The relationship between the stereochemistry of drugs and their therapeutic and toxic effects is discussed. Molecular structures that have the same chemical constituents and are related to each other as an object and its nonsuperimposable mirror image are enantiomers. A molecule that has at least one pair of enantiomers is a chiral compound, and a 50:50 mixture of enantiomers is a racemic mixture. The enantiomers of a chiral drug may vary in their interactions with chiral or asymmetric environments such as enzymes and receptors. These variations may lead to differences in pharmacology, pharmacokinetics, therapeutic effect, efficacy, and toxicity. For example, dextropropoxyphene is an analgesic, whereas its mirror image, levopropoxyphene, is an antitussive agent. Enantiomers may have identical efficacy and toxicity. They may have the same therapeutic and toxic effects but differ in the magnitude of these effects; one may possess all the pharmacologic activity and the other may be biologically inactive, or both may be pharmacologically active but have qualitatively different therapeutic and toxic effects. Continued research into the stereochemistry of drugs may lead to the development of newer, safer, and more efficacious drugs and other chemical compounds.

 

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