a Crystal structure of the IDO1–epacadostat complex (PDB code: 5WN8). The heme site is colored in green. The inhibitor epacadostat is displayed in yellow stick. The H-bonding interactions between epacadostat and the IDO1 protein matrix are indicated by the blue dotted lines, and the intramolecular H-bonds within epacadostat are indicated by the red dotted lines. b Schematic illustration of the IDO1–epacadostat interactions.