SATURDAY, March 31 (HealthDay News) -- A diabetes medication used by millions is now showing promise against a variety of different cancers.

Two new clinical studies to be presented Saturday at the annual meeting of the American Association for Cancer Research, in Chicago, found that metformin (also known by the brand name Glucophage, among others), appeared linked to a slowing in the rate of prostate cancer growth in certain patients, and in prolonging life for early-stage pancreatic cancer patients.

Other studies, done either in the lab or in animals, also hint that the drug might have an effect against liver or oral tumors, as well as certain forms of melanoma.

The findings have sparked interest in the cancer field and do seem promising, but much more research needs to be done before the drug can be recommended as a cancer treatment, experts said.

"There are very exciting clues from laboratory studies and population studies that metformin . . . may improve cancer outcomes or lower cancer risk," said Dr. Michael Pollak, professor of oncology and of medicine at McGill University in Montreal. "However, we need more laboratory and clinical studies to find the best dose to use, to understand in what disease situations it may help most, and also to determine if metformin itself or a metformin derivative would be most suitable for trials."

Metformin has also shown promise against colon and breast cancer, noted Pollak, who is a co-author on the prostate cancer trial.

For that trial, 22 men with prostate cancer received 500 milligrams of metformin three times a day after their diagnosis but before they were scheduled to undergo removal of the prostate gland, a procedure known as prostatectomy.

After an average treatment time of 41 days, men taking metformin showed a slowing in the growth of cancer cells in the prostate after it had been removed versus in the earlier biopsy samples, said study lead author Dr. Anthony Joshua, a staff medical oncologist with Princess Margaret Hospital/University Health Network in Toronto.

Not surprisingly, metformin also decreased blood sugar levels, insulin growth factor and body mass index (BMI, a measure of obesity).

None of the men in the study had diabetes, said Joshua, so "it remains to be seen who would benefit the most from metformin." The most appropriate patients may be those with diabetes, those who are at risk for the disease or those whose tumors are sensitive to metformin.

It's unclear exactly how metformin exerts its effect but it may reduce the amount of circulating insulin in the blood, and insulin can fuel the growth of prostate cancer cells, explained Joshua.

The drug may also interfere with a specific pathway linked to cancer growth, he added.

The results build on prior research done in the laboratory but are the first to be seen in humans. Experts note, however, that research presented at medical meetings is typically considered preliminary until published in peer-reviewed journals.

In another study, researchers at the University of Texas M.D. Anderson Cancer Center in Houston, reviewed records of 302 patients who had both diabetes and pancreatic cancer, two conditions that often go hand-in-hand. A total of 117 patients were taking metformin.

About 30 percent of those who had taken the drug were alive after two years, compared with 15.4 percent of those who had not taken metformin (the "control" group).

And patients on metformin lived an average of just over 15 months versus about 11 months for the control group, translating into a 32 percent reduced risk of dying.

But the survival benefit was seen only in patients whose cancer had not yet spread. The study was published in the journal Clinical Cancer Research.

"I think it's very hopeful," said Dr. Michael Pishvaian, assistant professor of hematology/oncology at Georgetown Lombardi Comprehensive Cancer Center in Washington, D.C. "Metformin may help keep blood sugar levels down [which improves prognosis for cancer patients] but it may also work in a number of different ways to combat cancer cell growth."

The next step is to test the concept in a prospective fashion, said Pishvaian, who was not involved with the study.

Three other studies, also released Saturday, suggest that metformin might be active against other cancer types:

• Researchers at the University of Maryland School of Medicine report that metformin appeared to slow the growth of liver tumors in mice.
• A team at the U.S. National Institute of Dental and Craniofacial Research said that, in mouse studies, metformin appears to reduce the number and size of oral cancer lesions.
• Research conducted at the Paterson Institute for Cancer Research in Manchester, England, finds that combining metformin with certain cancer drugs worked better to suppress a certain type of melanoma versus using the cancer drugs alone.

The liver cancer and oral cancer studies were published March 31 in Cancer Prevention Research, while the melanoma study appears in the March 31 issue of Cancer Discovery.

More information

The U.S. National Cancer Institute has more on prostate cancer.