Drugs Affecting Cholinergic Neurotransmission 2024 PDF

Drugs Affecting Cholinergic Neurotransmission (Cholinergic Agonists and Antagonists) Dr. Olson Text: Foye’s Principles of Medicinal Chemistry Introduction Neurons that release acetylcholine (ACh) are referred to as cholinergic, as are the receptors on whic...

Drugs Affecting Cholinergic Neurotransmission (Cholinergic Agonists and Antagonists) Dr. Olson Text: Foye’s Principles of Medicinal Chemistry Introduction Neurons that release acetylcholine (ACh) are referred to as cholinergic, as are the receptors on which the neurons synapse Cholinergic neurons are found in the autonomic nervous system (parasympathetic and sympathetic), somatic nervous system, and in a few neurons of the CNS Cholinergic receptors can be (1) muscarinic or (2) nicotinic, depending on their ability to naturally bind the alkaloid muscarine or nicotine, respectively Functions of ACh Parasympathetic nerve impulses stimulate contraction of smooth muscle in the GI and urinary tract, contraction of the ciliary muscles of the eye, relaxation of the smooth muscle of the blood vessels and heart, decreasing contractability and rate “Rest and Digest” Classes of Drugs Cholinomimetic (Parasympathomimetic) Agents: Chemical compounds that stimulate the parasympathetic nervous system Agonists of Cholinergic Receptors Inhibitors of Acetylcholinesterase, the enzyme that degrades ACh Cholinolytic (Parasympatholytic) Agents: Compounds that possess affinity for cholinergic receptors, but exhibit no intrinsic activity (cholinergic antagonists)  Introduction, Cont’d Deficiencies in ACh can produce/enhance a number of diseases… Could be treated by administering ACh. Why isn’t this approach likely to work? Administration of ACh also produces nonselective effects; could produce serious adverse effects Indications Muscarinic cholinergic agents (agonists) are used: 1. Post-surgically to reestablish smooth muscle tone of the GI and urinary tracts to relieve abdomen distension and urinary retention 2. In glaucoma, by enhancing outflow of aqueous humor thereby reducing intraocular pressure 3. Being evaluated for the treatment of cognitive disorders (i.e. Alzheimer’s) Muscarinic antagonists (anticholinergic drugs) are used: 1. To reduce smooth muscle spasms resulting from overstimulation of GI and urinary smooth muscles (“antispasmodics”) Overactive bladder  There is a lack of known antagonists exhibiting “very high subtype “neural” selectivity” There “are no muscarinic agonists with high selectivity” “heart” “bladder” Muscarinic Receptors are GPCRs ACh ACh causes dissociation Effector Responses Overview of GPCR Signaling  Nicotinic Receptor The majority of clinically effective cholinergic agents are muscarinic agonists or antagonists Nicotinic receptors are found in the skeletal neuromuscular junction, adrenal medulla and autonomic ganglia; implicated in myasthenia gravis (autoimmune disease) Pentameric, transmembrane ligand- ACh gated ion channels; ACh gatekeeper, modulate passage of ions, ACh principally K+ and Na+, through the channel ACh Neurochemistry ACh biosynthesized in cholinergic neurons by enzyme-catalyzed transfer of acetyl from acetyl coenzyme A (acetyl-S-CoA)to choline Choline comes from serine and predominantly from ACh esterase (AChE)-catalyzed hydrolysis of ACh in synaptic space Therapeutic agents based on regulation of ACh biosynthesis have not been successful Free ACh that is not bound to a receptor is hydrolyzed by AChE; this action terminates the action of ACh A number of useful therapeutics have been developed that inhibit AChE SAR and Drug Design To understand the design of synthetic cholinergic agents, an SAR considering the structure of acetylcholine and various cholinergic receptors must be understood Muscarinic Receptors: Basic model of the muscarinic receptor reflects the basic requirements for an ester functionality and a quaternary ammonium group separated by two carbons What types of bonds exist? SAR of Muscarinic Receptor Agonists 1. Ester: Engages muscarinic receptor through a hydrogen bond in the esteratic site 2. Quaternary Ammonium Salt: Engages muscarinic receptor through an ionic bond at the anionic site of the receptor 3. At least two alkyl groups bonded to the quaternary nitrogen must be methyls 4. Defined stereochemistry 5. All potent