Review Position of Hydrogen on a Chiral Carbon Aromaticity and Special Nitrogens in Rings

CHOLINERGIC DRUGS Cholinergic Drugs These are agents that interact with receptors utilizing the neurotransmitter (NT) acetylcholine (ACh). Cholinergic Synaptic Sites autonomic effector sites: innervated by post-ganglionic parasympathetic fibers autonomic ganglia and the adrenal medulla some CNS synapses skeletal muscle neuromuscular junction (motor nerves) Terminology The term mimetic is applied to agents that activate and lytic is applied to those that block the actions of NT receptors. Thus, cholinergic agonists are often referred to as parasympathomimetics or cholinomimetics while antagonists are referred to as parasympatholytics or cholinolytics.

RECEPTOR TYPES NICOTINIC RECEPTORS There are 2 main subtypes of cholinergic receptors Nicotinic (N) and Muscarinic (M)   NICOTINIC RECEPTORS There are at least 2 subtypes N1 and N2 The N1 receptor is found at the neuromuscular junction where its activation results in skeletal muscle contraction The N1 receptors are blocked by d-tubocurarine, succinylcholine and decamethonium The N2 receptors are found at the autonomic ganglia and are blocked by hexamethonium and trimethaphan MUSCARINIC RECEPTORS Currently there are 5 subtypes; M1 – M5 The muscarinic receptors are blocked by Atropine

Acetylcholine Biosynthesis and Effect of ACh on the Cholinergic System Ca2+ Na+ Muscarinic Receptor ACH Choline Acetyltransferase (CHAT) Acetylcholinesterase Acetyl CoA + Acetylcholine Action Potential Choline a b Na+ H+ Nicotinic Receptor ACH Choline Acetate Choline Presynaptic neuron Postsynaptic target

Interaction of Acetylcholine at Muscarinic Receptors Tyr381 VI I VII Thr189 V II IV III Asp105

Pharmacological Effects of Cholinergic Agonists Ophthalmic Effects Miosis Cardiovascular Effects Negative chronotropic Effects Negative inotropism (decreased contractility) Decreased SA and AV nodal conduction velocity Effect on ionic currents Gastrointestinal and Urinary Tract Effects Secretion of gastric acid, intestinal peristalsis, tone, and contraction amplitude. Stimulate urinary tract by increasing ureteral peristalsis

STRUCTURE ACTIVITY RELATIONSHIP (SAR)   ONIUM GROUP 1) Essential for intrinsic activity, contributes to affinity as well. 2) Optimal functional group for activity 3) Increase in size of methyl groups (on N) decrease activity ESTER GROUP 1) Contributes to binding affinity 2) Acetyl group not critical for activity and can be replaced 3) The single bonded oxygen appears to be important for high muscarinic activity. CHAIN 1) shortening or lengthening of chain decreases activity. 2) a - substitution decreases M activity more than it does nicotinic activity. 3) b - substitution decreases N activity more than it does M activity.

Individual Direct-Acting Cholinergic Agents Predict the selectivity of the following compounds at M and N receptors Individual Direct-Acting Cholinergic Agents Acetylcholine Chloride - Cardiac depressant and vasodilator - Bronchial constriction - Skeletal muscle contraction - Stimulates secretions from salivary and sweat glands - atropine blocks M effects but not those of N receptors PROBLEM: ACh is easily hydrolyzed by acetyl cholinesterase (AChE). In addition, an aq solution of ACh is unstable, thus prepare solution just before use.

