CHOLINERGIC AGENTS1. Cholinergic Agonists

Medicinal Chemistry II / lecture 13rd stage/ 2nd semester

5/February/2020

Nervous system

1- Central 2- Peripheral a- Somatic b- Autonomic -Sympathetic - Parasympathetic * -Enteric Neurotransmitters• Acetylcholine• Epinephrine• Norepinephrine• Cholinergic receptors• Muscarinic receptors• Nicotinic receptors

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Adrenergic and cholinergic innervations in sympathetic andparasympathetic nervous systems

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Introduction

• Cholinergic NS refer to the part of Nervous system that utilize Acetylcholine (Ach) as a neurotransmitter.

• It is key NT in the parasympathetic NS.• There are 2 major receptors in the Cholinergic NS, muscarinic

and nicotinic receptors.• Their name came from their natural agonists, muscarine and

nicotine.

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Muscarinic and nicotinic receptor

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Muscarinic receptor subtypes and functions

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Nicotinic receptor*They are agonized by nicotine.• They are located in the CNS, Autonomic NS and in neuromuscular junction, they are a part of a ligand gated ion channel receptors.• Physiological functions depend upon muscle-type or neuronal-type.• Muscle-type nicotinic AChRs are localized at neuromuscular junctions and allow muscle contraction and maintain muscle tone; (thus these are targets for muscle relaxants)• Neuronal type are involved in cognitive function, learning and memory, arousal, reward and motor control.

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Nicotinic receptor subtypes and functions

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NeurotransmitterAcetylcholine (Ach)

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Acetyl Co-A

ethanolamine

choline

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Metabolism of acetylcholine

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The esteratic subsite, where acetylcholine is hydrolyzed to acetate and choline, contains the catalytic triad of three amino acids: serine 200, histidine 440 and glutamate 327.

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Physico-chemical property of Ach• Prototype muscarinic (and nicotinic) agonist• ACh chloride (powder for injection) to be dissolved in sterile water for injection

shortly before use. • It is a short-acting miotic when introduced into the anterior chamber of the eye.• It cannot be administered topically, because it is not lipophilic enough to penetrate

the cornea.• In the presence of acid or base, as in GIT, the rate of hydrolysis is so fast that it

prevents oral dosing of Ach.• Can not be administered as IV dosage form because it will be degraded by a

hydrolyzing enzyme called butrylcholinesterase in plasma .

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Requirements for cholinergic agonists:• Stability to stomach acids and esterases• Selectivity for cholinergic receptors• Selectivity between muscarinic and nicotinic

receptors

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SAR of cholinergic agonists

• Cholinergic drugs mimic action of Ach on Muscarinic or nicotinic receptors and produce the same effect as Ach but in greater magnitude.

• A general strategy of making an agonist is to use the original compound, in this case Ach, as a framework

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1) Modification of quaternary Ammonium group:• Replacing nitrogen atom by arsenic, phosphorus, or sulfur resulted in

less active compounds and are not used clinically.• Replacing all three methyl groups on the nitrogen by larger alkyl

groups resulted in inactive compound.• Replacing all three methyl groups with ethyl groups resulted in

antagonist.• Replacement of only one methyl group by an ethyl or propyl group

affords a compound that is active, but much less so than acetylcholine.• Successive replacement of one, two, or three of the methyl groups

with hydrogen atoms to afford a tertiary, secondary, or primary amine, respectively, leads to successively diminishing muscarinic activity.

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• Summery:• If R = methyl,(CH3) --> active• If R = ethyl (C2H5) --> antagonist!• If R = propyl (C3H9) and higher alkyls -->

inactive• If only one of the R = ethyl or propyl activebut less potent than Ach• If any or all R = H →decreases activity

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2) Modification of the ethylene bridge:

• Methyl substitution in β carbon relative to N affords acetyl-β-methylcholine (methacholine), which has potency almost equivalent to that of acetylcholine and much greater muscarinic than nicotinic selectivity.

• Methyl substitution in α carbon relative to N affords acetyl-α-methylcholine, which has reduced muscarinic and nicotinic potency to that of acetylcholine. But has greater nicotinic than muscarinic selectivity. (This is not used clinically)

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• Summery:

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3) Modifications of the acyloxy group:

• Choline esters of aromatic or higher Mwt. acids possess cholinergic antagonist activity.

