The physiological disposition of 14 C-methsuximide in the rat.

1. (14)C-Methsuximide (N-methyl-(14)C-2-methyl-2-phenylsuccinimide) was rapidly absorbed from the small intestine of the rat (t(1/2) 17.4 min).2. The drug was rapidly and fairly evenly distributed throughout the body with peak blood and tissue levels occurring 1 h after oral administration. At all times, adrenals, body fat, kidneys and liver had higher levels of (14)C-methsuximide than other tissues and the drug freely traversed the blood-brain barrier. However, radioactivity disappeared rapidly from most tissues after the initial phase of the distribution.3. During 24 h, 26% of the orally administered radioactivity was recovered in urine and 29% appeared in expired air as (14)CO(2). The excretion of (14)CO(2) indicated N-demethylation of (14)C-methsuximide to 2-methyl-2-phenylsuccinimide. 2.7% of an administered dose of methsuximide was excreted unchanged in 24 h urine and 2.7% appeared as 2-methyl-2-phenylsuccinimide. 2-Methyl-2-phenylsuccinimide was also detected as a urinary metabolite of methsuximide in man.4. 2-Methyl-2-phenylsuccinimide possesses anticonvulsant activity and it is suggested that this metabolite contributes to the overall anticonvulsant activity and toxicity of methsuximide.5. Rat urine also contained a radioactive substance with similar chromatographic properties to one of the products of alkaline hydrolysis of methsuximide. This compound may arise from the spontaneous decomposition of the parent drug in vivo.