Pharmacokinetics of methsuximide and a major metabolite in dogs

The pharmacokinetics of methsuximide and its major metabolite 2-methyl-2-phenylsuccinimide were studied in dogs after single intravenous doses. Plasma methsuximide levels were described by a two-compartment open model, and those of the metabolite were described by a one-compartment open model. An expression was derived that describes both methsuximide and metabolite plasma levels after methsuximide administration. Excellent fits were obtained between observed data and those predicted from the model. The metabolite accounted for 40% of the overall elimination of methsuximide, and the half-life of the metabolite (15 hr) was much greater than that of the parent drug (1--3.5 hr). The results suggest that pharmacological effects after methsuximide administration may be due primarily to the metabolite, which may accumulate in the body during repeated doses.