Metformin Hydrochloride vs. Placebo in Overweight or Obese Patients at Elevated Risk for Breast Cancer

Inclusion Criteria:

Patients must be post-menopausal women; post-menopausal women are defined as: (1) those >= 50 years of age who had not menstruated during the preceding 12 months or who had castrate follicle-stimulating hormone levels (> 40 IU/L), (2) those who had undergone a bilateral oophorectomy

Patients must be at elevated risk for breast cancer based on strong family history or a history of breast biopsy documenting atypical hyperplasia anytime in the past; for this study strong family history is defined as having:

1 first-degree (parent, offspring, sibling) relative =< 50 years old when diagnosed with breast cancer, or
>= 2 first-degree relatives of any age when diagnosed with breast cancer, or
>= 2 second-degree (aunts, uncles, grandparents, grandchildren, nieces, nephews, or half-siblings) maternal or paternal relatives diagnosed with breast cancer and at least 1 diagnosed at =< 50 years of age

Patients must have a body mass index (BMI) >= 25.0 as calculated by the formula: weight in pounds / height squared x 703 = BMI; a BMI of:

18.5-24.9 is considered normal;
25.0-29.9 is considered overweight;
30.0+ is regarded as obese
Patients must be willing to complete a bilateral mammogram at baseline with repeat exam after 12 cycles of protocol therapy; patients who have had a mammogram within 1 month prior to registration to protocol therapy will not need to repeat the exam
Patients must be willing to provide a core tissue biopsy at baseline and with repeat tissue collection after 12 cycles of protocol therapy
White blood cell (WBC) >= 3.0 x 109/L
Granulocytes (polymorphs + bands) >= 1.5 x 109/L
Platelets >= 100 x 109/L
Hemoglobin >= 110 g/L
Aspartate aminotransferase (AST) =< 1.8 X upper limit of normal (ULN)
Alanine aminotransferase (ALT) =< 1.8 X ULN
Alkaline phosphatase =< 2 X ULN
Serum creatinine =< 115 umol/L (1.3mg/dL)
Serum bilirubin =< institution ULN (except for subjects with Gilbert's Disease who are eligible despite elevated serum bilirubin level)
12 hour fasting glucose level < 7.0 mmol/L
Eastern Cooperative Oncology Group (ECOG) performance status of 0 - 1 within 28 days of registration
Life expectancy of >= 5 years
Subjects must be accessible for treatment, adverse event tracking and follow-up as determined by the treating physician
Subject consent and authorization for the release of health information must be obtained according to local institutional guidelines

Exclusion Criteria:

No history of any malignancy except: adequately treated non-melanoma skin cancer, curatively treated in-situ cancer of the cervix, or other solid tumors curatively treated with no evidence of disease for >= 5 years
No known diabetes (type 1 or 2) or baseline fasting glucose >= 7.0 mmol/L
No known hypersensitivity or intolerance to metformin
No condition associated with increased risk of metformin-associated lactic acidosis (e.g. congestive heart failure defined as New York Heart Association [NYHA] class III or IV functional status, history of acidosis of any type; habitual intake of 3 or more alcoholic beverages per day)
No current treatment with metformin, sulfonylureas, thiazolidinediones or insulin for any reason
No breastfeeding
No concurrent or planned participation in randomized trials of weight loss or exercise interventions or trials targeting insulin, insulin-like growth factor 1 (IGF-1) or their receptors