By Kathleen Doheny
A study of nearly 4,000 diabetic men found that those taking metformin when diagnosed with prostate cancer were less likely to die of the cancer or other causes compared to men using other diabetes drugs.
"We demonstrated that metformin is associated with improved survival among diabetic patients with prostate cancer," said Dr. David Margel, a uro-oncologist at Rabin Medical Center in Petah Tikva, Israel, who conducted the research while at the University of Toronto.
"It's associated in a dose-response manner," he said. "The longer you were on metformin, the less likely you were to die of prostate cancer and of all causes."
Diabetes and prostate cancer are common in the United States. This year, about 239,000 new cases of prostate cancer will be diagnosed, and more than 29,000 men will die from it, according to the American Cancer Society.
Type 2 diabetes is rampant, and metformin is the drug most commonly prescribed to treat it. More than 61 million metformin prescriptions were filled in the United States last year. Brand names include Glucophage and Glumetza. The drug, in its generic forms and certain brand names, is relatively inexpensive.
Previous research has focused on whether metformin might reduce the risk of getting prostate cancer, but most studies were negative. Some experts believe the drug instead works to improve survival once the cancer occurs.
In the new study, published online Aug. 5 in the Journal of Clinical Oncology, Margel looked at more than 3,800 diabetic men aged 67 or older who lived in Ontario. About one-third were taking metformin at the study's start. Others were using different diabetes drugs.
The men took the metformin for a median of 19 months (half longer than that, half shorter) before the cancer was diagnosed and nearly nine months after.
During roughly four years of follow-up, Margel found those who took metformin had a 24 percent reduction in risk from prostate cancer death for every additional six months of use after their cancer diagnosis. The risk reduction of death from other causes was initially the same but declined over time.
In both instances, although an association was found between metformin and survival, a direct cause-and-effect relationship was not established.
No reduction in death risk was seen for patients taking any other diabetes drug.
Although other diabetes drugs work by increasing the body's insulin production, metformin is an "insulin sensitizer" that works by making the body more sensitive to the insulin already produced. Insulin is needed to move glucose into cells for energy.
Some research suggests that high insulin levels can influence cancer growth. Metformin, by not increasing the body's insulin production, may decrease cancer cells' growth, some experts say.
In their next study, the researchers plan to test metformin in patients with prostate cancer but not diabetes. "Metformin is very safe to use among nondiabetic patients," Margel said.
The findings point to a need for a large study in which men with early stage prostate cancer are assigned to a metformin group or placebo group, one expert said. Writing in an accompanying journal editorial, Kathryn Penney, an instructor in medicine at Brigham and Women's Hospital in Boston, said at least nine ongoing trials are looking at metformin in men with recurrent or advanced prostate cancer.
But these current trials might be starting too late, she said. Instead, a trial should look at metformin's effect at the time of diagnosis, when the disease is typically in early stages.
"If this trial showed a benefit, then yes, men without diabetes could be put on metformin at the time of prostate cancer diagnosis," she said.