Cholinergic Agonists: Key Structural Features

Cholinergic Agents

• Cholinergic agents are drugs and chemicals that act on cholinergic nerves or tissues they innervate.
• These agents can mimic or block the action of acetylcholine (ACh).
• Cholinergic agents have specific clinical indications and represent a relatively small class of therapeutic agents.
• Cholinergic agents are used to treat glaucoma, myasthenia gravis, and Alzheimer's disease.
• These are also used for atony of the smooth muscle of the GI or urinary system for urination initiation and bladder emptying in certain urinary disorders
• Parasympathetic responses include constricted pupils, constricted bronchioles, increased secretions in the lungs, decreased heart rate, dilated blood vessels, increased peristalsis and secretion in the gastrointestinal system, contracted bladder, and increased salivation.

Cholinergic Receptors

• Not all cholinergic receptors are identical.
• Two types of cholinergic receptors include nicotinic and muscarinic.
• The receptors are named after natural products showing receptor selectivity.
• Acetylcholine is the natural messenger for both receptor types
• Nicotine activates cholinergic receptors at nerve synapses and on skeletal muscle.
• L-(+)-Muscarine activates cholinergic receptors on smooth muscle and cardiac muscle.

Cholinergic Drugs Classification

• Cholinergic drugs are classified as agonists or antagonists.
• Cholinergic agonists are divided into muscarinic agonists (parasympathomimetics) and nicotinic agonists (ganglion stimulants).
• Muscarinic agonists can be direct-acting or indirect-acting.
• Indirect-acting muscarinic agonists can be reversible or irreversible.

Design of Cholinergic Agonists

• Requirements for cholinergic agonists include correct size, the correct pharmacophore which consists of an ester and quaternary nitrogen, increased stability to acid and esterases, and increased selectivity
• Rationales include protecting the ester from nucleophiles and enzymes
• Size protects the ester from hydrolysis
• A drug must be large enough to hinder hydrolysis, however, it must be small enough to fit the binding site.

SAR of Cholinocepter Agonists

• The rule of five indicates that there should be no more than five atoms between the nitrogen and terminal hydrogen atom for maximal muscarinic potency.
• Quaternary nitrogen gives maximal activity in the following order: 3°>2°>1°.
• At least two substituents on the nitrogen must be methyl; larger alkyl groups result in less activity.
• There should be a two-carbon unit between the oxygen and nitrogen atoms.
• Introduction of a methyl group in the alpha position increases selectivity as a nicotinic agonist, while a methyl group in the beta position results in muscarinic agonist activity.
• Higher esters as butyryl or propionyl lessen the activity, and can even act potentially as antagonists.
• The carbamic acid ester of choline (carbachol) is a potent cholinergic agent with both muscarinic and nicotinic activity.

Direct Cholinergic Agonists

• Esters include Acetylcholine, Methacholine, Carbachol, and Bethanechol.
• Alkaloids include Muscarine and Pilocarpine.
• Alkaloids are not metabolized by cholinesterases
• Contraindications for direct cholinergic agonists include asthma, coronary insufficiency, peptic ulcers, intestinal obstruction, and hyperthyroidism.

Acetylcholine

• Acetylcholine is a choline molecule acetylated at the oxygen atom
• The highly polar, charged ammonium group means that external drugs do not penetrate lipid membranes
• A external drug remains in extracellular space, failing to pass through the blood-brain barrier
• It stimulates the parasympathetic nervous system.
• Acetylcholine acts on muscarinic and nicotinic receptors
• Stimulation of muscarinic receptors causes cardiac depression, vasodilation of the arteries, short-acting miosis, and increased tone and motility of GIT and UB muscle fibers.

Disadvantages

• Non-selective
• Poorly absorbed from GIT
• Short duration of action due to rapid hydrolysis by esterases

Methacholine

• Methacholine is a synthetic choline ester that acts as a non-selective muscarinic receptor agonist.
• The β-methyl group offers more selectivity towards muscarinic receptors, and also sterically hinders the enzyme.
• Hydrolyzed by true cholinesterase only, making it more stable than ACh.
• Its muscarinic actions are more prominent on CVS than on GIT and the urinary tract.
• It is used in Bronchial challenge tests (methacholine challenge test).
• Its other therapeutic uses are limited to its adverse cardiovascular effects of bradycardia and hypotension

Carbachol

• Carbachol is a choline carbamate that isn't well absorbed in the gastro-intestinal tract and does not cross the blood-brain barrier.
• Non-selective in stimulating muscarinic and nicotinic receptors.
• It is stable towards hydrolysis by both true and pseudo cholinesterase.
• Primarily used for treating glaucoma and during ophthalmic surgery.
• Carbachol is usually administered topically ocularly or through intraocular injection.
• Since it is poorly absorbed through topical administration, benzalkonium chloride is mixed in to promote absorption.
• With topical ocular and intraocular administration, the principal effects include miosis and increased aqueous humor outflow.

