Moreover, targeting the IDO-catalyzed kynurenine pathway seems to be attractive for Treg depletion in cancers, as various IDO1 inhibitory drugs have been designed thus far, such as secondary sulfonamides (Compound 5d), GDC-0919, epacadostat, and NLG802, while none of them have been approved for formal clinical treatment of malignancies by the FDA [41, 152, 154, 186]. Here, IDO1 is linked to cancer.