Abstract B054: Intratumoral injection with stimulator of interferon genes (STING) agonist increases T- and B-cell infiltrates in canine osteosarcoma

Abstract We desperately need novel treatment approaches for osteosarcoma beyond cytotoxic chemotherapy and surgery. Our group has an ongoing NIH-funded clinical trial treating canine osteosarcoma with neoadjuvant intratumoral delivery of a stimulator of interferon genes (STING) agonist, cryotherapy, or combined cryotherapy with intratumoral STING agonist injection. Here we report the analysis of 10 dogs treated with neoadjuvant STING agonist alone. Dogs with confirmed localized osteosarcoma were treated under image guidance with intratumoral injection of 100 micrograms of the STING agonist ADU-S100 or R229 (enantiomer of ADU-S100) on days 1 and 8 prior to planned limb amputation on day 15. After recovery from amputation, dogs received standard of care treatment with adjuvant carboplatin per their treating oncologist. We obtained the amputated limb for complete analysis of the treated tumor and peripheral blood samples. Dogs were followed every 3 months with scans and relapse and survival data was captured. We analyzed resected tumors by immunohistochemistry to quantify T cell (CD3+), myeloid cell (IBA1+), and B-cell (CD20+) density comparing areas where the STING agonist was injected to pre-treatment biopsy samples and/or areas of the tumor distant to the injection sites. Cell density was quantified using HALO imaging-analysis software. Tumor areas with immune infiltrates were further characterized by nanoString gene expression profiling and bulk RNA-sequencing. Both treated and untreated areas of every tumor have significant IBA1+ myeloid cell infiltrates. Comparing STING-treated versus untreated areas, 80% of tumors showed 2 fold induction of CD3+ T cell density in STING treated areas (range 4.1-21 fold), and the ratio of T cells : myeloid cells was increased (P=0.004). CD20+ B-cell density also increased by an average of 10.4-fold (range 1.8-44.3-fold). Gene expression analysis indicates predominant effector CD8+ T cell populations, NK cells, activated B cells, and increased effector cell cytokines. We also observed increased regulatory/suppressive pathways such as IDO1, PD1, and TIM3. STING agonist injection increased activated T- and B-cell infiltrates in treated canine osteosarcomas suggesting augmented adaptive immune system activation against the tumor. Compensatory regulatory pathways also increased indicating the potential to further augment immune activation targeting these pathways. Citation Format: Michele Doucet, Stephanie Glavaris, Maggie Phillips, Emily Kulp, Sarah Powers, Kathy Gabrielson, Cory Brayton, Dara Kraitchman, Brian H. Ladle. Intratumoral injection with stimulator of interferon genes (STING) agonist increases T- and B-cell infiltrates in canine osteosarcoma abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pediatric Cancer Research; 2024 Sep 5-8; Toronto, Ontario, Canada. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl):Abstract nr B054.