followed by loss of consciousness, which in turn is followed by a series of tonic and then clonic spasms. The seizures generally last from 2 to 5 minutes.

The non-convulsive seizures or absence seizures (petit mal)

This type of epilepsy is most frequently found in children. It consists of a sudden brief loss of consciousness, characterized by blank spells or loss of speech. This seizure is for very short period of time which usually last from 1 to 30 seconds even many persons are not aware that they have had a seizure. In petil mall epilepsy there is no motor activity or some minor clonic motor activity exists.

Partial (or focal) Seizures:

Partial seizures have a focus (i.e., begin locally). Major types of focal (partial) epilepsy are simple focal (Jacksonian motor epilepsy) and complex (psychomotor or temporal lobe) focal seizures.

Simple focal (Jacksonian motor epilepsy) seizures:

Jacksonian epilepsy is rare and usually associated with lesion of a certain part of the brain (cerebral cortex). It is characterized by focal or local clonic type convulsions of localized muscle groups (for example, thumb, big toe, and so forth). The patient does not loss consciousness and therefore be able to tell what happened. The seizures normally last from 1 to 2 minutes.

Complex (Psychomotor or temporal lobe) focal seizures:

Psychomotor epilepsy is uncommon and is characterized by certain abnormal types of behaviour (for example, extensive swallowing or chewing). It mostly occurs in children through adolescence. The individual may experience an aura followed by confusion and bizarre (very strange or unusual behaviour) with perceptual alterations, such as hallucinations or a strong sense of fear. The seizures normally last from 1 to 2 minutes. The seizure may be misdiagnosed as a psychotic episode and difficult to treat.

Status Epilepticus:

A status epilepticus occurs whenever a seizure persists for at least 30 minutes or it repeated so frequently that recovery between attacks does not occur. It is a dangerous condition which may result in brain damage (cerebral necrosis) with severe morbidity or death. A status may be the patient’s first epileptic event or may be precipitated by suddenly discontinuing anticonvulsant therapy.

Each of the epilepsy types is characterized by a typical abnormal pattern in the Electroencephalography (EEG) which gives sudden excessive electrical activity in the brain.

Anticonvulsant Drugs

The proper diagnosis of seizure type is essential to the selection of an appropriate anticonvulsant drug. If used for the wrong seizure type, some anticonvulsant drugs actually increase seizure activity.

The ‘anticonvulsants’ are also called as ‘antiepileptic drugs’ and ‘antiseizure drugs’ that are used regularly to control and manage the neurological disorder caused by excessive nerve cell discharge in the brain. Generally the term anticonvulsant is used for an agent that blocks experimentally produced seizures in laboratory animals whereas; antiepileptic drugs are used medically to control the epilepsies.

For many years, treatment options for epilepsy were limited. However, over the last decade, many new pharmacological therapies have been introduced, and several more are in development.

Classification of Anticonvulsant Drugs:

The anticonvulsants are classified as follows:

• Barbiturates: Phenobarbital, Methabarbital.
• Hydantoins: Phenytoin, Mephenytoin, Ethotoin.
• Oxazolidinediones: Trimethadione, Paramethadione.
• Succinimides: Phensuximide, Methsuximide, Ethosuximide.
• Urea and monoacylureas: Phenacemide, Carbamazepine.
• Benzodiazepine: Clonazepam, diazepam.
• Miscellaneous: Primidone, Valproic acid, Gabapentin, Felbamate, Tiagabine, Lamotrigine, Zonisamide.

Mechanism of Action of Anticonvulsants:

The mode of action of anticonvulsants is not clear. However, it is believed that the anticonvulsants suppress seizures by depressing the cerebral (motor) cortex of the brain.

Gama amino butyric acid (GABA) is the major inhibitory neurotransmitter in the brain. Several of the anticonvulsant medicines work through the GABA system. A major mode of action of anticonvulsants (Benzodiazepine and barbiturate) can be positive allosteric modulation of GABAA receptors. The mechanism of action of barbiturates is based on their structure.

Phenobarbital acts by blocking voltage gated sodium channel, whereas calcium-T channel is blocked by 5,5-dialkyl substituted barbiturates members. Oxazolidine-2.4-diones and succinimides act via calcium T-type channel block. The major mode of action for phenytoin, carbamazepine, valproic acid, felbamate, lamoirigine and zonisarnide are reported to act by blocking voltage-gated sodium channel.

Structure – Activity Relationship of Anticonvulsants:

Several major groups of anticonvulsant drugs viz., Barbiturates, Hydantoins, Oxazolidine diones, Succinimides and Acetylureas have the common structural features as shown below. These groups have in common a similar heterocyclic ring structure with variety of substituents. Very small changes in structure can dramatically alter the mechanism of action and clinical properties of the compound.

• Both R and R’ should be hydrocarbon radicals.
• If both R and R’ are lower alkyls, then it is more active against absence seizure (petit mal) and not active against generalized tonic-clonic (grand mal) or partial seizures.
• If one of the hydrocarbon substituents is an aryl group, then the activity increases towards generalized tonic-clonic and partial seizures and no action against absence seizure.

Barbiturates

Phenobarbital:

• Phenobarbital is chemically 5-ethyl-5-phenyl-1,3-diazinane-2,4,6-trione.
• It is the drug of choice and is used virtually in all the three types of epileptic seizures viz, grand mal, petit mal and psychomotor.
• It binds to an allosteric site on the GABAA receptor, and it enhances the GABA receptor mediated current by prolonging the opening of the chloride channels. It leads to membrane hyperpolarization and ultimately synaptic inhibition and decreased neuronal excitability.
• It also blocks excitatory responses induced by glutamate,
• It has a long half-life time and therefore it will take a few weeks before reaching a therapeutic and effective level,
• The main side-effects of phenobarbitone are drowsiness, especially during the first week of treatment.

Uses:

• Phenobarbital is a barbiturate that is widely used as a sedative and an antiseizure medication.
• It is used in idiopathic generalized epilepsies.
• It is also reasonably effective in other generalized seizures and in partial seizures.

Methabarbital:

• Methabarbital is chemically, 5,5-diethyl-1-methyl-1,3-diazinane-2,4,6-trione.
• It is mostly demethylated to barbital in vivo. Also it possesses more sedating property than Phenobarbital.
• It could be safely recommended for grand mal seizures.
• It binds at a distinct binding site associated with a Clionopore at the GABA receptor, increasing the duration of time for which the Clionopore is open.

Uses:

• Methabarbital has similar properties to phenobarbital and is used in the treatment of epilepsy.
• It is also used for the treatment of short term insomnia.
• It belongs to Central Nervous System depressants group that induce drowsiness and relieve tension or nervousness.

Hydantoins

• The hydantoins are close structural relatives with barbiturate, only differing in lacking the 6-oxo group. They possess imidazoline-2, 4-dione heterocyclic system.
• Absence of one carbonyl group leads to weaker organic acids than the barbiturates. Hydantoins show their effect by activating Na*- K* dependent and Ca** dependent ATPase and increase Na* transport.
• Hydantoins are more active against partial and generalized tonic-clonic seizure rather than absence seizure.
• All of the clinically useful hydantoins used in treatment of generalized tonic-clonic seizure, possess an aryl substituent on the 5 position.
• Hydantoins with lower alkyl substituents are not active against absence seizure.

Phenytoin:

• Phenytoin is chemically, 5,5-diphenylimidazolidine-2,4-dione.
• Phenytoin potentially acts by promoting sodium efflux from neurons located in the motor cortex. This inhibits neuronal firing and results in the stabilization of neuronal membranes, thereby preventing the spread of seizure activity at the motor cortex.
• It acts as a anticonvulsant by blocking sodium channel and decreases presynaptic glutamic acid release. It also reduces glutamate-induced ischemic damage to neuron.

Uses:

• Phenytoin is an anticonvulsant that is used to treat a wide variety of seizures.
• It is useful against all types of seizures except absence seizure.
• It is also used as an anti-arrhythmic and a muscle relaxant.

Mephenytoin

• Mephenytoin is chemically, 5-ethyl-3-methyl-5-phenylimidazolidine-2,4-dione.
• It has similar mechanism of action as that of phenytoin.
• It is found to be relatively more toxic, therefore it is exclusively given to such patients who do not response to other treatments.

Uses:

• Mephenytoin is more active against partial and generalized tonic-clonic seizure.
• The adverse effect such as dermatitis, agranulocytosis or hepatitis associated with mephenytoin is higher than phenytoin.

Ethotoin

• Ethotoin is chemically, 3-ethyl-5-phenylimidazolidine-2,4-dione.
• It is recommended for the patients who are hypersensitive to phenytoin.
• It has similar mechanism of action as that of phenytoin.
• The adverse effect and toxicity are generally less severe than those associated with phenytoin, but it appears to be less effective.

Uses:

• Ethotoin is used against generalized seizures.

Oxazolidinediones

• Oxazolidine-2,4-diones are analogous to hydantoins, only replacement of the -NH group at position-1 with an oxygen atom.
• These drugs are more active against absence seizures, provided that branched atom of these compounds is substituted with lower alkyls.
• Aryl substituted oxazolidine-2,4-diones have shown activity against generalized tonic-clonic seizures.

Trimethadione

• Trimethadione is chemically, 3,5,5-trimethyl-1,3-oxazolidine-2,4-dione.
• It undergoes metabolism N-demethylation to active metabolite dimethadione. • Dimethadione blocks calcium-T channel and reduces T-type calcium currents in thalamic neurons, thereby stabilizing neuronal membranes.

Uses:

• Trimethadione is an anticonvulsant effective in absence seizures.
• It’s clinical use is limited because of dermatological and hematological toxicities.

Paramethadione:

• Paramethadione is chemically 5-ethyl-3,5-dimethyl-1,3-oxazolidine-2,4-dione.
• It blocks calcium-T channel and reduces T-type calcium currents in thalamic neurons. This inhibits corticothalamic transmission and raises the threshold for repetitive activity in the thalamus.

Uses:

• Paramethadione is an anticonvulsant effective in absence seizures.

Succinimides

As oxazolidine diones are toxic, an extensive search was carried out to replace them with less toxic drugs. Substitution of ring oxygen in the oxazolidine diones with a -CH2 group gave the antiseizure succinimides. The precise mechanism of action of succinimides is unknown, but it is proposed to act by decreasing the activity of T- type calcium channel. These drugs are used in the treatment of absence (petit mal epilepsy) seizure.

Phensuximide:

• Phensuximide is chemically 1-methyl-3-phenylpyrrolidine-2,5-dione.
• It suppresses the paroxysmal three cycles per second spike and wave EEG pattern associated with lapses of consciousness in absence (petit mal) seizures.
• N-demethylation occurs to yield the putative active metabolite.

Uses:

• Phensuximide occasionally is used for the treatment of absence seizures.
• The phenyl substituent leads to be active against generalized tonic-clonic and partial seizures.
• It also had some activity against maximal electroshock seizure.

Methsuximide:

• Methsuximide is chemically, 1,3-dimethyl-3-phenylpyrrolidine-2,5-dione.
• It increases the seizure threshold and suppresses the paroxysmal three cycles per second spike and wave ECG pattern seen with absence (petit mal) seizures.
• It is generally considered as more toxic than ethosuximide.

Uses:

• Methsuximide is used primarily for the treatment of absence seizures and complex partial seizure.
• It also had some activity against maximal electroshock seizure.

Ethosuximide:

• Ethosuximide is chemically, 3-ethyl-3-methylpyrrolidine-2,5-dione.
• It acts by blocking calcium T channel of the thalamic neurons. This results in decrease in burst firing of thalamocortical neurons, which stabilize the nerve activity in the brain and prevents seizures.
• It is more active and less toxic than trimethadione.

Uses:

• Ethosuximide is used in the treatment of absence (petit mal) seizures.
• With other antiepileptic drugs it can be used to treat other types of epilepsy.

