Enzalutamide resistance results in immunosuppressive alterations in prostate tumor immune microenvironment

Abstract Background: Emerging data suggested that enzalutamide-treated prostate cancer patients with increased programmed death-ligand 1 (PD-L1) expression may benefit from anti-PD-L1 treatment. Unfortunately, the Phase III IMbassador250 trial revealed that the combination of atezolizumab (PD-L1 inhibitor) and enzalutamide failed to extend overall survival in patients with castration-resistant prostate cancer (CRPC). The mechanisms underlying treatment failure remain unknown. In this study, we investigated the regulation of immunosuppressive signaling in enzalutamide resistant prostate cancer and characterized the immune infiltrating cells in murine prostate tumors. Methods: The expression of interferon gamma-related genes was determined using qRT-PCR and/or western blotting. Androgen receptor (AR) and AR-V7 levels were downregulated using specific siRNA. Myc-CaP cells were chronically exposed to increasing concentrations of enzalutamide (5-50 μM) for >12 months, and the cells resistant to enzalutamide were referred to as Myc-CaP MDVR. The gene-regulating mechanisms in drug-resistant prostate cancer cells were determined by RNA sequencing analyses. Myc-CaP and Myc-CaP MDVR xenograft tumors were established in FVB mice, and tumor-infiltrating lymphocytes were isolated using Ficoll. The stained immune cells were determined by flow cytometry, and the data were analyzed using Flowjo. Results: Immune-related signaling pathways (interferon alpha/gamma response, T cell activation, and cell chemotaxis pathways) were suppressed in C4-2B MDVR cells. However, PD-L1 expression was highly upregulated and negatively regulated by AR in C4-2B MDVR cells. Enzalutamide treatment decreased CD8+ T cell but increased monocytic myeloid-derived suppressor cell (M-MDSC) population and PD-L1 expression in murine Myc-CaP tumors. Consistently, chemotaxis and immune response-regulating signaling pathways were suppressed, and PD-L1 expression was increased in enzalutamide-resistant Myc-CaP MDVR cells. Notably, MDSC populations were significantly increased in Myc-CaP MDVR orthotopic tumors compared with those in Myc-CaP parental tumors. Co-culturing bone marrow cells with Myc-CaP MDVR cells significantly promoted MDSC differentiation and polarized macrophages from the M1 to M2 phase. Conclusions: Immunosuppressive signaling is activated in enzalutamide resistant prostate cancer cells. The activated immune-related signatures involved in T cells, MDSC, and macrophages may reduce the efficacy of immune checkpoint inhibitors in enzalutamide-resistant prostate cancer. Grant Support: This work was supported in part by grants NIH/NCI R37CA249108 (C, Liu) and R01CA251253 (C, Liu). Citation Format: Pengfei Xu, Joy C. Yang, Bo Chen, Christopher Nip, Jonathan E. Van Dyke, Christopher P. Evans, William J. Murphy, Chengfei Liu. Enzalutamide resistance results in immunosuppressive alterations in prostate tumor immune microenvironment. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5151.