Presently provided are IDO inhibitors and pharmaceutical compositions thereof, useful for modulating an activity of indoleamine 2,3-dioxygenase； treating indoleamine 2,3-dioxygenase (IDO) mediated inimunosuppression； treating a medical conditions that benefit from the inhibition of enzymatic activity of indoleamine-2,3-dioxygenase； enhancing the effectiveness of an anti-cancer treatment comprising administering an anti-cancer agent； treating tumor-specific immunosuppression associated with cancer； and treating immunosupression associated with an infectious disease. In one embodiment the compound is 1-(4-(1-hydroxy-2-(5H-imidazo[5,1-a]isoindol-5-yl)ethyl)piperidin-I-yl)-2-(3-hydroxyphenyl)ethanone.