Urea and Monoacylureas

Phenacemide:

• Phenacemide is chemically, N-carbamoyl-2-phenylacetamide.
• It acts on the Central Nervous System to reduce the number and severity of seizures.
• It binds to and blocks neuronal Na* channels or voltage sensitive Ca** channels.
• It blocks or suppresses neuronal depolarization and hyper synchronization.

Uses:

• Phenacemide is used to control certain seizures in the treatment of epilepsy.

Carbamazepine:

• Carbamazepine is chemically 5H-dibenz[b, f] azepine-5-carboxamide.
• It is chemically related to tricyclic antidepressants (TCA) with anticonvulsant and analgesic properties.
• It exerts its anticonvulsant activity by reducing polysynaptic responses and blocking post-tetanic potentiation.

Uses:

• Carbamazepine is used as an anticonvulsant to control grand mal and psychomotor or focal seizures.

Benzodiazepines

Clonazepam:

• Clonazepam is chemically, 5-(2-chlorophenyl)-7-nitro-1,3-dihydro-1,4-benzo-diazepin-2-one.
• It is a synthetic benzodiazepine derivative.
• It produces its anticonvulsant activity by acting selectively at benzodiazepine allosteric binding sites on GABAA receptors and enhances GABA receptor responses.

Uses:

• Clonazepam is an anticonvulsant used for several types of seizures such as myotonic or atonic seizures, photosensitive epilepsy and absence (petit mal) seizures.
• It is reasonably effective in generalized tonic-clonic (grand mal) or partial seizures.

Diazepam:

• Diazepam is a benzodiazepine derivative, chemically it is 7-chloro-1-methyl-5- phenyl-3H-1,4-benzodiazepin-2-one.
• It binds with GABA receptor located in the limbic system and the hypothalamus and inhibits the activities of GABA. This leads to open chloride channel increases influx of chloride ions into the neuron, results into membrane hyper polarization and a decrease in neuronal excitability.

Uses:

• Diazepam is the drug of first choice for the treatment of status epilepticus and administered by IV rout.
• It is also useful in treating generalized tonic-clonic (grand mal) seizure.
• It is also used as anti-anxiety and as hypno-sedative.

Miscellaneous

Primidone:

• Primidone is chemically, 5-ethyl-5-phenyl-1,3-diazinane-4,6-dione.
• It is an analog of phenobarbital with antiepileptic property.
• It’s mode of action is similar to phenobarbital, i.e. activation of GABAA and increased frequency of opening of the chloride channel within the receptor complex. This leads to an alteration in the electrical activity of the nerve cell membrane results in to hyper polarization and prevention of partial and tonic-clonic seizures.
• It produces its anti-seizure properties as such and through partially metabolism to phenobarbital and phenylethyl malonamide (PEMA), which may also contribute to its activity. Adverse effects are reported to be more frequent than with phenobarbital.

Uses:

• The efficacy of primidone is against all types of seizures except absence seizure.

Valproic acid:

• Valproic acid is chemically, 2-propylpentanoic acid.
• It is synthetic derivative of propylpentanoic acid.
• It may act by increasing GABA levels in the brain or by blocking voltage dependent sodium channels and disorganized electrical activity.
• As this drug undergoes extensive dissociation at physiological pH produce poor partitioning across BBB, hence less potent compared to other drugs.

Uses:

• Valproic acid has anticonvulsant properties and is used in the treatment of grand mal epilepsy, petit mal epilepsy and complex partial seizure.
• It is also used as a mood stabilizer.
• It possesses antineoplastic and antiangiogenesis activities.

Gabapentin:

• Gabapentin is chemically, 1-(aminomethyl) cyclohexaneacetic acid.
• It is synthetic GABA-mimetic analogue capable of penetrating the CNS.
• It is a water-soluble amino acid, act by altering the metabolism or release of GABA and decreased CNS disorganized electrical activity.
• It raises brain GABA levels in patients with epilepsy.
• It also acts by binding with calcium channels.

Uses:

• Gabapentin is used in refractory partial seizures and generalized tonic-clonic seizures.
• It also approved for the treatment of postherpetic neuralgia.

Felbamate:

• Felbamate is chemically 2-phenylpropane-1,3-diyl dicarbamate.
• It acts by interaction with strychnine-insensitive glycine recognition site of the N-methyl-D-aspartate (NMDA) receptor ionophore complex. Hence blocks the effect of excitatory amino acid and suppresses the neuronal disorganized electrical activity. It possesses GABA receptor blocking action and produces inhibitory effect on neuronal excitatory.
• It also acts by blocking sodium channels.

Uses:

• Felbamate is used in combination with other antiepileptic medications against generalized tonic-clonic and complex partial seizures.
• As it is associated with a serious risk of aplastic anaemia and acute liver failure, its use is restricted.

Tiagabine:

• Tiagabine is chemically, (3R)-1-[4,4-bis(3-methylthiophen-2-yl)but-3-enyl]piperidine- 3-carboxylic acid.
• It is a nipecotic acid derivative with an improved ability to cross the blood-brain barrier.
• It acts by blocking GABA receptor and inhibits GABA uptake by glial cells.

Uses:

• Tiagabine is used largely as an adjunctive agent in therapy of partial seizures in adults or children.

Lamotrigine:

• Lamotrigine is a triazine compound and is chemically, 6-(2,3-dichlorophenyl)-1,2,4- triazine-3, 5-diamine.
• It enhances the action of an inhibitory neurotransmitter, GABA, and reduces the pain- related transmission of signals along nerve fibers.
• It acts by blocking sodium channels and preventing excitatory neurotransmitters (glutamate) release and inhibits serotonin reuptake.
• It reduces neuronal cell death in ischemia by reducing glutamate release.

Uses:

• Lamotrigine is an anti-epileptic agent and mood stabilizer.
• It is effective against refractory partial seizures.
• It also has analgesic property.

Zonisamide:

• Zonisamide is chemically, 1,2-benzisoxazole-3-methanesulfonamide.
• It is sulfonamide derivative and acts by blocking sodium channel and calcium-T channels, thereby stabilizing neuronal membranes and decreased CNS disorganized electrical activity..

Uses:

• Zonisamide is new generation anticonvulsant used in combination with other antiepileptic drugs for use in the treatment of partial seizure.

Synthesis

Phenytoin

Carbamazepine

Ethosuximide

Multiple Choice Questions:

Question 1. The mechanism of action of antiseizure drugs is:

1. Enhancement of GABAergic (inhibitory) transmission
2. Diminution of excitatory (usually glutamatergic) transmission
3. Modification of ionic conductance
4. All of the above mechanisms

Answer. 4. All of the above mechanisms

Question 2. Ethusuximide is an a-disubstituted derivative of succinimide. It contains:

1. Two methyl groups
2. Two ethyl groups
3. One methyl and one ethyl group
4. One methyl and one propyl group

Answer. 3. One methyl and one ethyl group

Question 3. Valproic acid is:

1. 2-ethylpentoic acid
2. 2-methylpentoic acid
3. 2-propylpentoic acid
4. 2-butylpentoic acid

Answer. 3. 2-propylpentoic acid

Question 4. Which of the following antiseizure drugs produces enhancement of GABA-mediated inhibition?

1. Ethosuximide
2. Carbamazepine
3. Phenobarbital
4. Lamotrigine

Answer. 3. Phenobarbital

Question 5. Which of the following antiseizure drugs produces a voltage-dependent inactivation of sodium channels?

1. Lamotrigine
2. Carbamazepin
3. Phenytoin
4. All of the above

Answer. 4. All of the above

Question 6. The drug for partial and generalized tonic-clonic seizures is:

1. Carbamazepine
2. Valproate
3. Phenytoin
4. All of the above

Answer. 4. All of the above

Question 7. Indicate an anti-absence drug:

1. Valproate
2. Phenobarbital
3. Carbamazepine
4. Phenytoin

Answer. 1. Valproate

Question 8. The drug against myoclonic seizures is:

1. Primidone
2. Carbamazepine
3. Clonazepam
4. Phenytoin

Answer. 3. Clonazepam

Question 9. The anticonvulsant drug belongs to category benzodiazepines:

1. Primidone
2. Carbamazepine
3. Clonazepam
4. Phenytoin

Answer. 3. Clonazepam

Question 10. The anticonvulsant drug belongs to category hydantoin:

1. Primidone
2. Carbamazepine
3. Clonazepam
4. Phenytoin

Answer. 4. Phenytoin

Question 11. The most effective drug for stopping generalized tonic-clonic status epilepticus in adults is:

1. Lamotrigine
2. Ethosuximide
3. Diazepam
4. Zonisamide

Answer. 3. Diazepam

Question 12. Select the appropriate consideration for phenytoin:

1. It blocks sodium channels
2. It binds to an allosteric regulatory site on the GABA-BZ receptor and prolongs the openings of the Cl-channels
3. It effects on Ca2+ currents, reducing the low-threshold (T-type) current
4. It inhibits GABA-transaminase, which catalyzes the breakdown of GABA Unit 4 | 4.52

Answer. 1. It blocks sodium channels

Question 13. Phenytoin is used in the treatment of:

1. Petit mal epilepsy
2. Grand mal epilepsy
3. Myoclonic seizures
4. All of the above

Answer. 1. Petit mal epilepsy

Question 14. The drug of choice for partial seizures is:

1. Carbamazepine
2. Ethosuximide
3. Diazepam
4. Lamotrigine

Answer. 1. Carbamazepine

Question 15. The mechanism of action of carbamazepine appears to be similar to that of:

1. Benzodiazepines
2. Valproate
3. Phenytoin
4. Ethosuximide

Answer. 3. Phenytoin

Question 16. Indicate the drug of choice for status epilepticus in infants and children:

1. Phenobarbital sodium
2. Clonazepam
3. Ethosuximide
4. Phenytoin

Answer. 1. Phenobarbital sodium

Question 17. Which of the following anticonvulsant drug belongs to category Oxazolidine diones?

1. Paramethadione
2. Carbamazepine
3. Clonazepam
4. Phenytoin

Answer. 1. Paramethadione

Question 18. Which of the following anticonvulsant drug belongs to category monoacylurea?

1. Paramethadione
2. Carbamazepine
3. Clonazepam
4. Phenytoin

Answer. 2. Carbamazepine

Question 19. The drug of choice in the treatment of petit mal (absence seizures) is:

1. Phenytoin
2. Ethosuximide
3. Phenobarbital
4. Carbamazepine

Answer. 2. Ethosuximide

Question 20. Valproate is very effective against:

1. Absence seizures
2. Myoclonic seizures
3. Generalized tonic-clonic seizures
4. All of the above

Answer. 4. All of the above

Question 21. Indicate the antiseizure drug – a benzodiazepine receptor agonist:

1. Phenobarbital
2. Phenytoin
3. Carbamazepine
4. Clonazepam

Answer. 4. Clonazepam

Question 22. The most dangerous effect of antiseizure drugs after large overdoses is:

1. Respiratory depression
2. Gastrointestinal irritation
3. Alopecia
4. Sedation

Answer. 1. Respiratory depression

Chapter 5 Drugs Action On Central Nervous System

General Anesthetics

Introduction

Earlier the pain-producing surgical procedures and dental surgeries were undertaken without the aid of acceptable anesthetic agents. Various chemical methods were adopted at that time included intoxication with ethanol, hashish, or opium, whereas physical methods included packing a limb in ice, creating ischemic conditions with tourniquets, inducing unconsciousness by a blow to the head, or the most common technique, employing strong armed assistants to hold down the helpless patient during the entire surgical procedure.

Nitrous oxide also known as “laughing gas” was first time used by Hartford dentist Horace Wells as a surgical anesthetic in 1844. This gas is still commonly used today, especially in combination with other anesthetic and analgesic agents. Another agent cyclopropane was popularly used as general anesthetics but because of its explosive nature like diethyl ether it is no longer used. For many years, the inhalational anesthesia was used for all major surgical procedures, however recently intravenous anesthesia has become more commonly used technique.

The inhalational anesthetic agents used today are hydrocarbons and ethers with halogen like, Cl, Br, or F and intravenous anesthetics (short acting barbiturates e.g. Thiopental sodium) possess most of the ideal characteristics.

