CYP3A4-Mediated Oxygenation versus Dehydrogenation of Raloxifene

摘要

Raloxifene was approved in 2007 by the FDA for the chemoprevention of breast cancer innpostmenopausal women at high risk for invasive breast cancer. Approval was based in part on the improvednsafety profile for raloxifene relative to the standard treatment of tamoxifen. However, recent studies havendemonstrated the ability of raloxifene to form reactive intermediates and act as a mechanism-based inhibitornof cytochrome P450 3A4 (CYP3A4) by forming adducts with the apoprotein. However, previous studies couldnnot differentiate between dehydrogenation to a diquinonemethide and themore common oxygenation pathwaynto an arene oxide as themost likely intermediate to inactivate CYP3A4. In the current work,n18nO-incorporationnstudies were utilized to carefully elucidate CYP3A4-mediated oxygenation versus dehydrogenation ofnraloxifene. These studies established that 30n-hydroxyraloxifene is produced exclusively via CYP3A4-mediatednoxygenation and provide convincing evidence for the mechanism of CYP3A4-mediated dehydrogenationnof raloxifene to a reactive diquinone methide, while excluding the alternative arene oxide pathway.nFurthermore, it was demonstrated that 7-hydroxyraloxifene, which was previously believed to be a typicalnO2-derived metabolite of CYP3A4, is in fact produced by a highly unusual hydrolysis pathway from a putativenester, formed by the conjugation of raloxifene diquinone methide with a carboxylic acid moiety of CYP3A4, ornother proteins in the reconstituted system. These findings not only confirmCYP3A4-mediated dehydrogenationnof raloxifene to a reactive diquinone methide but also suggest a novel route of raloxifene toxicity.