The following is a summary of “Effect of Metformin Versus Placebo on New Primary Cancers in Canadian Cancer Trials Group MA.32: A Secondary Analysis of a Phase III Randomized Double-Blind Trial in Early Breast Cancer,” published in the December 2023 issue of Oncology by Goodwin, et al.

For a study, researchers sought to assess the impact of metformin on the risk of developing new primary invasive cancers in nondiabetic patients with high-risk breast cancer.

Between 2010 and 2013, 3,649 patients under 75 years with high-risk T1-3, N0-3 M0 breast cancer (any estrogen receptor, progesterone receptor, human epidermal growth factor receptor 2), and without diabetes were enrolled in the MA.32 randomized adjuvant breast cancer trial. Participants were randomly assigned to receive metformin 850 mg orally twice a day or a placebo for 5 years. The study aimed to identify new primary invasive cancers (outside the ipsilateral breast) as the first event. Time to events was described using the competing risks method. Two-sided likelihood ratio tests, adjusting for age, BMI, smoking, and alcohol intake, were employed to compare the metformin and placebo arms.

Out of 3,649 patients, 184 developed new invasive cancers (102 in the metformin group and 82 in the placebo group). The hazard ratio (HR) was 1.25 (95% CI, 0.94 to 1.68; P = .13). This included 48 contralateral invasive breast cancers (27 metformin vs 21 placeboes), HR 1.29 (95% CI, 0.72 to 2.27; P = .40), and 136 new nonbreast primary cancers (75 metformin vs 61 placebo), HR 1.24 (95% CI, 0.88 to 1.74; P = .21). Metformin did not demonstrate a significant reduction in the risk of new cancer development in nondiabetic patients with breast cancer.

The study concluded that metformin, when administered as adjuvant therapy, did not significantly impact the risk of developing new primary invasive cancers in nondiabetic patients with high-risk breast cancer.

Source: ascopubs.org/doi/full/10.1200/JCO.23.00296