Abstract 2925: Metformin can induce abscopal effects of local irradiation and suppress the spontaneous lung metastases in murine colon cancer model

Introduction: Neoadjuvant chemoradiation therapy (CRT) for locally advanced rectal cancer is effective for the reduction of local recurrence and improvement of postoperative QOL. However, CRT has little effects for metastatic lesions in distant organs, and does not prolong the prognosis. Abscopal effect refers to a phenomenon of tumor regression at a distant tumor away from the irradiated primary tumor, presumably through the host immune system. Recently, it has been reported that combination therapy with radiation and immune checkpoint inhibitor is effective for non-irradiated metastatic lesions. Metformin is an oral medicine which has been widely used for the treatment of type 2 Diabetes Mellitus. It has been reported to have anti-cancer effects partly through the activation of host immunity. However, anti-tumor effects of the combination therapy of local radiation and metformin have not been well investigated. Method: LuM1, a highly lung metastatic subclone of colon 26 , were subcutaneous injected by 1x106 cells at right flank of 7 weeks female BALB/c mice.11 days later, mice were divided into 4 groups; Control, metformin, radiation, and combination therapy with radiation and metformin groups. In irradiated groups, mice received 4Gy X-ray for 2 times every 2 days from 12 days. Radiation was selectively given to subcutaneous tumors using lead shield under general anesthesia. Metformin was injected into intraperitoneal every day at the dose of 250mg/kg/day. We recorded the body weight and the size of subcutaneous tumor every other day. At 28 days, mice were sacrificed and we measured the weight of primary tumor and the number of lung metastasis. We analyzed the phenotypes of splenocytes by flow cytometry. Results: In radiation groups with or without metformin, primary tumors size and weight were reduced as compared to non-irradiated groups(p Conclusion: Metformin may enhance abscopal effect of local radiation therapy with the reduction of the number of PMN-MDSC, which may suppress the growth of metastases in distant organs. Citation Format: Mineyuki Tojo, Hidenori Tsukui, Akira Saito, Kouhei Tamura, Yuki Kaneko, Yurie Futoh, Yuki Kimura, Kazuya Takahashi, Yuko Kumagai, Hideyuki Ohzawa, Hideyo Miyato, Koji Koinuma, Hisanaga Horie, Naohiro Sata, Joji KItayama. Metformin can induce abscopal effects of local irradiation and suppress the spontaneous lung metastases in murine colon cancer model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2925.