The diabetes drug metformin may lower the risk for breast cancer in postmenopausal women with diabetes, according to study results.
Metformin, a commonly used drug for treatment of type 2 diabetes, increases insulin sensitivity and improves glycemic control. Prior studies have suggested metformin may reduce breast cancer incidence, but results were mixed.
Rowan T. Chlebowski, MD, PhD, chief of medical oncology and hematology at Harbor-UCLA Medical Center and professor of medicine at David Geffen School of Medicine, and colleagues undertook this study to assess associations between diabetes, metformin use and breast cancer in postmenopausal women.
The researchers evaluated data from 68,019 postmenopausal women aged 50 to 79 years who participated in Women’s Health Initiative clinical trials. Women who already had breast cancer — as well as women who had developed diabetes before adulthood, suggesting they were type 1 diabetics — were excluded from the study.
During mean follow-up of 11.8 years, 11,290 of the patients were diagnosed with diabetes, 3,273 were diagnosed with invasive breast cancer and 754 were diagnosed with ductal carcinoma in situ.
Breast cancer incidences were confirmed by a review of central medical records and detailed pathology reports. Researchers used the Cox proportional hazards regression model to compare breast cancer incidence in women with diabetes who were metformin users or nonusers vs. breast cancer incidence in women without diabetes.
The results indicated there was no difference in breast cancer incidence among all women with diabetes and women without diabetes, the researchers said.
However, women with diabetes who were treated with metformin had lower incidences of invasive breast cancer than women who did not have diabetes (HR=0.75; 95% CI, 0.57-0.99), according to study results. Metformin users also were diagnosed with fewer ER- and PR-positive cancers, as well as fewer HER-2–negative cancers, the researchers wrote.
Women with diabetes who were treated with medications other than metformin had slightly higher breast cancer incidence than women who did not have diabetes (HR=1.16; 95% CI, 0.93-1.45).
Further studies designed to evaluate the use of metformin in the management and prevention of breast cancer — as well as metformin’s influence on breast cancer subtypes — are needed, Chlebowski and colleagues concluded.
In an accompanying editorial, Pamela J. Goodwin, MD, MSc, FRCP, professor of medicine for Mount Sinai Hospital at the University of Toronto, and colleagues called scientific understanding of the relationship between breast cancer, diabetes and diabetes treatments “a work in progress.”
“Are we making progress in understanding the relationships between diabetes, its treatment and breast cancer? Absolutely,” Goodwin and colleagues wrote. “Do we have sufficient evidence to change clinical practice in relation to either diabetes or breast cancer? Not yet.”
Disclosure: Dr. Chlebowski reports no relevant financial disclosures. Other researchers involved in the study report serving as consultants or advisers to Pfizer, Novartis and other companies, as well as stock ownership in Merck.