Bethanechol

• Bethanechol is a potent muscarinic agonist that is orally effective and administered by subcutaneous injection.
• It has increased hydrolytic stability due to carbamate and steric bulk.
• Bethanechol is a stimulant of GI tract smooth muscle and the urinary bladder.
• Used for the relief of post-surgical urinary retention and abdominal distention.
• Low toxicity, with no serious side effects.
• Use with caution in asthmatic patients.

Pilocarpine

• Pilocarpine's mechanism of action involves contraction of the ciliary muscle to enhance aqueous humor outflow.
• IOP decreases in 1 hour.
• It is indicated for both angle-closure and open-angle glaucoma.
• Oral pilocarpine (Salagen®) is used to treat xerostomia (dry mouth).

Indirect Cholinergic Agonists

• Indirect Cholinergic Agonists are also known as Acetylcholinesterase Inhibitors (AChEls)
• Acetylcholinesterase (AChE) increases the concentration of acetylcholine (ACh) in the synapse.
• This results in prolonging the action of ACh, producing both muscarinic and nicotinic responses.
• AChEls have therapeutic applications that include treatment of myasthenia gravis, atony in the gastrointestinal tract, and glaucoma
• Investigational therapy for Alzheimer's disease

Therapeutic applications

• Agricultural insecticides and nerve gas warfare agents.
• Cholinesterase Inhibitors slow or prevent degradation of ACh released into the synapse

Types of Cholinesterase Inhibitors

• Reversible inhibitors include Physostigmine, Neostigmine, Pyridostigmine, Edrophonium, and Tacrine.
• Irreversible inhibitors are organophosphates that are lipid-soluble and can cross the blood-brain barrier such as Isofluorphate, Parathion, Soman, and Sarin.

Acetylcholinesterase

• Acetylcholinesterase's active site includes an anionic binding region and an ester-binding region.
• The anionic binding region is similar to the cholinergic receptor site.
• Binding and induced fit strains Ach and weakens bonds
• The molecule is positioned for reaction with His and Ser.

Neostigmine

• Neostigmine is an anticholinesterase agent that does not enter the CNS
• Uses include an antidote for neuromuscular blockers, myasthenia gravis, open-angle glaucoma, and GI and urinary tract retention.
• It has greater miotic activity than physostigmine.

Adverse reactions

• Salivation
• Flushing
• Decreased BP
• Diarrhea

Pyridostigmine

• Pyridostigmine is used for myasthenia gravis treatment more often than neostigmine due to its longer duration of action.
• It has fewer side effects than neostigmine in myasthenia gravis.
• Antagonist of neuromuscular blocking agents.
• It is better used than Neostigmine as it is orally used.
• Available as oral tablets & syrup; injection.

Edrophonium

• Edrophonium has a rapid onset as its action starts in one minute with parental administration.
• It has a shorter duration than neostigmine and pyridostigmine.
• The site of action for Edrophonium is in the neuromuscular junction.
• It is considered a specific anticurare agent.
• It is the only one that has a direct cholinomimetic effect on the skeletal muscle.
• Edrophonium is a diagnostic agent for myasthenia gravis.

Tacrine

• Tacrine is a potent AChE inhibitor.
• FDA approved for the treatment of Alzheimer's disease.
• Increasing the level of Ach in the brain is an approach provided that muscarinic receptors are normal.

Side effects

• Hepatotoxicity
• Abdominal cramping

Irreversible AChE Inhibitors

• Irreversible AchE inhibiors are classified as Organophosphorous compounds
• AchE is inhibited irreversibly by phosphate esters that are highly toxic.
• If the enzyme is inhibited, acetylcholine accumulates and nerve impulses cannot be stopped, causing prolonged muscle contraction, leading to paralysis and death.
• The respiratory muscles are also affected
• These chemicals are nerve poisons and have been used in warfare and as agriculture insecticides.

Causes of Organic Phosphate Poisoning

• Inhalation of spray or dust of insecticides
• Skin contamination with agricultural workers
• Contamination of crops or food
• Accidental or intentional ingestion

Symptoms of Toxicity

• Nausea, vomiting
• Excessive sweating, salivation
• Miosis
• Bradycardia
• Low blood pressure and respiratory difficulty, often the cause of death.

Mode of Action of Organic Phosphate

• They form a phosphorylated serine at the esteratic site of the enzyme.
• The Ser-O-P bond is hydrolyzed much slower than Ser-O-C due to its greater energy, which gives it a long/extended duration up to several weeks.

