Effects of Radiotherapy, Immune Checkpoint Inhibitors, and PIM Kinase Inhibition in Castration-Resistant Prostate Cancer

INTRODUCTION/OBJECTIVE: Immune checkpoint inhibitors (ICIs) have limited efficacy in prostate cancer. Radiation therapy (RT) combined with ICIs and PI3K inhibition may enhance anti-tumor immune activity. METHODS: RT-induced cytosolic double-stranded DNA (dsDNA) was measured in human and mouse prostate cancer cell lines as the surrogate upstream marker consistent with cGAS-STING engagement. In a syngeneic mouse model for castration-resistant prostate cancer (Myc-CaP in FVB mice), triple therapies were studied: (1) anti-CTLA-4, anti-PD-1, and RT, and (2) PIM kinase inhibitor PIM447, anti-PD-1, and RT. Mass cytometry (CyTOF) was used to measure the tumor-immune profile. RESULTS: The peak induction of cytosolic dsDNA was at an RT dose of 13 Gy. In the mouse model, triple therapy with anti-CTLA-4, anti-PD-1, and RT doubled the median survival compared to monotherapy (32 days vs 11 to 22 days, p<0.006). Triple therapy with the PIM kinase inhibitor PIM447, anti-PD-1, and RT nearly tripled the median survival compared to PIM447 monotherapy (82 days vs 29 days, p=0.002). Mass cytometry analysis revealed that the combination of anti-PD-1 and RT reduced myeloid-derived suppressor cells and tissue-associated macrophages and enhanced CD8+ T-cell infiltration. DISCUSSION: Although prostate cancer is an immunocold entity, RT can trigger immune activation, consistent with engagement of the cGAS-STING signaling pathway (cytosolic dsDNA serving as a surrogate upstream marker). In triple therapy, RT can enhance the efficacy of ICI and PI3K-targeted drug therapy, significantly improving overall survival in a mouse model of CRPC. CONCLUSIONS: A combination of RT, ICIs, and PIM kinase inhibition may help overcome immune resistance in prostate cancer. This combination therapy approach supports further preclinical validation and careful clinical evaluation and warrants further clinical investigation to optimize treatment strategies for CRPC.