Single-dose pharmacokinetics and effect of food on the bioavailability of topiramate, a novel antiepileptic drug

Topiramate, a new antiepileptic drug effective in controlling partial-onset seizures, was administered to humans for the first time as single oral doses of 100 mg, 200 mg, 400 mg, 800 mg, and 1,200 mg in a phase I safety and pharmacokinetic study. Model-independent pharmacokinetic data analysis was performed on plasma concentration and renal excretion data for topiramate. Maximum plasma concentrations (C-max) were observed between 1.4 and 4.3 hours after administration. Mean values for plasma C-max and area under the concentration-time curve (AUC) increased linearly with dose; however, a greater-than-proportional increase in both parameters was observed, probably due to saturable binding of the drug to erythrocytes, Mean oral clearance (Cl/F) was 22.5 to 36.1 mL/min and mean volume of distribution (Vd/F) was 38.5 to 58.0 L. Approximately 50% of the dose was excreted renally and cumulative urinary excretion increased linearly and proportion ally over the 200 mg to 1,200 mg dose range. Elimination half-life (t(1/2)) values calculated from plasma (21.5 hrs) and urinary data (18.5 hrs) were consistent and independent of dose. Intersubject variability was low for all parameters. Renal clearance was 13.9 mL/min, suggesting that renal tubular reabsorption may be prominently involved in the renal handling of topiramate. The elimination profile of topiramate indicated that longer sampling times are necessary in future studies to more accurately determine the t(1/2). Food effect studies indicated a slight reduction in the rate (similar to 10% decrease in mean C-max and mean t(max) similar to 2 hours later) but not the extent of absorption when topiramate was given with food. Topiramate demonstrates a number of favorable pharmacokinetic features, including linear and predictable dose-concentration relationships, excretion mainly as unchanged drug by the kidney, a dose-independent t(1/2), low intersubject variability in pharmacokinetic parameters, and lack of clinically significant effect of food on bioavailability.