List the clinically relevant acetylcholinesterase inhibitors.
The clinically relevant acetylcholinesterase inhibitors include neostigmine, pyridostigmine, and edrophonium (Table 25.1). They are all quaternary amines that do not cross the blood–brain barrier. Physostigmine, a tertiary amine, has profound central effects and, for this reason, is not used to antagonize neuromuscular blockade.
The active portion of acetylcholinesterase includes an anionic and an esteratic subsite. The anionic subsite interacts electrostatically with the quaternary nitrogen of either choline or the acetylcholinesterase inhibitors, while the esteratic subsite interacts with edrophonium by hydro-gen bonding and with neostigmine by covalent bonding. The electrostatic and hydrogen bonds formed by edropho-nium are rapidly reversible, while the covalent bond of neostigmine requires more time (a half-life of 30 minutes) to break, resulting in hydrolysis of the neostigmine molecule into two fragments. Conversely, edrophonium’s interaction with acetylcholinesterase yields an intact edrophonium molecule, which is subsequently cleared by the kidney.