Enantiomers: implications and complications in developmental pharmacology

The majority of synthetic drugs with chiral centers are administered as racemates. Thus chemically, and to an even greater extent biologically, a racemic drug is not a single compound, but a 50:50 mixture of two enantiomeric drugs. No generalization can be made concerning whether and to what extent the activity, in either qualitative or quantitative terms, differs between enantiomers. It is not unusual for the enantiomers of a drug to have a high degree of enantioselectivity for one action but no enantioselectivity for another action. For instance S-propranolol is at least two orders of magnitude more potent than R-propranolol with regard to beta-adrenoceptor antagonism. However, the two enantiomers are equipotent with regard to their membrane stabilizing effect. It is often overlooked that enantioselectivity in the activity of enantiomers as determined in vitro cannot be extrapolated to the in vivo situation since enantioselective drug disposition can lead to an enantiomer ratio in vivo which differs substantially from that in the dosage form administered. Enantioselectivity in drug disposition seems to be the rule rather than the exception and, depending on whether the active or less active enantiomer is preferentially affected, there may be amplification or attenuation of in vivo as compared to the in vitro drug potency.