Cholinergic, Anticholinergic and Anticholinesterases

1. Cholinergic, Anticholinergic and Anticholinesterases Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Patel Nagar, Dehradun – 248001 Uttarakhand

2. Cholinergic System: Physiology Source: http://schoolbag.info/biology/humans/9.html Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

3. Cholinergic System Cholinergic Agonist Direct acting - act on the receptors to activate a tissue response (Receptor agonists ) Choline esters – Acetylcholine Bethanecol Alkaloids- Pilocarpine Indirect acting - inhibit the action of the enzyme Acetylcholinesterase. (Cholinesteraseinhibitors) Carbamates- Physostigmine, Neostigmine, Pyridostigmine, Edrophonium Phosphates- Isoflurophate Antidote- Pralidoximine Major uses = Stimulate bladder and GI tone, constrict pupils, neuro-muscular transmission Cholinergic Receptor • Acetylcholine with cholinergic receptors • Muscarinic receptors (M1-M5) Five types- M1-5 Excitatory(M1,3&4) or Inhibitory (M2)response • Found in smooth muscles and cardiac muscles. • Muscarine act as Agonist • Nicotinic receptors (N1&2) Two types- Neuronal (N1) and Muscle (N2) • Excitatory response • Found in skeletal muscles. • Nicotine. Act as Agonist Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

4. Glu 327 Glu 327 His 440 His 440 Hydrolysis of Acetylcholine by AChE Anionic site Anionic site Phe 338 Phe 338 ACh Ch Trp 86 Trp 86 Ser 203 Ser 203 Esteratic site Esteratic site Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

5. Cholinergic Agonist Acetylcholine • Synthesis • Fates • Easily hydrolysed in stomach (acid catalysed hydrolysis) • Easily hydrolysed in blood (esterases) • No selectivity between receptor types • No selectivity between different target organs Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

6. Cholinergic Agonist Muscarine and Nicotine as cholinergic agonists Nicotine Muscarine • Advantages • More stable than ACh • Selective for M or N receptor • Target Selective • Disadvantages • Side effects Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

7. Chemistry of Cholinergic Agonist Naturally Occurring Cholinergics Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

8. Carbacol and Bethanechol • Replacement of ester with urethane (Carbamate) group stabilises the carbonyl group toward hydrolysis. NH2 and CH3 are equal sizes. Both fit the hydrophobic pocket • Long acting cholinergic agonist. • Administered orally. • Not selective…… just used topically in glaucoma. • More stable. • More selective on muscarinic receptor. • Used to stimulate GIT and urinary bladder after surgery. Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

9. BethanecholChloride Synthesis: Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

10. SAR for Acetylcholine Ing’s rule of five: This rule suggests that there should be no more than five atom between the nitrogen and the terminal hydrogen atom for maximal muscarinic potency. • Nitrogen atom • A a)Amine is a Quaternary nitrogen • (Trimethylammonium group ) • b)Isosteric replacement of nitrogen with S or As, P, Se → less active; so N is very essential • c) Only those compounds which posses +ve • charge on the atom in place of nitrogen are active • B Replacement in the alkyl groups: • –one CH3 with ethyl → less active than Ach • –Me3 into Et3 → inactive compound where ethyl is a bulky group → steric hindrance which will make the cationic nitrogen away from the ionic sit of the receptor → incomplete binding • –CH3 with H → decreased activity • So, optimum activity is obtained with trimethyl ammonium head. Acetyloxy group Acetyl group is replaced with higher homologs( propionyl or butyryl groups) Less active than Ach  Reduction of the carbonyl group to CH2 → muscarinic activity but this reduction → CH2-O-CH2 → ↑ lipophilicity → CNS toxicity ↑ the size of alkyl (CH3) group → ↓ activity Replacement of O by S → ↓ activity Replacement of CH3 by NH2 → i.e. carbamylation instead of esterification → compound with longer duration. • Ethylene Bridge • A). Substitution with methyl group at α-carbon → increase nicotinic activity. • B) Introduction of methyl group at β-carbon → compounds with greater muscarinic activity than nicotinic, and greater stability / resist hydrolysis (long duration) • As a result of β-substitution → chiral carbon → 2-isomer • a) S-isomer: CH3 still performs D-R interaction (active at M receptors). • b) R-isomer : CH3 interfere with D-R interaction (less active). Summary 1. Tight fit between Ach and binding site. 3. Methyl groups fit into small hydrophobic pockets 2. Ester interacting by H-bonding 4. Quaternary nitrogen interacting by ionic bonding Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

11. SAR of Cholinocepter agonists • The molecule must posses a nitrogen atom capable of bearing a positive charge, preferably a quaternary ammonium salt. • For maximum potency the size of the alkyl groups substituted on the nitrogen should not exceed the size of a methyl group. • The molecule should have an oxygen atom, preferably an ester like O, capable of participating in a hydrogen bond. • There must be a two carbon unit between the O atom and the nitrogen atom. Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

