PDCD1 and metastatic melanoma: Despite these limitations, our current study sheds some light on potential reasons for the limited efficacy of ECHO-301/KEYNOTE-252 (NCT02752074), the first phase 3 randomized double-blind study of epacadostat in combination with anti-PD-1 antibody pembrolizumab in patients with unresectable or metastatic melanoma (19–21).