Researchers have found that the diabetes drug metformin, once hoped to hold enormous promise in treating breast cancer, does not prevent or stop the spread of the most common forms of the disease but may still have potential in HER2-positive breast cancer.
The randomised, double-blind trial enrolled 3600 patients who received two pills a day of either placebo or metformin. Overall, researchers found the addition of metformin to standard breast cancer treatments did not improve outcomes in the two most common types of breast cancer, hormone receptor-positive or negative.
“The results tell us that metformin is not effective against the most common types of breast cancer and any off-label use of this drug for the treatment of these common types of breast cancer should be stopped,” said Pamela Goodwin, a professor in the department of medicine at the University of Toronto’s Temerty Faculty of Medicine.
Prof Goodwin presented the findings at the 2021 San Antonio Breast Cancer Symposium.
While metformin was found not to be effective in treating the most common forms of breast cancer, there was evidence that use of metformin for five years might lead to a reduction in deaths from HER2-positive breast cancer, a less aggressive subtype which makes up about 20% of all breast cancers.
“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,” said Prof Goodwin. “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,” she added
Previous studies suggested metformin may also reduce the risk of development and increase survival of some cancers, including breast cancer.
Metformin was theorised to slow breast cancer growth by improving patient metabolism, notably insulin levels, leading to reduced cancer cell growth, or that it might impact cancer cells directly.
Next steps would be to prospectively test the impact of metformin in patients with HER2-positive breast cancer in a randomised clinical trial.
Source: University of Toronto
