Pharmacokinetics of felbamate, a novel antiepileptic drug: application of mixed-effect modeling to clinical trials.

Felbamate is a novel antiepileptic drug currently undergoing clinical trials in the United States. Serum felbamate concentration data from a phase II safety and efficacy trial were analyzed using NONMEM. A one-compartment, open model with first-order absorption and elimination was used. Body weight, sex, concurrent folic acid therapy, and phenytoin dose and dose:concentration ratio did not affect the estimates for felbamate clearance (Cl). Carbamazepine dose and dose:concentration ratio (CDCR) led to significant improvements in the objective function. The final models for felbamate clearance and volume of distribution (Vd) were as follows: Cl(L/hr) = 2.43 + 0.429*CDCR/240, Vd (L) = 51. The coefficient of variation of clearance was only about 12%, which may be indicative of the highly selective patient population. The clearance estimates are similar to those obtained in healthy volunteers and in patients receiving lower dosages of felbamate. The volume of distribution estimate, however, is slightly smaller than that reported previously. Valuable pharmacokinetic information can be obtained from the routine monitoring of serum concentrations during safety and efficacy trials.