Abstract PS4-07-16: Epigenetic Reactivation of estrogen receptor beta Enhances Antitumor Activity of a Novel estrogen receptor beta agonist CIDD-0149897 in Triple-Negative Breast Cancer

Abstract Background: Breast cancer (BC) is the most common cancer in women, with ∼40,000 annual deaths in the U.S. Triple-negative breast cancer (TNBC), comprising 15-24% of cases, is aggressive and linked to high mortality. TNBC lacks ER alpha (ERa), PR, and HER2 but uniquely expresses ER beta (ERb), a tumor suppressor. However, ERb-targeted therapies remain underdeveloped due to limited selective agonists and epigenetic silencing in advanced disease. We identified CIDD-0149897, a novel ERb agonist, and evaluated its activity alone and with HDAC inhibitors (HDACi) to enhance ERb expression and therapeutic efficacy in TNBC. Methods: CIDD-0149897 was evaluated in seven TNBC cell lines (SUM159, BT549, MDA-MB-468, HCC1806, HCC70, MDA-MB-231, E0771). ERb expression was measured by RT-qPCR and Western blot. Functional assays included cell proliferation, colony formation, invasion, and apoptosis following treatment with CIDD-0149897, HDACi, or combination therapy. Mechanistic studies involved RNA sequencing, reporter assays, immunohistochemistry, and pathway analysis. In vivo efficacy was assessed using TNBC xenograft and syngeneic mouse models. Results: Expression analysis of ERb across seven TNBC cell lines (SUM159, BT549, MDA-MB-468, HCC1806, HCC70, MDA-MB-231, and E0771) revealed detectable but variable levels of ERb mRNA and protein. Treatment with the selective ERb agonist CIDD-0149897 led to a dose-dependent decrease in cell viability across multiple TNBC models. Treatment with CIDD-0149897 significantly induced apoptosis. ERb reporter assays confirmed increased transcriptional activity following CIDD-0149897 exposure. Overexpression of ERb further sensitized cells to treatment, while ERb knockout attenuated response. HDAC inhibitors (HDACi) upregulated ERb mRNA and protein expression, with chromatin immunoprecipitation confirming increased acetylation at the ERb promoter. Combination treatment with CIDD-0149897 and HDACi synergistically inhibited proliferation, reduced clonogenic survival, and suppressed invasion more effectively than either agent alone. RNA-seq and RT-qPCR showed enhanced activation of ERβ target genes involved in apoptosis and cell cycle arrest. In vivo, monotherapy with CIDD-0149897 or HDACi modestly inhibited tumor growth in HCC-1806 xenograft and E0771 syngeneic models. However, combination therapy significantly suppressed tumor volume over time, prolonged survival, and reduced tumor cell proliferation (Ki-67 staining). Immunohistochemical analysis revealed upregulation of ERb and increased cleaved caspase-3 in tumors treated with the combination therapy. In the immunocompetent E0771 model, combination therapy not only inhibited tumor progression but also enhanced infiltration of immune cells, suggesting favorable modulation of the tumor immune microenvironment. Conclusion: These findings demonstrate that CIDD-0149897, in combination with HDAC inhibition, effectively reactivates ERb signaling and modulates the tumor microenvironment. This combinatorial strategy offers promising therapeutic potential for TNBC and provides critical insights that may guide the development of more effective, targeted treatments. Citation Format: U. Pratap, M. Mahajan, K. Nassar, T. Adeniran, N. Karinel, G. Sareddy, N. Mukherjee, S. Viswanadhapalli, S. McHardy, A. Brenner, R. Vadlamudi. Epigenetic Reactivation of estrogen receptor beta Enhances Antitumor Activity of a Novel estrogen receptor beta agonist CIDD-0149897 in Triple-Negative Breast Cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-07-16.