Ideal Characteristics General Anesthetics

The ideal anesthetic state is characterized by a loss of all sensations and includes analgesia and muscle relaxation. It should possess following characteristics, but currently there is no such ideal agent that fulfils all these characteristics.

• It should be non-reactive.
• It should be inexpensive.
• It should be non-toxic.
• It should be non-flammable / non-explosive.
• It should produce rapid and pleasant induction of surgical anesthesia.
• It should produce rapid and pleasant withdrawal from anesthesia.
• It should produce adequate relaxation of skeletal muscles.
• It should be potent enough to permit adequate oxygen supply in mixture.
• It should have wide margin of safety.
• It should be free of adverse effects.
• It should be chemically compatible with anesthetic devices.

General anesthesia is the induction of a state of unconsciousness with the absence of pain sensation over the entire body, through the administration of Anesthetic drugs. General anesthetic drugs produce controlled, reversible depression of the functional activities of the CNS producing loss of sensation and consciousness.

Purpose of General Anesthesia

• To get relief of pain (analgesia).
• To block memory of the procedure (amnesia).
• To produce unconsciousness.
• To inhibit normal body reflexes to make surgery safe and easier to perform.
• To relax the muscles of the body.

Mechanism of Action of General Anesthetic Agents

General anesthetic agents are positive modulators of the action of GABA on GABAA receptors by binding to allosteric binding sites. They act by facilitation of GABA receptors to promote chloride ion conductance and at therapeutic concentrations, some of the agents (e.g. ethanol and phenobarbital) depress the function of ionotropic glutamate receptors (excitatory), which may contribute to the overall anesthetic effect.

Stages of Anesthesia

After administration of general anesthetic agent (lipophilic and unionised) to a patient, it passes most rapidly into the central nervous system. As the blood concentration of the agent increases, penetration into the CNS increases which leads to increased depth of anesthesia. Guedel in 1937, has defined four stages of anesthesia as follows:

• Stage-1 (Stage of analgesia): This stage is the period from the beginning of administration of anesthesia to the maintain consciousness. Analgesia is produced and the patient progressively loses pain, therefore this stage is also called stage of analgesia. Since, in this stage higher cortical centers are depressed. This stage is also known as cortical stage
• Stage-2 (Stage of delirium or stage of excitement): This stage extends from the loss of consciousness through a stage of irregular and specific breathing to the re-establishment of regular breathing. There is loss of consciousness. Due to further depression of cortical centers, patient leads to the excitement and may shout or struggle violently. Hence this stage is called as the stage of excitement. In this stage patient may salivate, vomit or develop cough excessively. In this stage the respiration is normal and regular.
• Stage-3 (Stage of surgical anesthesia): In this stage excitement is lost and skeletal muscles are relaxed and hence most of the operative procedures are performed in this stage. This stage is divided into four different planes with progressive increase in depth of anesthesia and decrease in respiration and eye movement.
• Stage-4 (Stage of medullary paralysis or respiratory paralysis): The respiration ceases as the depth of anesthesia reaches to stage-IV. This may happen due to the overdose or toxic effect of the anesthetic. In this stage respiratory and cardiovascular system gets collapsed and the tissue rapidly becomes anoxic. There is no any eye movement.

To avoid the toxic effect of anesthesia, anesthetics are either administered by intravenous or rectal rout (basal anesthetics or fixed anesthetics), which lead to loss of consciousness before volatile anesthetics given and hence transition from complete consciousness to the surgical anesthesia will be rapid and safe.

Pre-anesthetic Medication

For the safe and effective response of general anesthetics, pre-anesthetic medications are generally recommended. These are:

• Hypnotics: Can be given one night before surgery, to assure good night sleep.
• Atropine (or hyoscine): Can be given two hours before surgery to prevent excess secretion of saliva or mucous which might obstruct the process is anesthesia.
• Morphine (or pethidine): It is given to minimize fear and anxiety.

Classification Of General Anestnetics

• Inhalation anesthetics: Halothane, Methoxyflurane, Enflurane, Sevoflurane, Isoflurane, Desflurane.
• Ultra short-acting barbitutrates: Methohexital sodium, Thiamylal sodium, Thiopental sodium.
• Dissociative anesthetics: Ketamine hydrochloride.

Inhalation Anesthetics

Halothane:

• Halothane is chemically, 2-bromo-2-chloro-1,1,1-trifluoroethane.
• It was introduced in 1956 as a non-flammable, non-explosive, halogenated volatile anesthetic that is usually mixed with air or oxygen.
• The presence of the carbon-halogen bonds contributes to its non-flammability, volatility, and high lipid solubility (Blood/Gas partition coefficient = 2.3).
• It activates GABAA and glycine receptors. It also acts as an NMDA receptor antagonist and inhibits nACh and voltage-gated sodium channels.

Structure Activity Relationship:

• The SAR studies conducted independently by Meyer and Overton in the 1880s showed a distinct positive correlation between anesthetic potency and solubility.
• The potency of alkanes, cycloalkanes, and aromatic hydrocarbons increases in direct. proportion to the number of carbon atoms in the structure upto a cutoff point 7. Within the n-alkane series, the cutoff number is 10, with n-decane showing minimal anesthetic potency.

Metabolism:

Uses:

• Halothane can be used to start or maintain anesthesia.
• One of its benefits is that it does not increase the production of saliva which can be particularly useful in those who are difficult to intubate.

General Structure-Activity Relationship of Alkanol Series:

• Similar increase in potency with increase in carbon length was seen in the n-alkanol series.
• In association, the n-alkanol with a given a number of carbons is more potent than the n-alkane with same chain length.

Effect of Halogenation on Ether:

• Decrease the flammability of the ether, enhances their stability and increases their potency.
• Higher atomic mass halogens increase potency more compared to lower atomic mass halogens. e.g. Replacing the fluorine (F) in desflurane with chorine (CI) to form isoflurane increases the potency more than four-fold.
• Replacing the chlorine (CI) with bromine (Br) increases the potency three-fold i.e. isoflurane.
• Unfortunately, halogenation also increases the toxicity.
• Halogenated methyl ethyl ether was found to be more stable and potent than halogenated diethyl ether. i.e. enflurane and isoflurane.
• For n-alkane series, fully saturating the alkane with fluorine (F) increases the potency. Number of carbon atoms was 2 to 4 which show highest potency. If it is more than 5, then the activity is diminished..
• The stereoisomer of isoflurane (+) and (-) has been isolated and tested for anesthetic potency. The (+) isomer was found to be 53% more potent than (-) isomer.
• The addition of double and triple bonds to anesthetics molecule decreases their potency.

Methoxyflurane:

• Methoxyflurane is chemically 2,2-dichloro-1,1-difluoro-1-methoxyethane.
• It acts as a positive allosteric modulator of the GABAA receptor, it also acts as an NMDA receptor antagonist.
• It should be administered with nitrous oxide to achieve a relatively light level of anesthesia, and a neuromuscular blocking agent given concurrently to obtain the desired degree of muscular relaxation.

Metabolism:

Uses:

• Methoxyflurane provides rapid short-term analgesia using a portable inhaler device.
• Currently, methoxyflurane is rarely used for surgical, obstetric, or dental anesthesia.

Enflurane:

• Enflurane is chemically 2-chloro-1-(difluoromethoxy)-1,1,2-trifluoroethane.
• It acts as a positive allosteric modulator of the GABAA, glycine, and 5-HT3 receptors.
• It is a fluorinated ether and very potent general anesthetic agent.
• It is more stable inhalation anesthetic which provides rapid adjustments of anesthesia depth with little change in pulse or respiratory rate.

Metabolism:

• Enflurane is metabolized via CYP2E1 to form a fluoride ion and difluoromethoxy difluoro- acetic acid metabolites.

Uses:

• Enflurane may be used for induction and maintenance of general anesthesia.
• It can also be used to provide analgesia for vaginal delivery.

Isoflurane:

• Isoflurane is an isomer of enflurane.
• It is chemically 2-chloro-2-(difluoromethoxy)-1,1,1-trifluoroethane.
• It is a fluorinated ether with general anesthetic and muscle relaxant activities.
• It reduces pain sensitivity (analgesia) and relaxes muscles.
• It likely binds to GABA, glutamate and glycine receptors, but has different effects on each receptor.
• It acts as a positive allosteric modulator of the GABAA receptors.
• It inhibits receptor activity in the NMDA glutamate receptor subtypes.
• It inhibits conduction in activated potassium channels.
• It also affects intracellular molecules and NADH dehydrogenase.

Metabolism:

Uses:

• Isoflurane is a general anesthetic.
• It can be used to start or maintain anesthesia.
• Often another medication is used to start anesthesia due to airway irritation with isoflurane.

Sevoflurane:

• Sevoflurane is chemically, 1,1,1,3,3,3-hexafluoro-2-(fluoromethoxy)propane.
• It is a fluorinated isopropyl ether with general anesthetic property.
• It acts as a positive allosteric modulator of the GABAA receptor, it also acts as an NMDA receptor antagonist.

Metabolism:

Uses:

• Sevoflurane is one of the most commonly used volatile anesthetic agents, particularly for outpatient anesthesia, across all ages, as well as in veterinary medicine.

Desflurane:

• Desflurane is chemically, 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane.
• It is a highly fluorinated methyl ethyl ether used for maintenance of general anesthesia.
• It acts on the lipid matrix of the neuronal membrane, resulting in disruption of neuronal transmission in the brain.
• It may also enhance the synaptic activity of the inhibitory neurotransmitter gamma- aminobutyric acid (GABA).
• It may activate GABA channels and hyperpolarize cell membranes.
• It also may inhibit certain calcium channels and therefore prevent release of neurotransmitters and inhibit glutamate channels.

Metabolism:

Uses:

• Drugs Acting on Central Nervous System (II)
• Desflurane is a non-flammable liquid general anesthetic administered via vaporizer. • It is indicated as an inhalation agent for induction of anesthesia for inpatient and outpatient surgery in adults.

Ultra Short-Acting Barbiturates

Thiopental sodium:

• Thiopental sodium is chemically, 5-ethyl-5(1-methylbutyl)-2-thiobarbiturate.
• Thiopental binds to the chloride ionophore site of the gamma-aminobutyric acid GABA/chloride ionophore receptor complex, thereby enhancing the inhibitory actions of GABAA in the brain which leads to synaptic inhibition, decreased neuronal excitability and induction of anesthesia.
• It also decreases glutamate responses.

Metabolism:

• Thiopental sodium is a barbiturate that is administered intravenously for the induction of general anesthesia of short duration.
• It helps patients to relax before receiving general anesthesia with an inhaled medication.

Methohexital sodium:

• Methohexital sodium is chemically sodium, 5-(hex-3-yn-2-yl)-1-methyl-2,6-dioxo- 5-(prop-2-en-1-yl)-1,2,5,6-tetrahydropyrimidin-4-olate.
• It binds to a distinct site which is associated with CI ionophores at GABAA receptors. This increases the length of time for which the CI ionopores are open, thus causing an inhibitory effect.

Metabolism:

• Metabolism of methohexital is primarily hepatic via demethylation and oxidation. Side-chain oxidation is the primary means of metabolism involved in the termination of the drug’s biological activity.

Uses:

• Methohexital sodium is an intravenous anesthetic with a short duration of action that may be used for induction of anesthesia.
• It has been commonly used to induce deep sedation or general anesthesia for surgery and dental procedures.

Thiamylal Sodium:

• Thiamylal sodium is chemically sodium – 4,6-dioxo-5-pentan-2-yl-5-prop-2-enyl-1H- pyrimidine-2-thiolate.
• It binds at a distinct binding site associated with a CI ionopore at the GABAA receptor, increasing the duration of time for which the CI ionopore is open. The post-synaptic inhibitory effect of GABA in the thalamus is, therefore, prolonged.

Uses:

• Thiamylal sodium is a barbiturate that is administered intravenously for the production of complete anesthesia of short duration.
• It has sedative, anticonvulsant, and hypnotic effects, and is used as a strong but short acting sedative.
• It is still in current use, primarily for induction in surgical anesthesia or as an anticonvulsant.

