Inclusion Criteria:

• Patients must have measurable stage III, measurable stage IVA, stage IVB (with or without measurable disease) or recurrent (with or without measurable disease) endometrial carcinoma
  • Histologic confirmation of the original primary tumor is required; patients with the following histologic epithelial cell types are eligible:
    • Endometrioid adenocarcinoma, serous adenocarcinoma, undifferentiated carcinoma, clear cell adenocarcinoma, mixed epithelial carcinoma, adenocarcinoma not otherwise specified (N.O.S.)
• Measurable disease is defined by Response Evaluation Criteria in Solid Tumors (RECIST) (version 1.1); measurable disease is defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded); each lesion must be >= 10 mm when measured by computed tomography (CT), magnetic resonance imaging (MRI) or caliper measurement by clinical exam; or >= 20 mm when measured by chest x-ray; lymph nodes must be >= 15 mm in short axis when measured by CT or MRI
• Patients must have a Gynecologic Oncology Group (GOG) performance status of 0, 1, or 2
• Absolute neutrophil count (ANC) greater than or equal to 1,500/mcl
• Platelets greater than or equal to 100,000/mcl
• Creatinine less than 1.4 mg/dl
• Bilirubin less than or equal to 1.5 x institutional/laboratory upper limit of normal (ULN)
• Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) less than or equal to 3 x ULN
• Alkaline phosphatase less than or equal to 2.5 x ULN
• Patients must NOT have received prior chemotherapy or targeted therapy, including chemotherapy used for radiation sensitization for treatment of endometrial carcinoma
• Patients may have received prior radiation therapy for treatment of endometrial carcinoma; prior radiation therapy may have included pelvic radiation therapy, extended field pelvic/para-aortic radiation therapy, and/or intravaginal brachytherapy; all radiation therapy must be completed at least 4 weeks prior to the first date of study therapy
• Patients may have received prior hormonal therapy for treatment of endometrial carcinoma; all hormonal therapy must be discontinued at least one week prior to the first date of study therapy
• Patients must be able to swallow and retain orally-administered medication
• Patients must have signed an approved informed consent and authorization permitting release of personal health information; individuals with impaired decision-making capacity are not eligible to participate on the study

Exclusion Criteria:

• Patients must NOT be taking metformin or have been on metformin in the past 6 months
• Patients with a history of other invasive malignancies, with the exception of non-melanoma skin cancer are excluded if there is any evidence of other malignancy being present within the last three years
• Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements
• Patients who are pregnant or nursing; if patients are of reproductive age and have not undergone hysterectomy, they must use an effective contraceptive method for the duration of this study
• Any 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)