Population analysis of the dose-dependent pharmacokinetics of zonisamide in epileptic patients

The pharmacokinetics of zonisamide was studied using routine therapeutic drug monitoring data from 68 epileptic patients. The 266 serum concentration data at steady-state after repetitive oral administration were analyzed using the nonlinear mixed effects model (NONMEM) program designed for estimation of population pharmacokinetic parameters. A one-compartment model with dose-dependent clearance was used for the pharmacokinetic analysis of zonisamide. The volume of distribution (V) was estimated to be 1.27 l/kg in a typical 33-kg patient, assuming that the bioavailability of orally administered zonisamide is 100%. The maximal daily dose to be cleared (Vmax) and the concentration giving half maximal clearance (a Michaelis-Menten constant) was 27.6 mg/d/kg and 45.9 micrograms/ml, respectively. The parameter of a power function of weight to adjust V and Vmax was estimated to be 0.741. In addition, Vmax for zonisamide appears to be 13% increased in patients receiving carbamazepine concurrently. The population pharmacokinetic parameters of zonisamide will be useful for designing dosage regimens in epileptic patients.