Diabetes Drug May Lower Breast Cancer Risk
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A diabetes drug may reduce the risk of breast cancer in some women, a new study suggests.

In the study, postmenopausal women with diabetes who took the drug metformin to control their blood sugar were 25 percent less likely to have breast cancer after 11 years, compared with diabetes-free women who did not take the drug.

Moreover, diabetic women in the study who took drugs other than metformin were slightly more likely to have breast cancer, compared with diabetes-free women of a similar age. (Studies have suggested a generally increased risk of breast cancer among women with diabetes, though not all research has shown this to be the case.)

The findings held even after the researchers took into account factors that could affect the participants' risk of breast cancer, such as their body mass indexes (BMI), physical activity levels, and how frequently they received mammograms.

The study adds to a growing body of research showing that the drug may have anti-cancer properties. A few previous studies have noted lower rates of breast cancer and pancreatic cancer among women with diabetes who take the drug. And studies in animals and on cells in lab dishes have suggested the drug may inhibit cancer cell growth.

However, the new study found an association, and not a cause-effect link, and so more research is needed to confirm the findings. Results from an upcoming trial sponsored by the National Cancer Institute will be part of that evidence, said Dr. Iuliana Shapira, an investigator at the Feinstein Institute for Medical Research in Manhasset, N.Y., who was not involved in the study. In the trial, women with early stage breast cancer, who don't have diabetes, will be randomly assigned to receive either metformin or a placebo, and will be followed for five years, Shapira said.

In the new study Dr. Rowan Chlebowski, of the UCLA David Geffen School of Medicine, and colleagues analyzed information from more than 68,000 postmenopausal women who participated in the ongoing study called the Women's Health Initiative, and were followed for an average of 11.8 years.

Over the study period, 11,290 women were diagnosed with diabetes, and 3,273 were diagnosed with invasive breast cancer.

It's not clear how metformin might reduce breast cancer risk, but the drug is known to increase the activity of an enzyme called AMP Kinase, Shapira said. This enzyme is an "energy sensor" for cells, Shapira said, and an increase in its activity may cause cancer cells to die by committing cellular suicide.

Metformin is not prescribed only for diabetes, it is also given to women with a condition known as polycystic ovary syndrome. A goal of future research is to see whether the drug could be given to women at risk for breast cancer as a way to prevent the disease, Shapira said.

The study was published on June 11 in the Journal of Clinical Oncology.

Pass it on:  Taking metformin is associated with a reduced risk of breast cancer.

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