CX-5461 Treatment Leads to Cytosolic DNA-Mediated STING Activation in Ovarian Cancer

SIMPLE SUMMARY: CX-5461 is an RNA polymerase I inhibitor that is in clinical trials for both advanced hematological cancers and solid tumors. Experimentally, this drug has been shown to induce a p53-independent DNA damage response through ATM and ATR kinase, and has particular activity against chemoresistant tumors. The current study shows for the first time that CX-5461 treatment in ovarian cancer cells induces the release of cytoplasmic DNA that stimulates cGAS–STING signaling, leading to the production of IFN type I in both cancer cells and xenografts in vivo. Because the cGAS–STING pathway is a key mediator of the immune response against cancer cells, this novel finding may lead to utilization of RNA Pol I inhibitors in combination with checkpoint inhibition. ABSTRACT: Epithelial ovarian cancer (EOC) is the deadliest of the gynecologic malignancies, with an overall survival rate of <30%. Recent research has suggested that targeting RNA polymerase I (POL I) with small-molecule inhibitors may be a viable therapeutic approach to combating EOC, even when chemoresistance is present. CX-5461 is one of the most promising POL I inhibitors currently being investigated, and previous reports have shown that CX-5461 treatment induces DNA damage response (DDR) through ATM/ATR kinase. Investigation into downstream effects of CX-5461 led us to uncovering a previously unreported phenotype. Treatment with CX-5461 induces a rapid accumulation of cytosolic DNA. This accumulation leads to transcriptional upregulation of ‘STimulator of Interferon Genes’ (STING) in the same time frame, phosphorylation of IRF3, and activation of type I interferon response both in vitro and in vivo. This activation is mediated and dependent on cyclic GMP–AMP synthase (cGAS). Here, we show THAT CX-5461 leads to an accumulation of cytosolic dsDNA and thereby activates the cGAS–STING–TBK1–IRF3 innate immune pathway, which induces type I IFN. CX-5461 treatment-mediated immune activation may be a powerful mechanism of action to exploit, leading to novel drug combinations with a chance of increasing immunotherapy efficacy, possibly with some cancer specificity limiting deleterious toxicities.