Purpose: To determine the relative bioavailability of two marketed, immediate-release methylphenidate tablets. The study used a replicated study design to characterize intrasubject variability, and determine bioequivalence using both average and individual bioequivalence criteria.
Methods: A replicated crossover design was employed using 20 subjects. Each subject received a single 20 mg dose of the reference tablet on two occasions and two doses of the test tablet on two occasions. Blood samples were obtained for 10 hr after dosing, and plasma was assayed for methylphenidate by GC/MS.
Results: The test product was more rapidly dissolved in vitro and more rapidly absorbed in vivo than the reference product. The mean Cmax and AUC(0-infinity) differed by 11% and 9%, respectively. Using an average bioequivalence criterion, the 90% confidence limits for the Ln-transformed Cmax and AUC(0-infinity), comparing the two replicates of the test to the reference product, fell within the acceptable range of 80-125%. Using an individual bioequivalence criterion the test product failed to demonstrate equivalence in Cmax to the reference product.
Conclusions: The test and reference tablets were bioequivalent using an average bioequivalence criterion. The intrasubject variability of the generic product was greater and the subject-by-formulation interaction variance was borderline high. For these reasons, the test tablets were not individually bioequivalent to the reference tablets.