Safety and Efficacy Study of the Trifunctional Antibody Ertumaxomab to Treat Patients With Advanced or Metastatic Breast Cancer

Inclusion Criteria: * Female gender, and if of child-bearing potential must have negative pregnancy test result within 2 days before enrolment and must agree to practice effective birth control during the study. * Aged 18 years and older. * Histologically or cytologically confirmed invasive breast cancer with stage IIIb or IV disease with documented progression. * Measurable disease according to RECIST. * Histologically documented advanced primary breast cancer or biopsy of metastatic site demonstrating HER-2/neu expression (HER-2/neu 1+ or 2+, determined by immunohistochemistry \[IHC\]). HER-2/neu 2+ patients must have a negative Fluorescence In Situ Hybridization \[FISH\] test result. * Hormone receptor status Estrogen Receptors (ERs) positive and/or Progesterone Receptors (PRs) positive. * No prior treatment with mouse or rat antibodies. * Life expectancy of at least six months (if the life expectancy of a patient is unspecified she will be allowed to enter the study). * An Eastern Cooperative Oncology Group (ECOG) performance score of £ 1. * Patients must have had disease progression after hormonal therapy including at least one aromatase inhibitor. * Adequate hematological, liver and kidney function: * Thrombocytes ³ 100000 / mm³ (= 100 x 109 /l) * Hemoglobin ³ 10 g/dl * Neutrophil count ³ 1500/mm³ (= 1.5 x 109 /l) * WBC ³ 3 X 109 /l * Serum glutamic oxaloacetic transaminase (SGOT) and serum glutamic pyruvate transaminase (SGPT) ≤ 2.5 x upper limit of normal (ULN) * Serum bilirubin ≤ 2 x ULN * Creatinine ≤ 1.5 x ULN or clearance ³ 60 ml/min * No life-threatening visceral disease. * No known brain or central nervous system metastases. * No symptomatic pleural effusions. * No symptomatic pericardial effusions. * No subjects whose only site of metastatic involvement is bone metastases with the exception of those with a measurable soft tissue component of the bone lesion seen with imaging that does not require palliative radiation intervention and/or the patient has a lytic bone lesion ³ 1 cm measured with radiography that can be followed for evidence of re-calcification. * No history of relevant cardiovascular disease: * LVEF within the institutional ranges of normal as measured by echocardiogram or MUGA scan * No prior uncontrolled or symptomatic congestive heart failure NYHA ³ 2 * No myocardial infarction within the past two years * No uncontrolled or symptomatic cardiac arrhythmias * No severe dyspnea. * No pulmonary dysfunction or need for continuous supportive oxygen inhalation. * No other concurrent uncontrolled co-morbid illness. * No other concurrent malignancy, except treated basal cell or squamous cell carcinoma of the skin, or in situ carcinoma of the cervix. * Patients with documented autoimmune diseases (such as lupus) are excluded from participation in the study unless a waiver is granted by the responsible medical monitor. * Patients with a human immunodeficiency virus, hepatitis B or hepatitis C positive status are excluded from participation in the study. * No prior or concurrent chemotherapy regimen for advanced or metastatic disease. * Prior neo-adjuvant or adjuvant chemotherapy is allowed provided it was stopped at least six months before study entry. * No concurrent hormone therapy (hormone therapy must be stopped at the screening visit). * At least 4 weeks since prior radiotherapy. * No concurrent immune therapy. * No concurrent corticosteroid therapy. * No regularly used medication for a health condition or comorbidity that might result in undue risk to the patient. * No prior investigational treatment for advanced or metastatic disease. * Able and willing to comply fully with the protocol.