Abstract PS17-09: Development of a potent mutant-ESR1 targeted agent, ERX-245, for treating metastatic therapy-resistant breast cancer

Background: ESR1 mutations are acquired following ERα targeted therapies and are a major determinant of therapy-resistance. These ESR1 mutations maintain ESR1 signaling, albeit in a ligand-independent manner. Effective drugs targeting these mutant (MT) ERα proteins represent a significant unmet clinical need. We had previously shown that ERX-11, an ESR1-coregulator binding inhibitor, could block the function of these MT ERα proteins. In this study, we sought to leverage recently published structures of MT ERα to develop more potent analogues of ERX-11. Methods: Virtual screening of >250,000 derivatives of ERX-11 was performed with simulated docking on the MT ERα to identify and design analogues of ERX-11. Several hundred analogues were synthesized and tested in vitro using multiple BC model cells that express wild type (WT) ESR1 or mutant (MT) ESR1 (Y537S or D538G). Mechanistic studies were performed using RNA-Seq, Western blotting, qRT-PCR and reporter gene assays. The in vivo efficacy of the most potent ERX-11 analogue ERX-245 was examined using xenograft, PDX and metastatic models of MT-ER driven BC. Results: From our virtual and functional screen, we identified an ERX-11 analogue, ERX-245 as the most potent hit to target MT-ERα. Docking studies modeled a better fit of ERX-245 into the ligand binding domain of both the Y537S and D538G MT-ERα. ERX-245 potently reduced (IC50 ~250 nM) the cell viability of both WT-ERα and MT-ERα driven BC cells but not ERα negative BC cells. ERX-245 significantly reduced the growth (colony formation, clonogenic and mammosphere assays) of MT-ERα BC cells. ERX-245 exhibited synergistic activity in combination with CDK4/6 inhibitors. In distinction to classic SERDs like fulvestrant (which degrade ERα with in 4h), ERX-245 treatment decreased MT-ERα protein levels over 24 hours. PK studies indicated that ERX-245 is more polar and has better solubility and pharmacokinetic properties than ERX-11. ERX-245 reduced tumor growth of subcutaneous xenograft and PDX models driven by MT-ERα as well as the proliferation of xenograft derived MT-ERα explant models. ERX-245 significantly reduced the invasive capability of MT-ERα BC cells in vitro and inhibited both the metastatic capability and growth of metastatic tumors derived from MT-ERα BC cells injected by intracardiac or intratibial routes. Conclusions: Taken together, these results indicate that ERX-245 is a potent and pharmacologically translatable analog of ERX-11, with activity against both primary and metastatic tumors driven by MT-ERα. Citation Format: Ganesh V Raj, Suryavathi Viswanadhapalli, Karla Parra, Shihong Ma, Tae-Kyung Lee, Xihui Liu, Kara Kassees, Weiwei Tang, Junhao Liu, Zexuan Liu, Uday P Pratap, Behnam Ebrahimi, Rajeshwar Rao Tekmal, Jung-Mo Ann, Ratna K Vadlamudi. Development of a potent mutant-ESR1 targeted agent, ERX-245, for treating metastatic therapy-resistant breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS17-09.