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Abstract Little improvement has been made in outcomes for pediatric patients with osteosarcoma in decades. Cytotoxic chemotherapy alone is insufficient for cure and upfront complete surgical resection is usually required to achieve long-term survival. Better systemic therapies are needed. Osteosarcoma is characterized by a high degree of genomic instability that would suggest these tumors have sufficient neoantigen burden to be immunogenic and responsive to immunotherapy, but trials of immune checkpoint inhibitors in osteosarcoma have shown poor effectiveness. Although immunotherapy drugs targeting the adaptive immune system have worked poorly, we hypothesize that an innate immune adjuvant - stimulator of interferon response cGAMP interactor 1 (STING) agonist - in combination with cytotoxic chemotherapy would improve chemotherapy effectiveness and immune-mediated tumor clearance, particularly given the abundance of innate immune cells in the microenvironment of these tumors. Using two different immune-competent in vivo murine models of osteosarcoma (K7M2 in BALB/c mice and F420 in C57BL/6 mice), we challenged mice with tumor cells via subcutaneous or intratibial injection. In all experimental replicates, we observed improved tumor regression in mice treated with intratumoral STING agonist followed by intraperitoneal treatment with carboplatin. Responses included complete tumor response in up to 40% of treated mice. Treatment with STING agonist alone or carboplatin alone only delayed tumor growth compared to untreated controls. Additionally, mice treated first with carboplatin and then with STING agonist demonstrated an initial modest response to treatment, but ultimately exhibited tumor progression, suggesting an importance to the sequence of STING agonist administration. In vitro treatment of K7M2 and F420 cells with STING agonist showed a tumor-intrinsic transient increase in expression of pro-inflammatory mediators Cxcl10, Ccl5, and Ifnb 4 hours after STING exposure. Flow cytometry analysis of K7M2 tumors demonstrated increased infiltration of STING/carboplatin-treated tumors with CD8+/IFNγ+ lymphocytes and CD11b+/MHCII+/F480+ tumor-associated macrophages. Interestingly, these tumors also demonstrated an increased quantity of CD11b+/MHCII-/F480-/Ly6Glo/Ly6C+ myeloid-derived suppressor cells despite their favorable clinical response. Our findings suggest that intratumoral STING agonist therapy stimulates tumor-intrinsic and tumor-extrinsic pro-inflammatory mediators that improve the tumor response to treatment with carboplatin. Ongoing studies are further investigating the tumor-intrinsic and extrinsic mediators that are driving this improved response to therapy. Citation Format: Erin E. Resch, Emily Kulp, Michele Doucet, Alice Recho, Brian H. Ladle. Antecedent STING agonist therapy improves tumor response to chemotherapy in murine models of osteosarcoma 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 5461.
Resch et al. (Fri,) studied this question.