Abstract 619: Selective estrogen mimics for the treatment of tamoxifen-resistant breast cancer

Endocrine-resistant breast cancer, whether de novo or acquired, is a major clinical obstacle. Although recent clinical trials have demonstrated the efficacy of 17β-estradiol (E2) or diethylstilbestrol (DES) following exhaustive use of antiestrogens, E2 treatment is associated with major side effects such as an increased risk of other gynecological cancers, deterring the clinical community from adopting it as a treatment strategy. Our lab has previously shown that Protein kinase C alpha (PKCα) overexpression predicts tamoxifen (TAM) resistance in the clinic (Tonetti DA et al., Br J Cancer. 2003) and may also predict a positive response to an estrogenic therapy (Chisamore MJ et al., Clinical Cancer Research. 2001). Further, the ectopic overexpression of PKCα in T47D breast cancer cells, led to a TAM-resistant, E2-inhibited phenotype in vivo which was accompanied by the translocation of estrogen receptor alpha (ERα) to extranuclear sites (Perez White B et al., Molecular Cancer. 2013). The purpose of this study was to identify novel selective estrogen mimics (SEMs), which could achieve the positive therapeutic effects of E2 treatment in TAM-resistant breast cancers, while minimizing the side effects. In vitro screening identified two SEMs, BTC and TTC-352, which displayed estrogenic activity in breast cancer cell lines. BTC and TTC-352 treatment resulted in significant tumor regression in two xenograft models of TAM-resistant, PKCα-overexpressing breast cancer. Similar to E2, T47D:A18/PKCα tumor regression was accompanied by translocation of ERα to extranuclear sites, possibly defining a mechanism through which these SEMs initiate tumor regression. SEM treatment, however, did not result in growth of parental, TAM-sensitive xenograft tumors. Endometrial thickening, caused by both E2 and TAM, is directly associated with gynecological carcinogenesis and uterine cancer. Interestingly, SEM treatment did not increase uterine weight in mice suggesting negligible hormonal stimulation in gynecological tissues. Both BTC and TTC-352 resulted in regression of two TAM-resistant breast cancer models, while displaying enhanced safety compared to E2 and TAM. These data suggest that further development of SEMs targeted to TAM-resistant breast cancer is a feasible therapeutic strategy. Citation Format: Mary Ellen Molloy, Bethany Perez White, Huiping Zhao, Bradley T. Michalsen, Hitisha K. Patel, Jiong Zhao, Rui Xiong, Marton I. Siklos, Gregory R.J. Thatcher, Debra A. Tonetti. Selective estrogen mimics for the treatment of tamoxifen-resistant breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 619. doi:10.1158/1538-7445.AM2014-619