Metabolic bioactivation of the estrogen receptors antagonist elacestrant

Abstract ID 131553 Poster Board 242 Elacestrant (ELT), an antagonist of estrogen receptors, has been granted approval by the United States Food and Drug Administration (FDA) in 2023 for the treatment of advanced or metastatic breast cancer that is positive for estrogen receptors (ER), negative for HER2, and carries ESR1 mutations, particularly in cases where the disease has progressed despite prior endocrine therapy. However, frequent and severe adverse effects associated with the use of ELT, notably liver injury, have been reported. The mechanisms of ELT-related hepatotoxicity remain largely unknown. The process of drug metabolism is known to play a crucial role in drug toxicity. Unfortunately, information regarding the metabolism of ELT, including its bioactivation pathways, is currently unavailable. In this study, we investigated the metabolism of ELT using human and mouse liver microsomes (HLM/MLM), both with and without trapping agents such as glutathione (GSH) or methoxyamine, which can capture different types of reactive metabolites. Using LC-MS-based metabolomic techniques, we identified a total of 22 stable metabolites, along with 9 ELT-GSH adducts and 10 ELT-methoxyamine adducts. Our findings indicate that CYP3A4 and CYP3A5 are primarily responsible for the formation of both stable and reactive metabolites, as demonstrated through experiments utilizing recombinant human P450 isoenzymes and the CYP3A inhibitor ketoconazole in HLM. Furthermore, our research revealed that ELT exhibits significant toxicity towards both human primary hepatocytes (with an IC50 of 7.0 μM) and HepG2 cells (with an IC50 of 6.0 μM). In summary, current work determined the stable and reactive metabolites of ELT in liver microsomes and CYP3A is mainly involved in their formation. Further studies are warranted to elucidate the relationship between reactive metabolites and ELT-induced liver toxicity, as well as the role of CYP3A in vitro and in vivo. Keywords: elacestrant, reactive metabolite, aldehyde, GSH-adduct, metabolomics Note: $, equal contribution This work was supported by the NIDDK (R01DK121970) and NICHD (R61/33HD099995) to Dr. Feng Li. Feng Li was also partially supported by the NICHD (P01 HD087157, R01HD110038) to Dr. Martin M. Matzuk.