A new study has reported that metformin, a drug used to treat type 2 diabetes, inhibits breast cancer in a mouse model of aggressive breast cancer, especially in mice fed a high calorie diet. The study was designed to evaluate the potential impact of calorie availability and metformin on tumor growth and metastasis. Calorie restriction has been shown to inhibit both the development and subsequent growth of mammary tumors in animal studies.

Metformin has long been safely used to increase insulin sensitivity in people with type 2 diabetes and has also been shown to reduce risk of cancer and cancer-related death in humans. Metformin mimics some aspects of caloric restriction in the body. In the study, mice were implanted with triple negative cancer cells and then divided into groups who were given either (1) a diet with restricted calories; (2) a standard maintenance diet (control diet); or (3) a high calorie diet with high sugar content. The diets were tested in conjunction with metformin for their effects on mammary tumor growth and metastases to the lung.

Metformin was found to be effective in suppressing insulin like growth factor-1 (IGF-1), insulin and glucose, especially in the animals who were given a high calorie diet. Long-term metformin treatment was found to produce moderate but significant reductions in mammary tumor growth, again most significantly in combination with the high energy diet. Compared to the standard maintenance diet, the high calorie diet was found to promote tumor growth, increase expression of the inflammatory adipokines leptin and resistin, and promote metastasis. However, metformin had no effect on inflammatory adipokine expression or the development of lung metastases. The authors conclude that metformin may have tumor suppressing activity in the presence of a combination of high calorie intake, metabolic syndrome, and diabetes, but may have little or no effect on metastasis driven by inflammation.