What is the prospect of indoleamine 2,3-dioxygenase 1 inhibition in cancer? Extrapolation from the past

Table 3 Outline of IDO1 selective inhibitor epacadostat

Assay	Enzyme source/cell			IC₅₀ (nM)	Ref
Enzymatic	His-tagged human IDO1			71.8	[79]
Cellular	CT26 colon carcinoma cells			76	[65]
	Pan02 pancreatic adenocarcinoma cells			27	[65]
	293/MSR-human IDO1 ^a			15.0	[79]
	293/MSR-mouse IDO1 ^a			52.4	[79]
	293/MSR-mouse IDO2 ^a			> 5000	[79]
	293/MSR-human TDO ^a			> 10,000	[79]
	Human dendritic cells			12.7	[79]
	Hela human cervical carcinoma cells			7.1	[79]
	SKOV-3 ovarian cancer cell			15.3	[84]
Type of inhibition	Ref	Ki	Ref	Binding sites	Ref
Competitive ^b	[85]	1 µM ^b	[85]	Heme iron ^c	[86]

^a HEK293/MSR cells were transfected with full-length human or mouse IDO1 or mouse IDO2 cDNA, or a human TDO expression vector; ^b Type of inhibition and Ki of 4-amino-1,2,5-oxadiazole-3-carboximidamide which shares same skeleton as epacadostat; ^c Epacadostat coordinates to the heme iron of human IDO1; IC₅₀ the concentration that exerts 50 % of the maximal inhibition effect on IDO1

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