A multiple‐dose safety and bioequivalence study of a narrow therapeutic index drug: A case for carbamazepine

Background and Objectives Carbamazepine is among those drugs that have been considered to have a narrow therapeutic plasma concentration range, that is, a narrow therapeutic index. Although the US Food and Drug Administration has approved new generic products based on standard single-dose bioequivalence studies, several state formularies, including the New Jersey Drug Utilization Review Council, have recently established additional criteria for acceptance of bioequivalence of narrow therapeutic index drugs, limiting the use of some approved generic drugs in specific states. To further validate the adequacy of single-dose studies for the determination of bioequivalence of narrow therapeutic index drugs, a multiple-dose study was conducted that more closely reflected therapeutic use. Methods A single-center, multiple-dose, randomized, open-label, 2-way crossover bioequivalence study was conducted in 32 fasting volunteers at steady state. Subjects received the test and reference products as a 200 mg carbamazepine tablet 3 times a day in a crossover fashion. Concentrations of carbamazepine and carbamazepine-10,11-epoxide in plasma were measured by a validated specific HPLC method. Results A total of 28 subjects completed the study. Pharmacokinetic parameters and measures of fluctuation for both products at steady state were similar, with 90% and 95% confidence intervals falling within 90% and 110%. Conclusion The multiple-dose study provided reliable safety and bioequivalence data under rigorous statistical conditions and confirmed bioequivalence of test and reference products determined by a single-dose study. Clinical Pharmacology & Therapeutics (1999) 65, 389–394; doi: