1. CHOLINERGIC AGENTS 1. Cholinergic Agonists
Medicinal Chemistry III / lecture 1
Shokhan J. Hamid
14/10/2018

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

4. Adrenergic and cholinergic innervations in sympathetic and parasympathetic nervous systems

5. 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.

6. Muscarinic and nicotinic receptor

7. Muscarinic receptor subtypes and functions

8. 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.

9. Nicotinic receptor subtypes and functions

10. Neurotransmitter Acetylcholine (Ach)

11. ethanolamine
Acetyl Co-A
choline

14. Metabolism of acetylcholine

15. 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 .

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

17. 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

18. 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 agonists.
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.

19. 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 active
but less potent than Ach
If any or all R = H →decreases activity

20. 2) Modification of the ethylene bridge:
Methyl substitution in β carbon relative to N affords acetyl‐β‐methylcholine (methacholine), which has muscarinic 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)

21. Summery:

22. 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.

23. SAR summary A “rule of five” idea states that there should be no more than 5 atoms between the Nitrogen and the terminal Hydrogen

25. 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

26. Methacholine:
Methacholine is non selective muscarinic agonist, can exist as (S) and (R) enantiomers.
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 not hydrolyzed 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 in asthma patients for purpose of verifying the diagnosis of asthma

27. 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.

28. Metabolism of acetylcholine

29. Bethanecol Chloride: Orally active Selective for muscarinic receptor
Used to stimulate GI tract and urinary bladder after surgery (urinary retention)

30. 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 and to terminate acute angle closure attacks.
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.

31. Cevimeline hydrochloride:
Cevimeline is a cholinergic agonist which binds to the 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.

32. 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

33. 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

34. mechanism of action of acetylcholinesterase

35. 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

36. 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."

37. 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.

38. Acetylcholinesterase Inhibitors
1. Reversible AChEIs - Physostigmine - Neostigmine - Pyridostigmine - Edrophonium chloride - Ambenonium Chloride - Demecarium Bromide - Metrifonate - Tacrine HCl - Donepezil - Rivastigmine - Galantamine

39. 3. Antidotes for 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)

40. Reversible AChEIs
Half life for the methylcarbamated enzyme = ~ 15 minutes

41. 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 investigated for use in the treatment of Alzheimer's disease.

42. Neostigmine: Fully ionised Cannot cross BBB No CNS side effects
More stable to hydrolysis
The methylsulfate salt is used postoperatively as a urinary stimulant and in the diagnosis and 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.

43. 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)

45. Reversible AchEIs for treating Alzheimer's disease (AD)
Patients with AD are reported to have reduction in acetylcholine, serotonin, norepinephrine, dopamine, and glutamate levels

46. Galantamine Dual cholinergic action
1- By binding to nicotinic receptors
2- By inhibiting AChE No hepatotoxicity
Tablet, AD

47. 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 glaucoma.

48. 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

49. END