Abstract 3191: The effect of metformin on the tumor immune microenvironment
Authors: Saito Akira , Ohzawa Hideyuki , Kaneko Yuki , Tamura Kohei , Futoh Yurie , Takahashi Kazuya , Kimura Yuki , Tojo Mineyuki , Kumagai Yuko , Miyato Hideyo , Sata Naohiro , Kitayama Joji 
Published: July 2021

Background: Although metformin (Met) is well known to have anti-tumor effect, the detailed mechanisms remain unknown. Tumor-associated macrophages (TAMs) play pivotal roles to regulate tumor cell behavior in tumor microenvironment. Here, in this study, we investigated the effect of Met on TAMs and its effect on the prognosis of cancer patients.

Methods: CD14(+) monocyte purified from healthy donor were cultured with M-CSF for 5 days followed by additional 2 day-culture with IL-10+IL-4 with or without Met, and their functions were examined in vitro. Also, we selected the patients with diabetes and colorectal cancer (CRC) who received curative surgery in our Department and examined the impact of Met intake on patient outcome. Moreover, expression of macrophage and lymphocyte markers were immunohistochemically examined in surgically removed 40 CRC tissues derived from patients who had taken Met or not for diabetes. In addition, stromal fibrosis was evaluated with Masson's Trichrome staining.

Results: Monocytes stimulated with IL-10+IL-4 highly expressed CD163, CD206 and PD-ligands and potently suppressed T cell proliferation stimulated by anti-CD3 mAb, which were greatly inhibited by Met (500μM&lt;). A total of 1781 patients underwent curative colectomy from January 2009 to June 2019 in Jichi Medical University Hospital. Among them, 267 patients had type 2 diabetes mellitus at the time of surgery, and 53 patients were treated with drugs including Met. The postoperative disease-free survival rate was significantly improved in the Met intake group compared with the non-intake group. In immunostaining the density of CD3 (+) and CD8 (+) TIL was significantly higher in Met intake than in non-intake group (CD3: 146.6/HPF vs 121.9/HPF, p &lt;0.05, CD8: 99.2/HPF vs 59.9/HPF, p &lt;0.01) and CD8/CD3 significantly increased. (74.9% vs 50.7%, p &lt;0.01). On the other hand, the density of CD68 (+) TAM was significantly higher in the Met intake group (CD68: 101.1/HPF vs 81.2/HPF, p &lt;0.01), but that of CD163 (+) M2 type TAM tended to be lower (CD163: 57.0/HPF vs 66.3/HPF, p = 0.1), and the CD163 (+)/CD68 (+) ratio was markedly reduced (56.2% vs 77.8%, p &lt;0.01). In addition, the Fibrosis score tended to decrease in the Met intake group (p = 0.051).

Conclusion: Met suppresses density of M2-macrophages and stromal fibrosis and increases the number of tumor-infiltrating T cells in tumor microenvironment, which might lead to the improvement of patient outcome.

Citation Format: Akira Saito, Hideyuki Ohzawa, Yuki Kaneko, Kohei Tamura, Yurie Futoh, Kazuya Takahashi, Yuki Kimura, Mineyuki Tojo, Yuko Kumagai, Hideyo Miyato, Naohiro Sata, Joji Kitayama. The effect of metformin on the tumor immune microenvironment [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 3191.