Abstract 4227: Chromatin remodeling enhances the anti-cancer effects of estrogen therapy in ER+ breast cancer

Abstract Resistance to endocrine therapies in estrogen receptor alpha (ER)-positive breast cancer can be driven by ER amplification or overexpression. We hypothesized that the anti-cancer effects of the estrogen 17b-estradiol (E2) could be enhanced through combination with chromatin remodeling agents. We evaluated the effects of E2 and the histone deacetylase inhibitor (HDACi) entinostat in long-term estrogen-deprived (LTED) ER+ breast cancer cells, cells engineered to overexpress exogenous ER, and mice bearing endocrine-resistant patient-derived xenografts (PDX). Assayed endpoints included apoptosis, growth, DNA damage (gamma H2AX), histone post-translational modification, ER-inducible transcripts and proteins, cell cycle and replication status, chromatin accessibility by ATAC-seq, and transcriptomic changes. HDAC inhibition with entinostat increased histone H3 acetylation (H3K27ac) in cell lines and PDX models. Combination treatment with E2 and entinostat induced higher levels of apoptosis compared to single-agent treatments in LTED cells. In ER-overexpressing cells, single-agent E2 or entinostat inhibited growth that was increased with combination treatment. Entinostat and E2 each induced DNA damage and ER target gene expression. Pre-treatment with entinostat halted cell cycling while addition of E2 stalled cells in S-phase. ATAC-seq showed entinostat increases chromatin accessibility in regions containing ER target genes. In mice, entinostat slowed tumor growth rate, while single-agent E2 and the combination of E2/entinostat induced complete tumor regression. The drug combination synergized to promote more rapid tumor regression than E2 alone, and the combination delayed time to tumor recurrence compared to single-agent E2. We conclude that combination therapy with chromatin remodeling agents and E2 synergistically inhibits ER+ breast cancer cell growth and induces tumor regression in mice. Efficacy is a result of increased DNA damage, elevated chromatin accessibility, treatment-induced ER activation, cell cycle stalling, and subsequent cell death. The novel combination of an HDACi and E2 has the potential to be an effective therapy for patients with endocrine-resistant ER+ breast cancer. Citation Format: Anneka L. Johnson, Alexa Warren, Nicole Traphagen, Alyssa Roberts, Patricia Muskus, Barbara Karakyriakou, Eugene Demidenko, Fred W. Kolling, Owen Wilkins, Kimberly Birmingham, Brock Christensen, Todd W. Miller. Chromatin remodeling enhances the anti-cancer effects of estrogen therapy in ER+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4227.