Estimation of Population Pharmacokinetic Parameters of Free-Phenytoin in Adult Epileptic Patients

Background concentrations of free-phenytoin (F-PHT) obtained in adult epileptic patients receiving PHT in monotherapy were analyzed to estimate the Michaelis–Menten pharmacokinetic parameters. s -state F-PHT serum concentrations, PHT dosing history, and associated information were collected prospectively. The maximum metabolic rate (Vm) and Michaelis–Menten constant (Km) of F-PHT and their interindividual variability data were estimated using nonlinear mixed effects modeling (NONMEM). s -nine patients with two or more available steady-state F-PHT serum concentrations (total of 63 dose/serum concentration pairs) met the inclusion criteria. Patients were taking PHT (100–500 mg/day) in monotherapy. The population estimates of F-PHT for Vm and Km were 9.1 mg/kg/day and 7.3 mg/L, respectively. The model was prospectively evaluated in a small group (seven) of additional patients. sions commended daily dose in this population to achieve a F-PHT concentration of 1.5 mg/L is 6.1 mg/kg.