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SAR-Cholinergic-drugs.pdf

This document summarizes the structure activity relationship of directly acting cholinergic drugs. It describes how modifications to different parts of the structure, including the ester group, ethylene bridge, and quaternary ammonium group, can impact cholinergic activity. Replacing the ester with an ether or ketone group produces stable compounds with potent activity. Increasing the carbon chain length of the ethylene bridge or replacing the quaternary ammonium nitrogen with sulfur decreases activity. The quaternary ammonium group is essential for muscarinic activity.

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SAR (Structure activity relationship ) of directly acting cholinergic Drugs The general structure of directly acting cholinergic drugs is represented below
The acetoxy group The higher analog of methyl group i.e. propionyl or butyryl groups are less active than acetyl group. Choline ester of aromatic or higher molecular weight carboxylic acid possess anticholinergic activity. Carboxylate esters are susceptible to hydrolysis by cholinesterase. However, carbamate esters of choline are more stable to hydrolysis e.g. In carbachol. Carbachol (Stable to hydrolysis) 3 + 3 3 3 Acetylcholine (susceptible to hydrolysis)
Replacement of ester group either by ether or ketone group produce chemically stable and potent compound. Hence neither the ester group or carbonyl group is required for activity. Choline ethyl ether possess cholinergic activity The ethylene bridge Increasing the carbon chain length from 2-C to more than 2 carbons i.e. 3/4/5 decreases the activity. Replacement of the hydrogen atom from the ethylene bridge by methyl group leads to equal or greater cholinergic activity. Presence of groups larger than methyl decrease the activity.
The α and β methyl substituted derivatives offers receptor selectivity towards muscarinic receptor. e.g. β-methyl choline (methacholine) acts selectively on muscarinic receptor while α-methyl choline acts on nicotinic receptor. β-methyl choline α-methyl choline  The S (+) enantiomer of methacholine is equipotent with acetylcholine while R (+) enantiomer is 20 time less potent.
Quaternary ammonium group The quaternary ammonium group is essential for muscarinic activity. Replacement of nitrogen atom from quaternary ammonium group by Sulphur or arsenic or selenium decrease the activity. Compound containing primary or secondary or tertiary amino groups are less active than the compound containing quaternary ammonium group. Replacement of methyl group by ethyl or larger alkyl group produce inactive compounds.