The Effect of Metformin on Survival Outcomes of Non-Metastatic Breast Cancer Patients with Type 2 Diabetes

BACKGROUND: There are still inconsistencies about the role of metformin on breast cancer. This study was designed to assess metformin’s effect on the prognosis of female breast cancer patients with type II diabetes. METHODS: The present research was carried out as a retrospective cohort study between 2003 and 2014. Breast cancer patients with pre-existing type II diabetes mellitus were included. Overall survival (OS) and relapse-free survival (RFS) were measured as the main endpoints. Kaplan-Meier estimate was used to calculate survival rates. RESULTS: 217 patients were included with a mean age of 53.32±11.10 years. 148 (68.2%) patients were prescribed metformin and 69 (31.8%) took other antidiabetic drugs (non-metformin group). Five-year OS and RFS rates for all patients were 82.5% (95% CI: 76.0%-87.4%) and 71.1% (95% CI: 64.2%-77.0%) respectively. Log-rank test showed that the metformin group had a significant advantage over the non-metformin group in terms of both OS and RFS rates (P<0.001 for both). Five-year OS and RFS rates for metformin group were 91.9% (95% CI: 85.4%-95.6%) and 82.8% (95%CI: 75.5%-88.2%) respectively; the same rates for non-metformin group were 59.1% (95% CI: 43.9%-71.5%) and 39.3% (95%CI: 25.1%-53.1%) (P<0.001 for both). Results of proportional hazards model, after adjustment for body mass index, age, and tumor stage, depicted an independent prognostic value of metformin use with multivariate hazard ratio of 0.15 (95% CI: 0.07-0.32) for OS and 0.23 (0.14-0.40) for RFS compared to non-metformin group (P<0.001 for both). CONCLUSION: This study indicated that using metformin for diabetic breast cancer patients is associated with favorable results regarding recurrence and survival rates.