IDO1 and melanoma: The phase I/II ECHO-202/KEYNOTE-037 trial (NCT02178722) revealed that epacadostat (a selective IDO1 enzyme inhibitor) plus pembrolizumab generally had a manageable safety profile and encouraging anti-tumor activity in several advanced solid tumors like melanoma, NSCLC, squamous cell carcinoma of the head and neck, renal cell carcinoma, urothelial carcinoma, and endometrial adenocarcinoma [128].