• The ester group isn’t mandatory as quaternary amine group but an oxygen atom is required in this region.

• Replacing methyl with amine group results in carbamate (carbachol) which is more resistant to hydrolysis than ester group.

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SAR summaryA “rule of five” idea states that there should be no more

than 5 atoms between the Nitrogen and the terminalHydrogen

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Specific muscarinic agonists

1. Methacholine chloride 2. Carbachol chloride 3. Bethenechol chloride 4. Pilocarpine hydrochloride 5. Cevimeline hydrochloride

• Mechanism of Action (MOA) : They act directly by binding to muscarinic receptor as a agonist and produce the same effects as Ach

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• Methacholine:• Methacholine is non selective muscarinic agonist, can exist as (S) and (R)

isomer.• Although the chemical is used as the racemic mixture, its muscarinic activity

resides principally in the (S)-isomer.• The (S)/(R) ratio of muscarinic potency for these enantiomers is 240:1.• Acetylcholinesterase hydrolyzes the S-(+)-isomer much slower (approximately

half the rate) than acetylcholine.• The R-(–)-isomer is more resistant to hydrolysis by AChE and even acts as a

weak competitive inhibitor of the enzyme. • This stability toward AChE hydrolysis as well as the AChE inhibitory effect of the

R-(–)-enantiomer may explain why racemic methacholine produces a longer duration of action than acetylcholine.

• Use – It is used to induce bronchospam for purpose of verifying the diagnosis of asthma

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• Carbachol Chloride• Carbachol is a potent cholinergic agonist possessing both muscarinic

and nicotinic activity• Carbachol is used topically for glaucoma• Carbachol is less readily hydrolyzed by gastric acid, AChE, or

butyrylcholinesterase than acetylcholine.• Carbachol forms a carbamyl ester in the active site of AChE, which is

hydrolyzed more slowly than an acetyl ester. • This slower hydrolysis rate reduces the amount of free enzyme and

prolongs the duration of action.

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• Bethanecol Chloride:• Orally active• Selective for muscarinic receptor• Used to stimulate GI tract and urinary bladder after

surgery (urinary retention)

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• Pilocarpine:• It is a plant derived alkaloid whose structure does not match the

established SAR but still acts like a cholinomimetic.• Pilocarpine is marketed as tablets (Salogen), an ophthalmic solution,

and gel. • It penetrates the eye and is the miotic of choice for open-angle

glaucoma.• It also is used for the treatment of xerostomia (dryness of the mouth)

caused by radiation therapy of the head and neck, Sjogren's syndrome, or as a side effect of some psychotropic drugs.

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• Cevimeline hydrochloride:• Cevimeline is a cholinergic agonist which binds particularly to the M1 and M3

muscarinic receptor subtype, which results in an increase secretion of exocrine glands, such as salivary and sweat glands.

• Cevimeline hydrochloride is available as an oral capsule for the treatment of xerostomia (dry mouth) associated with Sjögren's syndrome.

• Before its approval, pilocarpine was the only drug for this condition.

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• Uses of cholinergic agonists:• Nicotinic selective agonists:• Treatment of myasthenia gravis - lack of acetylcholine at skeletal muscle causing weakness• Muscarinic selective agonists- Treatment of glaucoma- Switching on GIT and urinary tract after surgery- Decreases heart muscle activity and decreases

heart rate

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Acetylcholinesterase inhibitors

Cholinesterases

1- Acetylcholinesterase (AChE):• Associated with glial cells in the synapse• Catalyzes the hydrolysis of Acetylcholine (serine hydrolase)

2- Butyrylcholinesterase (BuChE):• Located in human plasma (also called pseudocholinesterase)• Broad substrate specificity• Can hydrolyze dietary ester and drug molecules in the blood

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mechanism of action of acetylcholinesterase

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AChEIs• Commonly referred to as anticholinesterases.• Classified as indirect cholinomimetics.

• Principle mechanism of action does not involve binding to cholinergic receptors

• They act by interfering with the metabolism of ACh• Response is non-selective resulting in activity at both muscarinic and

nicotinic receptors

• AChE inhibitors are useful in the treatment of myasthenia gravis (muscular fatigue / weakness), atony in the gastrointestinal tract and glaucoma.

