Presently provided are inhibitors of cellularly expressed TDO2 and pharmaceutical compositions thereof, useful for modulating an activity of tryptophan 2, 3 dioxygenase; treating immunosuppression; treating a medical conditions that benefit from the inhibition of tryptophan degradation; enhancing the effectiveness of an anti-cancer treatment comprising administering an anti-cancer agent; and treating tumor-specific immunosuppression associated with cancer.