TDO2 and cancer: The imbalances of Trp metabolism in cancer have drawn great interest in targeting the kynurenine pathway for therapy, especially the rate-limiting enzymes indoleamine-2,3-dioxygenase 1 (IDO1), IDO2, tryptophan-2,3-dioxygenase (TDO) and kynurenine monooxygenase (KMO) 114, 115.