(ChemotherapyAdvisor) – In postmenopausal women with diabetes, use of oral metformin was associated with lower incidence of invasive breast cancer, according to results reported from the Women’s Health Initiative (WHI) clinical trials in the Journal of Clinical Oncology online June 11.

Of the 68,019 postmenopausal women included in the WHI, 3,401 had diabetes at study entry; diabetes incidence status was collected throughout the follow-up period. Over a mean of 11.8 years, 3,273 cases of invasive breast cancer were diagnosed. Medication information was collected at baseline and years 1, 3, 6, and 9, the investigators noted.

Incidence of breast cancer in women with diabetes vs those without diabetes was found to differ by type of diabetes medication (P=0.04). “Women with diabetes receiving medications other than metformin had a slightly higher incidence of breast cancer (HR 1.16), and women with diabetes who were given metformin had lower breast cancer incidence (HR 0.75),” they wrote. This association was observed both for estrogen receptor positive and progesterone receptor positive cancers as well as those negative for human epidermal growth factor receptor 2.

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“Our findings are of most direct relevance to women with diabetes, most of whom were overweight or obese,” the authors concluded. “However, consideration of the totality of available evidence does provide support for the ongoing clinical studies of metformin, including a prospective, full-scale, multicenter adjuvant trial and proof of principal studies in prevention settings.”