Phase II Study With the Trifunctional Antibody Ertumaxomab to Treat Metastatic Breast Cancer Progressing After Endocrine Treatment

Inclusion Criteria: * Signed and dated informed consent form * Women ≥ 18 years, negative pregnancy test at screening life expectancy of at least 6 months * Locally advanced (stage IIIb) or metastatic (stage IV) and not curable adenocarcinoma of the breast * Measurable disease, defined as at least one lesion that is measurable in one dimension (RECIST) * HER-2/neu expression 1+ or 2+ / FISH negative * Estrogen Receptors (ERs) and/or Progesterone Receptors (PRs) positive * Prior adequate endocrine therapy for advanced or metastatic disease * Disease progression during or after endocrine therapy * No prior treatment with mouse or rat antibodies * ECOG performance score of ≤ 1 * Adequate hematological, liver and kidney function Exclusion Criteria: * Women who are pregnant or breast-feeding * Known HIV infection or Presence of autoimmune disease or other Concurrent non-malignant co-morbidities that are uncontrolled * History or symptoms indicative of brain or CNS metastases * Prior diagnosis of any malignancy not cured by surgery alone less than 5 years before study entry (except in situ carcinoma of the cervix or adequately treated basal cell carcinoma of the skin) * Documented acute or chronic infection requiring antibiotic treatment * Any concurrent chemo-, hormonal, immuno- or corticoid therapy * Any prior chemotherapy for advanced or metastatic disease * Any concurrent investigational treatment for advanced or metastatic disease * History of relevant cardiovascular disease as follows: * Left ventricular ejection fraction (LVEF) below the institution's lower limit of normal, based on echocardiography (ECG) at rest * Uncontrolled or symptomatic congestive heart failure (New York Heart Association (NYHA) \> 2 * Uncontrolled or symptomatic arrhythmia and/or angina pectoris * Myocardial infarction during the last 2 years