Phase II Study of Metformin for Reduction of Obesity-Associated Breast Cancer Risk

High adiposity is a major risk factor for a number of chronic diseases, including type 2 diabetes, cardiovascular diseases, and certain types of cancer, including postmenopausal breast cancer. The increased postmenopausal breast cancer risk in women with high adiposity is likely to be attributed to multiple metabolic disturbances including altered circulating sex steroid hormones, hyperinsulinemic insulin resistance, altered expression and secretion of adipokines from adipose tissue, increased production of pro-inflammatory cytokines, and increased oxidative stress. Metformin, a widely used antidiabetic drug, exerts favorable effects on multiple metabolic disturbances which may lead to reduction of breast cancer risk in women with high adiposity. In addition, metformin may exert a direct effect in mammary tissue through the activation of the AMP-activated protein kinase signaling pathway, leading to an antiproliferative effect and induction of apoptosis. Recent case control and cohort studies found that treatment with metformin appears to substantially reduce the risk for development of cancer in diabetics, including breast cancer. There are a number of ongoing clinical trials of metformin in breast cancer patients. However, applicability of these trials to at risk healthy women requires further research and the concurrent or prior cancer treatments in these trials hinder the evaluation of metformin as a single agent for breast cancer risk reduction. In addition, recent clinical and animal studies suggest that metformin may only exert tumor suppressive effects in metabolic phenotypes of high adiposity and metabolic disturbances. A Phase II randomized, double-blind, placebo-controlled trial of metformin in overweight/obese premenopausal women who have metabolic syndrome will be conducted. This study population is at increased risk for postmenopausal breast cancer and has a high prevalence of metabolic disturbances. The overall objective of this study is to determine its potential effects on reduction of obesity-associated breast cancer risk.