Targeting Tryptophan Dioxygenase Degradation for Suppression of Tumor Immune Evasion

The tryptophan degradation pathway is used to prevent unrestrained immune activation in healthy cells. However. tumors hijack this mechanism to escape immune surveillance. The key enzyme of this pathway. tryptophan 2.3-dioxygenase (TDO) and indoleamine 2.3-dioxygenase (IDO). are established immune checkpoint proteins. Tumors enhance their expression to block T cell proliferation and induce T cell death. thereby avoiding immune system surveillance and increasing tumor cell migration capacity. Thus. inhibiting these checkpoint enzymes in cancer cells by small molecule-based therapies has emerged as a potential immunotherapeutic strategy. There is a critical need for new agents developed from an innovative approach with a solid understanding of their chemistry and biology. From a joint basic science-clinical study we identified a non-catalytic L-Trp binding site in human TDO. which binds L-Trp surprisingly much tighter than the catalytic heme site. This biological role of this newly discovered L-Trp binding site is that it is a regulatory place in TDO for protein stability. This site serves as a degradation signal when empty whereas degradation-resistance when loaded with L-Trp. This finding has inspired us to propose a central hypothesis that this newly discovered signaling site is an Achilles??? heel of TDO for drug development. This application will fill the critical need to evaluate its biomedical potential. Towards this end. we will design compounds with a novel mode of action that destabilizes the signaling site of TDO or bind without enhancing the protein stability. These agents will not target the catalytic activity of TDO but disrupt its degradation resistance signal. We will assess the effects of the promising compounds to the human TDO in cellular and animal models to valid the innovative approach and target. In the end. this work will open the door for designing revolutionarily new checkpoint inhibitors targeting checkpoint protein human TDO.