Purity >98%. Epacadostat is an inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1) with an IC50 value of 7.1 nM in HeLa cells that demonstrates little activity against the related enzymes IDO2 or tryptophan 2,3-dioxygenase (TDO). It has been shown to promote T and natural killer-cell growth, to increase IFN-Gamma production, and to reduce conversion to regulatory T (Treg)-like cells in a coculture system of human allogeneic lymphocytes with either dendritic cells or tumor cells. Epacadostat can also inhibit tumor growth in tumor-bearing mice in a lymphocyte-dependent manner.