 IDO is an enzyme that converts the essential amino acid tryptophan to kynurenine. The availability of tryptophan is essential to sustaining both helper T cell and effector T cell activation. Overexpression of IDO by tumor cells or antigen presenting cells serves to arrest T cell activation thus acting as an immunosuppressive enzyme. Epacadostat INCB024360 is an inhibitor of the enzyme indoleamine 2,3-dioxygenase 1 IDO1 that has shown promise in the treatment of solid tumors and lymphomas in ongoing Phase III studies. 