TDO2 and neoplasm: Specific to GBM, although increased expression of upstream enzymes IDO and TDO have been observed, along with the accumulation of Kyn12,57, targeting these pathways individually did not result in notable changes in the immune profile of these tumors or independent anti-tumor activity in preclinical models10,13, and unfortunately, as these investigations predicted, did not result in clinical benefit58,59.