IDO1 inhibition suppresses tumor growth in immunocompetent, but not immunodeficient, mice. Female C57BL/6 (A) or Balb/c nu/nu (B) mice bearing PAN02 pancreatic cancer cells were treated with 25 mg/kg (▴) or 100 mg/kg (●) INCB024360 orally twice a day or vehicle (■). Mean tumor volumes (mm³) ± SEM (n = 9-11 mice/group) are shown from the initiation of dosing (∼ 90-120 mm³). Both groups are significantly significant (*P < .01) compared with vehicle on the last day of the C57BL/6 study. (C) Plasma was harvested and trp and kyn levels were determined by liquid chromatography–dual mass spectrometry (LC-MS/MS). Kyn/trp ratios and associated percentage of inhibition were calculated.