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Metformin may not boost breast cancer survival among older women with diabetes, new research reveals

Wednesday, May 22 2013 | Comments
Evidence Grade 0 What's This?
Use of metformin does not appear to improve survival rates among older women with incident diabetes and breast cancer, according to a new study.

Using administrative health care databases in Ontario, Canada, researchers conducted a population-based study of 2,361 women aged >=66 years with recent-onset diabetes who received a breast cancer diagnosis between April 1, 1997, and March 31, 2008. The women were followed through death or March 30, 2010, whichever came first. Mean follow-up was 4.5 years.

The primary outcome was all-cause mortality, and the secondary outcome was breast-cancer specific mortality.

In total, there were 1,101 (46.6%) deaths during follow-up, including 386 (15.1%) breast cancer-specific deaths.

Despite previous studies linking metformin use with a reduction in breast cancer risk and breast cancer survival, the current study showed no statistically significant association between cumulative duration of past metformin use and all-cause or breast cancer-specific survival (hazard ratio [HR], 0.97; 95% CI, 0.92-1.02 and HR, 0.91; 95% CI, 0.81-1.03, respectively).

However, the authors noted, the "possible 9% decrease in breast cancer-specific mortality per additional year of cumulative metformin use" translates into a potential 38% decrease over 5 years.

There was a significant increase in breast-cancer specific deaths per additional year of exposure to thiazolidinediones (HR, 1.52; 95% CI, 1.02-2.26) and a significant reduction in all-cause mortality per additional year of sulfonylurea exposure (HR, 0.95; 95% CI, 0.89-0.99).

Because of the small number of women with long-term metformin exposure and the short follow-up for breast cancer-specific mortality (3.7 years), the researchers said the study might have lacked the power to detect a statistically significant outcome.

"Further work is required to examine the effect of metformin in a younger population of patients with breast cancer and diabetes as well as in a prevalent diabetic population, although careful attention will be needed to minimize indication bias in such a cohort and in nondiabetic populations," they wrote.

"A better understanding of metformin's effect on breast cancer is essential to help address the disparity in cancer outcomes between patients with and without diabetes, as well as to guide diabetes treatment strategies in this population."

The study results were published online April 30 ahead of print in the journal Diabetes Care by Lega I, et al.

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