Inclusion Criteria:

1. Postmenopausal overweight or obese women with a history of biopsy-proven hormone
receptor-positive breast cancer and clinical evidence of metastatic disease.
Overweight is defined as body mass index (BMI) of 25 - 29.9 kg/m2 while obese is
defined as BMI >/= 30 kg/m2. Postmenopausal status is defined by one of the
following: a) no spontaneous menses for over 1 year, in women >/=55 years; b) no
spontaneous menses within the past 1 year in women < 55 years with postmenopausal
gonadotrophin levels (LH and FSH levels > 40 IU/L) or postmenopausal estradiol levels
(by local laboratory range); or c) bilateral oophorectomy.

2. Prior hormonal therapy for metastatic breast cancer is allowed. Patients who develop
progressive metastatic disease on a non-steroidal aromatase inhibitor are eligible.
Patients who develop metastatic disease while receiving a non-steroidal aromatase
inhibitor in the adjuvant setting are eligible.

3. One prior chemotherapy line for metastatic breast cancer is allowed if there is
evidence of progressive disease. Patients treated with chemotherapy to best response
and no evidence of progression are not eligible.

4. Prior tamoxifen, LH/RH agonist, anastrozole or letrozole therapy in the adjuvant
and/or neoadjuvant settings is allowed. Prior adjuvant and/or neoadjuvant
chemotherapy is allowed.

5. Patients must have: [1] at least one lesion that can be accurately measured in at
least one dimension >/= 20 mm with conventional imaging techniques or >/= 10 mm with
spiral CT or MRI; or [2] bone lesions: lytic or mixed (lytic + sclerotic) in the
absence of measurable disease; the following will be considered disease progression
among these patients: a) the appearance of one or more new lytic lesions in bone; b)
the appearance of one or more new lesions outside of bone; c) unequivocal progression
of existing bone lesions.

6. Localized radiotherapy, which does not influence the signal of evaluable lesion, is
allowed prior to the initiation of study medications.

7. ECOG performance status
8. Absolute neutrophil count (ANC) >/= 1000/microliter, platelets >/= 75,000/microliter,
hemoglobin >/= 8.5 gm/dL; creatinine clearance >60 mg/dL; bilirubin < 1.5 mg/dL ( 3 × ULN for patients known to have Gilbert Syndrome); ALT <3 x upper limit of normal
(or of normal; calcium
9. Fasting serum cholesterol 2.5 × ULN. In case one or both of these thresholds are exceeded, the patient can only
be included after initiation of statin therapy and when the above mentioned values
have been achieved.

10. Bisphosphonate treatment is permitted for the management of bone loss and/or bone
metastases.

11. Patients must be competent to give informed consent and to state that they understand
the investigational nature of the proposed treatment.

Exclusion Criteria:

1. HER2-overexpressing breast cancer (IHC 3+ staining or in situ hybridization
positive).

2. Diabetes mellitus on active treatment or hemoglobin A1C >/= 6.5% or random plasma
glucose > 200 mg/dL in patients without known diabetes.

3. Treatment with metformin in the 30 days prior to enrollment.

4. Known hypersensitivity or intolerance to metformin.

5. Previous treatment with exemestane or mTOR inhibitors.

6. Known hypersensitivity to mTOR inhibitors, e.g. sirolimus (rapamycin).

7. History of acromegaly, Cushing's syndrome, Cushing's disease, Addison's disease
(treated or untreated).

8. Patients with unstable angina, uncontrolled ischemic cardiac disease or symptomatic
congestive heart failure (e.g. Class III or IV New York Heart Association's
Functional Classification).

9. Other investigational drugs within the past 3 weeks or concurrently.

10. Patients with known chronic liver diseases (e.g., chronic active hepatitis, and
cirrhosis).

11. Another malignancy within 5 years prior to registration, with the exception of
adequately treated in-situ carcinoma of the cervix, uteri, basal or squamous cell
carcinoma or non-melanomatous skin cancer.

12. Radiotherapy within four weeks prior to registration except in case of localized
radiotherapy for analgesic purpose or for lytic lesions at risk of fracture which can
then be completed within two weeks prior to registration. Patients must have
recovered from radiotherapy toxicities.

13. History of brain or other central nervous system metastases.

14. Bilateral diffuse lymphangitic carcinomatosis.

15. Presence of life-threatening metastatic visceral disease, defined as extensive
hepatic involvement, or any degree of brain or leptomeningeal involvement (past or
present), or symptomatic pulmonary lymphangitic spread. Subjects with discrete
pulmonary parenchymal metastases are eligible, provided their respiratory function is
not compromised as a result of disease.

16. Patients receiving concomitant immunosuppressive agents or chronic corticosteroids
use, at the time of study entry except in cases outlined below: Topical applications
(e.g. rash), inhaled sprays (e.g. obstructive airways diseases), eye drops or local
injections (e.g. intra-articular) are allowed. Patients on stable low dose of
corticosteroids for at least two weeks before enrollment are allowed.

17. Any severe and / or uncontrolled medical conditions such as: Unstable angina
pectoris, symptomatic congestive heart failure, myocardial infarction prior to enrollment, serious uncontrolled cardiac arrhythmia; Uncontrolled diabetes
as defined by fasting serum glucose > 1.5 × ULN; Acute and chronic, active infectious
disorders (except for Hep B and Hep C positive patients) and nonmalignant medical
illnesses that are uncontrolled or whose control may be jeopardized by the
complications of this study therapy; Impairment of gastrointestinal function or who
have gastrointestinal disease that may significantly alter the absorption of study
drugs (e.g., ulcerative disease, uncontrolled nausea, vomiting, diarrhea,
malabsorption syndrome); Active skin, mucosa, ocular or GI disorders of Grade > 1

18. Significant symptomatic deterioration of lung function. If clinically indicated,
pulmonary function tests including measures of predicted lung volumes, DLco, O2
saturation at rest on room air will be considered to exclude restrictive pulmonary
disease, pneumonitis or pulmonary infiltrates.

19. Patients being treated with drugs recognized as being strong inhibitors or inducers
of the isoenzyme CYP3A (Rifabutin, Rifampicin, Clarithromycin, Ketoconazole,
Itraconazole, Voriconazole, Ritonavir, Telithromycin) within the last 5 days prior to
registration.