ABSTRACT
Bioavailability is defined by the U.S. Food and Drug Administration (FDA) as “the rate and extent to which the active ingredient or the active moiety is absorbed from a drug product and becomes available at the site of action. Whereas the measurement of drug concentrations in biological fluids provides information on the bioavailability of drugs intended to be absorbed into the systemic circulation, for drug products that are not intended to be absorbed into the bloodstream, bioavailability may be assessed by measurements intended to reflect the rate and extent to which the active ingredient or active moiety becomes available at the site of action” (1,8). The latter approach may be accomplished by using pharmacodynamic endpoints in certain cases where the direct chemical measurement of the active drug substance in biological fluids is not feasible.
