(UroToday.com) CheckMate 9KD is a phase 2 trial of nivolumab (anti-PD-1) combined with either rucaparib, docetaxel, or enzalutamide for mCRPC. Historically, nivolumab monotherapy has shown minimal clinical activity in unselected populations of patients with mCRPC, potentially secondary to an immunosuppressive prostate tumor microenvironment. PARP inhibitors, like rucaparib, increase cellular DNA damage, particularly in tumors with DNA repair defects, leading to genomic instability and cell death. This cellular DNA damage produces tumor neoantigens, leading to increased immune priming and subsequently promoting immune cell infiltration, adaptive regulation of PD-L1, and other changes that contribute to a more immunoresponsive tumor microenvironment. Consequently, dual PD-(L)1 and PARP inhibition is a plausible therapeutic strategy for mCRPC. At the 2021 ASCO annual meeting, Dr. Russell K. Pachynski and colleagues presented the final results for cohort A1 (nivolumab + rucaparib for post-chemotherapy mCRPC) of CheckMate 9KD.