Abstract 446: Embryonic ectoderm development protein (EED) inhibitor APG-5918 exhibits potent antitumor activity and synergizes with androgen receptor (AR) inhibitor enzalutamide in preclinical prostate cancer (PCa) models

Abstract Background: Frequent PRC2 dysregulation in PCa is associated with poor prognosis. As a core PRC2 component, EED is essential for histone methyltransferase activity. The potential antitumor effects of APG-5918 in combination with enzalutamide were evaluated in PCa models. Methods: Human AR-dependent LNCaP and AR-independent castration-resistant C4-2B and 22Rv1 PCa cell lines were used for in vitro antiproliferation assays. Cell viability was assessed using CellTiter-Glo® luminescent assay. Subcutaneous LNCaP cell-derived xenograft (CDX) and 22Rv1 CDX models in castrated nude mice were established to evaluate in vivo antitumor effects. Results: APG-5918 showed superior inhibitory effects on cellular proliferation in vitro, with IC50 values of 10.16 nM in LNCaP, 26.32 nM in C4-2B, and 70.32 nM in 22RV1 cells after 14-day continuous treatment. APG-5918 appeared to be more potent than two EZH2 inhibitors (tazemetostat, PF-06821497) and EED inhibitor ORIC-944 under the same experimental condition. APG-5918 in combination with enzalutamide synergistically suppressed cellular proliferation. APG-5918 induced dose-dependent cell cycle arrest in LNCaP and C4-2B cells, increasing the proportion of cells in G0/G1 phase while decreasing those in S- and G2/M-phase by flow cytometry. Combining enzalutamide with APG-5918 deepened G0/G1 cell cycle arrest. In the LNCaP CDX model, APG-5918 100 mg/kg administered orally once daily for 28 days exerted significant antitumor activity (T/C = 40.78%; p < 0.001 vs. vehicle), whereas enzalutamide 20 mg/kg showed limited antitumor activity (T/C = 81.56%). In the 22Rv1 CDX model, APG-5918 100 mg/kg or 50 mg/kg had superior antitumor activity, with T/C values of 41.57% (p < 0.001 vs. vehicle) and 33.78% (p < 0.001 vs. vehicle), respectively, whereas enzalutamide 30 mg/kg for 41 days had minimal antitumor effects (T/C = 87.03%). No significant differences in body weight changes were observed between treatment and vehicle groups (all p values > 0.1 vs. vehicle), indicating a favorable safety profile. Western blot analysis showed that APG-5918 downregulated key on-target pharmacodynamic markers (e.g., H3K27me3, EED, EZH2, SUZ12) and AR pathway proteins. APG-5918 also suppressed oncogenic driver ERG, DNA methylation factors (UHRF1, DNMT1), and antiapoptotic protein MCL-1, and reduced levels of pRb, CDK4, and cyclin D1—crucial mediators of cell cycle regulation. APG-5918 combined with enzalutamide further enhanced the downregulation of PRC2 components, AR pathway proteins, cell cycle regulators, oncogenic drivers, and DNA methylation factors. Conclusions: APG-5918 alone or in combination with enzalutamide exhibited antitumor activity in preclinical PCa models, supporting EED inhibition (± AR antagonists) as a therapeutic strategy for PCa. Citation Format: Yan Yin, Zhiyan Liang, Baisong Li, Yun Yang, Dajun Yang, Yifan Zhai. Embryonic ectoderm development protein (EED) inhibitor APG-5918 exhibits potent antitumor activity and synergizes with androgen receptor (AR) inhibitor enzalutamide in preclinical prostate cancer (PCa) models [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 446.