Prospective, placebo-controlled, randomized study of metformin for breast cancer prevention in overweight/obese women

Background: In several epidemiologic studies, the oral anti-diabetic medication metformin is associated with lower cancer incidence and diminished tumor progression. The molecular mechanism of metformin anti-cancer activity remains unclear, particularly in the preventive setting. This study investigated the effect of metformin on inflammatory and proliferative pathways in cancer-free breast tissue of overweight and obese women at high risk for breast cancer, in an effort to better understand how we might prevent obesity- related carcinogenesis in the breast. Methods: The study cohort included 24 obese or overweight (BMI ≥ 25), postmenopausal women at increased risk for breast cancer based on family history, risk models indicating lifetime risk ≥ 20%, atypical hyperplasia, or history of DCIS or breast cancer treated over 5 years ago. Subjects were randomized to metformin (850 mg BID) or placebo for 12 months. Breast tissue core biopsy and blood collection were performed at baseline and after 12 months. Reverse Phase Protein Array (MD Anderson RPPA Core Facility) was used to determine the proteomic profiling of the breast tissue biopsies (5 replicates for each subject) at both time points. 297 unique antibodies were analyzed by Array-Pro Analyzer 6.3 then by SuperCurve_1.5.0 via SuperCurveGUI_2.1.1. All relative protein level data points were normalized for protein loading and transformed to linear values. Serum biomarkers including IGF-I, IGF-II, Leptin, IL6, IL10, adiponectin, leptin, and IGFBP3 were measured using commercial enzyme-linked immunosorbent assays (ELISA). Statistical analysis was performed using R platform. Wilcox and T tests are used to check the difference for each protein between the two time points in each group. P values below 0.05 were deemed significant. Results: Of 24 women randomized, 18 completed the study, including 9 treated with metformin and 9 treated with placebo. Participants receiving metformin showed no significant changes in the measured serum biomarkers between pre- and post-treatment, or in comparison with the placebo group. Proteomic analysis revealed a change in both protein level as well as protein phosphorylation status in the metformin-treated group. Metformin treatment induced downregulation of regulators of cell proliferation and survival (Ets1, P=0.02; Abl1, P=0.04; Smad3, P=0.01; 14-3-3β, P=0.02; TG2, P=0.03, Beclin1, P=0.04; pT1135-RICTOR, P=0.03) in breast tissue. Several of the downregulated targets belong to two major pathways: mTOR and TGFβ. Increase in proteins involved in metabolic and AMPK pathways (PDHK1, P=0.007; pS79-ACC, P=0.02; Mnk1, P=0.02) was detected in the metformin-treated group post- treatment; however, no change was detected in the placebo-treated group. Conclusion: Our preliminary data suggest that metformin treatment has a direct effect on breast tissue of overweight or obese, postmenopausal woman at increased risk for breast cancer by reducing pro-survival pathways, such as mTOR and TGFβ, and activating metabolic pathways, such as AMPK. Further investigation of the specific targets activated or inhibited in the cancer-free breast tissue by metformin is being performed to broaden our understanding of the protective mechanism of this antidiabetic agent. Citation Format: Tarah Ballinger, Natascia Marino, Rana German, Jason True, Xiaoyu Lu, Sha Cao, Anna Maria Storniolo, Lida Mina. Prospective, placebo-controlled, randomized study of metformin for breast cancer prevention in overweight/ obese women [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-13-03.