Abstract 6539: RNAi mediated silencing of STAT3 in tumor-associated immune cells induces robust anti-tumor effects in immunotherapy resistant tumors

Abstract Malignant tumors are often associated with an immunosuppressive tumor microenvironment (TME), rendering most of them resistant to standard-of-care immune checkpoint inhibitors (CPIs). Signal transducer and activator of transcription 3 (STAT3), a ubiquitously expressed transcription factor, has well-defined immunosuppressive functions in several leukocyte populations within the TME. Since the STAT3 protein has been challenging to target using conventional pharmaceutical modalities, we investigated the feasibility of applying systemically delivered RNA interference (RNAi) agents to silence its mRNA directly in tumor-associated immune cells. In preclinical murine tumor models, chemically stabilized acetylated small-interfering RNAs (siRNAs) selectively silenced Stat3 mRNA in multiple relevant cell types in TME and tumor-draining lymph nodes (TDLN), reduced STAT3 protein levels, and increased cytotoxic T-cell infiltration. In murine models of CPI-resistant tumors, RNAi-mediated Stat3 silencing resulted in tumor growth inhibition, which was further enhanced and resulted in tumor regressions in combination with CPIs. Our findings suggest that targeting STAT3 directly and reducing STAT3 protein in the suppressive immune cell populations in the TME and TDLN may overcome some of the current limitations of immunotherapy and be of therapeutic benefit especially for cancer patients who failed to respond to standard immunotherapy treatments alone. A human active STAT3 conjugate is currently in Phase 1 clinical trial for treatment of immunotherapy refractory cancer types (NCT06098651). Citation Format: Shanthi Ganesh, Minju Kim, Jenny Lee, Xudong Feng, Harini Sivagurunatha Krishnan, Rene Rijnbrand, Amber Beaudry, Jennifer A. Lockridge, Vinita Uttamsingh, John Hanrahan. RNAi mediated silencing of STAT3 in tumor-associated immune cells induces robust anti-tumor effects in immunotherapy resistant tumors abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6539.