cholinergic agonists have five atoms between the quaternary nitrogen and the terminal hydrogen atom (Ing’s Rule of Five)  Cα Cβ ACh Stereochemistry The stereochemistry of ACh resides in the different arrangements of its atom in space around σ bonds What type of isomer is this? Synclinal is most preferred conformation in solution Anticlinal most preferred conformation bound to receptor, but also least energetically favorable  Cα Cβ ACh Stereochemistry Even though ACh is achiral, many synthetic and naturally occurring agonists and antagonists possess chirality; usually, one enantiomer is many times more active than the other The stereochemistry of acetylcholine resides in the different arrangements in space of its atoms by virtue of rotation about  bonds (i.e., conformational isomerism). ACh Mimetics – Muscarinic Agonists Interaction of cholinergic agonists with muscarinic receptors produces well-defined pharmacological responses dependent on tissue or organ where receptor is located, including smooth muscle contraction, vasodilation, decreased heart rate, etc. Reminder: Why is ACh poor therapeutic (three reasons)? Why can’t ACh be administered orally? SAR of ACh Need therapeutic alternatives to ACh to agonize muscarinic receptors SAR has defined the design of all muscarinic agonists used as therapeutic agents Three Regions: (1) Acyloxy, (2) Ethylene and (3) Quaternary Ammonium SAR of ACh Quaternary Ammonium Only compounds possessing a positive charge on the atom in the position of the nitrogen have muscarinic activity Analogues in which all three methyl groups were replaced by larger alkyl groups are inactive Analogues in which all three methyl groups were replaced with ethyl groups produced a cholinergic antagonist Replacement of one methyl with H, ethyl, or propyl compromises activity  SAR of Ethylene Bridge As chain length is increased, agonist activity of ACh analogues is decreased Ing’s Rule of Five: There should be no more than five atoms between the N and the terminal H for maximum muscarinic potency Although larger analogues can bind to the muscarinic receptor, they have antagonist activity  SAR of Ethylene Bridge Replacement of the hydrogen atoms of the ethylene bridge with alkyl groups larger than methyl significantly compromises agonist activity Acetyl-β-methylcholine: Greater muscarinic than nicotinic activity; equipotent to ACh on muscarinic receptors Acetyl-α-methylcholine: Greater nicotinic than muscarinic activity; reduced activity vs. ACh at muscarinic receptor Note: Addition of methyl groups to the ethylene bridge produce chiral molecules Stereochemistry and Acetyl-β-Methylcholine mAChRs display stereoselectivity for the enantiomers of methacholine S-Enantiomer equipotent to ACh R-Enantiomer ~20x less potent AChE hydrolyzes methacholine (S-enantiomer) much slower than ACh; ~1/2 the rate The R-enantiomer is not hydrolyzed by AChE and actually acts as a weak inhibitor of the enzyme Rank the durations of action of the following compounds from shortest to longest: S-(+)- Methacholine, ACh, Racemic Methacholine  SAR of Acyloxy Group As predicted by Ing’s Rule of Five, replacement of the acetyl group with larger homologues (ethyl, propyl, butyl) produces less potent analogues of ACh Aromatic choline esters function as antagonists Because of the chemical instability of ACh to hydrolysis, analogues more stable to hydrolysis were explored Carbamic acid esters (carbamates) produce potent agonists of both muscarinic and nicotinic receptors Carbachol is less susceptible to hydrolysis Carbamic acid ester of methacholine; by AChE and gastric acid due to carbamate Specific for muscarinic receptor; being less electrophilic administered orally Ether and Ketone Analogues Choline ethyl ether exhibits significant muscarinic activity and is chemically stable, but has not been used clinically Potent ketones can be accomplished when derivatives possess a carbonyl on the carbon δ to the quaternary nitrogen, the same position as the carbonyl in ACh Do choline ethyl ether and the alkylaminoketones have a longer or shorter duration of action than ACh? Is Ing’s Rule of Five met? Practice Problem True or False: Ing’s Rule of Five applies to the natural agonist muscarine?  