CTLA4 and cancer: While inhibitors of CTLA-4 and PD-L1 have been tested in clinical trials for PCa, the only ICB therapy currently approved in mCRPC with high MSI or dMMR is the PD-1 inhibitor pembrolizumab (Keytruda) that blocks the binding of PD-1 on T-cells to its receptor PD-L1, which is upregulated in some cancer cells [193].