Dissociative Anesthetics

Ketamine Hydrochloride:

• Ketamine hydrochloride is chemically 2-(2-chlorophenyl)-2-(methylamino) cyclohexan-1-one;hydrochloride.
• It interacts with N-methyl-D-aspartate (NMDA) receptors, opioid receptors (μ and σ), monoaminergic receptors, muscarinic receptors and voltage sensitive Ca+ ion channels and thereby reducing pain perception, inducing sedation, and producing dissociative anesthesia.
• Unlike other general anesthetic agents, ketamine does not interact with GABA receptors.

Metabolism:

• Ketamine presents a mainly hepatic metabolism and its major metabolite is nor-ketamine. The biotransformation of ketamine corresponds to N-dealkylation, hydroxylation of the cyclohexone ring, conjugation to glucuronic acid and dehydration of the hydroxylated metabolites for the formation of cyclohexene derivatives.

Uses:

• Ketamine hydrochloride is a cyclohexanone derivative used for induction of anesthesia.
• Anesthesia in children, as the sole anesthetic for minor procedures or as an induction agent followed by muscle relaxant and tracheal intubation.
• It can be given to asthmatics or people with chronic obstructive airway disease.
• It acts as a sedative for physically painful procedures in emergency departments.

Synthesis

Halothane

Methohexital Sodium

Ketamine Hydrochloride

Multiple Choice Questions:

Question 1. The state of “general anesthesia” usually includes:

1. Analgesia
2. Loss of consciousness, inhibition of sensory and autonomic reflexes
3. Amnesia
4. All of the above

Answer. 4. All of the above

Question 2. The IUPAC name of Halothane is

1. 2-bromo-2-chloro-1,1,1-trifluoroethane
2. 2-bromo-2-chloro-1,1, -difluoroethane
3. 2-bromo-1,1,1-thrifluoroethane
4. None of above

Answer. 1. 2-bromo-2-chloro-1,1,1-trifluoroethane

Question 3. Inhaled anesthetics and intravenous agents having general anesthetic properties:

1. directly activate GABAA receptors
2. facilitate GABA action but have no direct action on GABAA receptors
3. reduce the excitatory glutamatergic neurotransmission
4. increase the duration of opening of nicotine-activated potassium channels

Answer. 1. directly activate GABAA receptors

Question 4. Indicate the anesthetic, which is an inhibitor of NMDA glutamate receptors:

1. Thiopental
2. Halothane
3. Ketamine
4. Sevoflurane

Answer. 3. Ketamine

Question 5. Halothane contains the haloatoms

1. Bromine, chlorine and fluorine
2. Bromine, chlorine and iodine
3. Bromine and iodine
4. Chlorine and fluorine

Answer. 1. Bromine, chlorine and fluorine

Question 6. An ideal anesthetic drug would:

1. induce anesthesia smoothly and rapidly and secure rapid recovery
2. possess a wide margin of safety
3. be devoid of adverse effects
4. All of the above

Answer. 4. All of the above

Question 7. Which of the following general anesthetics belongs to inhalants?

1. Thiopental
2. Desfluran
3. Ketamine
4. Propofol

Answer. 2. Desfluran

Question 8. Which of the following inhalants is a gas anesthetic?

1. Halothane
2. Isoflurane
3. Nitrous oxide
4. Desflurane

Answer. 3. Nitrous oxide

Question 9. Indicate the intravenous anesthetic, which is an ultra-short-acting barbiturate:

1. Fentanyl
2. Thiopental
3. Midazolam
4. Ketamine

Answer. 2. Thiopental

Narcotic And Non-Narcotic Analgesics

Introduction

Pain is the most common complaint for which patients seek treatment. Pain has been classified into various types such as physiological (e.g. touching a hot object or getting a cut), inflammatory (e.g. infection and tissue injury), and neuropathic (e.g. injury to the PNS or CNS).

Agents that decrease pain are referred to as analgesics, or analgetics or pain-killing drugs. As these drugs act as pain relieving agents, they are also called antinociceptives (reduced sensitivity of pain).

There are number of classes of drugs which relieve pain. Mainly these drugs are categorized into two classes, (a) morphine and related compounds and (b) antipyretics and anti-inflammatory analgesics.

Non-Steroidal Anti-Inflammatory Agents (NSAID), have primarily a peripheral site of action and are useful to relive mild to moderate pain without loss of consciousness. These agents often have an anti-inflammatory effect associated with their pain killing action.

Historically, opioid analgesics have been called narcotic analgesics. Narcotic analgesics are selective CNS depressant that cause sleep or loss of consciousness (narcosis) in conjugation with their analgesic effect. These are extremely potent analgesics and are effective for the relief of severe pain. Not all opioid analgesics cause necrosis.

The side effects caused by opioid analgesics are respiratory depression, nausea and drowsiness. Long term administration may lead to tolerance, psychological and/or physical dependence called as addiction.

Because of addiction properties, opioid class has been the most problematic for use in the proper management of pain. The term opiophobia was coined to describe the reluctance of physicians to prescribe opioid drugs in adequate amounts or for long enough periods.

Opioid Receptors:

The neuronal located proteins to which opioid agent binds and initiate biological response are called as opioid receptors. There are three major types of receptors classified by the order in which they are cloned.

• Delta (8) receptors are PO1 receptors.
• Kappa (K) receptors are PO2 receptors.
• Mu (u) receptors are PO3 receptors.

All of these receptors are located in human brain or spinal cord tissues and each has a role in the mediation of pain. These receptors have subtypes that provide varying degrees of analgesia, euphoria or dysphoria, central nervous system depression, and perhaps, the potential for tolerance.

Orphan opioid receptor (fourth receptor) has been identified and cloned (OP4) based on homology with cDNA sequence of the known (μ, 8 and K) opioid receptors. Despite the homology in cDNA sequence with known opioid receptors, this new receptor did not bind the classical peptide or non-peptide agonists or antagonists with high affinity.

Mechanism of Action of Narcotic Analgesics

The signal transition mechanism for (u, 8 and k) opioid receptors is through G-proteins. Activation of opioid receptors is linked through the G-protein to an inhibition of adenylate cyclase activity. This results into decrease in cAMP production, efflux of K* and closure of voltage gated Ca** channel leads to hyperpolarization of the nerve cell and strong inhibition of nerve firing.

Classification Of Narcotic Analgesics

• Natural alkaloids: Morphine, Codeine.
• Semi-synthetic analogues: Hydromorphone, oxymorphone, oxycodone.
• Synthetic agents: Meperidine, Methadone, Phenazocine, Pentazocine, Fentanyl Dextropropoxyphene, Nalorphine, Naloxone, Naltrexone.

Morphine and Related Drugs:

The prototypic narcotic analgesic is (-)-morphine, the principal alkaloid obtained from juice or latex from the unripe seeds of the poppy plant, (Papaver somniferum). The first alkaloid to be isolated was morphine (10% in opium), which then underwent structural modifications to produce a number of derivatives of morphine. It also contains codeine. (0.5%), thebaine (0.2%), papaverine (1%) and noscapine (6%).

Morphine Analogues

Structure-Activity Relationship

• Morphine is a prototype opioid.
• It is selective for μ opioid receptor.
• Structure of morphine composed of five rings (fused) i.e. a benzene ring (A), two partially unsaturated cyclohexane rings (B and C), a piperidine ring (D) and a dihydrofuran ring (E).
• Morphine molecule has 5 chiral centers with absolute stereochemistry, 5(R), 6(S), 9(R), 13(S) and 14(R).
• Naturally occurring morphine is levo (-) rotatory.
• Dextro (+) morphine has been synthesized and it is devoid of analgesic and other opioid activity.
• Any major change will cause changes in the affinity and intrinsic activity of the new compound at each opioid receptor type.
• Thus opioid receptor selectivity profile of new compound may be different than the structure from which it was made (e.g. from μ 8 or K).
• It may also have different physiological properties such as solubility, partition coefficient, pKa etc. results in different pharmacokinetic characteristics and can affect its in-vitro activity profile.

Ring-A:

• Ring ‘A’ and basic ‘N’ exist in protonated (ionized) form, required for opioid analgesic activity.
• Aromatic ring ‘A’ and cationic ‘N’ may be connected by ethyl linkage (i.e. 9, 10 position of B-ring) or propyl linkage (either edge of the piperidine ring that forms the D-ring).
• ‘A’ ring and basic ‘N’ are necessary components for every potent μ- agonist known. (iv) These two alone are not sufficient for μ-activity, however an additional pharmacophores are required such as,
• • Rigid structure (i.e. fused ring A, B, and D)
  • 3-OH and tertiary ‘N’ either greatly enhance or are essential for activity.

N-atom:

• Tertiary ‘N’ group has good opioid activity.
• Size of ‘N’ substitutions indicate agonist or antagonist properties.
• N-CH3 substitution has good agonist activity.
• Increase in ‘N’ substitution to 3-5 carbon (unsaturated or smaller carbocyclic ring), antagonist at some or all receptor types.
• Longer substitution on ‘N’ returns agonist properties to the opioid.
• N-phenyl ethyl substitution opioid is usually 10-fold more potent as a μ-agonist than the N-CH3 analogue.
• N-CH2-CH=CH2 substitution leads to μ-antagonist activity (Naloxane).

3-phenolic hydroxyl group:

• Substitution of 3-H instead of 3-OH decreases the activity 10 times.
• Substitution of 3-CH3 in place of 3-OH decreases analgesic activity, the compound so formed possesses anti-tussive activity (i.e. Codeine).
  (Codeine is weak μ-agonist and it undergoes slow o-demethylation to morphine).

• Substitution of ester (CH3-CO-) at position 3 decreases the activity.
• Diacetyl group at 3 and 6 position leads to formation of Heroin, which is synthesized in 1874 and marketed in 1898 by Friedrich Bayer Co. in Germanay.
• • Heroin is preferable to morphine as it does not disturb digestion or produce habit readily.
  • It has low affinity for u-receptors.
  • It has high lipophilicity compared to morphine and easily cross blood-brain barrier.
  • In body (including the brain), serum and tissue esterase hydrolyse 3-acetyl group to produce 6-acetyl morphine leads to increase μ-receptor activity in excess of morphine.
  • Administration of heroin by IV route rapidly converted to a potent μ-agonist provide a ‘euphoric rush’ – popular drug of abuse.
  • Repeated use of heroin develops tolerance, physical dependence and acquisition of the drug habit.
  • Self administration of heroin with unclean or shared hypodermic needle, results in transmission of HIV, hepatitis or other infectious disease.

C-ring:

• 6-keto decreases the activity in pure morphine only.
• 7,8-dihydro (no double bond between 7 and 8 carbon atom) with 6-keto derivative of morphine increases activity by 8-10 times than morphine (i.e Hydromorphone).

• Substitution of 3-OCH; derivative of hydromorphone (hydrocodone) is more active than codeine.
• Substitution of 6-H decreases the activity.

14a-hydroxy-6-keto derivatives:

• Substitution of B-OH at 14 position increases the analgesic activity (oxymorphone).
• Substitution of 3-OH and N-CH3 derivative is 10 times more potent than morphine.

• Substitution of 3-OCH3 derivative of oxymorphone (Oxycodone) is as potent as morphine when given parenterally, but oral dose is better than parenteral compared to morphine.
• Substitution of N-cyclobutyl methyl and reduction of 6-keto to 6a-OH of oxymorphone (Nalbuphine) acts through K-receptor and possess approximately half analgesic potency of morphine. Nalbuphine acts as a μ-receptor antagonist.

• Replacement of the potent narcotic agonist oxymorphone’s N-methyl group with an allyl group (-CH2-CH=CH2) (Naloxone) or methyl cyclopropyl group (Naltrexone) acts as pure opioid antagonist. Both the drugs act as antagonist of all types of opioid receptor.

• Substitution of -OH group at C14 and keto group at C6 is necessary for pure antagonist activity.

