Ovarian cancer (OC) is the most lethal gynecologic malignancy and the fifth leading cause of cancer death in the United States. Metformin, an antidiabetic drug, has been shown to decrease the risk of mortality in cancer patients, particularly those with OC. Though the drug acts through inhibition of the PI3K/mTOR/AKT pathway, which is commonly altered in OC, knockout of key proteins in the pathway does not attenuate metformin’s antidiabetic effects. Further, intravenous metformin administration is less effective than oral, implying that some crucial interaction takes place in the gut. The gut is home to some 1013 bacterial cells, collectively known as the microbiota. These bacteria play important roles in fermentation of fiber, producing short-chain fatty acids (SCFA) that have positive health benefits, including prevention of metabolic syndrome, bowel disorders, and certain cancers. Mechanistically, SCFAs inhibit the PI3K/mTOR/AKT pathway, similar to metformin. Conditions such as obesity, metabolic syndrome, and diabetes significantly alter gut microbiota composition, possibly leading to a reduction in beneficial SCFAs. However, in type II diabetes, treatment with metformin leads to an increase in SCFA-producing bacteria, suggesting that metformin may positively influence the microbiota to improve patient metabolic profiles. In OC, metformin may play a similar role in promoting the presence of SCFA-producing bacteria. We hypothesized that metformin increases the abundance of SCFA-producing bacteria in the gut, with which it acts synergistically to decrease growth of OC cells in combination with OC chemotherapy. To investigate our hypothesis, we have employed the OC patient-derived xenograft (PDX) mouse model established at Mayo Clinic. Immunocompromised SCID mice received OC cells from Mayo Clinic patients and the tumors were allowed to grow to an appropriate size before treatment was started. Cage mates were split into one of four treatment arms: a control (no treatment) group, metformin only, carboplatin/paclitaxel (CP) chemotherapy only, or CP with metformin. Mice received 2mg/ml metformin in their water continuously and/or 51mg/kg carboplatin and 15mg/kg paclitaxel intravenously weekly. Stool was collected weekly prior to tumor injection, during tumor growth, and through the four weeks of treatment. DNA was extracted from stool and bacterial 16S rRNA amplicon sequencing was used to determine microbiota profiles. Mice with chemosensitive tumors treated with only CP chemotherapy had significant reductions in tumor growth along with significantly less microbial gut diversity than pretumor and pretreatment mice. Chemosensitive mice treated with metformin alone did not lose significant microbial diversity but did not have significant reductions in tumor growth as compared to the control mice. CP in combination with metformin both reduced tumor growth and protected against loss of microbiota diversity. Further, mice treated with only metformin had significant increases in abundance in the Firmicutes genera Ruminococcus and Oscillospira, SCFA producers that are associated with leanness and health. Mice treated with both metformin and CP had significant increases in another SCFA-producing Firmicute, Moryella. Finally, mice with chemoresistant tumors showed improved tumor growth inhibition with combination CP/metformin therapy, suggesting that metformin-associated changes in the gut microbiota may increase sensitivity to CP. These data support the hypothesis that metformin protects against detrimental loss of gut microbiota diversity induced by chemotherapy treatment, enriching beneficial SCFA producers. This work implies a role for the gut microbiota in OC treatment and provides support for metformin’s potential use as a beneficial addition to chemotherapy treatment.

Citation Format: Dana M. Walsh, Ismail Mert, Chen Jun, Xiaonan Hou, Saravut John Weroha, Marina Walther-Antonio. Metformin alters the gut microbiota of ovarian cancer patients treated with carboplatin/paclitaxel chemotherapy and enhances sensitivity in resistant tumors. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr A70.