As a result, inhibiting enzymes such as indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO) could reduce tryptophan catabolism, lower kynurenine concentrations, and modulate the Trp-kynurenine axis, thereby restoring anti-tumor immunity and improving the body’s ability to fight cancer [82]. Here, TDO2 is linked to cancer.