Kyn is generated via two major routes: in peripheral tissues, controlled by the rate-limiting enzymes indoleamine 2, 3-dioxygenase 1 (IDO1) and indoleamine 2, 3-dioxygenase 2 (IDO2), and the hepatic route, in which tryptophan 2, 3-dioxygenase (TDO) is the rate-limiting enzyme