The chemopreventive action of equol enantiomers in a chemically induced animal model of breast cancer.

We describe for the first time the chemopreventive effects of S-(-)equol and R-(+)equol, diastereoisomers with contrasting affinities for estrogen receptors (ERs). S-(-)equol, a ligand for ERbeta, is an intestinally derived metabolite formed by many humans and by rodents consuming diets containing soy isoflavones. Whether the well-documented chemopreventive effect of a soy diet could be explained by equol's action was unclear because neither diastereoisomers had been tested in animal models of chemoprevention.