Abstract B032: Induction of the unfolded protein response unveils a vulnerability of advanced prostate cancer cells to BH3 mimetics

Abstract Background: Despite recent therapeutic advances, advanced prostate cancer (PCa) remains lethal as tumors develop resistance to current treatments. Novel and more effective therapeutic strategies to induce cell death in these tumors are urgently needed. Our group recently reported that NXP800, a drug in clinical development, drives unfolded protein response (UPR) and targets AR and E2F, decreasing the growth of castration-resistant PCa (CRPC) models in vitro and in vivo. BH3 mimetics are small molecules that inhibit antiapoptotic BCL-2 family proteins, thereby promoting apoptosis, and have shown particular promise in hematological malignancies. However, their efficacy in CRPC has been limited, likely due to functional redundancies among antiapoptotic proteins such as MCL1, BCLXL, and BCL2. Objective: We investigated the potential of combining NXP800 with BH3 mimetics targeting MCL1 (S63845) or BCLXL (A-1331852) to drive cell death by inducing the intrinsic apoptosis pathway in CRPC models. Methods: Cell viability and caspase 3/7 activity were assessed by luminescence assays, while additional apoptosis markers were evaluated by western blot following treatment with NXP800, S63845, and A-1331852, as single agents or in combination. To identify key mediators of the synergistic effects, an siRNA screen targeting BH3-only proteins was performed in CRPC cells before treatment with the single agents or their combination. To assess the molecular consequences of NXP800 treatment in vivo, RNA-seq was performed on tumors from CRPC-bearing mice treated with NXP800 (35 mg/kg daily for 5 days), with particular focus on genes involved in the intrinsic apoptosis pathway. Results: NXP800 synergized with MCL1 and BCLXL inhibitors in CRPC cells, inducing apoptosis as evidenced by caspase 3/7 activation and PARP cleavage. Co-silencing of the mitochondrial pore–forming proteins BAX and BAK, as well as treatment with the pan-caspase inhibitor Q-VD-OPh, prevented cell death induced by NXP800 in combination with BH3 mimetics, indicating that the effect is caspase-dependent and involves activation of the intrinsic apoptosis pathway. Blocking NXP800-induced eIF2α phosphorylation using ISRIB abolished the synergistic effect observed with BH3 mimetics. Thapsigargin, which induces the unfolded protein response via SERCA inhibition, recapitulated the synergy and triggered apoptosis in combination with BH3 mimetics. RNA-seq analysis of LNCaP95 xenograft tumors treated with NXP800 revealed induction of specific BH3-only proteins whose silencing (in vitro) prevented caspase 3/7 activation and abolished the synergistic cell death observed with NXP800 in combination with MCL1 or BCLXL inhibition. Conclusion: NXP800 sensitizes CRPC cells to BH3 mimetics by inducing UPR and dysregulating BH3-only proteins. These findings highlight the potential of combining UPR-inducing agents with BH3 mimetics as a therapeutic strategy in CRPC. Citation Format: Juan M. Jiménez-Vacas, Jonathan Welti, Denisa Bogdan, Ines Figueiredo, Bora Gurel, Wanting Zeng, Tomas Goldsmith, Souvik Das, Joe Taylor, Nicholas Waldron, Claudia Bertan, Suzanne Carreira, Wei Yuan, Paul Workman, Steven P. Balk, Johann de Bono, Adam Sharp. Induction of the unfolded protein response unveils a vulnerability of advanced prostate cancer cells to BH3 mimetics [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 B032.