This study will evaluate the safety and tolerability of CR011-vcMMAE in patients with locally advanced and metastatic breast cancer. CR011-vcMMAE will be administered intravenously (IV) once every 3 weeks (q3w) to cohorts of 3-6 patients to confirm the maximum tolerated dose (MTD) in breast cancer patients. Once the MTD is confirmed, up to 25 patients with locally advanced or metastatic breast cancer will be enrolled into the Phase II portion of the trial to further evaluate the safety and efficacy of CR011-vcMMAE.

Main Inclusion Criteria:

• Females with confirmed breast cancer

• Age ≥ 18 years

• Eastern Cooperative Oncology Group (ECOG) Performance Status 0 to 2

• Locally advanced or metastatic disease who have received at least two prior chemotherapeutic regimens for breast cancer, as follows:

  • At least one regimen must have been for locally advanced or metastatic disease
  • Subject must have received an anthracycline, a taxane, and capecitabine in any combination unless the subject was intolerant to or not a candidate for any of these agents
  • Hormonal therapy, biologic therapy (such as trastuzumab or bevacizumab), or immunotherapy do not count as one of the 2 regimens
  • Subjects with HER2/neu overexpressing tumors must have been treated with trastuzumab except in situations where the subject was intolerant to or not a candidate for trastuzumab

• Documented progressive disease within 6 months of the last regimen

• Adequate bone marrow, renal and liver function

• Signed informed consent

Main Exclusion Criteria:

• Chemotherapy, radiation, hormonal therapy, or bevacizumab within two weeks, or trastuzumab or investigational therapy within three weeks before treatment start
• Neuropathy > NCI-CTCAE Grade 1
• Active brain metastases.
• New York Heart Association class III or IV heart disease
• Unstable angina
• Uncontrolled arrhythmia
• A marked baseline prolongation of QT/QTc interval
• Pregnant or breast-feeding women, and women of childbearing age and potential who are not willing to use effective contraception