What can we do to decrease the ease of hydrolysis of acetylcholine? Less easily hydrolyzed Less easily hydrolyzed Possess the same pharmacology as Ach Selective for M receptors Pilocarpine (Diocarpine, Miocarpine) Non-selective muscarinic agonist It has a 3S, 4R stereochemistry Used in the treatment of open angle glaucoma

Indirect Acting Cholinergic Drugs Two common enzymes hydrolyze ACh 1) Acetylcholinesterase (AChE) Compounds which inhibit AChE prolong the life of ACh, thus they produce similar effects as direct agonist.   2) Butyrylcholinesterase / Pseudocholinesterase ( BuChE ) - located in human plasma - biologic functions unknown -hydrolyzes a broader group of esters (AChE hydrolyzes only acetyl & propionyl gps )

Replacement of the Ester Functional Group Three different functional groups interact with the esteratic site in AChE Ester Inhibition: Is completely reversible   Carbamate Inhibition: Is semi-reversible Effective carbamylation is in the order shown below. Why? Organophosphate Ester Inhibition AChE inhibition by organophosphate results in phosphorylation which is irreversible

REVERSIBLE AChE INHIBITORS 1. PHYSOSTIGMINE (Eserine) - NATURAL PROD Functional groups include: carbamate ester, pyrrolidine. Arrange the N atoms in physostigmine according to their pKas.   NEOSTIGMINE BROMIDE ( PROSTIGMINE ) Carbamate ester, quaternary ammonium salt Dimethyl group provides more chemical stability than monomethyl or H Can be given orally and pH has no effect on activity. Why? Used in Myasthenia gravis to prevent atony of skeletal and GI musculature. ( Myasthenia gravis is a degenerative neuromuscular disease) 3. PYRIDOSTIGMINE BROMIDE ( MESTINON ) less toxic than Neostigmine most widely used anti-AChE agent to treat MG 

REVERSIBLE AChE INHIBITORS (Contd) 4. AMBENONIUM CHLORIDE (Mytelase) Used in the treatment of MG Is poorly absorbed from GI ( why ? ) Is not hydrolyzed by Cholinesterases Metabolism: unknown 5. DEMECARIUM BROMIDE ( HUMORSOL ) Aqueous solution is stable Long acting miotic Used to treat wide angle glaucoma.  6. EDROPHONIUM CHLORIDE ( TENSILONON ) An anticholinesterase without the carbamate functional group Therefore, does not react with the receptor. Often used as a potential diagnostic agent for MG  

IRREVERSIBLE AChE INHIBITORS ORGANOPHOSPHATE ESTER INHIBITORS Used as both drugs and nerve poisons Used also as agricultural insecticides Long lasting anticholinesterase When A=S, referred to as a Phosphothionate and must be metabolized to the active compound by desulfuration. MOA: Bind irreversibly to AChE or BuChE ISOFLUOROPHATE (Floropryl) Irreversible inhibitor of AChE and effect can last for days or even weeks. Decomposes in water but stable in oil Can be absorbed readily through the skin. Is used clinically in the treatment of glaucoma. ECHOTHIOPHATE ( Phospholine ) Unlike isofluorophate when used topically it is not readily absorbed, thus has limited in vivo distribution used in the treatment of glaucoma.

IRREVERSIBLE AChE INHIBITORS INSECTICIDES MALATHION Insoluble in water Rapidly hydrolyzed in mammals to inactive metabolite. But undergoes desulfuration in insects to form an active metabolite. PARATHION (Thiophos) Low volatility with a stable aq solution – More polar Probably the cause of most common accidental OP poisoning and death.

Antidote for Organophosphate Poisoning Pralidoxime chloride ( Protopam, 2 - PAM) Serves as antidote for irreversible anti AChE Serves as effective antagonist for some carbamates such as neostigmine & pyridostigmine bromide. Little or no effect if not used before 36h of poisoning because of aging Phosphorylation of AChE Mechanism of Reversing Phosphorylation: Require the use of a better nucleophile

Newer Anticholinesterase Agents Rivastigmin Tartrate (Exelon®)   Centrally selective and long-lasting anticholinestarase activity Is a pseudo-irreversible carbamate AChE inhibitor (Inhibits the enzyme for as long as 10 h due to slow dissociation.) Also shows selectivity for AChE as compared to BuChE. Donepezil (Aricept®) Is a reversible non-competitive inhibitor of AChE. About 1000X more selective for AChE than BuChE shows greater affinity for brain over peripheral AChE.. Slows down the loss of cognitive functions and hence is used in Alzheimer’s disease. Readily penetrates the CNS and is 100% bioavailable by mouth.

