Abstract P6-04-05: ERX-41, a novel drug to target and block mutant-ERα-coregulator driven signaling

Background: A significant proportion of breast cancers (BC) that express estrogen receptor α (ERα) will initially respond to antiestrogens or aromatase inhibitors. However, endocrine therapy resistance is common with progression to incurable metastases. A significant proportion (~39%) of therapy resistant tumors acquires ERα mutations which enables estrogen-independent constitutive interaction of ERα with coregulators, transcription and resistance to endocrine therapy. Drugs that specifically target and block mutant-ERα (MT ER)-coregulator driven signaling are urgently needed to treat therapy-resistant BC. Methods: Using an innovative strategy of oligobenzamide-based peptidomimetics (small synthetic organic molecules), we developed an estrogen receptor modulator ERX-41 that disrupt the interactions between MT ERα and critical coregulators. Effect of ERX-41 was evaluated using BC models that express wild type (WT) ERα (MCF-7, ZR-75) and BC models with acquired resistance (MCF-7-TamR, MCF-7-LTLT), and engineered models that express ERα mutations (MCF-7-MT ERα-D538G, MCF-7-MT ERα-Y537S, ZR-75-MT ERα-D538G, ZR-75-MT ERα-Y537S). Mechanistic studies were performed using RNA-Seq, Mass spectrometry, immunoprecipitation, Western blotting, RT-qPCR and reporter gene assays. The in vivo efficacy of ERX-41 was examined using xenograft, and patient-derived explant (PDEX) models. Results: ERX-41 exhibited broad and potent activity (IC50 between 50-125 nM) against both WT and MT ERα driven BC models in in vitro assays. ERX-41 had no activity against benign breast epithelial cells. Mechanistic studies confirmed that ERX-41 modulates multiple ERα functions including alterations in ERα transcription, down regulation of ERα and ERα-signaling. Using immunoprecipitation mass spectrometry (IPMS) analyses, we found that ERX-41 disrupts the interaction of ERα with a number of protein binding partners. RNA-Seq analyses of ERX-41 treated BC cells indicated upregulation of endoplasmic reticulum stress pathways. Furthermore, mechanistic studies confirmed that anti-proliferative effect of ERX-41 is directly related to its ability to induce endoplasmic reticulum stress and subsequent unfolded protein response. Treatment of ERX-41 decreased the proliferation (Ki-67 staining) and increased apoptosis (TUNEL staining) in PDEX models. ERX-41 showed potent activity against WT ERαand MTERα xenograft models but no effect on mouse liver or body weight. Histologic evaluation of these tumors showed dramatically decreased Ki-67 proliferation indices. Conclusions: Collectively, our data indicate that ERX-41 could effectively target both the WT and MT ERα driven signaling and could be clinically translated for the treatment of therapy-resistant BC. Supported by NIH grant CA223828. Citation Format: Ratna Vadlamudi, Suryavathi Viswanadhapalli, Xihui Liu, Tae-Kyung Lee, Gangadhara R Sareddy, Shihong Ma, Mengxing Li, Ben Ho Park, Rajeshwar R Tekmal, Jung-Mo Ahn, Ganesh V. Raj. ERX-41, a novel drug to target and block mutant-ERα-coregulator driven signaling [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-05.