DISEASE CHARACTERISTICS:

• Histologically or cytologically confirmed primary adenocarcinoma of the breast

  • Locally advanced or metastatic disease not amenable to curative surgery or radiotherapy

• Tamoxifen-resistant disease, defined as 1 of the following:

  • No response to initial therapy (primary resistance)
  • Disease relapse or progression after showing an initial response to therapy (secondary resistance)

• Disease progression, as documented by 1 of the following:

  • CT scan, MRI, or x-ray
  • Increase in the number of bone lesions
  • Increased pain in an area of known bony metastasis AND ≥ 2 serial tumor marker elevations

• Measurable disease, defined as ≥ 1 unidimensionally measurable target lesion ≥ 20 mm by conventional techniques OR ≥ 10 mm by MRI or spiral CT scan

  • Not in a previously irradiated area
• No rapidly progressive disease in major organs (i.e., lymphangitic spread or bulky liver metastasis)

• No known brain or leptomeningeal metastases requiring active therapy

  • Previously treated asymptomatic stable CNS metastases allowed provided patient does not require corticosteroids for CNS metastases

• Hormone receptor status:

  • Estrogen and/or progesterone receptor positive disease

PATIENT CHARACTERISTICS:

Age

• 18 and over

Sex

• Male or female

Menopausal status

• Not specified

Performance status

• ECOG 0-2 OR
• Karnofsky 60-100%

Life expectancy

• At least 3 months

Hematopoietic

• Absolute neutrophil count ≥ 1,500/mm^3
• Platelet count ≥ 100,000/mm^3
• Hemoglobin ≥ 9.0 g/dL

Hepatic

• ALT and AST ≤ 1.5 times upper limit normal (ULN) (3 times ULN if liver metastases are present)
• Bilirubin ≤ 1.5 times ULN

Renal

• Creatinine normal OR
• Creatinine clearance > 60 mL/min

Cardiovascular

• Ejection fraction normal by echocardiogram or MUGA

• None of the following cardiovascular conditions within the past 6 months:

  • Myocardial infarction
  • Severe or unstable angina
  • Symptomatic congestive heart failure
  • Cerebrovascular accident or transient ischemic attack
• Deep venous thrombosis or other clinically significant thromboembolic event within the past 6 months allowed provided patient is clinically stable on anticoagulation therapy

Pulmonary

• Pulmonary embolus within the past 6 months allowed provided patient is clinically stable on anticoagulation therapy

Gastrointestinal

• No malabsorption syndrome
• No gastrointestinal (GI) tract disease that would preclude ability to take oral medication
• No requirement for IV alimentation
• No uncontrolled inflammatory GI disease (e.g., Crohn's disease or ulcerative colitis)
• Able to swallow and retain oral medication

Other

• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective barrier contraception during and for ≥ 2 weeks after completion of study treatment
• No other malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or carcinoma in situ of the cervix
• No history of allergic reaction attributed to compounds of similar chemical or biological composition to lapatinib
• No other severe acute or chronic medical or psychiatric condition or laboratory abnormality that would preclude study participation

PRIOR CONCURRENT THERAPY:

Biologic therapy

• Prior trastuzumab (Herceptin®) in combination with chemotherapy in the adjuvant setting only is allowed
• No prior trastuzumab in combination with hormonal therapy
• No concurrent trastuzumab

Chemotherapy

• See Biologic therapy
• Prior cumulative doxorubicin dose ≤ 450 mg/m^2

Endocrine therapy

• See Disease Characteristics
• See Biologic therapy
• At least 14 days since prior and no concurrent dexamethasone or dexamethasone equivalent dose > 1.5 mg/day

Radiotherapy

• See Disease Characteristics
• More than 2 weeks since prior radiotherapy

Surgery

• More than 4 weeks since prior surgery
• More than 6 months since prior coronary or peripheral artery bypass grafting
• No prior surgical procedure affecting absorption

Other

• No prior epidermal growth factor receptor- or HER2/neu-targeting therapies

• At least 7 days since prior and no concurrent CYP3A4 inhibitors, including any of the following:

  • Clarithromycin
  • Erythromycin
  • Troleandomycin
  • Itraconazole
  • Ketoconazole
  • Voriconazole
  • Fluconazole (doses ≤ 150 mg/day allowed)
  • Fluvoxamine
  • Nefazodone
  • Verapamil
  • Diltiazem
  • Cimetidine
  • Aprepitant
  • Proton pump inhibitors
  • H2 blockers
  • Grapefruit or grapefruit juice
  • Bitter orange

• At least 14 days since prior and no concurrent CYP3A4 inducers, including any of the following:

  • Phenytoin
  • Carbamazepine
  • Phenobarbital
  • Oxcarbazepine
  • Efavirenz
  • Nevirapine
  • Rifampin
  • Rifabutin
  • Rifapentine
  • Hypericum perforatum (St. John's wort)
  • Modafinil
• At least 6 months since prior and no concurrent amiodarone
• No concurrent gastric pH modifiers within 1 hour before and after lapatinib administration
• No concurrent combination antiretroviral therapy for HIV-positive patients
• No other concurrent antineoplastic agents
• No other concurrent investigational agents
• Concurrent zoledronate for bone metastases or hypercalcemia allowed
• Concurrent oral anticoagulants (e.g., warfarin) allowed provided there is increased vigilance in INR monitoring