Background: INCB024360 is a potent, selective inhibitor of IDO1. As the catabolism of tryptophan (Trp) to kynurenine (Kyn) by IDO1 inhibits immune responses and IDO1 expression is elevated in many human cancers, IDO1 inhibition may potentiate effective antitumor immune responses. Methods: This dose-escalation study in adult pts with advanced malignancies used a 3+3 design to determine MTD, toxicity, PK, PD, and tumor response rate. Daily doses of INCB024360 were evaluated in 28-day cycles in 8 cohorts (50 mg once daily; 50 mg, 100 mg, 300 mg, 400 mg, 500 mg, 600 mg, or 700 mg BID). Treatment continued until disease progression or unacceptable toxicity. PK and PD samples were drawn on days 1 and 15. Results: 52 pts have been treated. Tumor types included colorectal (56%), melanoma (12%), and other (33%). The most common adverse events (≥20%) were fatigue, nausea, decreased appetite, vomiting, constipation, abdominal pain, diarrhea, dyspnea, back pain, and cough. The most common grade 3 or 4 adverse events were abdominal pain, hypokalemia, and fatigue (9.6% each). One DLT each was observed at 300 mg BID (grade 3 radiation pneumonitis) and 400 mg BID (grade 3 fatigue); no DLTs were observed in the 18 pts treated with 600 mg or 700 mg BID. There were no objective responses. At 56 days, stable disease was seen in 15 patients and lasted ≥112 days in 7 patients. Significant dose-dependent reductions in plasma Kyn/Trp ratios and Kyn levels were detected at all doses and in all pts. Maximal effects were observed at doses ≥300 mg BID. With repeat dosing, 700 mg BID provided an average plasma concentration ~5-fold the projected IC₉₀. Overall, doses ≥300 mg BID achieved greater than 90% inhibition of IDO1 throughout the dosing period. Conclusions: INCB024360 was generally well tolerated at doses of up to 700 mg BID and there appears to be no correlation of dose with toxicity. Doses ≥300 mg BID were capable of >90% inhibition of IDO1 activity and found to effectively normalize plasma Kyn levels. The recommended dose as monotherapy is 600 mg BID. Clinical trial information: NCT01195311.