Possible HER2 advantage
However, the exploratory analysis suggested there may be an advantage for patients with HER2-positive disease, but primarily those who had at least one C allele of a prespecified ATM associated rs11212617 SNP. These patients achieved a higher pathologic complete response rate with metformin than that of those without the allele.
There were 620 patients with HER2-positive disease analyzed, with 99.4% receiving chemotherapy and 96.5% receiving trastuzumab. There were 99 IDFS events, and 47 OS events.
In the entire HER2-positive cohort, patients who received metformin had fewer IDFS events (HR, 0.64; P = .026) compared with the placebo arm. Mortality was lower with metformin (HR for overall survival, 0.53; P = .038).
“Subjects with HER2-positive breast cancer, notably those with at least one C allele of the ATM-associated rs11212617 SNP, experienced improved IDFS and overall survival with metformin,” Dr. Goodwin concluded. “However, no P-value ‘spend’ was allocated to this comparison. As a result, it requires replication in a prospective trial focusing on the HER2-positive population.”
More research?
Stephanie Bernik, MD, chief of breast surgery, Mount Sinai West, and associate professor of breast surgery, Icahn School of Medicine at Mount Sinai, New York, was approached by this news organization for an independent comment.
“It has long been known that obesity, which often correlates with diabetes, increases a woman’s risk of breast cancer,” she said.
“This study tried to show that using a medication that helps control insulin levels, even in those without diabetes, might decrease one’s risk of breast cancer,” she said. “Unfortunately, using metformin had no effect on outcomes in this study, even though it has shown promise in other studies. Perhaps more research needs to be carried out to try to pinpoint which mechanisms of action, if any, might be helpful to combat cancer in those with and without diabetes.”
The study was funded by the Canadian Cancer Trials Group, Cancer Therapy Evaluation Program, Breast Cancer Researcher Foundation, Susan G. Komen for the Cure, Canadian Cancer Society, Apotex, Swiss Cancer Research, and the Canadian Breast Cancer Foundation. Dr. Goodwin has no disclosures.
A version of this article first appeared on Medscape.com.