9511Background: Indoleamine 2,3-dioxygenase 1 (IDO1) enzyme-induced immunosuppression in the tumor microenvironment supports tumor cell evasion of immune surveillance. Epacadostat (E) is a potent and selective oral inhibitor of the IDO1 enzyme. ECHO-204 is an open-label, phase (P) 1/2 study of E plus PD-1 inhibitor nivolumab (N) in patients (pts) with advanced cancer. Updated efficacy and safety/tolerability outcomes for the P2, advanced (unresectable or stage IV) melanoma cohort are reported. Methods: Pts received E (100 or 300 mg PO BID) plus N (240 mg IV Q2W). Response was assessed every 8 weeks (RECIST v1.1). Safety/tolerability was assessed in pts receiving ≥1 study treatment dose. PD-L1 expression was assessed (Dako 28-8 assay; PD-L1 positive: ≥1% tumor cell staining). Results: As of 29 Oct 2017, all of the planned 50 pts were enrolled in the P2 expansion cohort (n = 40 treatment naive for advanced disease). Median (range) duration of follow-up was 417 days (70 to 617 days). Across all pts, overall ...