Potential Intrinsic Subtype Dependence on the Association Between Metformin Use and Survival in Surgically Resected Breast Cancer: A Korean National Population-Based Study.

PURPOSE/OBJECTIVE(S) Numerous studies have suggested that metformin treatment can increase breast cancer survival; however, it is unclear whether its effects interact with intrinsic subtype or diabetic status. Therefore, we conducted a large nationwide study to assess this in women with surgically resected invasive breast cancer. MATERIALS/METHODS Patients with newly diagnosed breast cancer between 2007 and 2016 were identified using the national health insurance claims database of South Korea. Metformin or other drug exposure was defined as medication for ≥ 90 days. Breast cancer subtypes were classified into four groups based on hormonal therapy and anti-HER2 treatment. RESULTS A total of 117,333 patients were included (median follow-up duration, 90 months). Type 2 diabetes mellitus (T2DM) affected significantly overall survival (OS, 7-year, 89.7% vs. 92.4%, P < 0.001). A significant interaction was found between the use of metformin and insulin in patients with diabetes (P = 0.018). Thus, the subsequent analysis was limited to these patients and propensity score-matching was performed. We found significantly increased OS in patients treated with metformin (7-year OS, 88.3% vs. 85.6%, P < 0.001). Interestingly, a significant effect was observed in the hormonal therapy+/HER2 targeted therapy- group (P < 0.001), whereas no specific association was observed in the hormonal therapy-/HER2 targeted therapy- group (P = 0.220). CONCLUSION Metformin administration may be associated with reduced mortality in patients with surgically resected breast cancer, particularly the luminal A type. Clinical trials investigating metformin as a combination agent in breast cancer should stratify patients by intrinsic subtype, the presence of T2DM, and insulin use.