CHOLINERGIC BLOCKING AGENTS 1. Postglanglionic parasympathetic nerve terminals in smooth muscles – blocked by Atropine 2. Autonomic ganglia: - blocked by hexamethonium 3. Neuromuscular junction: - blocked by d - tubocurarine   We will now consider #1 also referred to as parasympatholytics, cholinolytics, antimuscarinic (block muscarinic receptors) CLINICAL SIGNIFICANCE OF ANTICHOLINERGICS 1) improve cardiac rate output (used in bradycardia ) 2) lower motility of GI tract (antispasmodic effect) 3) reduce salivation, perspiration, acid & gastric secretions (antisecretory effect) 4) produce dilation of the pupil of the eye (mydriatic effect ) GENERAL MECHANISM OF ACTION: - occupy AChR and prevent ACh from binding to the receptor -These compounds have some component of ACh, with additional (often bulky groups) that extend beyond receptor area occupied by ACh & impart affinity.

STRUCTURE-ACTIVITY RELATIONSHIP ANTIMUSCARINIC PHARMACOPHORE A; B = Bulky groups such as Aromatic or cycloalkyl groups, D = H, -OH, CONH2 CATIONIC HEAD: A positively charged N atom either as quaternary ammonium salt, or becomes positively charged at physiological pH cationic head improves affinity but does not appear to be absolutely necessary. HYDROXYL GROUP ( OH ) - enhances affinity but is not a requisite for activity. - appears to be involved in H-bonding to an electron rich center on the receptor ESTER GROUP ( COOR) present in many potent molecules but is not necessary for the activity.   CYCLIC SUBSTITUENTS: At least one cyclic substituent (phenyl, cyclohexyl, etc) is present in most compounds. Substituents on the ring do not contribute to activity. STEREOCHEMICAL REQUIREMENTS Alcohol Portion: only small differences between R & S isomers. Acid Portion: shows significant differences CONNECTING CHAIN : Usually a 2-carbon chain

CHOLINERGIC BLOCKING AGENTS A AMINO ALCOHOL ESTERS. a) ATROPINE: Is a naturally occurring alkaloid --Standard anticholinergic agent b) CYCLOPENTOLATE ( CYCLOGYL ) - 1/2 as active as Atropine c) DICYCLOMINE ( BENTYL ) -used 1oly for spasmolytic effect on various smooth muscle spasms especially those of GI origin.   d) GLYCOPYRROLATE ( ROBINUL ) - used as an adjunct in the management of peptic ulcer and other GI ailments associated with hyperacidity, hypermotility and spasm. - Little or no CNS effect ( why ?) - high doses can block ganglia & myoneural junction.

CHOLINERGIC BLOCKING AGENTS B. AMINOALCOHOL ETHERS a) BENZTROPINE (Cogentin) -Useful in Parkinsonism because of its anticholinergic properties -Relieves tremors, rigidity associated with Parkinsonism -Has sedative properties as well b) CHLORPHENOXAMINE HYDROCHLORIDE (Phenoxene) Due to presence of Cl and CH3 gps, this drug has reduced antihistamine effect but has increased anticholinergic action. Useful in the symptomatic treatment of Parkinson’s disease  

CHOLINERGIC BLOCKING AGENTS C. AMINOALCOHOLS a) TRIHEXYPHENIDYL HYDROCHLORIDE (ARTANE) Notice the presence of the piperidine ring - 1/2 antispasmodic action as atropine - useful in the treatment of Parkinson’s Disease D. AMINOAMIDES a) ISOPROPAMIDE IODIDE ( DARBID ) - only slightly soluble in H2O ( why? ) - long duration of action - used in adjunctive therapy in the treatment of peptic ulcer, and other GI ailments E. DIAMINES a) ETHOPROPAZINE ( PARSIDOL ) - Has a phenothiazine ring system - useful in the symptomatic treatment of Parkinson’s disease - common side effects include drowsiness and dizziness.