Diabetes Drug Reduces Breast Cancer Incidence

Metformin is an inexpensive, well-tolerated drug that is prescribed to patients with type 2 diabetes to control blood glucose levels.  Several preclinical studies suggest that metformin can slow the growth of breast cancer cells grown under laboratory conditions.  These findings, along with results from observational studies, provided the impetus for studying the effect of metformin in the clinical setting.  A recent report published in the Journal of Clinical Oncology suggests that metformin reduces breast cancer incidence in postmenopausal women with diabetes.

Using data collected from 68,019 postmenopausal women enrolled in the Women’s Health Initiative clinical trials, researchers studied the relationship between breast cancer, diabetes, and metformin.  Over the course of the study, 11,290 women were diagnosed with diabetes and 3,273 were diagnosed with invasive breast cancer.  As a whole, women with diabetes were just as likely to be diagnosed with invasive breast cancer as women without diabetes.  However, when the diabetes group was stratified by the type of medication they took, and interesting result emerged.  Diabetic women who took metformin were 25% less likely to be diagnosed with invasive breast cancer than women without diabetes.  On the other hand, diabetic women who took medications other than metformin had a slightly higher risk of being diagnosed with invasive breast cancer, compared to women without diabetes.  Importantly, the frequency of screening mammograms was controlled in this study, and results were adjusted for weight loss.

A randomized-controlled trial is currently being conducted to test the effects of metformin in non-diabetic women with early stage breast cancer.  The study will determine whether taking two metformin pills for five years reduces recurrence or has an impact on overall survival.  Listen to Pamela Goodwin, MD discuss this trial here.