Abstract B024: Novel therapeutics targeting BET-mediated oncogenesis in lethal prostate cancer

Abstract Background Resistance to current therapies is a significant challenge in advanced prostate cancer. The bromodomain and extra-terminal (BET) subclass of proteins are known to contribute to cancer progression and resistance to standard therapies. To date, intervention strategies for targeting BET proteins on bromodomains using small-molecule inhibitors, specifically acetylated lysine mimics, have been developed to block the binding of BET protein bromodomains to acetylated lysine. Unfortunately, clinical trials have yielded disappointing results characterized by limited efficacy and significant dose-limiting toxicities. Methods: We use docking, surface plasmon resonance (SPR), and affinity precipitation assays to discover the sequence of amino acids for the critical binding moieties on extra-terminal (ET) of BET domain. High-throughput virtual screening is utilized to identify ET-BET binding inhibitors. SPR were used to evaluate the binding affinity and anti-tumor effects in enzalutamide-(MDVR) and darolutamide-resistant (DaroR) cells. Mechanism of action (MOA) of the selected compound BETi-10 was evaluated by transcriptomic sequencing, qRT-PCR, western blotting, and luciferase reporter assay. Alteration of chromatin accessibility by BETi-10 in MDVR cells was determined via ATAC sequencing. Ex vivo and in vivo anti-tumor effects of BETi-10 in LuCaP35CR PDX were evaluated. Results Based on the structure of ET-BET protein interface, ten compounds have been identified through computational screening to effectively inhibit the ET-domain binding. Biological screening on enzalutamide- and darolutamide-resistant cell lines revealed that BETi-10 as the most potent inhibitory compound. RNAseq analysis showed that the top downregulated pathways by BETi-10 include Myc and E2F pathways. ATAC-seq and ChIP-seq analysis has shown that BETi-10 decreases the genome-wide chromatin accessibility in MDVR cells. Ex vivo and in vivo study in LuCaP35CR PDX model demonstrated that BETi-10 effectively inhibits the growth of organoids and tumor, synergizes with enzalutamide and has better safety profile than current BET inhibitor. Conclusions In this study, we have developed BETi-10, a first-in-class small-molecule inhibitor targeting the ET domain of BET proteins. Our findings show that targeting BET/BRD4 by BETi-10 suppresses resistant tumor viability in vitro and in vivo, providing a novel therapeutic strategy for advanced prostate cancer patients. Citation Format: Shu Ning, Zachary Schaaf, Masuda Sharifi, Pui-Kai Li, Allen Gao. Novel therapeutics targeting BET-mediated oncogenesis in lethal prostate cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Innovations in Prostate Cancer Research and Treatment; 2026 Jan 20-22; Philadelphia PA. Philadelphia (PA): AACR; Cancer Res 2026;86(2_Suppl):Abstract nr B024.