IDO1 and neoplasm: Furthermore, there is increasing evidence to suggest that there is redundancy between the IDO1 and TDO2 tryptophan catabolising enzymes – IDO1 inhibition leads to an increase in systemic tryptophan, which in turn activates a large pool of TDO2 in the liver [29] elevating systemic kynurenine, which is likely to counteract local tumour IDO1 inhibition.