Epidemiology studies have indicated that the administration of Metformin, a drug commonly used in the therapy of type 2 diabetes, is associated with reduced incidence and severity of several types of cancers. Interestingly, in most reports the effects of Metformin were observed in sites of neoplasia usually embedded in the white adipose tissue (WAT) such as the breast, the digestive tract, the pancreas and the prostate. We have reported that Metformin is active in preclinical models of breast cancer and can target neoplastic, endothelial and other tumor-associated WAT microenvironment cells. In our previous studies in orthotopic models of breast cancer, Metfomin reduced local and metastatic tumor progression, reduced tumor microvessel density and altered the vascular pericyte/endothelial cell ratio, so that cancer vessels showed a dysplastic phenotype (Orecchioni et al, 2015). In these orthotopic breast cancer models we have more recently found that Metformin can also inhibit GM-CSF release from WAT progenitors, so that circulating GM-CSF levels, proposed to regulate WAT-embedded myeloid suppressor cells, are reduced. Few months ago Eikawa et al (PNAS 2015) have shown that Metformin enables normal but not T-cell-deficient mice to reject solid tumors, likely by increasing CD8+ PD-1-negative tumor-infiltrating lymphocytes (TILs) and protecting them from apoptosis and exhaustion. We report here that in immune competent, orthotopic models of triple negative breast cancer (4T1) and B-cell lymphoma (A20), treatment with Metfomin significantly reduced the number of intratumoral CD8+PD-1+ TILs, and of CD4+CD25brightCD127-low/neg intratumoral regulatory T cells (Tregs). In these models, anti-PD-L1 treatment was more effective in reducing tumor size when administered in the presence of concomitant Metformin treatment. Conversely, prolonged administration of Metformin after anti-PD-L1 treatment increased tumor growth. These effects of Metformin were observed only in intratumoral TILs and intratumoral Tregs, as the number of circulating CD8+PD-1+ T cells and Tregs were not reduced by Metformin administration. Taken together, our data indicate that, in addition to the direct effect on cancer cells, Metformin has a complex activity also on microenvironment cells and on the immune response against neoplasia. Further studies are ongoing to define a) whether the effects of Metformin on immune cells are related to the canonic targets of Metformin in cancer cells, namely AMPK and the Complex 1 of the respiratory chain, and b) the most effective schedule of administration of metformin and checkpoint inhibitors.

Citation Format: Stefania Orecchioni, Giovanna Talarico, Patrizia Mancuso, Valentina Labanca, Francesca Reggiani, Francesco Bertolini. Metformin reduces intratumoral CD8+PD-1+ and Treg lymphocytes in orthotopic models of breast cancer and lymphoma, and has paradoxic effects on anti-PD-L1 treatment. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4003.