Isofluorophate

• Isofluorophate has been used in glaucoma.
• Extreme caution must be applied during handling.
• Contact with eyes, nose, mouth and skin should be avoided because it is readily absorbed.

Nerve Agents and Insecticides

• Nerve agents (War gases): Tabun (GA), Sarin (GB), Soman (GD), and Cyclosarin (GF).
• Insecticides: parathion, methyl parathion and Malathion

Antidote for Phosphate Poisoning

• Anti-cholinergic drugs counteract the effects of excess acetylcholine, Atropine is an example antidote that can be used in conjunction with pralidoxime or other pyridinium oximes.
• Reactivation of the enzyme is possible if action is taken soon after exposure to these poisons.
• Several compounds, such as cholinesterase reactivators, can provide a nucleophilic attack on the phosphorylated enzyme and cause regeneration of the free enzyme
• Substances such as choline, hydroxylamine, hydroxamic acid, and pyridine-2-aldoximemethiodide or -chloride (2-PAM) can be administered.
• Pyridine nitrogen provides a positive charge to fit to the anionic site, and the N-OH occupies the optimal position for nucleophilic attack of the phosphorous atom

Muscarinic Antagonists

• Muscarinic antagonists are compounds with high affinity for the M-receptor with no intrinsic activity.
• Muscarinic antagonists are competitive (reversible) antagonists of ACh.
• Antagonism responses include decreased contraction of GI and urinary tract smooth muscles, dilation of pupils, reduced gastric secretion & saliva secretion.

Therapeutic Uses

• Treatment of smooth muscle spasms (antispasmodic)
• Shutting down digestion for surgery
• Ophthalmic examinations (mydriatic)
• Treatment of gastric ulcers
• Reduction of nasal and upper respiratory tract secretions in cold and flu

SAR for Antagonists

• Anticholinergic compounds have some similarity to ACh, but with additional substituents that enhance binding to the receptor
• A and B contain at least one aromatic moiety capable of Van der Waals interactions to the receptors and cycloaliphatic or other hydrocarbon for hydrophobic bonding interactions.
• C may be OH or CONH₂ to form H-bonding.
• N is a quaternary ammonium function or a tert. Amine that is protonated.
• The chain includes esters, ethers, or hydrocarbons.

Atropine

• Atropine (hyoscyamine) contains tropic acid and tropine (tropanol).
• Atropine functions as an antispasmodic because of its marked depressant effect on smooth muscle.
• It produces a mydriatic effect.
• Atropine can be administered in small doses before general anesthesia to lessen oral and air passage secretions.
• It is a specific antidote for organophosphorous compounds.

Side effects

• Prolonged pupillary dilation
• Loss of visual accommodation
• Arrhythmias
• Relative positions of ester and nitrogen similar in both molecules
• Amine and ester are important binding groups (ionic + H-bonds)
• Aromatic ring of atropine is an extra binding group (vdW)
• Atropine binds with a different induced fit - no activation
• Atropine binds more strongly than acetylcholine

Types of Antimuscarinic drugs

• Natural: Atropine (DL-hyoscyamine) and Scopolamine (L-hyoscine).
• Synthetic

Structural analogue of Atropine

• Tertiary ammonium compounds: Homatropine
• Quarternary ammonium compounds: Atropine methyl nitrate, Homatropine methyl bromide, Methscopolamine bromide, Ipratropium, and Tiotropium. Structurally Nonrelated to Atropine:
• Tertiary amines: Pirenzepine, Dicyclomine, Cyclopentolate, Oxyphencyclimine, and Tropicamide.
• Quarternary amines: Methantheline, Propantheline, Isopropamide, and Glycopyrrolate

Synthetic Anticholinergic Agents

• Wide range of undesirable effects of solanaceous alkaloids prompt the synthesis of new, specific cholynolytic agents useful as antispasmodics, antiparkinsinians, and neuromuscular blocking agents.

Aminoalcohol esters

• Quaternary ammonium compounds: One of the major developments in the aminoalcohol esters was the introduction of quaternary ammonium compounds which unlike others of their kind, do not pass through the BBB, making them more favorable

Methantheline

• Methantheline is synthetic antispasmodic drug.
• It is used to treat excessive sweating.
• Used to relieve cramps or spasms of the stomach, intestines, or bladder.
• This also treats intestine or stomach ulcers, intestine problems, pancreatitis, gastritis, or urinary problems.

Dicyclomine

• Dicyclomine has fewer side effects than atropine.
• It is used for its spasmolytic effect on various smooth muscle spasms.

Explore the crucial structural features of cholinergic agonists for maximal activity. Learn about the impact of methyl groups, the 'rule of five', and the importance of protecting the ester group. Identify the correct pharmacophore and match structural modifications to receptor selectivity.