Abstract 2244: ASO-mediated STAT3 knockdown relieves immunosuppression sensitizing tumors to immunotherapies

Abstract To improve the clinical outcome of cancer immunotherapies, strategies that modulate the immunosuppressive tumor microenvironment (TME) are currently under evaluation. Our study aims to dissect the immune-modulatory effects of danvatirsen, an antisense oligonucleotide (ASO) targeting the transcription factor Signal Transducer and Activator of Transcription 3 (STAT3) currently being studied in combination with pembrolizumab in the PEMDA-HN trial, a randomized Phase 2 trial in recurrent/metastatic head and neck squamous cell carcinoma (NCT05814666). In response to cytokines and growth factors, STAT3 regulates genes involved in cell differentiation, survival, inflammation, and angiogenesis. In tumors STAT3 is often hyperactivated, both in tumor and TME cells, and correlates with poor prognosis. Danvatirsen is a 16-mer generation 2.5 ASO designed by Ionis Pharmaceuticals to specifically target human STAT3 mRNA for degradation. Previous preclinical and clinical data have demonstrated tumor uptake of danvatirsen and STAT3 protein reduction in post-treatment biopsies. To better elucidate the mechanism, we performed in vitro and in vivo studies to assess STAT3 knockdown (KD) and effects on immune cell phenotype. Danvatirsen strongly reduces STAT3 mRNA and protein in vitro, both in human cancer cells and Peripheral Blood Mononuclear Cells (PBMCs), as well as in vivo, with a protein KD of 75% in human tumor xenografts implanted in immunodeficient mice. Interestingly, we found that danvatirsen increases the killing activity of PBMCs co-cultured with lung or liver human cancer cell lines, in line with the immunostimulatory effects of STAT3 KD. Moreover, we confirmed that danvatirsen affects the polarization of in vitro-differentiated pro-tumoral M2 macrophages towards an anti-tumoral M1 phenotype. A mouse surrogate STAT3 ASO administered in vivo reduced tumor STAT3 mRNA and protein levels by 60% and 75%, respectively, in the mouse breast cancers 4T1 and EMT6, and in the mouse lung cancer LL2. STAT3 KD was also observed in immune cells in circulation or infiltrating 4T1 tumors, with 33% and 55% KD in tumor-infiltrating T cells and macrophages, respectively. STAT3 ASO treatment in vivo remodels the TME composition, leading to pro-tumoral M2 macrophage repolarization towards an anti-tumoral M1 phenotype, among other effects. Those changes were associated with tumor growth inhibition (TGI) of 30% in 4T1-tumor bearing mice. When combined with anti-PD-1, STAT3 ASO reduced tumor growth by 55% compared to the control, and showed an additive effect to anti-PD-1 treatment alone, where TGI was 25%, supporting the combination of danvatirsen and pembrolizumab. Similar results were obtained in the EMT6 model. In conclusion, these data showed that ASO-mediated STAT3 KD relieves immunosuppression thereby sensitizing tumors to immunotherapies. Citation Format: Veronica Finisguerra, Marie-Aline C. Neveu, Joppe Lochten, Dido Van de Velde, Natalie De Geest, Marie Wallays, Marijke Pellens, Michel Janicot, Andrew Denker, Tiziana Girardi. ASO-mediated STAT3 knockdown relieves immunosuppression sensitizing tumors to immunotherapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2244.