Metformin and survival in diabetic patients with breast cancer.

BACKGROUND: Recent population studies provide clues that the use of metformin may be associated with reduced incidence and improved prognosis of breast cancer. This drug is widely used in the treatment of type 2 diabetes, where it is often referred to as an 'insulin sensitizer' because it not only lowers blood glucose, but also reduces the hyperinsulinemia associated with insulin resistance. Metformin inhibits the growth of cancer cells in vitro and tumors in vivo. However, there are little clinical data to support this.
OBJECTIVE: The aim of the study was to determine whether metformin use was associated with better clinical outcomes in diabetic patients with breast cancer receiving adjuvant chemotherapy. PATIENTS AND METHODS: In this retrospective study, we reviewed the records of 460 consecutive patients with pathologically proved stage I-III breast cancer. These patients received adjuvant chemotherapy during the period from January 2008 to December 2008. Patients were compared by groups that consisted of 25 diabetic patients taking metformin, 14 diabetic patients not taking metformin, and 400 nondiabetic patients.
RESULTS: In Kaplan-Meier analysis, metformin use during adjuvant chemotherapy significantly improved survival outcomes in diabetic patients' disease-free survival and overall survival. In multivariate Cox regression, metformin usage was a predictive factor that decreased the risk for breast cancer mortality.
CONCLUSION: There is an association between metformin use and better survival outcome in diabetic breast cancer patients who received metformin during adjuvant chemotherapy. The choice of antidiabetic pharmacotherapy may influence prognosis of this group.