Although approved for the treatment of hormone-dependent breast cancer as well as for the prevention of breast cancer in high-risk women, the selective estrogen receptor modulator (SERM) tamoxifen has been associated with an increased risk of endometrial cancer in women. With an understanding of the potential carcinogenic mechanisms of these compounds, SERMs could in principle be designed or selected for use that avoids these problems. Acolbifene (EM-652) is a fourth-generation SERM and the active form of the ester prodrug EM-800. As a pure antagonist of breast tumor development and growth, acolbifene does not stimulate endometrial tissue. However, acolbifene was found in this investigation to form two kinds of quinone methides, either through chemical or through enzymatic oxidation. One was a classical acolbifene quinone methide, which was formed by oxidation at the C-17 methyl group, and the other was a diquinone methide involving the oxidation of two phenol groups. The half-life of the classical quinone methide was determined to be 32 ± 0.4 s at physiological pH and temperature. The quinone methides reacted with glutathione (GSH) to form five mono-GSH conjugates and five di-GSH conjugates. The majority of GSH conjugates resulted from reaction of the classical acolbifene quinone methide with GSH. Incubations of acolbifene with GSH and either tyrosinase or human and rat liver microsomes also produced acolbifene quinone methide-GSH conjugates. In addition to reaction with GSH, the classical acolbifene quinone methide was also shown to react with deoxynucleosides. One of the major deoxynucleoside adducts was identified as the deoxyadenosine adduct resulting from reaction of the classical acolbifene quinone methide with the exocyclic amino group of adenine. Acolbifene could also induce DNA damage in the S30 breast cancer cell line. These data imply that the classical electrophilic acolbifene quinone methide might contribute to the potential toxicity of acolbifene.
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