3,4-Epoxide Bridge (Morphinans):

Removal of 3,4-epoxide bridge (4-5 ether bridge) in the morphine structure results in compounds called as morphinans.

• Morphinan can be prepared by synthetic route only, as it is very difficult to remove it from morphine analogue.
• The synthetic procedure yields compounds as racemic mixtures in which only the levo (-) isomers act as opioid analgesic, whereas dextro (+) isomers act as anti-tussive.
• Morphinan derivative,
• • Levorphenol possesses 8 times more potent analgesic activity as compared with morphine due to increased μ-receptor affinity and increased lipophilicity.
  • Butorphenol have mixed agonist/antagonists activity. It is a u-receptor antagonist and K-receptor agonist.

Benzomorphans:

• Benzomorphans are synthetic compounds that lack both the epoxide E ring and C ring of morphine, having only A, B and D rings, retain opioid activity.

• They are named chemically as benzomorphan or use a different nomenclature system as 2, 6-methano-3-benzazocine.
• Analgesic activity increases in order of substitution of OH>H>NH2, NO2, F and Cl at 2′ position.
• Trimethyl substitution at N=CH3, R5-CH3 and Rg-CH3 possesses 3 times more potent analgesic activity than dimethyl Rs-H and R9 CH3.
• Substitution of R-CH3 increases the analgesic activity.
• Substitution of R-OH having the same activity as substitution of -OH at 14th position of morphine leads to decease in analgesic activity.

• Pentazocine is a weak μ-receptor antagonist, whereas it is a k-receptor agonist and thus produces analgesia.
• Phenazocine (N-Phenylethyl substituted benzomorphan) is 10 times more potent than morphine as a μ- agonist.
• Cyclazocine (N- Cyclopropyl methyl substituted benzomorphan) is a narcotic antagonist. It possesses considerable hallucinogenic properties.

4-Phenylpiperidines:

• 4-phenylpiperidines possess only A and D ring analogues of morphine.
• The first agent among this class, meperidine, was synthesized in 1937 by Eislab, who was attempting to prepare antispasmodic agents, but serendipitously observed the opioid activity.
• Meperidine is proved to be a typical μ-agonist, with approximately 1/4th potency of morphine. It have very short duration of action because of its esterases hydrolysis to a zwitter ionic metabolite.

• Replacement of ethyl group of meperidine with isopropyl group (properidine), have 15 times more potent activity than meperidine.
• Reversed esters of meperidine have greater potency. 1,2,5- trimeperidine (propionoxy compound) is 7.5 times more potent than meperidine.
• Replacement of N-CH3 group of meperidine with N-p-amino phenyl ethyl group (Anileridine) is 4 times more potent than meperidine.

Anilinopiperidines:

• Structural modification of the 4-phenylpiperidines results into discovery of new compounds, 4-anilidopiperidine or the fentanyl.
• In fentanyl, phenyl and acyl groups are separated from the ring by nitrogen.
• Fentanyl and its derivatives are μ-receptor agonists.
• They are powerful analgesic (50 times stronger than morphine) with minimum side effects.
• Its short duration of action makes it well suited for use in anesthesia. pa

• Substitution of -CH3 at 3-postion of piperidine ring with addition of a small oxygen containing group at the 4-position of the piperidine ring, leads to the formation of analogues with extreme potent analgesic activity. (i.e. Lofentanyl – 8,400 times potent than morphine).
• Alfentanyl, sufentanyl and remifentanyl are the other examples of fentanyl derivatives, which are having highly safety margin than other μ-agonists.

Diphenylheptanone:

• German scientists synthesized another series of open-chain compounds as potential antispasmodics.
• When the nitrogen ring of morphine is opened, the analgesic activity is virtually abolished.
• Further modifications by scientists made the compound to possess both analgesic and spasmolytic activity.

• Methadone was the first analgesic agent..
• Levo isomer of methadone and levo-isomethadone are twice active as their recimic mixture.
• Reduction of keto group leads to decrease in activity.
• Removal of any phenyl group leads to decrease in activity.
• Replacement of dimethylamino group with pyrrolidyl group gives 3/4th activity as methadone.

Metabolism of Methadone:

Metabolism of Opioids (In liver):

Morphine sulphate:

• The prototypic narcotic analgesic is (-)-morphine, the principal alkaloid obtained from the Opium poppy (Papaver somniferum).
• Morphine was isolated as a pure alkaloid by a German Pharmacist, Serturner in 1803. Morphine sulphate is chemically, (4R,4aR,75,7aR,12bS)-3-methyl-2,4,4,7,7,13- hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinoline-7,9-diol;sulphuric acid.
• It exerts the major effects by interacting with opioids receptors (μ, 8 and K) in the CNS.
• It activates 7 TM GPCRs located presynaptically and postsynaptically along pain transmission pathways.

Uses:

• Morphine is used as narcotic analgesic.
• It is used as pre-anesthetic medication.
• It can also be used for the treatment of diarrhoea.
• It produces sleep and sedation.

Codeine:

• Codeine is an alkaloid that occurs in opium, but the amount present is usually too small to be of commercial importance.
• Conversion of the 3-OH to a 3-OCH3, yields codeine, reduces activity to 15% of morphine.
• It is chemically, (4R,4aR,75,7aR,12bS)-9-methoxy-3-methyl-2,4,4a,7,7a,13-hexahydro-
• 1H-4,12-methanobenzofuro[3,2-e]isoquinoline-7-ol.
• It is available as a sulphate and phosphate salt and also as the free base and as tablets, elixir and solution for injection.
• The 3-methoxy group protects the 3-position from glucuronide as occurs with morphine.

Uses:

• Codeine is used as an analgesic, antitussive and cough suppressor.
• It also possesses antidiarrheal and antihypertensive activity.
• It has hypnotic and sedative property and can be used as antianxiety drug.

Meperidine Hydrochloride:

• Meperidine hydrochloride is chemically ethyl-1-methyl-4-phenylpiperidine-4- carboxylate; hydro-chloride.
• It has very short duration of action and largely metabolized in the liver with only a small quantum of it~ 5% gets excreted unchanged.
• Importantly, the ‘esterases’ predominantly cause cessation of the ester linkage (as ethyl ester at para-position) to leave as residue the inactive-carboxylate analogue.
• It also undergoes N-demethylation to yield the corresponding product known as ‘normeperidine’ – a metabolite which gets accumulated after a prolonged medication with meperidine.

Uses:

• Meperidine is a narcotic analgesic that can be used for the relief of most types of moderate to severe pain, including postoperative pain and the pain of labour.
• It can also use as a premedication before anesthesia.

Anileridine Hydrochloride:

• Anileridine is chemically, ethyl 1-[2-(4-aminophenyl)ethyl]-4-phenylpiperidine-4- carboxylate; hydrochloride.
• It is a synthetic analgesic drug and is a member of the piperidine class of analgesic agents.

Uses:

• It differs from pethidine (meperidine) in that the n-methyl group of meperidine is. replaced by an N-aminophenethyl group, which increases its analgesic activity.

Diphenoxylate Hydrochloride:

• Diphenoxylate is chemically, ethyl-1-(3-cyano-3, 3-diphenylpropyl)-4-phenyl- isonipecotate.
• Diphenoxylate is a narcotic antidiarrheal drug related chemically to loperamide.
• It reduces bowel contractions and consequently the frequency and fluidity of bowel movements.
• Although diphenoxylate is chemically related to narcotics, it does not have pain relieving (analgesic) actions like most other narcotics.
• In higher doses, like other narcotics, diphenoxylate can cause euphoria (elevation of mood) and physical dependence.

Uses:

• It is a synthetic analogue of pethidine (meperidine) with some analgesic activity, but is mostly used in the treatment of diarrhoea associated with gastroenteritis, irritable bowel, acute infections, hyper motility, ulcerative colitis and sometimes even food poisoning.
• It prevents hyper gastrointestinal propulsion by reducing intestinal motility.

Loperamide Hydrochloride:

• Loperamide hydrochloride is loperamide, a synthetic, piperidine derivative.
• It is chemically, 4-(4-chlorophenyl)-4-hydroxy-N,N-dimethyl-a,a-diphenyl-1-piperidine butyramide hydrochloride.
• It acts on the μ-receptors in the intestinal mucosa.
• It slows intestinal motility by acting on the nerve endings and/or intramural Ganglia embedded in the intestinal wall.
• The prolonged retention of the feces in the intestine results in reducing the volume of the stools, increasing viscosity, and decreasing fluid and electrolyte loss.

Uses:

• Loperamide hydrochloride is used as opioid agonist.
• It is used in symptomatic relief of acute non-specific diarrhoea and of chronic diarrhoea associated with inflammatory bowel disease.

Fentanyl citrate:

• Fentanyl citrate is chemically N-phenyl-N-[1-(2-phenylethyl)piperidin-4-yl] propanamide-N-(1-phenylethyl-4-piperidinyl) propionanilide, citrate.
• It is a very potent synthetic opiate, which can be used, as an analgesic.
• It is structurally related to phenylpiperidines (e.g. meperidine) and produces strong analgesia, similar to morphine.
• It possesses an inherent rapid onset and short duration of action.

Uses:

• Fentanyl is 80 times more potent than morphine as analgesic primarily employed as an analgesic for the arrest of pain after all types of surgical procedures.
• It is used as an aid for induction and maintenance of inhalation anesthesia.
• It may be employed also as an adjuvant to all such drugs mostly used for regional and general anesthesia.

Methadone Hydrochloride:

• Methadone hydrochloride is a synthetic narcotic drug.
• It is chemically, 6-(dimethylamino)-4,4-diphenylheptan-3-one.
• Methadone shows optical activity. Among the optical isomers, l-methadone is a more potent analgesic, while d-isomer is antitussive.
• It is more active and more toxic than morphine.

Uses:

• Methadone hydrochloride is used for the relief of many types of pain.
• It is also used in the treatment of some heroin addicts.
• It is used as a narcotic substitute in addiction treatment because it prevents morphine abstinence syndrome.

Propoxyphene Hydrochloride:

• Propoxyphene hydrochloride is chemically, [(25,3R)-4-(dimethylamino)-3-methyl-1,2- diphenylbutan-2-yl] propanoate; hydrochloride.
• This agent mimics the effects of the endogenous opiate dextropropoxyphene, by binding to μ-receptors located throughout the central nervous system.

Uses:

• Propoxyphene hydrochloride is used as narcotic analgesic.

Pentazocine:

• Pentazocine is a novel drug possessing of both opioid agonistic and antagonistic properties.
• It is an agonist at the 8 and K opioid receptors and has a weak antagonist action at the μ-receptor.
• It presumably acts on K-receptors to produce analgesia and sedation.
• Like other narcotics, it produces analgesia, sedation and respiratory depression.
• The bioavailability of pentazocine after oral administration is only 20-50% due to the first pass metabolism.
• It gets metabolized extensively in the liver; and subsequently, excreted by the urinary tract.

Uses:

• Pentazocine is used in oral and parenteral forms as an analgesic for moderate-to- severe pain.

Levorphanol Tartarate:

• Levorphanol tartarate is a synthetic phenanthrene with potent opioid analgesic activity.
• It mimics the actions of endogenous peptides at CNS opioid receptors, thereby producing the characteristic morphine-like effects on the μ-opioid receptor.

Uses:

• Levorphanol tartarate has morphine like effects such as analgesia, euphoria, sedation, respiratory depression, miosis, bradycardia and physical dependence.

Narcotic Antagonists

• Narcotic antagonists are drugs which block the “high” and other effects of narcotics. They also precipitate withdrawal symptoms in the narcotic addict.
• This feature of narcotic antagonists makes them extremely useful in treating overdoses.
• They are structurally related to morphine with the exception of the group attached to nitrogen, hence they act by competing for the same analgesic receptor sites. Research is currently going on to determine the usefulness of antagonists as maintenance drugs.
• Present narcotic antagonists (such as naloxone and cyclazocine) have too brief effect and too many side effects to be completely satisfactory.
• A new drug, naltrexone, appears to be more promising since its effects last longer, and it appears to be more acceptable to the treatment clients.
• Narcotic antagonists prevent or abolish excessive respiratory depression caused by the administration of morphine or related compounds.
• They are also used to treat asphyxia neonatorum and for the diagnosis of possible narcotic addiction.

