Effect of metformin on survival outcomes in diabetic patients with triple receptor-negative breast cancer

Background: Recent observational studies have shown that metformin use in diabetic patients decreases both cancer incidence and mortality. Metformin use is also independently predictive of pathologic complete response. In the current study, the authors explored the association between metformin use and survival outcomes in patients with triple receptor-negative breast cancer (TNBC) who were receiving adjuvant chemotherapy. Methods: The Breast Cancer Management System database of The University of Texas MD Anderson Cancer Center identified 1448 women who received adjuvant chemotherapy for TNBC between 1995 and 2007. Patients were categorized by diabetes status and metformin use. The Kaplan-Meier product-limit method was used to calculate distant metastasis-free survival (DMFS), recurrence-free survival (RFS), and overall survival (OS). Cox proportional hazards models were fit to determine the association between metformin use and survival outcomes. Results: The study cohort was comprised of 63 diabetic patients receiving treatment with metformin, 67 diabetic patients not receiving metformin, and 1318 nondiabetic patients. Patients in the diabetic groups tended to be older (P =.005); more diabetic patients were postmenopausal (P =.0007), black (P =.0001), and obese (P <.0001). At a median follow-up of 62 months, there were no significant differences with regard to 5-year DMFS (P =.23), RFS (P =.38), and OS (P =.58) between the 3 groups. Compared with the metformin group, patients who did not receive metformin (hazard ratio [HR], 1.63; 95% confidence interval [95% CI], 0.87-3.06 [P =.13]) and nondiabetic patients (HR, 1.62; 95% CI, 0.97-2.71 [P =.06]) tended to have a higher risk of distant metastases. Conclusions: The findings of the current study suggest that metformin use during adjuvant chemotherapy does not significantly impact survival outcomes in diabetic patients with TNBC.