IDO1 and cancer: However, recent studies5, 13 demonstrating that TDO-mediated Trp metabolism represents an alternative route to IDO activity employed by tumour cells generated growing interest in TDO as therapeutic target for cancer immunotherapy in general and, in particular, liver cancers.19 Thus, as the available IDO inhibitors do not suppress TDO activity, there could be a need to develop specific TDO inhibitors.15