Discovery of TREX1 Antagonists for Cancer Immunotherapy

SummaryTREX1, the major cytosolic exonuclease, plays an important role in preventing aberrant immune responses to self-DNA in normal cells. However, multiple lines of evidence indicate that TREX1 dampens the intrinsic immune response to tumors by preventing activation of the DNA-sensing cGAS/STING pathway in tumor cells, which is required for activation of T cells and cross presentation of tumor antigens by dendritic cells. Moreover, TREX1 expression is upregulated by DNA damaging treatments that are being investigated as ‘in situ vaccines’ to improve responses to existing immune checkpoint inhibitors; e.g., radiotherapy in combination with PD-1 blockers.We are proposing to develop small molecule inhibitors of TREX1 for cancer immunotherapy. In Phase I, we will lay the groundwork for a successful TREX1 lead discovery program incorporating both high throughput screening (HTS) and virtual screening (VS). We will develop a robust, HTS- compatible enzymatic assay for TREX1, which will serve as the primary biochemical tool for discovering inhibitors and developing them into lead molecules, and perform a pilot screen with a 20K diversity library to validate the assay. We will also use advanced computational methods to develop a pharmacophore model for the TREX1 active site and perform a virtual screen of lead like molecules, which will be assessed with the HTS assay. Successful completion of these aims will put us in a strong position to pursue a full scale TREX1 lead development program in Phase II, powered by the proven preclinical discovery platform that we have assembled for the related target cGAS, under separate SBIR grants. To our knowledge, there are no small molecule TREX1 inhibitors; using both HTS and VS will maximize the Type I IFNs TREX1 acts as a checkpoint on antitumor immunity by preventing activation of the cGAS/STING pathway by cytosolic DNA. chemical space that we can sample, an important consideration for an unexplored target, both from a scientific and intellectual property perspective.The potential medical impact of TREX1 blocking drugs is broad and fundamental, as they could potentially be used to improve responses to existing checkpoint blockers, all of which act downstream; to allow lower dosing of cytotoxic therapies; or as monotherapy to stimulate a T cell response in immunogenically ‘cold’ tumors.Narrative TREX1, the major cytosolic exonuclease, is an unexplored immune checkpoint at the very top of the pathway that leads to activation of an anti-tumor immune response. We propose to develop small molecule inhibitors of TREX1 for evaluation as a new type of cancer immunotherapy drug.