Phase 2 Study of the Monoclonal Antibody MGAH22 (Margetuximab) in Patients With Relapsed or Refractory Advanced Breast Cancer

Inclusion Criteria: * Histologically or cytologically confirmed invasive carcinoma of the breast * Treatment with at least two prior systemic therapies for advanced (unresectable locoregional or metastatic) disease * Evidence of HER2 oncoprotein expression at the 2+ level by central laboratory. Patients whose tumors exhibit 2+ staining by IHC are eligible for the study. * Patients whose tumors score 1+ by conventional IHC, are non-amplified by FISH testing, and whose tumors score \> or = 10.5 by HERmark® testing, are eligible for the study. * Evidence of lack of HER2 oncogene amplification as determined by FISH testing by central laboratory * Performance Status of 0 or 1 * Life expectancy at least 6 months * Measurable disease (by RECIST 1.1) * Acceptable laboratory parameters and organ reserve * Baseline left ventricular ejection fraction \> or = 50% * Anti-cancer therapy (including conventional cytotoxic chemotherapy and/or biological therapy) and radiotherapy must be completed and any associated toxicities resolved to \</= Grade 1 levels or baseline levels and at least 2 weeks must have elapsed before enrollment. Treatment with monoclonal antibodies must be completed at least 14 days before entry. Must have completed immunosuppressive medications or vaccinations before enrollment. * Patients who are estrogen receptor+ and/or progesterone receptor+ and who are receiving anti-hormone therapy for at least three months may continue to receive such therapy during the course of…