Study of CR011-vcMMAE to Treat Locally Advanced or Metastatic Breast Cancer

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