• Also useful as agricultural insecticides and nerve gas warfare agents.• Treatment of Alzheimer’s disease and other cognitive disorders

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• Myasthenia gravis is a chronic autoimmune neuromuscular disease that causes weakness in the skeletal muscles, which are responsible for breathing and moving parts of the body, including the arms and legs. The name myasthenia gravis, which is Latin and Greek in origin, means "grave, or serious, muscle weakness."

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Theory of AchE inhibitors

• During hydrolysis of Ach, the AchE gets acylated, it needs to be hydrolyzed by water to be regenerated in free from, otherwise it can’t function again.

• If instead of acetyl group there is carbamate group then hydrolysis will be resisted.

• The AchE which is not hydrolyzed cannot be used again. • Thus goal of AchE inhibitor is to provide such hydrolysis

resistant functional group such as carbamates or phosphate esters.

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Acetylcholinesterase Inhibitors1. Reversible AChEIs

- Physostigmine- Neostigmine- Pyridostigmine- Edrophonium chloride- Ambenonium Chloride - Demecarium Bromide- Metrifonate- Tacrine HCl- Donepezil- Rivastigmine- Galantamine

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2. Irreversible AChEIs:- Isofluorphate- Echothiophate Iodide- Hexaethyltetraphosphate (HETP) and Tetraethyl pyrophosphate- Malathion- Parathion- Schradan

3. Antidotes for irreversible AChEIs- Pralidoxime (2-PAM, 2-pyridine aldoxime methyl chloride)

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Reversible AChEIs

• Half life for the methylcarbamated enzyme = ~ 15 minutes

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Physostigmine• Physostigmine is a tertiary amine rather than a quaternary ammonium

salt, it is more lipophilic than other AChEIs and can diffuse across the blood-brain barrier.

• It is used when CNS effects are desired, as in the treatment of severe anticholinergic toxicity.

*Rarely used. *Injectable dosage form.

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• Neostigmine:• Fully ionised• Cannot cross BBB• No CNS side effects• More stable to hydrolysis• Injectable and oral dosage form• The methylsulfate salt is used postoperatively as a urinary stimulant and in

the treatment of myasthenia gravis.

• Pyridostigmine bromide It appears to function in a manner similar to that of neostigmine and is the most widely used anticholinesterase agent for treating myasthenia gravis.

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• Demecarium Bromide: is a long-acting miotic used to treat wide-angle glaucoma

• Ambenonium Chloride: Ambenonium chloride has a long DOA is used for the treatment of myasthenia gravis in patients who do not respond satisfactorily to neostigmine or pyridostigmine.

• Edrophonium Chloride: edrophonium has a more rapid onset and shorter duration of action than neostigmine, pyridostigmine, or ambenonium.

• Used for diagnosis of myasthenia gravis (IV)

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Reversible AchEIs for treatingAlzheimer's disease (AD)

• Patients with AD are reported to have reduction in acetylcholine, serotonin, norepinephrine, dopamine, and glutamate levels

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Galantamine

• Dual cholinergic action1- By binding to nicotinic receptors2- By inhibiting AChE - No hepatotoxicity- Tablet, AD

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Irreversible inhibitors of AChE

• Designed based on chemical logic that phosphate esters are more stable to hydrolysis than carboxylate ester or an amide

• Rate of hydrolysis of phosphorylated enzyme is much slower due to aging (t1/2 for diethyl phosphate is about ~8h)

• These agents are used as insecticides.

• Echothiophate is used by topical application to treat chronic glaucoma.

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Irreversible inhibitors of AChE as insecticides

• Irreversible AChEI insecticides is beneficial to agricultural production throughout the world

• To be used with extreme caution in the presence of humans and other mammals to prevent inhalation of the vapors and their absorption through the skin.

• Both routes of exposure cause a number of poisoning accidents every year, some of which are fatal

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Pralidoxime: antidote for irreversible ACHEIs

• Pralidoxime is administered subcutaneously, intramuscularly, or intravenously, and it must be given within a short period of time after enzyme phosphorylation, generally a few hours after exposure, for it to be effective due to the aging process of the phosphorylated enzyme.

• Little reactivation is likely if given 36 hours after exposure. • If the phosphorylated AChE has aged, 2-PAM will not regenerate the

enzyme.

• For this reason, as well as because new phosphate ester AChEIs capable of aging rapidly are being developed as insecticides and chemical warfare agents.

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END

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