5.12: Indirect-Acting Cholinergic Agonists: Chemistry and Structure-Activity Relationship
Indirect-acting cholinergic agonists are agents that interact with the acetylcholinesterase enzyme in the synaptic cleft, preventing the breakdown of acetylcholine into choline and acetate. Consequently, the concentration of acetylcholine in the synaptic cleft increases. These agonists can be classified into reversible and irreversible inhibitors based on their duration of action.
Reversible inhibitors display short to medium durations of action. Short-acting agents include simple alcohols with quaternary ammonium groups, such as edrophonium. Intermediate-acting agents possess medium durations of action and are carbamic acid esters containing tertiary or quaternary ammonium groups. Physostigmine, a natural carbamate, features a tertiary amine group, while neostigmine, a synthetic carbamic ester, has a quaternary ammonium group.
Reversible inhibitors display short and medium durations of action. Short-acting agents include simple alcohols with quaternary ammonium groups, such as edrophonium. Intermediate-acting agents possess medium durations of action and are carbamic acid esters containing tertiary or quaternary ammonium groups. Physostigmine, a natural carbamate, features a tertiary amine group, while neostigmine, a synthetic carbamic ester, has a quaternary ammonium group.
Irreversible inhibitors are phosphoric acid derivatives with multiple substituents, where one substituent serves as a labile group. Echothiophate, for instance, comprises two alkoxy groups and one labile thiocholine group. Similarly, sarin, a toxic nerve gas, contains one alkoxy group, one alkyl group, and a labile halogen group.
