Metformin in diabetes linked to lower breast cancer risk

In women with type 2 diabetes, long-term metformin (Glucophage, Merck Serono) use is associated with a lower risk of breast cancer, according to research published in the June issue of Diabetes Care, as reported by HealthDay News.

Michael Bodmer, MD, of University Hospital Basel, Switzerland, and colleagues analyzed data from breast cancer cases and controls drawn from 22,621 women with type 2 diabetes who used oral antidiabetes drugs. The 305 cases had a mean age of 67.5 at cancer diagnosis, and were matched to 1,153 controls without cancer.

The researchers found that the long-term use of at least 40 prescriptions of metformin, which exceeded 5 years’ worth, was associated with a lower risk of breast cancer compared with those not taking the drug (adjusted odds ratio, 0.44). Short-term metformin use and use of sulfonylureas or other antidiabetes drugs were not associated with a substantially different cancer risk.

“We did not find any alteration of breast cancer risk in relation to short-term use of metformin or any other use of another oral antidiabetic drug,” Christoph R. Meier, PhD, of the Basel Pharmacoepidemiology Unit at the University of Basel, told Formulary. “The findings fit in well with previous animal models and studies in humans. However, observational studies can provide evidence, but they cannot prove a causal association between exposure (metformin) and outcome (breast cancer).”

According to the authors, several clinical studies in pre- and postmenopausal women with or without type 2 diabetes revealed a significantly higher risk of breast cancer in association with hyperinsulinemia and insulin resistance.

“Furthermore, high fasting insulin levels and obesity have been associated with poor cancer-related outcome. Both the reduction of hyperinsulinemia and growth inhibition via AMP-activated protein kinase activation might explain why metformin therapy seems to be associated with a decreased risk of breast cancer,” they wrote. “This has been hypothesized by various authors and is supported by the current findings.”

Dr Meier said the increasing body of literature suggests that metformin may be an interesting drug as an add-on therapy for breast cancer.

“We are now waiting for the results of ongoing clinical trials investigating the potential role of metformin as an add-on therapy to standard chemotherapy,” he pointed out. “Whether metformin is indeed beneficial for prophylaxis of breast cancer clearly needs further studies at this point in time. However, since these studies need a large number of included subjects for a long period of time (our results suggest years of treatment), such trials are difficult to accomplish.”

The study was funded by a grant from Europe-based Merck Serono, which manufactures metformin.