Type 2 diabetes ("diabetes"), a disease characterized by insulin resistance and abnormally high levels of glucose in the blood, is associated with poorer survival among women with breast cancer. One study reported that breast cancer patients with high fasting blood sugar levels at diagnosis were almost twice as likely to experience a recurrence as patients with normal levels. Metformin has long been safely used to increase insulin sensitivity in people with type 2 diabetes and has also been shown to reduce the risks of cancer and cancer-related death. Generally speaking, diabetics who use metformin have better outcomes than those who do not. On the other hand, there is some evidence that the use of insulin (especially long-acting types such as Lantus), or sulfonylurea derivatives such as Glipizide, could reduce breast cancer-specific survival, although these findings are controversial. Now a new study has reported that the risks of breast cancer recurrence and death among women enrolled in Medicare are lowest in those using metformin.

Metformin and breast cancer

Metformin reduces the production of glucose in the liver, enhances the uptake of glucose in muscles, and reduces the level of circulating glucose. Metformin also reduces circulating insulin levels and insulin like growth factor-1 (IGF-1), increases insulin sensitivity, and reduces insulin resistance-associated excess levels of circulating insulin in the blood. High fasting insulin appears to be an independent risk factor for poor outcomes in women with breast cancer.

Most studies that have examined the association have reported that diabetic women taking metformin have a lower risk of breast cancer than those not treated with metformin. One study reported that perimenopausal and postmenopausal women treated with metformin for type 2 diabetes were 23% less likely to develop breast cancer than similar diabetic women not on metformin. Metformin reduces cancer cell proliferation, including breast cancer stem cell proliferation, in those with insulin resistance.

Metformin mimics some aspects of caloric restriction in the body. Metformin has been shown to induce more favorable responses to chemotherapy and radiotherapy. Metformin has been shown to improve survival of diabetic women with HER2/neu overexpressing (HER2+) breast cancer and suppress metastasis of triple negative (ER-/PR-/HER2-) breast cancer cells.

Latest research finds metformin reduces recurrence and death

The study referenced above was designed to investigate the influence of diabetes treatment on breast cancer relapse and survival. To conduct the study, the authors used data from the Surveillance, Epidemiology and End-Results (SEER)-Medicare database. The study included 14,766 women aged 66 to 80 newly diagnosed with stage I or II breast cancer during the period 2007 to 2011, of whom 627 experienced a recurrence and 237 died from breast cancer during the study period. Medicare Part D claims data was used to obtain information concerning diabetes-related medications. Metformin was used by 2,558 of the study population.

Treatment with metformin was found to be associated with 31% lower risk of recurrence and 49% lower risk of breast cancer-specific death. On the other hand, use of sulfonylureas was associated with 1.49 times higher risk of breast cancer-specific death and use of insulin was associated with 2.58-fold higher risk of death from breast cancer. The authors comment that further research may be justified to determine whether metformin (1) is a preferred treatment for diabetes among breast cancer survivors; and (2) is beneficial for breast cancer patients without diabetes.

Please see our article on type 2 diabetes for more information.