CTLA4 and neoplasm: Checkpoint inhibitors including nivolumab (anti-programmed cell death protein 1 (PD-1)), pembrolizumab (anti-PD-1), and ipilimumab (anti-cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4)) are also tested in clinical trials for anti-tumor activity against CRPC [16,17].