The purpose of this pilot study is to use a pre surgical intervention model to evaluate the biologic effects of metformin in women with newly diagnosed early invasive breast cancer. Metformin is a drug commonly used to treat patients with diabetes. This model will be used to evaluate the effects of metformin.

Breast cancer is the most common cancer and the second leading cause of cancer death among women in the United States. Several epidemiologic studies have found an increased risk of breast cancer in women with type 2 diabetes. This association appears to be mediated by a state of hyperinsulinemia and insulin resistance. Population based studies have also found a decreased risk of cancer in diabetic patients taking metformin, an oral biguanide derivative. Proposed mechanisms include its effect on lowering serum insulin levels as well as modulation of cellular signaling pathways which inhibit tumor cellular proliferation.

Inclusion Criteria:

• Histologically-confirmed operable invasive breast cancer or ductal carcinoma in situ (DCIS) who undergo core needle biopsy followed by surgical excision at least 2 weeks after enrollment
• Body mass index > 25
• Age ≥ 21 years
• No prior chemotherapy, radiation therapy, or surgery within 6 months of study entry
• Signed informed consent

Exclusion Criteria:

• History of diabetes mellitus requiring medical therapy
• Treatment with other investigational drugs within 6 months of study entry
• Significant renal impairment with a creatinine > 1.4 mg/dl
• Other serious intercurrent medical illness

Of the 35 patients enrolled, 33 completed the trial and were considered evaluable. Patients were evaluable if they had sufficient pre and post treatment tumor tissue for analysis, and demonstrated at least 75% compliance with metformin, as measured by weekly telephone interviews and pill counts.

Up to 4 weeks

The definition of adverse event and/or serious adverse event, used to collect adverse event information, is the same as the ClinicalTrials.gov definitions.