Effects of Chirality on Toxicity

Chirality is a prominent feature of the living world, and also occurs in man-made chemicals, especially pharmaceuticals and agrochemicals. The significance of chirality has long been recognized in relation to the biological activity of natural compounds and synthetic drugs. At a molecular level, chirality is ubiquitous in the building blocks of proteins, carbohydrates, nucleic acids, lipids and steroids. Thus, stereoselectivity can be expected in the interactions of chiral chemicals with biological molecules. It is not uncommon in racemic mixtures of chiral drugs or agrochemicals to have only one of its two (or more) enantiomers responsible for most or all of the desired activity. The other enantiomer(s) are all too often assumed to be inactive of little or no important activity. The last 30 years has seen a significant increase in published work highlighting the important relationship between molecular geometry and bioactivity, particularly for chiral pharmaceuticals and agrochemicals. However, a sizable number of chiral drugs and agrochemicals are still available as racemates with relatively little or no information with regard to the toxicological properties of the individual enantiomers. The persistent reluctance to acknowledge the risks associated with the chirality of a chemical is no longer justified. Chiral technology has developed to a point where we are allowed several options in enantiomer resolution and preparation techniques, which in turn offer new avenues for human and environmental toxicologists to explore the stereochemical properties of these ubiquitous agents. This chapter discusses some of the toxicological complexities that could result from chirality, in the hope of highlighting the importance of enantioselective considerations in both mammalian and ecotoxicology. Keywords: chiral; chirality; enantiomers; enantioselectivity; stereoisomers; stereoselectivity; racemate; racemic mixtures