Abstract P3-05-14: Modeling chemoendocrine therapy for ER+/p53wt luminal breast cancer

Abstract

Abstract Background ER+ breast cancers are predominantly p53 wildtype (wt). Despite this genotype, these cancers are relatively resistant to chemotherapy-induced apoptosis in the presence of estrogen. Thus, current clinical practice is to administer hormonal therapy and chemotherapy sequentially rather than concurrently. Using the ER+/p53wt cell line MCF-7, we previously demonstrated that ER bound by either the ER agonist estradiol or partial antagonist tamoxifen inhibits the expression of a set of proapoptotic p53 target genes, whereas the full ER antagonist fulvestrant, a selective ER downregulator (SERD), removes the ER-mediated suppression of these genes, sensitizing breast cancer cells to p53-mediated cell death .We hypothesize that effective chemoendocrine therapy requires complete ER antagonism resulting in ER downregulation, increased p53 activity, and ultimately apoptosis. Methods: To model mechanisms of tumor sensitivity and resistance to therapy in vivo, therapeutic experiments were performed in immunodeficient mice bearing ER+/p53wt tumor xenografts derived from a 61-year-old African-American female with grade 2 invasive ductal carcinoma. The mice were randomly assigned to the following treatment groups: A) fulvestrant B) tamoxifen, C) doxorubicin, D) fulvestrant plus doxorubicin, E) tamoxifen plus doxorubicin, and F) control. Tumors were measured weekly for 42 days to determine treatment effects on growth. In addition, RNA was harvested from tumors at an early time point for RNA sequencing to determine genes regulated by the treatments. Results: Combination therapy comprising doxorubicin plus fulvestrant resulted in tumor regression compared with single-agent or doxorubicin plus tamoxifen treatment, which inhibited the growth of tumors but did not lead to their regression. Conclusions: Current paradigms for the treatment of ER+ breast cancer either in the adjuvant or advanced setting have involved the sequential use of endocrine therapy and chemotherapy. This protocol is partially based on evidence of potential antagonism between tamoxifen and chemotherapy as observed in early studies. Our published work in vitro and the preclinical study presented here suggest that complete ER antagonism with SERDs such as fulvestrant is required to overcome the ability of ER to block p53-mediated apoptosis. Our results suggest treatments involving fulvestrant concurrent with chemotherapies involving p53 activation should be considered for the treatment of patients with ER+/p53wt breast cancer. Citation Format: Shiliang A Cao, Elgene Lim, Myles Brown, Shannon T Bailey. Modeling chemoendocrine therapy for ER+/p53wt luminal breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-05-14.