Abstract 2925: Metformin can induce abscopal effects of local irradiation and suppress the spontaneous lung metastases in murine colon cancer model
Authors: Tojo Mineyuki , Tsukui Hidenori , Saito Akira , Tamura Kouhei , Kaneko Yuki , Futoh Yurie , Kimura Yuki , Takahashi Kazuya , Kumagai Yuko , Ohzawa Hideyuki , Miyato Hideyo , Koinuma Koji , Horie Hisanaga , Sata Naohiro , KItayama Joji 
Published: July 2021

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&lt;0.05). Metformin alone did not change the growth of primary tumor. The number of lung metastasis in non-irradiated lesion was not changed in metformin alone as well as radiation groups. However, the number of lung metastases tended to be reduced in the combination therapy with radiation and metformin (control vs combination=41.5±25.6 vs 29.4±10.6,p=0.08). Among the 4 treatment groups, phenotype of splenocytes showed no differences in the ratios of CD3, CD4, CD8, PD1 positive cells and Treg(CD4+CD25+FoxP3+) in CD45(+) cells. However, the ratio of PMN-MDSC(Gr-1 high Ly6c low) in CD45(+) cells was reduced only in combination therapy in splenocytes.( control vs combination=8.0±4.5% vs 4.9±1.9%, p&lt;0.05)

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.