Summary of ACh SAR 1. The molecule must possess a nitrogen atom capable of bearing a positive charge, preferably a quaternary ammonium salt 2. For maximum potency, the size of the alkyl group situated on the nitrogen should not exceed the size of a methyl group 3. The molecule should have an oxygen atom, preferably an ester-like oxygen, capable of participating in a hydrogen bond 4. There should be a two-carbon unit between the oxygen and nitrogen atoms; Ing’s Rule of Five must be met Clinically Relevant Therapeutics Methacholine Chloride (Provocholine): Marketed as the racemic mixture though the S-enantiomer is 240x more potent. [Do you remember why?] Selective muscarinic agonist [What chemical feature determines muscarinic specificity?] Carbachol Chloride (Isopto Carbachol) Carbamate analogue of ACh No selectivity for muscarinic or nicotinic receptors More resistant to acid, base, or enzyme-catalyzed (AChE) hydrolysis Exhibits weak AChE inhibitory activity Longer duration of action [Do you remember why?] Clinically Relevant Therapeutics Bethanechol Chloride (Urecholine) Carbamate analogue of methacholine Selective for muscarinic receptors; exhibits no activity at nicotinic receptors Administered orally Pilocarpine HCl (Isopto Carpine) Unique, muscarinic agonists that does not adhere to traditional SAR Administered orally, ophthalmically, and as gel Where would you hypothesize that pilocarpine is metabolized? What is the basic functional group with pKa of 6.8? How many stereocenters are present in pilocarpine? Clinically Relevant Therapeutics Cevimeline HCl (Evoxac) Nonclassical muscarinic antagonist Quinuclidine derivative Administered orally Where would you hypothesize that cevimeline is metabolized? Hint: It’s Phase II. How many acidic/basic functional groups are in cevimeline? AChE Inhibitors (AChEIs) Another means of producing a cholinergic effect is to interfere with the mechanism of ACh breakdown/metabolism By inhibiting the rapid hydrolysis of ACh (AChE inhibition), the concentration of ACh in the synapse will increase, resulting in sustained muscarinic and nicotinic effects AChE Inhibitors (AChEIs) AChEIs, or anticholinesterases, are indirect cholinomimetics because they do no act at the cholinergic receptor, but produce the same effect by increasing the synaptic concentration of ACh Indications: Myasthenia gravis, glaucoma, Alzheimer’s and other cognitive disorders, insecticides, chemical warfare agents Binding of ACh to catalytic site of AChE What types of binding interactions do you see?  AChE Mechanism of Hydrolysis Hydrolysis of ACh Enzyme Regeneration: Important for the development of inhibitors If the enzyme becomes acylated by a functional group (i.e., carbamyl or phosphate) that is more stable toward hydrolysis than an ester, the Acetylated AChE is inactive; cannot bind to another molecule of ACh enzyme remains inactive longer Reversible AChEIs Type I: Reversible AChEIs are substrates for and react with AChE to form an acetylated enzyme that is more stable than the ACh acetylated enzyme, but still capable of undergoing hydrolytic regeneration Aryl Carbamates (i.e. Physostigmine) Alkyl Carbamates (i.e. Carbachol, Bethanechol) MOA: Hydrolysis of carbamate leads to transesterification of the serine residue, forming a carbamylated enzyme Regeneration of active AChE by hydrolysis of carbamate is much slower than an acetylated enzyme (~15 minutes vs. ~0.2 milliseconds) Reversible AChEIs Aryl carbamates have better affinity (stronger inhibition) for AChE’s compared to alkyl carbamates Aryl carbamates are also superior to alkyl carbamates because the resulting anion (from hydrolysis) is better stabilized  Physostigmine Identify the carbamate. MOA: AChE Carbamylation Because physostigmine is a tertiary amine with a pKa = 8.2 rather than a quaternary ammonium salt, it is more lipophilic than most AChEIs and can diffuse across the BBB Metabolic Route:  Other Aryl Carbamates Metabolic Route: Neostigmine (Prostigmin): Chemical Features: Carbamate Aryl Quaternary Ammonium Salt Distance between quaternary ammonium salt and carbamate is approximately Type of Metabolism? the same as in ACh Can Neostigmine cross the BBB? Other Aryl Carbamates Pyridostigmine (Mestinon): Charged N in pyridine ring No CNS activity Orally effective Metabolism analogous to neostigmine (Practice) Edrophonium Chloride (Enlon): Quaternary ammonium salt Does Enlon carbamylate/acetylate AChE? Irreversible