Nalorphine Hydrochloride:

• Nalorphine hydrocholoride is N-allylmorphine. In morphine tertiary nitrogen is attached to an allyl (-CH2CH=CH2) group.
• It is a narcotic antagonist with some agonist properties. It is an antagonist at μ-opioid receptors and an agonist at K-opioid receptors.
• Nalorphine hydrochloride is white colored, odorless, crystalline powder.
• It darkens on exposure to light.
• It is soluble in water and dilute alkali hydroxide solution, but insoluble in chloroform and ether.
• It must be kept in tightly closed light resistant containers.

Uses:

• Nalorphine is a narcotic antagonist used to treat narcotic-induced respiratory depression.
• It is administered by intravenous injection for treating the overdosage of morphine, pethidine, methadone and levorphanol.
• Nalorphine precipitates withdrawal symptoms and produces behavioral disturbances in addition to the antagonism action.

Levallorphan Tartarate:

• Levallorphan is available as tartarate salt.
• Levallorphan tartarate occurs as white colored, odorless, crystalline powder.
• It melts at 175°C and is slightly soluble in water, but insoluble in ether and chloroform.

Uses:

• Levallorphan is a potent narcotic antagonist used in the treatment of narcotic induced respiratory depression.

Naloxone Hydrochloride:

• Naloxone is (5R, 9R, 135, 14S)-N-allyl-4, 5-epoxy-3, 14-dihydroxymorphinan-6-one.
• It is a derivative of 7, 8-dihydro-14-hydroxymorphinone having an allyl group at the nitrogen.
• It is administered by IV or IM (low oral bioavailability and slow action) and has a relatively short half-life (1 hour).
• It is a specific narcotic antagonist which, unlike nalorphine, possesses no morphine- like properties. It is considered to be an effective antagonist for mixed agonist- antagonist like pentazocine.
• It may also reverse some of the adverse effects of narcotic antagonists having agonist actions owing to its lack of respiratory depressant property.
• It has been found to reverse narcotic analgesic and possesses little analgesic properties of its own.

Uses:

• Naloxone is a pure antagonist with no morphine like effects.
• It blocks the euphoric effect of heroin when given before heroin.

Synthesis

Fentanyl Citrate

Methadone Hydrochloride

Multiple Choice Questions:

Question 1. The drug naloxone

1. produces morphine like activity
2. produces respiratory depression
3. induces constipation
4. precipitates withdrawal symptoms in morphine addicts

Answer. 1. produces morphine like activity

Question 2. Kappa and delta agonists:

1. inhibit postsynaptic neurons by opening K+ channels
2. close a voltage-gated Ca** channels on presynaptic nerve terminals
3. both (a) and (b)
4. Inhibit of arachidonate cyclooxygenase in CNS

Answer. 2. close a voltage-gated Ca** channels on presynaptic nerve terminals

Question 3. Pentazocine is a benzomorphan derivative. It has alkyl group at C-3 position, identify it

1. – CH = (CH3)2
2. -CH2-CH=C(CH3)2
3. = CH2-CH2-CH(CH3)2
4. – CH(CH3)2

Answer. 2. -CH2-CH=C(CH3)2

Question 4. 3-etherification of morphine molecule causes

1. Morphine antagonism
2. No change in activity
3. Decrease of analgesic and addiction
4. Increase of analgesic and addiction.

Answer. 3. Decrease of analgesic and addiction

Question 5. Naltrexone is morphine

1. Agonist
2. Antagonist
3. Partial antagonist
4. All of above

Answer. 2.Antagonist

Question 6. The activity of the one of the following drug is dependant on phenyl-N-alkyl piperidine moiety:

1. Meperidine
2. Imipramine
3. Diazepam
4. Chlorpromazine

Answer. 1. Meperidine

Question 7. Which of the following analgesics is a phenanthrene derivative?

1. Fentanyl
2. Morphine
3. Methadone
4. Pentazocine

Answer. 2. Morphine

Question 8. Pentazocine belongs to the class

1. Benzomorphans
2. Morphinans
3. Phenyl piperidine
4. Azepine

Answer. 1. Benzomorphans

Question 9. Which one is the pure opioid antagonist among the following?

1. Nalorphine
2. Nalbuphine
3. Naloxone
4. Levallorphan

Answer. 3. Naloxone

Question 10. Tick narcotic analgesic, which is a phenylpiperidine derivative:

1. Codeine
2. Dezocine
3. Fentanyl
4. Buprenorphine

Answer. 3. Fentanyl

Question 11. Petazocine belongs to

1. N-methylmorphinan
2. Benzomorphan
3. Meperidine
4. Methadone

Answer. 2. Benzomorphan

Question 12. Morphine and heroin differs from each other in respect of

1. Methyl group on nitrogen
2. Acetyl group at C3 and C6
3. Absence of double bond between C4 and C5
4. Absence of D-ring

Answer. 2. Acetyl group at C3 and C6

Question 13. N,N-Dimethyl-(1-methyl-1-oxo-3,3-diphenyl hexyl) ammonium chloride is the chemical name for

1. Methadone hydrochloride
2. Meperidine hydrochloride
3. Alpha proline hydrochloride
4. Darvon

Answer. 1. Methadone hydrochloride

Question 14. Which of the following opioid analgesics is a strong μ-receptor agonist?

1. Naloxone
2. Morphine
3. Pentazocine
4. Buprenorphine

Answer. 2. Morphine

Question 15. Indicate the narcotic analgesic, which is a natural agonist:

1. Meperidine
2. Fentanyl
3. Morphine
4. Naloxone

Answer. 3. Morphine

Question 16. Select the narcotic analgesic, which is an antagonist or partial μ-receptor agonist:

1. Fentanyl
2. Pentazocine
3. Codeine
4. Methadone

Answer. 2. Pentazocine

Question 17. Which of the following agents is a full antagonist of opioid receptors?

1. Meperidine
2. Buprenorphine
3. Naloxone
4. Butorphanol

Answer. 3. Naloxone

Question 18. Which of the following opioid analgesics is used in obstetric labor?

1. Fentanyl
2. Pentazocine
3. Meperidine
4. Buprenorphine

Answer. 3. Meperidine

Question 19. Which of the following opioid agents is used in the treatment of acute opioid overdose?

1. Pentazocine
2. Methadone
3. Naloxone.
4. Remifentanyl

Answer. 3. Naloxone.

Anti-Inflammatory Agents

Introduction

Non-steroidal anti-inflammatory drugs (NSAIDs) are successfully used to treat a wide range of painful conditions and for the management of edema and tissue damage resulting from inflammation. Most of the drugs under this category are used in the treatment of fever as they possess antipyretic activity in addition to having analgesic and anti-inflammatory actions. Non-steroidal anti-inflammatory drugs (NSAIDs) reduce inflammation which helps to ease joint pain and stiffness. They are used in rheumatoid arthritis, osteoarthritis, ankylosing spondylitis and dysmenorrhea therapy.

Inflammation

The inflammatory response involves the migration of immune system cells into a damaged tissue. In some cases, this is beneficial especially for fighting infection, but in many cases, the inflammatory response increases the damage to the tissue such as in case of asthma, several forms of arthritis. In addition to this, inflammation may also be a step on the pathway towards certain cancers, especially colon cancer.

There are four signs of inflammation:

• Redness: Due to local vessel dilatation.
• Heat: Due to local vessel dilatation.
• Swelling: Due to influx of plasma proteins and phagocytic cells into the tissue spaces.
• Pain: Due to local release of enzymes and increased tissue pressure.

Role of Cyclooxygenase (COX):

Humans and most other mammals have two genes (isoenzyme) for cyclooxygenase, COX-1 and COX-2. Both are quite similar in structure, with only subtle differences. They catalyze the same reactions, although COX-2 works with a wider range of substrates.

COX-1 is constitutively expressed in most of the tissues such as gastrointestinal (GI) tract, the kidneys, and the circulatory system. In contrast, COX-2 is inducible, especially by inflammatory stimuli and found in few cell types such as macrophages, leukocytes, fibroblasts and endothelial cells.

COX-1 is responsible for generating the prostaglandins required for protection of the gastrointestinal tract, platelet aggregation, renal electrolyte haemostasis and renal blood flow maintanace while COX-2 is responsible for the increased prostaglandin synthesis associated with inflammation, fever, and pain responses.

General Structure of Prostaglandins (PG):

Prostaglandins and related molecules are called eicosanoids. The term eicosanoid is derived from “eicosa” meaning “twenty”, referring to the 20 carbons in most of the molecules. Prostaglandin is a naturally occurring 20-carbon cyclopentano fatty acid derivative. Most prostaglandins are synthesized from arachidonic acid.

Mechanism of Action of NSAIDs:

Inflammation can be treated with two major classes of anti-inflammatory drugs: steroids (corticosteroids), and non-steroids. The steroids are compounds with glucocorticoid activity, and include the physiological glucocorticoid, cortisol, and synthetic glucocorticoid analogs such dexamethasone.

Glucocorticoids inhibit inflammatory responses by several mechanisms, and are more powerful drugs than NSAIDs. One mechanism is phospholipase A2 inhibition; this inhibits both prostaglandin and leukotriene synthesis, and therefore has a stronger effect than COX inhibition alone. In addition, glucocorticoids have other effects, unrelated to eicosanoid pathways.

The non-steroidal compounds are called Non-Steroidal Anti-Inflammatory Drugs (NSAIDs). Most NSAIDs inhibit both COX-1 and COX-2 with varying degree of selectivity. Selective COX-2 inhibitor may eliminate the side effects associated with NSAIDs due to COX-1 inhibition, such as gastric and renal effect. Some of the most widely used drugs, including aspirin, ibuprofen, and naproxen fall into this class.

Possible side-effects of NSAIDs include

• Stomach upsets
• Heartburn
• Indigestion
• Rashes
• Headaches
• Wheeziness
• Fluid retention, which can cause swelling of the ankles.

General Structure And Properties Of Nsaids

In general, NSAIDs structurally consist of an acidic moiety (carboxylic acid, enols) attached to a planar, aromatic functionality. Some analgesics also contain a polar linking group, which attaches the planar moiety to an additional lipophilic group. This can be represented as follows:

The NSAIDs are characterized by the following chemical/pharmacological properties:

• All are relatively strong organic acids with pKa in the 3.0-5.0 range. Most, but not all, are carboxylic acids. Thus, salt forms can be generated upon treatment with base and all of these compounds are extensively ionized at physiologic pH. The acidic group is essential for COX inhibitory activity.
• The NSAIDs differ in their lipophilicities based on the lipophilic character of their aryl groups and additional lipophilic moieties and substituents.
• The acidic group in these compounds serves a major binding group (ionic binding) with plasma proteins. Thus all NSAIDS are highly bound by plasma proteins (drug interactions).
• The acidic group also serves as a major site of metabolism by conjugation. Thus a major pathway of clearance for many NSAIDS is glucuronidation (and inactivation) followed by renal elimination.

Classification Of Anti-Inflammatory Agents

• Salicylic acid derivatives: Sodium salicylate, Aspirin, Diflunisal, Salsalate, Sulphasalazine.
• p-Amino phenol derivatives: Paracetamol, Phenacetin.
• Pyrazolidine dione derivatives: Phenyl butazone, Oxyphenbutazone, Sulphin- pyrazone.
• Anthranilic acid derivatives: Mefenemic acid, Flufenemic acid, Meclofenamate.
• Arylalkanoic acid derivatives:
  • Indole acetic acid derivatives: Indomethacin.
  • Indene acetic acid derivatives: Sulindac.
  • Pyrrole acetic acid derivatives: Tolmetin, Zomepirac.
  • Aryl and heteroaryl acetic/propionic acid derivatives: Ibuprofen, Diclofenac, Naproxen, Caprofen, Fenoprofen, Keto-profen, Flurbiprofen, Ketorolac, Etodaolac.
• Oxicams: Piroxicam, Meloxicam, Tenoxicam.
• Selective COX-2 inhibitors: Celecoxib, Rofecoxib, Valdecoxib.
• Miscellaneous: Nimesulide.

