6010 Background: Indoleamine 2,3-dioxygenase 1 (IDO1) is a tryptophan-catabolizing enzyme that induces immune tolerance by T-cell suppression. IDO1 overexpression has been associated with poor survival in SCCHN. Epacadostat (E) is a potent, selective oral IDO1 inhibitor. ECHO-202/KEYNOTE-037 is an open-label, phase I/II (P1/2) study evaluating E plus PD-1 inhibitor pembrolizumab (P) in multiple tumor types. We report preliminary P1/2 efficacy, safety, and tolerability findings in the SCCHN cohort as of a 29OCT2016 data cutoff. Methods: Eligible adult patients (pts) had metastatic SCCHN and received ≥1 prior chemotherapy regimen that included a platinum agent. Prior checkpoint inhibitor therapy (tx) was not permitted, and pts with carcinoma of the nasopharynx or salivary gland were excluded. In P1 dose escalation (3+3+3), pts received E (25, 50, 100, or 300 mg PO BID) + P (2 mg/kg or 200 mg IV Q3W); MTD was not exceeded. E (100 mg BID) + P (200 mg Q3W) dosing was selected for P2 cohort expansion. Response was assessed in RECIST 1.1 evaluable pts. Results: A total of38 pts (P1, n = 2; P2, n = 36) were evaluated. Median age was 63 years, 87% of pts were men, 95% were white, and 66% received prior cetuximab. Of 36 efficacy-evaluable pts, 81% (n = 29) received 1–2 prior lines of tx and 19% (n = 7) received ≥3 prior lines of tx. ORR (CR+PR) and DCR (CR+PR+SD) for pts with 1–2 prior tx were 34% (2 CR, 8 PR) and 62% (8 SD), respectively; for pts with ≥3 prior tx, ORR and DCR were 14% (1 PR) and 43% (2 SD). Response was observed regardless of HPV status. At data cutoff, 9/11 responses were ongoing (range, 1+ to 563+ days). PFS and biomarker analyses are ongoing. The most common TRAEs in all 38 pts were fatigue (24%), nausea (11%), and decreased weight (11%). Grade ≥3 TRAEs occurred in 11% of pts; only increased amylase and lipase (both asymptomatic) were grade ≥3 TRAEs that occurred in > 1 pt. TRAEs led to discontinuation in 1 pt (increased amylase and lipase). Conclusions: In pts with advanced SCCHN, E + P was generally well tolerated and associated with encouraging response rates, particularly in pts with 1–2 prior lines of tx. A phase III SCCHN study is planned. Clinical trial information: NCT02178722.