Trimethaphan camsylate (Arfonad) and human plasma cholinesterase

These results for the first time document that the brief duration of action of the ganglionic blocker, Arfonad (trimethaphan camsylate), is not due to inactivation by plasma cholinesterase. Inhibition of the enzyme in vitro by Arfonad was not altered by prior incubation of the drug in plasma at 37 degrees C for up to 16 hours nor by treatment with NaOH at 100 degrees C for 10 minutes. Both procedures inactivated succinylcholine. These results in vitro were confirmed in vivo by finding that the toxicity of Arfonad in mice was not significantly altered by these procedures whereas they rendered succinylcholine inocuous. Arfonad is a noncompetitive whereas succinylcholine is a competitive inhibitor of plasma cholinesterase using benzoylcholine as the substrate. The camphorsulfonate moiety of Arfonad was inactive in vitro but caused mild tremors in mice at relatively high doses.