Abstract 884: Cooperative actions of a novel ERβ ligand, OSU-ERb-12, and Enzalutamide in advanced prostate cancer
Authors: Gray Jaimie S. 
Published: June 2022

Abstract
Background: Historically, estrogens were used therapeutically in prostate cancer (PCa) but discontinued due to serious side effects. Given estrogen receptor alpha (ERα) and beta (ERβ) exert opposing actions, it is unclear how estrogen actions control PCa growth. The OSU Drug Development Institute developed a novel, highly selective, ERβ ligand (OSU-ERB-12 (ERB12)) which has excellent in vivo PK/PD properties.
Methods: A combination robotic high-throughput drug viability screen was undertaken in androgen deprivation therapy (ADT)-sensitive and resistant PCa cell models that combined multiple concentrations of ERB12 with 30 compounds including Enzalutamide (ENZA), and epigenomic regulators; potent combinations were examined by clonogenic assays. The ERB12 and ENZA combination was examined in 22RV1 by RNA-Seq and ATAC-Seq and CRISPR-editing undertaken to delete ERα or ERb.
Result: All cell lines (LNCaP, 22RV1, PC-3, and DU145) were sensitive to high doses of ERB12 alone (1-10μM) whereas each cell line displayed unique responses to ERB12 drug interactions. 22RV1 cells responded with the greatest additive effects to combinations with ENZA and HDAC or DNMT inhibitors. Clonogenic assays also demonstrated significant suppression of colony formation by ERB12 combined with all three classes of drug. Also, 22RV1 cells with CRISPR-deleted ERα grew slower whereas faster with ERβ deletion. ATAC-Seq in 22RV1 cells revealed that the combination of ENZA (1μM) with ERB12 (100nM), but not 17b-estradiol (E2) + ENZA), led to unique gain of nucleosome free (NF) regions enriched for nuclear receptor motifs (e.g. AR, RARa, PPARa) and loss of CTCF motifs. These regions also significantly overlapped with ChromHMM-defined Active and Poised Enhancers. Parallel RNA-Seq also identified a unique ERB12/ENZA-dependent transcriptome significantly associated with repression of androgen and MYC signaling. MYC repression was confirmed at the protein level. Integration of ATAC-Seq and RNA-Seq data revealed antagonism between ERB12 and E2 treatments notably with ERB12/ENZA leading to gain of NF regions and increased expression of coregulators (e.g. RUNX1 and NCOA3) that were repressed by E2/ENZA. Finally, transcript-aware analyses identified unique and common ERB12/ENZA driven splice variation including coregulators NCOR2 and PRAME.
Conclusion: Our data suggest that there is a cooperative action between ERB12 and ENZA in limiting cell proliferation and altering nucleosome positioning and leading to a unique transcriptome. ERB12, is an orally-available and well-tolerated drug that appears to antagonize the AR driven signaling pathways that lead to lethal prostate cancer. We propose that ERB12 could allow patients to take a lower, and more well-tolerated dose of ADT drugs, which at high dose can be very harsh on patients.
Citation Format: Jaimie S. Gray. Cooperative actions of a novel ERβ ligand, OSU-ERb-12, and Enzalutamide in advanced prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 884.