The existence of sequence effect in cross-over bioequivalence trials

A generic drug product (T) in order to be approved for marketing authorization a bioequivalence trial is required. In the trial the generic product is compared to the innovator product (R) in terms of the pharmacokinetic parameters AUC and Cmax. The regulatory requirement for bioequivalence is that the 90% confidence intervals for the ratio (T/R) of the generic to innovator product pharmacokinetic parameter averages lies within the limits (80%, 125%). The design of the trial is usually a two-period crossover. This design has the limitation that if the statistical analysis reveal significant sequence effect then the bioequivalence results may be biased and their interpretation is difficult. The sequence effect is confounding with the unequal residual effect and with the formulation by period interaction. Since the existence of the sequence effect questions the quality of the trial, the applicant should provide possible explanations and information on the subjects, the trial conditions, the clinical settings and the assay methodology. An additional statistical analysis on the data from the first period of the trial may support the bioequivalence. If it is proven that the sequence effect is a true effect then the generic may be approved for marketing authorization.