AChEIs Phosphate esters are very stable to hydrolysis, being even more stable than many amides Act as inhibitors by the same mechanism as the carbamate inhibitors, except that they form phosphate esters with the enzyme. The rate of hydrolysis of the phosphorylated enzyme is much slower than that of the carbamylated enzyme, and its rate is measured in hours Because the duration of action of these compounds is much longer than that of carbamate esters, they are referred to as irreversible inhibitors of AChE. Topical only, due to toxicity Therapeutic effect can last up to 4 weeks More commonly used as warfare agents and insecticides Acetylcholine Antagonists – Muscarinic Antagonists Muscarinic antagonists have high affinity for muscarinic receptors, but have no intrinsic activity (“bind and block”) Commonly referred to as anticholinergics, antimuscarinics, cholinergic blockers, antispasmodics, or parasympatholytics Muscarinic antagonists are competitive antagonists of ACh, thus the pharmacological effect of muscarinic antagonists is opposite ACh Commonly used to treat smooth muscle spasms associated with increased tone of GI tract (motion sickness), overactive bladder, nasal and upper respiratory tract secretions (OTC cold and flu) Clinically Relevant Agents SAR Atropine provided the structural model that has guided the design of muscarinic antagonists Although the amine functional groups is separated from the ester by >2 carbons, the conformation of the tropine ring orients the two functional groups so the distance between them resembles ACh SAR of Muscarinic Antagonists 1. Substituents R1 and R2 should be carbocyclic or heterocyclic for maximal antagonist potency. The rings may be identical, but the more potent compounds have different rings. These rings may be fused together. 2. The R3 substituent may be a hydrogen, hydroxyl, hydroxymethyl, carboxamide, or a component of an R1/R2 ring system. Hydroxyl and hydroxymethyl groups are the most potent. The hydroxyl group presumably increases binding strength by participating in a hydrogen bond interaction at the receptor. 3. The X substituent in the most potent anticholinergic agents is an ester, but an ester functional group is not necessary for activity (could be absent or an ether). SAR of Muscarinic Antagonists 4. The N Substituent is a quaternary ammonium salt in the most potent anticholinergic agents but is not required for activity. Tertiary amines can also possess antagonist activity after being protonated. Alkyl substituents on the nitrogen are usually methyl, ethyl, propyl or isopropyl. 5. The distance between the ring-substituted carbon and the amine nitrogen is not critical. The length of the alkyl chain may be 2-4 carbons. The most potent anticholinergic agents have two methylene units. Final Comments Muscarinic Antagonists Antagonists compete with agonists (ACh) for a common receptor binding site R1 and R2, particularly the added size (sterics), enable antagonists to outcompete Ach Effective when administered orally or parenterally Anticholinergic agents possessing a quaternary ammonium functional group are generally not well absorbed from the gastrointestinal tract due to their ionic character, but can be used in the treatment of ulcers Antagonists having a tertiary nitrogen are much better absorbed and distributed following all routes of administration and are especially useful when systemic distribution is desired Case Study PJ was brought to the emergency department where you work by his roommates. PJ is a 29-yo graduate student at the local university who has been studying for his PhD. His housemates say he consumes 6-12 beers per day and uses marijuana. In the ED, he is agitated, confused, and combative. His friends also state that he recently ingested a handful of brown, kidney shaped seeds. PJ’s vital signs show a temperature of 102 oF, tachycardia, hypertension, and unresponsive, dilated pupils. Urine was collected for drug screening and serum for liver function tests. Tests reveal that he has atropine and scopolamine in his urine. A diagnosis of anticholinergic overdose was made, and PJ was given a gastric lavage of saline and activated charcoal. The physician wants to administer a cholinergic agonist to counter the effect of the anticholinergic overdose. Evaluate structures 3 and 4 for use in this case.