Metformin intervention in obese non-diabetic patients with breast cancer: phase II randomized, double-blind, placebo-controlled trial

Previous observational studies have suggested that metformin in diabetes patients may reduce breast cancer risk more than the reductions from other anti-diabetes medications. This randomized, double-blind, placebo-controlled trial was performed to evaluate the efficacy of metformin for controlling physical and metabolic profiles related to prognosis and adverse events in non-diabetic breast cancer patients. Female breast cancer patients (N = 105), at least 6 months post-mastectomy, with obesity (≥25 kg/m²) and/or pre-diabetes (fasting blood sugar levels ≥100 mg/dL), were randomly assigned to three groups (placebo, metformin 500 mg, and metformin 1000 mg) stratified by tamoxifen use. A linear mixed model for repeated measurements among three groups and ANOVA for profile differences during 6 months of treatment were used for the intention-to-treat analysis. The metformin 1000 mg group had a significantly greater decline in glucose and HbA1c levels between treatment weeks 0 and 6 month (p = 0.008 and 0.009, respectively), and the declines increased with an increase in body mass index (BMI) level (p interaction with BMI = 0.007 and 0.067, respectively). A marginally significant different effect from the metformin 1000 mg treatment was detected for glucose and HbA1c levels (p interaction = 0.084 and 0.063, respectively) in the intention-to-treat analysis. Metformin 1000 mg treatment had a favorable effect on controlling glucose and HbA1C levels in obese non-diabetic breast cancer patients, indicating prognostic importance. Further trials are needed to elucidate the risk–benefit ratio of long-term use of metformin.