ABSTRACT
Bioavailability assessment of new drug products and formulations requires monitoring drug concentration in the plasma or urine to establish the time course of drug in the body. Enantiomers may display stereoselective absorption and disposition when the processes are highly structurally, and thus stereochemically specific. The time course of stereoisomer effects is affected by their pharmacokinetic disposition and bioavailability. Sources of stereoselective absorption would include diastereoisomer/enantiomer-specific membrane transport, presystemic metabolism and degradation, and dissolution. Stereoisomers which possess structural similarities to endogenous substrates and nutrients may demonstrate differences in absorption rates. Metabolic patterns for stereoisomers are not necessarily similar and are also reflected in intrinsic hepatic clearance differences. The similarity in structural and chemical features of stereoisomers suggests a strong potential for them to interact and alter the pharmacokinetics as well as pharmacodynamics of each other. Several drug classes including beta blockers and calcium channel blockers are administered orally as racemates and have low oral bioavailability due to hepatic first-pass biotransformation.
