Although data from a phase I/II dose-escalation trial combining epacadostat with nivolumab 3 mg/kg every 2 weeks indicate that anti-PD-1 combined with IDO1 inhibition has antitumor activity and an acceptable toxicity profile in humans [37], it was announced in April 2018 that the phase III ECHO-301/KEYNOTE-252 study of pembrolizumab (anti-PD-1) plus epacadostat in advanced melanoma did not meet its first primary endpoint of progression-free survival (PFS) (HR 1.00). Here, PDCD1 is linked to melanoma.