Table 1.

Feature of IDO1, IDO2, and TDO

Main features	IDO1	IDO2	TDO
Distribution	Ubiquitously distributed throughout the body, including brain, lung, intestine, spleen, stomach, placenta, pancreas, cortex, medulla, adrenal ,urinary bladder, prostate [30]	Liver, testis, and thyroid, myeloid and plasmacytoid dendritic cells [30]	Liver and brain [30]
Enzyme inducer	Interferon-γ (IFN-γ), lipopolysaccharide (LPS) and tumor necrosis factor (TNF), interleukin (IL)-β, IL-6 [30]	Liver, kidney and epididymis in mice Dendritic cells (DCs) and monocytes [31]	Tryptophan, glucocorticoids, corticosterone, catecholamines [30]
Enzyme inhibitor	1-methyl-l-tryptophan, Epacadostat (INCB24360), Navoximod (GDC-0919) and D-1-methyl-tryptophan (D-1MT), α-cyclohexyl-5H-imidazo (NLG-919) and Linrodostat (BMS-986205) [30]	D-1MT appears to be a more effective inhibitor of IDO2 than 1-Methyl-L-tryptophan (L-1-MT), 1-alkyl-tryptophan, tenatoprazole [30]	6-Fluoro-3-[(1E)-2-(3-pyridinyl) ethanoyl)-1H-indole (680C91) Tryptamine, 5-hydroxytryptophan (5-HTP) and melatonin, 2-(3,4- Dihydroxyphenyl)-3,6,7-trihydroxy-2,3-dihydro-4H-chromen-4-one(NSC36398) [30]

IDO, indoleamine-pyrrole 2,3-dioxygenase; TDO, tryptophan 2,3dioxygenase.