PDCD1 and metastatic melanoma: Furthermore, the phase III Keynote-252/ECHO-301 clinical trial that evaluated combination treatment with the potent IDO enzyme inhibitor, epacadostat, and pembrolizumab [anti-programmed cell death 1 (PD-1)] in patients with unresectable or metastatic melanoma, failed to meet its primary end point as compared to individuals treated with pembrolizumab as a monotherapy (5).