Indoleamine 2,3-dioxygenase 1 Focused Library
Indoleamine 2,3-dioxygenase (IDO) is a heme enzyme that initiates the oxidative degradation of the least abundant, essential amino acid, l-tryptophan, along the kynurenine pathway. The local cellular depletion of l-tryptophan that results may enable the host to inhibit the growth of various infectious pathogens in vivo. However, over the past decade, it has become increasingly apparent that IDO also represents an important immune control enzyme. Thus, cells expressing IDO, are capable of suppressing local T cell responses to promote immune tolerance under various physiological and pathophysiological conditions of medical importance, including infectious diseases, foetal rejection, organ transplantation, neuropathology, inflammatory and auto-immune disorders and cancer.
IDO represents an ideal target for therapeutic intervention. Inhibition of inappropriate IDO activity in tumours in vivo may attenuate the ability of tumours to evade immune surveillance and promote clearance. In neurological disorders characterized by increased levels of the excitatory neurotoxin, quinolinic acid there is considerable interest in the utility of kynurenine-3 mono-oxygenase inhibitors to block the production of quinolinic acid, while increasing kynurenic acid, an antagonist of quinolinic acid toxicity. 
 N. J. C. King and S. R. Thomas, “Molecules in focus: Indoleamine 2,3-dioxygenase,” Int. J. Biochem. Cell Biol., vol. 39, no. 12, pp. 2167–2172, 2007, doi: 10.1016/j.biocel.2007.01.004.