Abstract 2075: Systemic small molecule TREX1 inhibitors to selectively activate STING in the TME of metastatic disease
Authors: Francica Brian , Burdette Dara , Clark Ryan , Cope Jamie , Freund David , Holtz Anja , Prasit Peppi , Whiting Chan , Dubensky Thomas W. 
Published: June 2022

Abstract
Genetic evidence from human disease and mouse genetic knock-out studies identify the Stimulator of Interferon Genes (STING) pathway as a critical innate immune sensor for the development of immunity. Tumor cells can evolve to avoid immune recognition through inactivating the STING pathway by diverse mechanisms, indicating that it is important to generating tumor-specific immunity. However, the clinical activity of STING agonists given by intratumoral (IT) administration has not compared well to preclinical studies. The scientific hypothesis supporting these first clinical studies is that localized CD8+ T cell priming would have activity against distal non-injected tumors, but findings that tumors in advanced malignancies have unique antigenic repertoires suggests that this approach may have limited activity against distal tumors. Global innate activation in metastases may therefore be necessary to prime a broadly active CD8+ T cell population targeting diverse antigens, in addition having the benefit of reversing the immune suppressive tumor microenvironment (TME). However, ubiquitous expression of STING presents a significant challenge to achieving a therapeutic index with systemic delivery of direct STING agonists. Selective activation of the STING pathway may be achieved through targeted inhibition of TREX1, a cytosolic DNA exonuclease that modulates cGAS/STING signaling. Expression of TREX1, in contrast to STING, is increased in tumor cells due to elevated levels of cytosolic DNA resulting from genetic instability, DNA repair mutation, inflammatory mediators or DNA-modifying anti-cancer therapies. These observations provide the principal scientific rationale to selectively activate the STING pathway in metastatic disease through targeted inhibition of TREX1. Utilizing published TREX1 X-ray crystal structures to guide medicinal chemistry, we discovered small molecule inhibitors of TREX1 and transformed these molecules from &gt;100 µM leads into a series with drug-like physicochemical properties and picomolar potency against both human and mouse TREX1. We evaluated the activity of lead molecules in cell-based assays, in which TREX1 inhibition enhanced cGAS/STING signaling, and advanced molecules with desired pharmacokinetic profiles to mouse tumor studies. We observed significant anti-tumor activity in mice with CT26 tumors given a combined therapy of low dose doxorubicin to induce dsDNA breaks and increase TME TREX1 expression along with lead series TREX1 molecule inhibitors. Recognizing that TREX1 is a DNA repair enzyme, we also showed that TREX1 inhibitors were cytotoxic in DNA repair deficient human tumor cell lines, informing advancement of this new class of STING therapeutics as a clinical approach to both activate the cGAS/STING pathway to initiate immune recognition, as well as to inhibit DNA repair orthogonal to existing tumor-driver DNA repair mutations.
Citation Format: Brian Francica, Dara Burdette, Ryan Clark, Jamie Cope, David Freund, Anja Holtz, Peppi Prasit, Chan Whiting, Thomas W. Dubensky. Systemic small molecule TREX1 inhibitors to selectively activate STING in the TME of metastatic disease [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2075.