The impact of stereochemistry on the bioavailability of drug molecules has been emphasized by Jamali (1992). Typically, stereoisomers have very similar physicochemical properties, and passive processes such as diffusion across the gastrointestinal membrane are not governed by any specific enantioselective mechanisms. However, the drug absorption process is likely to be stereospecific when mediated by carrier molecules. In addition, when chiral excipients are used in the formulation of enantiomers, interaction between them may result in stereoselective release from the dosage form (Duddu et al., 1993). Many enantioselective drugs are usually marketed as racemates, although their therapeutic benefits are attributed only to specific enantiomers.
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