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