TDO2 and metastatic melanoma: Although Epacadostat failed to demonstrate additional benefits when combined with pembrolizumab in a randomized phase III study for metastatic melanoma, a review of past research suggests several key factors that may have contributed to the treatment failure, including the compensatory activation of TDO following IDO1 blockade by Epacadostat and the non‐enzymatic roles of IDO1.