IDO1 and melanoma: In support of this hypothesis, epacadostat, which inhibits the enzyme IDO-1 and prevents depletion of the essential amino acid tryptophan from the TME, is exhibiting striking results in combination with anti-PD-1 therapy in patients with melanoma, lung cancer, RCC, or head and neck carcinoma [58–62], arguing that targeting T cell checkpoints and immunosuppression by the TME is an efficacious combination.