12. Therapeutic uses of Cholinergic Agonist • Nicotinic agonists • Treatment of myasthenia gravis • Lack of acetylcholine at skeletal muscle causing weakness • Muscarinic agonists • Treatment of glaucoma • Switching on GIT and urinary tract after surgery • Treatment of certain heart defects. Decreases heart muscle activity and decreases heart rate Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

13. Anticholinergic Agents Nondepolarizing Atracurium Vecuronium Tubocurarine Depolarizing Succinylcholine Centrally acting Dextromethorphan M1- Pirenzepine, Trihexyphenidyl/ Benzhexol M3- Darifenacin Atropine Scopolamine Ipratropium Diphenhydramine Hexamethonium Trimetaphan Mecamylamine Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

14. Anticholinergic Agents • Natural alkaloids- Atropine (spasmolytic, mydriatic), Hyoscine (Scopolamine), Scopoderm TTS (antiemetic) • Semisynthetic derivatives-Mydriatics: Homatropine, GI spasmolytics: Hyoscine butyl bromide • Synthetic compounds-GI spasmolytics: Oxyphenonium • Antiulcus drugs: Pirenzepine (M1-blockers) • Antiasthmatics: Ipratropium and Tiotropium • Antidisurics: Flavoxate, Oxybutynyne, Trospium • Mydriatics: Tropicamide • Antiparkinsonian (central M-cholinolytics): Benztropine, Biperiden, Trihexyphenidyl Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

15. Solanaceous Alkaloids Found mainly in Henbane (Hyoscyamus niger), Deadly nightshade (Atropa belladonna), and Jimson weed (Datura stramonium) Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

16. SAR of Atropine • Substituent P and Q should be carbocyclic or heterocyclic rings for maximum antagonist potency. • Generally one ring is aromatic and the other is saturated possessing only one olefinic double bond. • Substituents P and Q can be fused into aromatic tricyclic ring system. • Substituent R can be a hydrogen atom, a hydroxyl group, a hydroxymethyl group, a carboxamide or it can be a component of one of P or Q ring system. • When R substituent is either hydroxyl group or hydroxymethyl group the antagonist is usually more potent than the same compound without this group. • When substituent S is an ester, then the compound is most potent. But this is not essential for activity. Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

17. Nicotinic Antagonists Classification: • Ganglionic blocking agents (N1-receptor antagonist) • Skeletal neuromuscular blocking agents (N2-receptor antagonist) • Nondepolarizing (Competitive Neuromuscular Blocking Agents) : inactive on N2-receptor • Depolarizing (Noncompetitive Neuromuscular Blocking Agents) : to activate N2-receptor Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

18. Tubocurarine and Metocurine • Tubocurarine • Plant alkaloid. Isolated from Chondodendrontomentosum. • • Causes muscle paralysis (arrow poison). • • only one quaternary ammonium group. • Administered intravenously and has a long duration of action. Primarily excreted as unchanged drug in the urine and bile. • • Therapeutic Use: As a muscle relaxant in various surgical procedures. • Metocurine • A semi-synthetic analog of tubocurarine. • As a bis-quaternary ammonium compound, it is about four times more potent than the parent compound R=H Tubocurarine R=CH3 Metocurine Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

19. Cholinesterasetypes • Acetylcholinesterase • Located in synapses • Substrate selectivity: • ACH • Plasma cholinesterase • Located in plasma (non-neuronal) • Substrate selectivity: • ACH • Succinylcholine • Local anesthetics (procaine) Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

20. Acetylcholinesterase inhibitors • Quaternary ammonium alcohol • Simplest structures • Bind to anionic site and block ACh binding • Reversible • Non-covalent Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

21. Chemistry of Cholinesteraseinhibitors Reversible Anticholinestrase Irreversible Anticholinestrase • Organophosphates • Irreversible binding to Cholinesterase active site • Longer acting • Used in the treatment of glaucoma Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

22. Most basic Nitrogen; protonated at physiological pH. Acetylcholinesterase inhibitors • Carbamates • Quaternary or tertiary ammonium groups • Reversible • Covalent modification to AChE Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

23. Therapeutic uses of Cholinesterase inhibitors • These agents indirectly increase the synaptic concentration of acetyl choline by inhibiting enzyme Cholinestrase and have following uses: • Improve muscle strength in Myasthenia gravis. • Used in open-angle glaucoma to decrease intraocular pressure by stimulating contraction of ciliary muscle and sphincter of iris. • Alzheimer disease and similar cognitive disorders. Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

24. Parathion Malathion Ecothiopate Organophosphates • Used to treat glaucoma • Topical application • Quaternary N is added to improve binding interactions • Results in better selectivity and lower, safer doses • Relatively harmless to mammals • Agents act as prodrugs in insects • Metabolised by insects to produce a toxic metabolite Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand

25. Acetylcholinesterase inhibitors Dr. ArchanaGahtori Division of Pharmaceutical Sciences Shri Guru Ram Rai Institute of Technology & Science Dehradun – 248001 Uttarakhand