Enhancing the Effectiveness of Chemotherapy in Osteosarcoma by Targeting Tumour‐Associated Macrophages (TAMs) through STING Activation

Abstract Osteosarcoma is an aggressive pediatric, adolescent, and young adult bone cancer with an immunosuppressive tumor microenvironment (TME) that limits immunotherapy efficacy. Tumor‐associated macrophages, key players in immunosuppression and metastasis, are abundant in the osteosarcoma TME. The cGAS/STING pathway has emerged as a target for enhancing anti‐tumor immunity. Here, this work investigates whether STING stimulation could reprogramme macrophages toward a tumoricidal M1‐like phenotype and enhance doxorubicin efficacy against osteosarcoma. These results show that while doxorubicin induces cell death of osteosarcoma cells, it fails to activate STING in macrophages. However, pre‐treatment of macrophages with a STING agonist enhances M1‐like polarization of macrophages when indirectly co‐cultured with chemotherapy‐treated osteosarcoma cells, regardless of the original macrophage phenotype. Importantly, this work observes a loss of STING protein when cells are excessively stimulated with a STING agonist and sequential dosing offered no advantage over a single treatment. Finally, this work demonstrates that the combined therapy of doxorubicin and a single dose of neoadjuvant STING agonist synergistically increases osteosarcoma cell death via M1‐macrophages compared to either therapy alone. These findings highlight the therapeutic potential of STING agonists to reprogram macrophages within the TME, and improve chemotherapy efficacy, offering a promising new strategy to enhance osteosarcoma treatment options.