Mephobarbital and phenobarbital plasma concentrations in epileptic patients treated with mephobarbital.

Plasma mephobarbital and phenobarbital concentrations were determined in 11 epileptic patients receiving mephobarbital alone or in combination with other antiepileptic drugs. The analysis was carried out by a selective ion monitoring (SIM) mass fragmentography technique following formation of N-propyl derivatives of both drugs. The plasma concentrations of phenobarbital ranged from 4 to 32 micrograms/ml and those of mephobarbital from 0.2 to 1.7 micrograms/ml. Differences in the metabolism rates of the drugs accounted for the plasma concentration differences; mephobarbital is metabolized more rapidly than phenobarbital. Phenobarbital concentrations obtained by SIM mass fragmentography were similar to those obtained by gas-liquid chromatographic on-column methylation, which quantitates only "total phenobarbital" (mephobarbital plus phenobarbital).