Abstract A003: Organoid–Macrophage Co-culture Model uncovers Macrophage Polarization and Chemoresistance Mechanisms in Metastatic Castration-Resistant Prostate Cancer

Abstract Metastatic castration-resistant prostate cancer (mCRPC) is an aggressive disease characterized by therapeutic resistance and poor response to immunotherapy that involves interactions with the tumor immune microenvironment. Increasing evidence implicates tumor-associated macrophages (TAMs) as key mediators of tumor progression, immune evasion, and drug resistance; however, the paucity of physiologically relevant human models has hindered mechanistic investigation of macrophage–tumor interactions in mCRPC. To address this, we developed a 3D organoid–macrophage co-culture model using LuCaP167 patient-derived xenograft organoids to recapitulate the prostate tumor microenvironment ex vivo. Confocal microscopy revealed dynamic, spatially resolved macrophage–tumor interactions within a Matrigel matrix, demonstrating progressive macrophage infiltration and intimate engagement with organoids over time. Migration assays confirmed time-dependent recruitment of macrophages toward CRPC organoids, accompanied by an increase in organoid size, suggesting bidirectional signaling that promotes tumor growth and microenvironmental remodeling. Our study further demonstrates that CRPC organoids actively polarize monocyte-derived macrophages toward an M2-like, immunosuppressive phenotype. Flow cytometry analysis of co-cultured macrophages revealed a time-dependent increase in CD206 and CD163 expression, indicating a progressive shift from naïve M0 to tumor-promoting M2 polarization. Functionally, M2-polarized macrophages, but not M1 macrophages, enhanced organoid survival and reduced sensitivity to docetaxel and cabazitaxel, establishing a direct mechanistic link between macrophage reprogramming and therapeutic resistance. This humanized 3D platform provides a translationally relevant system to interrogate macrophage-driven mechanisms of immune suppression and chemoresistance in CRPC. By enabling real-time, functional, and phenotypic assessment of tumor–immune interactions, this model serves as a powerful preclinical tool to guide the development of next-generation immunomodulatory and combination therapeutic strategies targeting the prostate tumor microenvironment. Citation Format: Kinjal Bhadresha, Jessica D. Kindrick, Roger L. Depaz, Cindy H. Chau, William D. Figg. Organoid–Macrophage Co-culture Model uncovers Macrophage Polarization and Chemoresistance Mechanisms in Metastatic Castration-Resistant 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 A003.