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Abstract Metformin, an anti-diabetic drug, is known to have anti-tumor effects. We examined the outcome of 177 patients with type 2 diabetes mellitus (T2DM) who received surgery for breast cancer. Among them, 49 patients were treated with drugs including metformin. In those patients, recurrence in distant organs was less frequent and postoperative disease-free survival (DFS) tended to be better than those without metformin intake. In patients who received preoperative systemic therapy (PST), rate of pathological complete response (pCR) was significantly more frequent in patients with metformin treatment (p 0.05). Multiplex immunohistochemical staining of resected tumors revealed that the density of tumor associated macrophages (TAM), especially of CD68(+)CD163(+) M2-type TAM, was significantly lower in tumors with metformin treatment. In contrast, the rate of CD8(+) phenotype in CD3(+) tumor infiltrating lymphocytes (TILs) was significantly higher in metformin group. The results suggest that metformin can change the immune microenvironment from pro-tumorigenic to anti-tumorigenic status, which leads to the favorable outcome of the patients with breast cancer and T2DM. Citation Format: Satomi Shiba, Michiko Harao, Kasumi Ogihara, Takayo Fukuda, Masako Sakuragi, Kitayama Joji, Naohiro Sata. Metformin may improve the outcome of patients with breast cancer and type 2 diabetes mellitus through the effect of tumor immune microenvironment abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-08-06.
Shiba et al. (Thu,) studied this question.