Abstract 6509: Hypofractionated radiotherapy as a vaccine in combination with the TLR7/8 agonist resiquimod leads to significant abscopal tumor response and persistence antitumor immunity

High doses of ionizing radiation (IR) as delivered with hypofractionated radiotherapy (HFRT) can generate immunogenic cell death and elicit adaptive anti-tumor immunity through cross priming of tumor associated antigens. Resiquimod (R-848), a TLR7/8 agonist, generates a systemic antitumor immunity resulting in tumor regression and enhanced T-cell effector function in cutaneous T-cell lymphomas clinically. We hypothesized that combining HFRT with immunotherapy would boost the antitumorigenic effects of either monotherapies, and if so, could lead to a readily translated clinical modality. B16F10 (murine melanoma) and SCC7 (murine squamous cell carcinoma) cells were used as to grow flank subcutaneous syngeneic tumor models in C57BL/6J and C3H/HeJ mice respectively. Our studies indicate that intratumoral administration of resiquimod combined with HFRT to those tumors leads to a significantly stronger antitumor response than either single treatment in both tumor models. The combination therapy in the primary tumor led to complete tumor regression of the SCC7 tumors in 86% of the treated mice to just 14% rejection in the HFRT group alone. Tumor re-challenge of those mice led to tumor rejection in 83% in the combination group compared to 0% of the HFRT group indicating a potent systemic effect of the combined therapy. Furthermore, the combination therapy led to significant reduction of the index (treated) but moreover in the abscopal (untreated) tumors compared to either monotherapy confirming a systemic antitumor effect in this dual-tumor in vivo model. RT-qPCR analysis revealed an increase expression of Th-1 signature (IFNγ, IL-12, Prf1, GzmB) in both primary and abscopal tumors and flowcytometric analysis of the abscopal B16F10 tumors revealed an increase in CD8+ infiltration. A more thorough flowcytometric analysis following staining with the gp100 tetramer revealed an increased cross-priming against gp100 (PMEL) and enhanced gp100-specific T-cell infiltration within the tumor. Moreover, splenocytes that were re-stimulated in vitro with the gp100 peptide in an IFNγ ELISPOT assay, they increased in mice treated with the dual therapy. Finally, an in vitro study of B16F10 cells, which express TLR7/8 receptors, revealed a substantial increase in IFNβ and CCL5 expression with the dual treatment. Knocking down TLR7 and Tmem173 (STING) in those cells with siRNAs completely reversed the phenotypic IFN type I expression signature. In conclusion, the combination of HFRT with Resiquimod leads to very potent antitumor effects in both primary and abscopal melanoma and squamous cell carcinoma tumors driven by an enhanced systemic immunologic response. Our findings provide the rational for translating therapies combining radiotherapy and intratumoral resiquimod into early-phase clinical trials. Citation Format: Ilias V. Karagounis, Stefano Pierini, Noelle Francois, Christoforos Thomas, Xiaowei Xu, Constantinos Koumenis, Andrea Facciabene, Amit Maity. Hypofractionated radiotherapy as a vaccine in combination with the TLR7/8 agonist resiquimod leads to significant abscopal tumor response and persistence antitumor immunity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6509.