Potent and selective competitive indoleamine 2,3-dioxygenase (IDO1) inhibitor. Inhibits IDO1 in enzymatic (IC50=72nM) and in cellular tests (IC50=7.1nM), suppresses tryptophan catabolism in vivo and impedes tumor growth. Displays high selectivity over other related enzymes such as IDO2 or tryptophan 2,3-dioxygenase (TDO). Exhibits potent in vitro and in vivo immunomodulating and antineoplastic activities. By inhibiting IDO1 and decreasing kynurenine levels in tumor cells, Epacadostat increases and restores the proliferation and activation of various immune cells, including dendritic cells (DCs), NK cells and T-lymphocytes, as well as interferon (IFN) production and reduces tumor-associated regulatory T cells (Tregs).