A landmark clinical trial finds that a hoped-for treatment for early-stage breast cancer isn’t the answer in most cases.

The international trial tested the inexpensive diabetes medication metformin and found that it did not stop or prevent the spread of the most common types of breast cancer, despite hope that it might do so.

“The results tell us that metformin is not effective against the most common types of breast cancer and any off-label use for this drug for the treatment of these common types of breast cancer should be stopped,” said study leader Dr. Pamela Goodwin. She is a medical oncologist and clinician scientist at Sinai Health Lunenfeld-Tanenbaum Research Institute in Toronto.

The randomized, double-blind trial was the largest of its kind to date, tracking more than 3,600 breast cancer patients from Canada, the United States, Switzerland and the United Kingdom.

Previous observational and preclinical studies had suggested metformin might help reduce development of some cancers and increase survival. Researchers had theorized that the drug, which is used to treat diabetes or high blood sugar, might improve patient metabolism and insulin levels, leading to reduced growth of cancer cells, or that it might affect cancer cells directly.

In the study, trial patients were treated with two pills a day. Some received metformin and others received an inactive placebo. However, adding this drug to standard breast cancer treatments did not improve outcomes for either hormone receptor-positive or -negative cancers, the findings showed.

The researchers did find one positive outcome in a less common but aggressive type of breast cancer called HER2-positive breast cancer. The investigators found evidence that taking metformin for five years might lead to a reduction in deaths in these patients. About 20% of all breast cancers are of this type.

A potential next step will be to conduct a clinical trial of metformin in patients with HER2-positive breast cancer.

“Metformin is not beneficial for use in most common breast cancers, but in the cases of HER2-positive breast cancer, our findings suggest it may be beneficial,” Goodwin said in a news release from the Lunenfeld-Tanenbaum Research Institute. “These results need to be replicated in future research before metformin is used as a breast cancer treatment, however, it could provide an additional treatment option for HER2-positive breast cancer.”

The findings were published May 24 in the Journal of the American Medical Association.

Senior investigator Dr. Wendy Parulekar said the trial illustrates the importance of international collaboration to test new treatment approaches.

The trial was run by the Canadian Cancer Trials Group (CCTG), under the umbrella of the Breast International Group network.

“The results of all phase III trials inform current treatment standards and generate hypotheses to be tested in future studies,” Parulekar said. “CCTG is grateful to all the patients and families, health care teams, granting agencies and collaborators who enabled the successful conduct of the trial.”

More information

The U.S. National Library of Medicine has more on metformin.

SOURCE: Lunenfeld-Tanenbaum Research Institute, news release, May 24, 2022

Source: HealthDay