Stereoisomers in psychiatry: The case of escitalopram

Many medications in common clinical use consist of "mirror image" isomers that differ only in the direction in which they rotate plane-polarized light. These stereoisomers exist as mixtures of "right" and "left" handed molecules that are the product of chemical syntheses. However, the biochemical milieu of the human body is a highly stereospecific environment where the fit of medication and receptor may depend on the shape of the molecule in 3-dimensional space. Recent advances in chemistry have allowed the more ready preparation of single isomers of various drugs that were previously available only as racemic mixtures. For those compounds in which the isomers differ in stereospecificity, this separation into single isomers can result in significant changes in potency, tolerability, and efficacy. This article reviews some basic information about stereochemistry and describes the development of a new single isomer antidepressant, escitalopram, which is one of the components of the widely used selective serotonin uptake inhibitor citalopram.