Salicylic Acid Derivatives (Salicylates)

• Salicylates have potent anti-inflammatory activity with mild analgesic and antipyretic activities.
• These compounds mainly act on COX-1 and bind higher affinity to COX-1.
• The therapeutic and some of the toxic actions of salicylates (aspirin) are related to its ability to inhibit COX-1 in various tissues and participate in trans-acetylation reactions in vitro.

Metabolism of salicylic acid derivatives: The initial route of metabolism of these derivatives is their conversion to salicylic acid, which is excreted in urine as free acid (10%) or undergoes conjugation with either glycine to produce the major metabolites of salicylic acid (75%) or with glucuronic acid to form glucuronide (15%).

In addition, small amount of metabolites resulting from microsomal aromatic hydroxylation leads to gentisic acid.

Structural – Activity Relationship (SAR) of Salicylates:

• The active moiety of salicylates is salicylate anion, side effects of aspirin, particularly GIT effects appear to be associated with the carboxylic acid functional group.
• Reducing the acidity of the carboxy group results in a change in the potency. of activity. Example: The corresponding amides (salicylamide) retains the analgesic action of salicylic acid, but is devoid of anti-inflammatory properties.
• Substitution on either the carboxyl or phenolic hydroxyl group may affect the potency and toxicity. Benzoic acid itself has only week activity.
• Placement of the phenolic hydroxyl group at meta or para position to the carboxyl group abolish the activity.

Sodium Salicylate:

• Sodium salicylate is the sodium salt of salicylic acid.
• Sodium salicylate is chemically sodium; 2-hydroxybenzoate.
• It is an organic molecular entity.
• It irreversibly acetylates COX-1 and COX-2, thereby inhibiting prostaglandin synthesis and associated inflammation and pain.
• This salicylate produces the same adverse reactions as Aspirin, but there is less occult gastrointestinal bleeding.

Uses:

• Sodium salicylate is a non-steroidal anti-inflammatory agent, used to treat inflammation and pain.

Aspirin:

• Acetylsalicylic acid (aspirin) is an acetyl derivative of salicylic acid.
• It is chemically 2-acetyloxybenzoic acid.
• It acts as an inhibitor of cyclooxygenase which results in the inhibition of the biosynthesis of prostaglandins.
• It binds to and acetylates serine residues in cyclooxygenase.
• It can be prepared by the reaction between salicylic acid and acetic anhydride. In this reaction, the hydroxyl group on the benzene ring in salicylic acid reacts with acetic anhydride to form an ester functional group. Thus, the formation of acetyl salicylic acid is referred to as an esterification reaction.

Uses:

• Aspirin is the prototypical analgesic used in the treatment of mild to moderate pain.
• It has anti-inflammatory and antipyretic properties.
• It also acts as antirheumatic.
• It also inhibits platelet aggregation and is used in the prevention of arterial and venous thrombosis.

p-Amino Phenol Derivatives (Anilides)

• These drugs are having somewhat different mechanism of action than other NSAIDs.
• They are believed to act as scavengers of hydroperoxide radicals (Hydroperoxide radicals have a stimulating effect on COX).
• Therefore the anilides have no anti-inflammatory action.
• The lack of an acidic functionality and COX inhibitory activity imparts several advantages including limited gastric irritation, ulceration, respiratory effects and little effect on platelets.

Metabolism of p-aminophenol Derivatives:

These drugs undergo hydrolysis to yield aniline derivatives that produce directly or through their conversion to hydroxylamine derivatives, such as acetaminophen that undergoes rapid first pass metabolism in the GIT to o-sulphate conjugate.

The N-hydroxylamine is then converted into a reactive toxic metabolite, acetimino- quinone, which produces toxicity to the kidney and liver in conjugation with hepatic glutathione to form mercapturic acid or cysteine conjugates.

Structure – Activity Relationship of p-amino Phenol Derivatives:

• Etherification of the phenolic function with methyl or propyl groups produces derivatives with greater side effects than ethyl derivatives.
• Substituents of the nitrogen atom, which reduce the basicity, also reduce activity unless the substituent is metabolically labile. e.g. acetyl groups.
• Amides derived from aromatic acid. e.g. N-phenyl benzamides that are less active or inactive.

Phenacetin (Acetophenetidin):

• Phenacetin is chemically N-(4-ethoxyphenyl) acetamide.
• It is a phenylacetamide that was formerly used in analgesics, but nephropathy and methemoglobinemia led to its withdrawal from the market.

Uses:

• Phenacetin is an analgesic and an antipyretic with similar effectiveness as an aspirin.
• It has a greater potential for toxicity (hemolytic anaemia and methemoglobinaemia) than paracetamol.

Paracetamol (p-acetaminophen):

• Paracetamol is chemically N-(4-hydroxyphenyl)acetamide.
• Paracetamols produce antipyresis by inhibition of prostaglandin synthesis and release in the central nervous system (CNS) and by acting on the hypothalamic heat- regulating centre.
• It produces analgesia by elevating the pain threshold.
• It may inhibit the nitric oxide (NO) pathway mediated by a variety of neurotransmitter receptors including N-methyl-D-aspartate (NMDA), resulting in elevation of the pain threshold.
• Hepatic necrosis and death have been observed following over dosage.
• It may cause liver, blood cell, and kidney damage.

Uses:

• Acetaminophen is a p-aminophenol derivative with analgesic and antipyretic activities.
• Acetaminophen has weak anti-inflammatory properties and is used as a common analgesic.

3,5-Pyrazolidinedione Derivatives

Structure – Activity Relationship of 3,5-Pyrazolidinediones:

• Replacement of one of the nitrogen atom in the pyrazolidinediones with an oxygen atom yields isoxazole analogues, which are as active as pyrazolidinedione derivatives.
• In 3,5-pyrazolidinedione derivatives, pharmacological activities are closely related to their acidity, the dicarbonyl function at the 3rd and 5th positions enhance the acidity of hydrogen atom at the 4th position.
• Presence of a keto group in the y-position of the butyl side chain produces the active compound.
• Decreasing or eliminating acidity by removing the acidic proton at 4th position (e.g. 4, 4-dialkyl derivatives) abolishes anti-inflammatory activity. Thus, if the hydrogen atom at the 4th position of phenyl butazone is replaced by substituents, such as a methyl group, anti-inflammation activity is abolished.
• If acidity is enhanced too much, anti-inflammatory and sodium-retaining activities decrease; while other properties, such as the uricosuric effect increase.
• Introduction of polar function in these alkyl groups give mixed results. The -hydroxy-n-butyl derivative possesses pronounced uricosuric activity, but give fewer anti-inflammatory effects.
• Substitution of 2-phenyl thio ethyl group at the 4th position produces antigout activity (sulphinpyrazone).
• Presence of both the phenyl groups is essential for neither anti-inflammatory nor analgesic activity.
• m-substitution of aryl rings of the phenyl butazone gives uniformly inactive compounds.
• p-substitution, such as methyl, chloro, nitro, or OH of one or both rings retains activity.

Phenylbutazone:

• Phenylbutazone is chemically, 4-butyl-1,2-diphenylpyrazolidine-3,5-dione.

Uses:

• Phenylbutazone is a pyrazole derivative that has antipyretic, analgesic, and anti- inflammatory actions.
• It is especially effective in the treatment of ankylosing spondylitis.
• It is also useful in arthritis, acute superficial thrombophlebitis, painful shoulder, and Reiter’s disease.

Antipyrine:

• Antipyrine is chemically, 2, 3-dimethyl-1-phenyl-3-pyrazolin-5-one.

Uses

• Antipyrine has analgesic, anti-inflammatory and antipyretic activities.
• It exerts a paralytic action on the sensory and the motor nerves, resulting in some anesthesia and vasoconstriction, and it also exerts a feeble antiseptic effect.

Anthranilic Acid Derivatives (Fenamates)

• The anthranilates are primarily anti-inflammatory with some analgesic and antipyretic activity and are non-COX selective.
• The anthranilates are used as mild analgesics and occasionally to treat inflammatory disorders.
• Diclofenac is used for rheumatoid arthritis, osteoarthritis and post-operative pain and mefenamic acid as an analgesic for dysmennorhea.
• The utility of this class of agents is limited by a number of adverse reactions including nausea, vomiting, diarrhoea, ulceration, headache, drowsiness and hematopoietic toxicity.

Structure – Activity Relationship of Anthranilic Acid Derivatives (Fenamates):

• The position of the carboxyl function is important for the activity of anthranilic acid derivatives that are active, whereas the 3 and 4 amino benzoic acid analogues are not active.
• Replacement of carboxylic acid function with the isosteric tetrazole results in the retention of anti-inflammatory activity.
• Placement of substitution on the anthranilic acid ring generally reduces the activity.
• Substitution on the N-aryl ring can lead to conflicting results. In the ultraviolet erythema assay for anti-inflammatory activity, the order of activity was generally 3′> 2′>4′ for mono-substitution with CF3 group (flufenamic acid) being particularly potent. The opposite order of activity was observed in rat paw oedema assay, the 2′-Cl derivatives being more potent than 3′-Cl analogues.
• In disubstituted derivatives, where the nature of the two substitutes is the same, 2′,3′-disubstitution appears to be the most effective (mefenemic acid).
• The NH moiety of anthranilic acid is essential for the activity as the replacement of NH function with O, CH2, S, SO2, N-CH3, or NCOCH3 functionalities significantly reduced the activity.

Mefenamic acid:

• Mefenamic acid is anthralinic acid NSAID.
• It is chemically 2-(2,3-dimethylphenylamino) benzoic acid.
• It is an inhibitor of cyclooxygenase.
• It inhibits the activity of the enzyme COX-1 and COX-2, resulting in a decreased formation of precursors of prostaglandins and thromboxanes, thereby inhibiting platelets aggregation.

Uses:

• Mefenamic acid is used as an analgesic and anti-inflammatory agent.
• It also possesses antipyretic property.

Metabolism:

• Its metabolism occurs through regioselective oxidation of 3-methyl group and glucuronidation of mephanamic acid. Majority of the 3-hydroxy methyl metabolites and dicarboxylic acid products are excreted.

Meclofenamate Sodium:

• Meclofenamate sodium is anthralinic acid NSAID.
• It is chemically, sodium 2-(2,6-dichloro-3-methylphenylamino)benzoate.
• It inhibits the activity of the enzyme COX-1 and COX-2, resulting in a decreased formation of precursors of prostaglandins and thromboxanes thereby inhibiting platelets aggregation.

Uses:

• Meclofenamate sodium is used as an analgesic and anti-inflammatory agent.
• It also possesses antipyretic property.

Arylalkanoic Acids

Structure – Activity Relationship of Arylalkanoic Acids:

• The largest group of NSAIDS is represented by the class of arylalkanoic acids.
• Drugs of this class share a number of common structural features.
• The centre of acidity is usually located one carbon atom adjacent to a flat surface represented by an aromatic or hetero aromatic ring.
• The distance between these centres is crucial, because increasing this distance to two or three carbons generally decreases activity.
• All agents possess a centre of acidity, which can be represented by a carboxylic acid (R=COOH) and hydroxamic acid (R=CONHOH), a sulphonamide (R=SO2NH2), phenol or a terazole.
• Substitution of a methyl group on the carbon atom separating the aromatic ring leads to enhancement of anti-inflammatory activity.
• Groups larger than methyl decrease activity, but incorporation of this methyl group as part of an alicyclic ring system does not drastically affect activity.

