Capecitabine, Cyclophosphamide, Lapatinib Ditosylate, and Trastuzumab in Treating Patients With HER2-Positive Metastatic Breast Cancer

PRIMARY OBJECTIVES: I. To estimate the progression free survival (PFS). SECONDARY OBJECTIVES: I. To evaluate the overall response rate (ORR). II. To evaluate the clinical benefit rate (CBR; complete response, partial response, and stable disease for \>= 24 weeks). III. To estimate the overall survival (OS). IV. To assess the safety and tolerability. OUTLINE: Patients receive capecitabine orally (PO) once daily (QD), cyclophosphamide PO QD, and lapatinib ditosylate PO QD on days 1-21 and trastuzumab intravenously (IV) on day 1. Courses repeat every 21 days in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up every 3 months for 1 year.