Testing the Addition of Tazemetostat to the Immunotherapy Drug, Pembrolizumab (MK-3475), in Advanced Urothelial Carcinoma

This phase I/II trial studies the side effects and best dose of tazemetostat and how well it works when given together with pembrolizumab in treating patients with urothelial carcinoma that has spread to nearby tissue or lymph nodes (locally advanced ) or from where it first started (primary site) to other places in the body (metastatic). Tazemetostat may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Immunotherapy with monoclonal antibodies, such as pembrolizumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread. Giving tazemetostat and pembrolizumab may work better in treating patients with urothelial carcinoma compared to pembrolizumab without tazemetostat.