ERBB2 and breast cancer: Can Metformin Be Used as a Generic Therapy for All Breast Cancer Sub-Types? Xiao and colleagues looked at specific breast cancer subtypes and found that the nondiabetic metformin group of patients with Luminal A (ER+/PR+), Luminal B (high ki67), and luminal B (HER-2/NUE+) had better prognosis compared to the nondiabetic group not treated with metformin.