Lapatinib in Treating Patients With Advanced or Metastatic Breast Cancer That Overexpresses HER2

DISEASE CHARACTERISTICS: * Histologically confirmed breast cancer * Advanced or metastatic disease * No effective curative therapy available * Bone-only disease allowed * Tumor HER2 overexpression * HER2 3+ expression by immunohistochemistry OR \> 2-fold (HER2 2+) gene amplification by fluorescence in situ hybridization * Evaluable disease * Measurable disease is not required * No progressive brain metastases * Hormone receptor status not specified PATIENT CHARACTERISTICS: Inclusion criteria: * ECOG performance status 0-2 * Life expectancy \> 3 months * Female * Menopausal status not specified * Absolute neutrophil count ≥ 1,000 cells/mm\^3 * Hemoglobin ≥ 9 g/dL * Platelet count ≥ 75,000 cells/mm\^3 * Total bilirubin normal * AST and ALT ≤ 3 x upper limits of normal (ULN) (≤ 5 x ULN with liver metastases) * Creatinine normal OR creatinine clearance ≥ 40 mL/min * INR ≤ 1.5 * Potassium normal * Magnesium normal * Not pregnant * Negative pregnancy test * Fertile patients must use effective contraception prior to and during study therapy * Cardiac ejection fraction ≥ 50% * Consents to 2 tumor fine needle aspiration biopsies for biomarker analysis * Lung-only disease or sites otherwise deemed high-risk for biopsy, the requirement for biopsy will be waived Exclusion criteria: * History of significant cardiac disease including any of the following: * Congestive heart failure * Symptomatic cardiac arrhythmias * Unstable angina * Uncontrolled prior lapatinib ditosylate therapy toxicity ≥ grade 2 * Allergic reactions to IV contrast dye despite standard prophylaxis * History of malabsorption syndrome or disease significantly affecting gastrointestinal function or major resection of the stomach or small bowel that could affect absorption, distribution, metabolism, or excretion of study drug * Conditions that would impair the patient's ability to swallow and retain oral medication * Concurrent disease or condition that would make the patient inappropriate for study participation or would interfere with the patient's safety * Psychological, familial, sociological, or geographical conditions that do not permit compliance with the protocol PRIOR CONCURRENT THERAPY: * Prior lapatinib ditosylate or trastuzumab allowed * At least 4 weeks since prior and no concurrent chemotherapy or investigational anticancer agents * At least 2 weeks since prior and no concurrent hormonal therapy * At least 2 weeks since prior and no concurrent lapatinib ditosylate prohibited medications, including CYP3A4 inhibitors or inducers, all herbal supplements, and gastric pH modifiers * More than 4 weeks since prior radiotherapy * No aspirin or plavix therapy within 7 days prior to tumor biopsy * No concurrent coumadin * Low molecular weight heparin allowed provided it can be held at least 24 hours prior to tumor biopsy * Concurrent gonadal suppression agents (i.e., Zoladex or Lupron) or palliative bisphosphonates (i.e., Zometa) allowed