Importance of CYP2D6 metabolism in the in vitro antiestrogenic activity of toremifene and tamoxifen.

Abstract

e13067 Background: Toremifene (TOR) and tamoxifen (TAM) are approved treatments for advanced breast cancer. It has been suggested that poor metabolizers of TAM have worse clinical outcomes than patients who exhibit normal TAM metabolism. TOR and TAM are extensively metabolized by the human cytochrome P450 system (TAM - 3A4 and 2D6; TOR - 3A4). To gain a better understanding of CYP2D6 status and the potential impact on breast cancer activity, we investigated the estrogen and antiestrogen actions of TOR and TAM and their NDM, 4-OH, and 4-OH-NDM metabolites in Ishikawa endometrial and MCF-7 breast cancer cells in vitro. Methods: Estrogen receptor binding affinity was evaluated in vitro using purified ER-LBD and 3H-estradiol. Transactivation activity was determined in HEK-293 cells. TOR, TAM and their N-desmethyl (NDM), 4-OH, and 4-OH-NDM metabolites were evaluated at up to [10 μM] in Ishikawa endometrial and MCF-7 breast cancer cells. Results: TAM and metabolites had similar binding affinity to TOR analogs. The 4-OH and 4-OH-NDM metabolites bound to ER subtypes with 5-50-fold greater affinity and were 50-100 more efficient at ER transactivation than TOR and TAM. Endoxifen (a CYP2D6 product) exhibited the greatest potency of all compounds tested. Estrogen induced Ishikawa cell growth with maximal effects observed at 0.1 nM. TOR and TAM prevented estrogen-mediated agonist effects with similar activity. TOR and TAM had similar antiestrogen activity against Ishikawa cell growth, however, 4-OH metabolites of TAM and TOR are up to 377-fold more potent than their parent drugs. These data are consistent with in vivo studies in intact rats demonstrating TOR and TAM block estrogenic effects on the uterus. TOR and TAM and metabolites were also antagonists in MCF-7 cells, with the order of antiestrogenic potency being 4-OH and 4-OH-NDM metabolites > TOR/TAM and NDM metabolites. Conclusions: Our results provide mechanistic insight into the potential contributions of the 4-OH and 4-OH-NDM metabolites to TOR and TAM activity and perhaps an explanation for the importance of CYP2D6 metabolism to TAM but not TOR clinical outcomes.