Indole Acetic Acid Derivatives:

Structure – Activity Relationship of Indene and Indole Acetic Acid Derivatives:

• The carboxyl group is essential for anti-inflammatory activity.
• Placement of other acidic functionalities instead of the carboxyl group decreases activity and the amide derivatives are inactive.
• Alkyl group especially -CH3 at 2nd position is much active than aryl substituted analogues.
• The 5th position of the indole ring is most flexible with regard to the nature of substituents that enhance activity. Substituents such as methoxy, fluoro, dimethylamino, methyl, allyloxy, and acetyl are more active than the unsubstituted indole ring.
• The presence of indole ring nitrogen is not essential for activity because the corresponding 1-benzylidenylindene analogue (sulindac) is also active.
• Acylation of the indole nitrogen with aryl/alkyl carboxylic acids results in the decrease of activity.
• Presence of substituents on the N-benzoyl derivatives in the p-position with F, CI, CF3, or S-CH3 groups provide greatest activity.

Indomethacin:

• Indomethacin is chemically, 1-(p-chloro benzoyl)-5-methoxy-2-methylindole-3-acetic acid.
• It inhibits the enzyme COX necessary for the formation of prostaglandins and other autocoids.

Uses:

• It is a more potent antipyretic than either aspirin or acetaminophen.
• It possesses approximately 10 times the analgesic potency of aspirin.
• It is used as anti-inflammatory and analgesic in rheumatic arthritis, spondylitis, and to lesser extent in gout.
• The most frequent side effects are gastric distress and headache.
• It has also been associated with peptic ulceration, blood disorders, and possible deaths.
• It is recommended only for patients who cannot tolerate aspirin and in place of phenylbutazone in long-term therapy.

Indene Acetic Acid Derivatives:

Sulindac:

• Sulindac is a prodrug.
• It is chemically 5-fluoro-2-methyl-1[(4 methyl sulphinyl) phenyl methylene] indene-3- acetic acid.

Metabolism:

• Sulindac reaches peak blood levels within 2 to 4 hours and undergoes a complicated, reversible metabolism. The parent sulfinyl has a plasma half-life of 8 hours. It forms active metabolites of sulphide having plasma half-life of 16.4 hours. In addition to it, sulindac is oxidized to corresponding sulfoxide. The more polar and inactive sulphoxide is virtually the only form excreted.

Uses:

• Sulindac has analgesic, antipyretic, and anti-inflammatory properties.
• It is recommended for rheumatoid, osteoarthritis and ankylosing spondylitis.
• It is used in the treatment of muscular skeletal disorders and acute gouty arthritis.
• It may produce gastric bleeding, nausea, diarrhoea, dizziness as adverse effects, but with a lower frequency than with aspirin.

Pyrrole Acetic Acid Derivatives:

Structure – Activity Relationship of Pyrrole Acetic Acid Derivative:

• Replacement of the p-tolyl group (tolmetin) with a p-chloro benzoyl moiety produced little effect on activity.
• Introduction of a methyl group in the 4th position and 5-p-chloro benzoyl analogues (zomepirac) proved to be four times potent as tolmetin.

Tolmetin Sodium:

• Tolmetin sodium is chemically, sodium-2-(1-methyl-5-(4-methylbenzoyl)-1H-pyrrol- 2-yl) acetate.
• It inhibits the enzyme prostaglandin synthase, thus prevent synthesis of the inflammatory prostaglandin E2 (PGE2) from the precursor prostaglandin H2 (PGH2).

Uses:

• Tolmetin sodium has antipyretic, analgesic, and anti-inflammatory actions.
• It is employed in the treatment of rheumatic and musculoskeletal disorders.
• The drug is, however, comparable to indomethacin and aspirin in the control and management of disease activity.

Zomepirac:

• Zomepirac is chemically, 1,4-dimethyl-5-(p-chloro benzoyl)pyrrole-2-acetic acid.
• It has associated with fatal and near-fatal anaphylactoid reactions.

Uses:

• Zomepirac is recommended in greater degree of analgesia for severe pain.
• It is used as an analgesic and an anti-inflammatory drug.
• It is four times as potent as tolmetin.

Aryl and Heteroaryl Acetic/Propionic Acid Derivatives:

Ibuprofen:

• Ibuprofen is chemically 2(p-isobutyl-phenyl)-propionic acid.
• The activity resides in the (s)-(+) isomer, not only in Ibuprofen, but also throughout the arylacetic acid series.
• These isomers are the more potent inhibitors of prostaglandin synthase.
• The precursor Ibufenac (acetic acid in place of propionic acid), has abandoned hepatotoxicity and is less potent.

Uses:

• Ibuprofen is an anti-inflammatory drug that possesses antipyretic and analgesic action and is used for the treatment of rheumatoid arthritis and osteoarthritis.

Diclofenac:

• Diclofenac is chemically o-(2,6-dichloro anilino)-phenyl acetic acid.
• It is available in sodium and potassium salt form.
• It acts by inhibiting COX and decreased prostaglandin production.
• It binds and chelates both COX-1 and COX-2, thereby blocking the conversion of arachidonic acid to pro-inflammatory-prostaglandins.

Uses:

• Diclofenac sodium is used in the treatment of rheumatic arthritis, osteoarthritis and ankylosing spondylitis.
• The potassium salt is faster acting and is used for the management of acute pain.

Naproxen:

• Naproxen is chemically, (±) 2-(6-methoxy-2-naphthyl)-propionic acid.
• The effectiveness of naproxen is partly due to its ability to inhibit COX-1 and COX-2, resulting in a decreased formation of precursors of prostaglandins and thromboxanes.
• It decreases in the formation of thromboxane A2 synthesis, by thromboxane synthase, thereby inhibiting platelet aggregation.
• It irreversibly blocks the enzyme cyclooxygenase (prostaglandin synthase), which catalyzes the conversion of arachidonic acid to endoperoxide compounds.

Uses:

• Naproxen is fairly comparable to aspirin both in the management and control of disease symptoms.
• It has lesser frequency and severity of nervous system together with milder GI-effects. It possesses analgesic, anti-inflammatory, and antipyretic actions.
• It is used in the treatment of rheumatic arthritis, dysmenorrhea, and acute gout.

Ketorolac:

• Ketorolac is chemically 5-benzoyl-2,3-dihydro-1H-pyrrolizine-1-carboxylic acid.
• It is non-selective and it inhibits the enzymes COX-1 and COX-2.
• The inhibition of COX-2, prevents conversion of arachidonic acid to pro-inflammatory prostaglandins, whereas the inhibition of COX-1 prevents the normal steady-state production of prostaglandins that play housekeeping roles in the protection of the gastrointestinal tract.

Uses:

• Ketorolac is a potent analgesic indicated for the treatment of moderately severe and acute pain.
• It also possesses analgesic and antipyretic activities.
• Because of a number of potential side effects, its administration should not exceed 5 days.

Oxicams

• Oxicams are COX-2 selectivity than many other NSAIDs, particularly meloxicam.
• These agents have utility in treatment of rheumatoid arthritis and osteoarthritis.

Structure Activity Relationship of Oxicams:

• The most active analogues have substituent CH3 on the nitrogen and electron withdrawing substituents on the anilide phenyl groups, such as – Cl and -CF3.
• The introduction of heterocyclic ring in the amide chain significantly increases the anti-inflammatory activity. Example: 2-thiazolyl derivative sudoxicam is more potent than indomethacin.
• The most active benzothiazine have acidities in the pKa range of 6-8.

Piroxicam:

• Piroxicam is chemically, 4-hydroxy-2-methyl-N-2-pyridinyl-1,2-benzothiazine-3- carboxamide-1,1-dioxide.
• It binds and chelates both isoforms of COX-1 and COX-2 and prevent conversion of arachidonic acid into prostaglandin.

Uses:

• Piroxicam has anti-inflammatory, antipyretic and analgesic properties.
• It employed for acute and long-term therapy for the relief of symptoms of osteoarthritis and rheumatoid arthritis.
• It also possesses uricosuric action and has been used in the treatment of acute gout.

Selective COX-2 Inhibitors

• Cyclooxygenase-2 (COX-2) is an inducible enzyme release in response to injury or inflammation.
• COX-2 inhibitors are the new-generation NSAIDs that may selectively block the COX-2 isoenzyme without affecting COX-1 function.
• This may result in control of pain and inflammation with a lower rate of adverse effects compared with older non-selective NSAIDs.
• Rapidly evolving evidence suggests that COX-2 enzyme has a diverse physiologic and pathologic role.

Celecoxib:

• Celecoxib is chemically, 4-[5-(4-methylphenyl)-3-(trifluoromethyl)pyrazole-1-yl]- benzenesulfonamide.
• It has a central pyrazole ring with two adjacent phenyl substituents.
• One phenyl ring contains a methyl group at p-position, whereas other contains a polar sulfonamide moiety at p-position.
• The sulfonamide have ability to bind with a distinct hydrophilic region that is present on COX-2, but not COX-1.

Rofecoxib

• Rofecoxib is chemically, 3-(4-methylsulfonylphenyl)-4-phenyl-2H-furan-5-one.
• It has a central furanose ring and two adjacent phenyl substituents.
• One phenyl ring contains a methyl sulfone group, whereas other phenyl ring is unsubstituted.
• It binds to and inhibits the enzyme COX-2 resulting in an inhibition of the conversion of arachidonic acid to prostaglandins.
• It has greater potency and a longer half-life than celecoxib.

Valdecoxib

• Valdecoxib is chemically, 4-(5-methyl-3-phenyl-1,2-oxazol-4-yl)-benzenesulfonamide.
• It is an aryl sulfonamide derivative like celecoxib.
• It acts by inhibiting prostaglandin synthesis by blocking COX-2 thereby preventing the conversion of arachidonic acid to prostaglandins, which are involed in the regulation of pain, fever and inflammation.
• At therapeutic plasma concentrations valedecoxib does not inhibit COX-1.

Uses:

• Valdecoxib is non-steroidal anti-inflammatory drugs that exhibit anti-inflammatory, analgesic and antipyretic properties.
• These drugs are also recommended for rheumatoid arthritis, osteoarthritis and juvenile arthritis.
• It is also recommended to treat painful menstruation and menstrual symptoms.

Miscellaneous

Nimesulide:

• Nimesulide is chemically N-(4-nitro-2-phenoxyphenyl)-methanesulfonamide.
• It inhibits the COX (COX-2) mediated conversion of arachidonic acid to pro- inflammatory prostaglandins.

Uses:

• Nimesulide has anti-inflammatory activity.
• It is recommended in the treatment of acute pain.

Synthesis

Mafenamic Acid

Ibuprofen

Multiple Choice Questions:

Question 1. Non-narcotic analgesics are mainly effective against pain associated with:

1. Inflammation or tissue damage
2. Trauma
3. Myocardial infarction
4. Surgery

Answer. 1. Inflammation or tissue damage

Question 2. IUPAC name of the sulindac analogue is

1. (Z)-5-fluoro-2-methyl-1-[(p-methyl sulfinyl)phenyl] methylene-1H-indene-3- acetic acid
2. (E)-5-fluoro-2-methyl-1-phenyl methylene-1H-indene-3-acetic acid
3. (Z)-5-fluoro-2-methyl-2-[(p-methyl sulfinyl)phenyl] methylene-1H-indene-4- acetic acid
4. (R)-5-fluoro-2-methyl-1-1-phenyl methylene-1H-indene-3-acetic acid

Answer. 1. (Z)-5-fluoro-2-methyl-1-[(p-methyl sulfinyl)phenyl] methylene-1H-indene-3- acetic acid

Question 3. Non-narcotic analgesics are all of the following drugs EXCEPT:

1. Paracetamol
2. Acetylsalicylic acid
3. Butorphanol
4. Ketorolac

Answer. 3. Butorphanol

Question 4. Which isomer of ibuprofen is more active?

1. (S) (-) isomer
2. (S) (+) isomer
3. (R) (+) isomer
4. (R) (-) isomer

Answer. 2. (S) (+) isomer

Question 5. Select the non-narcotic drug, which is a para-aminophenol derivative:

1. Analgin
2. Aspirin
3. Baclophen
4. Paracetamol

Answer. 4. Paracetamol

Question 6. Phenylbutazone is the acidic drug. It is due to

1. Easily replaceable hydrogen
2. CO-CH2-CO moiety
3. Two keto groups
4. Two nitrogen atoms