Abstract A069: Epigenetic reprogramming restores STING pathway activation in high-grade serous ovarian cancer

Abstract High-grade serous ovarian carcinoma (HGSC), which accounts for ∼70% of ovarian carcinoma cases, is typically diagnosed at an advanced stage and carries a poor prognosis. Although immunotherapies can induce durable responses, overall response rates in HGSC remain modest, highlighting the need for new strategies to enhance antitumor immunity. Emerging evidence suggests epigenetic mechanisms regulate innate immune pathways, including Stimulator of Interferon Genes (STING), a key mediator of tumor immunosurveillance. Analysis of TCGA and GTEx datasets showed that STING expression is reduced in HGSC, particularly in TP53-mutant tumors, and that higher STING levels correlate with improved overall, progression-free, and disease-free survival. Compared to normal fallopian tube epithelial cells (FT190), HGSC cells exhibited reduced CXCL10 and CCL5 expression and impaired STING pathway activation, as evidenced by lower phospho-TBK1, phospho-IRF3, after dsDNA stimulation. STING knockout confirmed that cytokine production was STING-dependent. Interrogation of TCGA epigenetic data and methylation-specific qPCR revealed hypermethylation of the STING promoter in HGSC samples and cell lines (OVCAR3 and OVCAR5). Based on these findings, we hypothesized that DNA methyltransferase inhibitors (DNMTis) could restore STING expression through epigenetic reprogramming. Indeed, DNMTi treatment significantly reduced promoter methylation and dose-dependently increased STING and downstream chemokine (CXCL10, CCL5) expression. Given that these chemokines attract immune cells, we further analyzed TCGA tumors and observed enrichment of CD4+ and CD8+ T cell infiltration signatures in cases with high STING expression, suggesting that restoring STING may help reprogram the tumor microenvironment and sensitize HGSC to STING agonists. STING agonists have been in clinical trials in solid tumor studies, but responses to date have been suboptimal. Intravenous formulation of next generation STING agonist, CPD1 (Curadev Pharma) has demonstrated high levels of systemic safety in non-human primates and is currently undergoing dose escalation in patients with advanced solid cancers (MSKCC) and our first-in-human AML (UMGCCC). Initial studies with STING agonist CPD1 alone was not effective at inducing pathway activation. We therefore tested whether epigenetic re-activation of STING expression was necessary to restore signaling. We treated HGSC cells with DNMTi, STING agonist, or the combination. Only the combination significantly increased phospho-TBK1, phospho-IRF3, CXCL10 and CCL5 and induced apoptosis-dependent cell death. Combination therapy also markedly suppressed oncogenic phenotypes, including proliferation and colony formation. Together, these results demonstrate that epigenetic silencing contributes to defective STING signaling in HGSC. As inhibiting DNA methylation restores both STING expression and responsiveness to agonist stimulation, combining DNMTi with STING agonists represents a promising therapeutic strategy to enhance antitumor immunity in HGSC. Citation Format: Saranya Rajendran, Elnaz Abassi Farid, Ishani Chattopadhayay, Shu Zhang, Lalitha Holaly Sastry, Qiqi Xie, Jia Shen, Sheng Liu, Jun Wan, Stephen B. Baylin, Feyruz V. Rassool, Kenneth P. Nephew. Epigenetic reprogramming restores STING pathway activation in high-grade serous ovarian cancer abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85 (18Suppl): Abstract nr A069.