The study is just one of dozens under way worldwide examining the drug — which costs just pennies a pill — as a treatment for breast, colon, prostate, pancreatic, and ovarian cancers.
Metformin (also known as glucophage) has been approved to control blood sugar in the U.S. since the mid-1990s, and it has been used in Europe for more than 50 years.
In the study, Mayo clinic researchers followed 61 women with diabetes taking metformin who also had ovarian cancer, and 178 ovarian cancer patients without diabetes who were not taking the drug.
Even though the metformin-takers and non-takers were matched for age, ovarian cancer stage, and other key variables likely to impact survival, the women who took the drug had improved survival.
After five years, 67% of women who took metformin had not died from ovarian cancer, compared to 47% of women without diabetes who did not take the drug. The researchers also found that among women with diabetes who took insulin or other diabetes drugs, 43% of those taking insulin and 34% of those taking other diabetes drugs had not died from ovarian cancer.
"The survival difference in the two groups was striking," says researcher Sanjeev Kumar, MBBS.
William Cliby, MD, agrees. He directs the department of gynecologic surgery at the Mayo Clinic. While it was not clear if taking the diabetes drug caused the women with diabetes to live longer, Cliby says the data certainly pointed in that direction. "Every way we looked at this, the metformin group always did better," he says.
The researchers say the findings illustrate the need for clinical trials to determine if the diabetes drug really does have a place in the treatment of ovarian cancer.
Until these trials are done, metformin is not likely to be included in the drugs now used by patients with the disease, even though it has been used by tens of millions of people and has a long safety record.
If metformin really does fight cancer, it may do so by lowering circulating insulin levels in the blood. Brawley says circulating insulin is believed to promote the growth of certain types of cancer, including breast cancer.
The breast cancer research and advocacy group Susan G. Komen for the Cure has spent $10 million on studies investigating metformin.
Chandini Portteus, who is vice president of research, evaluation, and scientific programs for the organization, says the group is sponsoring four separate clinical trials involving the drug.
"I happen to be very hopeful that metformin will prove to be a useful cancer treatment," says American Cancer Society chief medical and scientific officer Otis Brawley, MD.
Published on-line, journal Cancer, Dec. 2012