Stereochemistry and bioequivalence

Despite the fact that many important drugs are chiral, for a variety of reasons they are marketed as racemates (i.e., an equal proportion of two enantiomers). Although enantiomers of racemic drugs often differ from one another in their pharmacodynamic and pharmacokinetic properties, bioequivalence assessments are made using nonstereospecific assays. Such an approach may provide a poor assessment of therapeutic equality. This can be true particularly for drugs with complicated pharmacokinetics and those that exhibit extensive stereoselectivity in their disposition kinetics. Accordingly, examples of bioequivalence studies based on stereospecific assays have started to appear in the literature. Fortunately, facile stereospecific assays have become available in the last several years for many drugs. Consequently, regulatory agencies have started to take the issue of stereochemistry into consideration in the assessment of bioequivalence, particularly from the standpoint of generic substitution.