Inclusion Criteria:

- Female, 18 years of age or older

- Histologically confirmed HER2-positive metastatic breast cancer

- Trastuzumab-resistance

- Measurable disease, according to RECIST guidelines

- ECOG performance status less than or equal to 1

- Life expectancy greater than 3 months

- No prior treatment with temsirolimus, everolimus, rapamycin, or any other mTOR
inhibitor

- At least 4 weeks must have lapsed between prior investigational therapy,
chemotherapy or radiotherapy, and the first dose of ridaforolimus

- Left ventricular ejection greater than or equal to 50%

- Adequate cardiovascular function

- Adequate hematological, hepatic, and renal function

- Serum cholesterol less than or equal to 350 mg/dL and triglycerides less than or
equal to 400 mg/dL

- Negative pregnancy test within 7 days prior to first dose of study drug and must use
an approved contraceptive method from screening to 30 days after the last dose of
study drug

- Availability and patient consent to obtain archival tissue samples

- Signed informed consent

Exclusion Criteria:

- Inadequate recovery from any prior surgical procedure or having undergone any major
surgery within 2 weeks before trial entry

- Grade 1 or Grade 2 hypersensitivity reactions to prior trastuzumab therapy if these
reactions prevented further trastuzumab administration

- Grade 3 or Grade 4 hypersensitivity reaction to prior trastuzumab

- Symptomatic intrinsic lung disease or extensive tumor involvement of the lungs,
resulting in dyspnea at rest

- Known allergy to macrolide antibiotics

- Pregnant or breast-feeding

- Know history of HIV

- Diagnosis of brain metastasis or leptomeningeal carcinomatosis within 3 months

- Other malignancies within the past 3 years, except for adequately treated carcinoma
of the cervix or basal or squamous cell carcinoma of the skin

- Active infection requiring prescription intervention

- Newly diagnosed or poorly controlled Type 1 or 2 diabetes

- Other concurrent illness which, in the Investigator's judgment, would either
compromise the patient's safety or interfere with the evaluation of the safety of the
study drug

- Concurrent treatment with medications that strongly induce or inhibit cytochrome
P450.

- Any condition that renders patient unable to fully understand and provide informed
consent and/or comply with the protocol