Abstract 5574: Development of novel breast cancer therapeutics for anti-estrogen resistant breast cancer.

Abstract

Abstract Breast cancer (BC) is the most common cause of cancer deaths among women across the globe. About 70% of the BCs express estrogen receptor (ER) and therapies interfering with estrogen mediated signaling pathways form the bedrock of ER positive (ER+) BC treatment. However, about 50% of advanced ER+ BC patients fail to respond and eventually develop resistant to current anti-estrogen treatments, the selective ER modulators (SERMs eg., Tamoxifen) and the selective ER down-regulators (SERDs eg., Fulvestrant). In this study, we took a new approach to counter hormonal resistance through a new class of drugs that, in part, replicate a physiological mechanism for regulation of ER activity. Breast cancer susceptibility gene-1 (BRCA1) is known to inhibit ER activity and over 10 years of functional interactions studies, between BRCA1 and ER, from our lab led us to develop a class of small molecule inhibitors that recapitulate BRCA1 mediated inhibition of ER activity. We computed a partial structure for BRCA1 in complex with the E2-bound ER, using the Global Range Molecular Matching program. Based on BRCA1: E2-bound ER model and employing stringent criteria suitable binding sites on E2-bound ER for small molecules were identified and in silico screening of small molecules was conducted from which top 40 hits out of 1990 compounds were selected and tested for bioactivity in ER+ MCF-7 BC cells. Of the 6 bioactive compounds, 4 compounds (A2,A7, A11 andB1) were found to inhibit E2 (10nm) stimulated ER activity, using ERE-TK-Luc reporter assay by ≥50% at concentration less than 5μM. Furthermore, these compounds showed no toxicity at concentrations < 100μM. A7 was the most potent inhibitor, with IC50 and IC80 of 2 and 4μM respectively. In addition, these compounds were specific to ER and did not inhibit progesterone receptor (PR) activity. These compounds significantly inhibited E2 stimulated proliferation in ER+ cell lines such as MCF-7 and Du145, however no effect on the proliferation of MDA MB-231 an ER negative BC cell line was observed. We further show that A7 significantly inhibited ER activity and proliferation in LCC2 and LCC9 cells, anti-estrogen resistant variants of MCF-7. Investigation into the mechanism of action using chromatin-immunoprecipitation (ChIP) assays revealed that A7 prevents ER binding to estrogen responsive element (ERE) within E2 inducible pS2 gene promoter. Taken together, these results indicate the potential new class of compounds that can be developed into novel drugs to treat the BCs resistant to anti-estrogens, including Tamoxifen and related drugs, Fulvestrant, and aromatase inhibitors (such asanastrazole). Citation Format: Yongxian Ma, Anju Preet, Changyan Hu, Akshay Kumar, James Ma, York Tomita, Eliot M. Rosen. Development of novel breast cancer therapeutics for anti-estrogen resistant breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5574. doi:10.1158/1538-7445.AM2013-5574 Note: This abstract was not presented at the AACR Annual Meeting 2013 because the presenter was unable to attend.