Background: Breast cancer (BC) is the most common cancer in American women. Majority of BC (70%) is estrogen receptor alpha (ERα) positive and these tumors initially respond to ER-targeted therapy, however, acquired therapy-resistance limit the utility of ERα-targeted therapy using aromatase inhibitors and antiestrogens. Importantly, both therapy-sensitive and therapy-resistant tumors retain ESR1 signaling, via interaction with critical oncogenic coregulator proteins. We recently developed a small organic molecule, ESR1 coregulator binding inhibitor ERX-11. The objective of this study is to develop better analogues of ERX-11 using medicinal chemistry approaches.

Methods: Virtual screening of a quarter million compounds and medicinal chemistry approaches were used to design new analogues of ERX-11 and identified ERX-245 as potent analogue. Effect of ERX-245 was evaluated in vitro using multiple BC models that express wild type (WT) ERα (MCF-7, ZR-75) and BC models with acquired resistance (MCF-7-Tam, MCF-7-LTLT). Mechanistic studies were performed using RNA-Seq, Western blotting, qRT-PCR and reporter gene assays. The in vivo efficacy of ERX-245 was examined using xenograft, and xenograft-derived explant (XDEx) models.

Results: We initially performed an intensive virtual screening of a quarter million compounds and selected several candidates that showed strong binding energy to ERα. We then designed and developed several analogues of ERX-11 using modeled structural interactions. Using this approach, we identified ERX-245 as a potential lead compound for interaction with ERα. Like the parental ERX-11, ERX-245 significantly reduced the cell viability of both WT and therapy-resistant BC cells with an IC50 of 300-500 nM with minimal activity in ER-negative models such as TNBC cells. In long-term colony formation assays, ERX-245 significantly reduced the colony formation ability of both ER-WT and therapy-resistant BC cells. In ERE reporter assays, ERX-245 significantly reduced the estrogen-mediated reporter activity. ERX-245 significantly reduced the invasion of endocrine-resistant BC cells. RNA sequencing revealed unique pathways blocked by ERX-245. PK studies indicated that ERX-245 is more polar and has better solubility and pharmacokinetic properties compared to ERX-11. Treatment of ERX-245 decreased the proliferation and increased apoptosis (TUNEL staining) in xenograft derived explant (XDEx) models. ERX-245 also showed potent activity against WT-ERα and therapy-resistant xenograft models.

Conclusions: Collectively, these results suggest that the ERX-11 analogue ERX-245 has potential to therapeutically target endocrine therapy resistant BC.

Citation Format: Suryavathi Viswanadhapalli, Shihong Ma, Tae Kyung Lee, Xihui Liu, Kara Kassees, Uday P. Pratap, Junhao Liu, Weiwei Tang, Rajeshwar R. Tekmal, Jung-Mo Ahn, Ganesh V. Raj, Ratna K. Vadlamudi. Preclinical evaluation of estrogen receptor coregulator binding inhibitor ERX-245 in breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5676.