DISEASE CHARACTERISTICS:

• Histologically or cytologically confirmed stage IV breast cancer

  • Patients without histologic or cytologic confirmation of metastatic disease must have unequivocal evidence of metastasis by physical examination or radiologic study

• Primary tumor or metastasis HER2-positive by one of the following:

  • Immunohistochemistry (3+ positive using the DAKO Herceptest or the CB-11 antibody)
  • Fluorescence in situ hybridization (positive by either the Vysis Pathvysion™ method or the Ventana INFORM™ method)
• Measurable or evaluable disease

• No active brain metastases or leptomeningeal carcinomatosis

  • Previously treated CNS metastases allowed provided there are no active symptoms from the CNS disease

• Hormone receptor status:

  • Not specified

PATIENT CHARACTERISTICS:

Age:

• 18 and over

Sex:

• Male or female

Menopausal status:

• Not specified

Performance status:

• ECOG 0-1 OR
• Karnofsky 70-100%

Life expectancy:

• More than 6 months

Hematopoietic:

• Absolute neutrophil count at least 1,500/mm^3
• Platelet count at least 100,000/mm^3

Hepatic:

• Bilirubin no greater than 1.5 mg/dL
• AST/ALT no greater than 3 times upper limit of normal

Renal:

• Creatinine no greater than 2.0 mg/dL

Cardiovascular:

• LVEF at least 50% by echocardiogram or nuclear scintigraphy (e.g., MUGA scan or radionuclide ventriculography)
• No acute changes on electrocardiogram
• No symptomatic congestive heart failure
• No unstable angina pectoris
• No cardiac arrhythmia

Other:

• Not pregnant
• Negative pregnancy test
• Fertile patients must use effective contraception
• Prior grade 1 or 2 allergic reactions to trastuzumab (Herceptin) allowed if these reactions did not prevent further administration
• No prior grade 3 or 4 allergic reactions attributed to compounds of similar chemical or biological composition to study agents
• No contraindication to warfarin or other warfarin products
• No other uncontrolled concurrent illness
• No ongoing or active infection
• No psychiatric illness or social situation that would preclude study compliance

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• No more than 2 prior trastuzumab-containing regimens for metastatic disease

Chemotherapy:

• No more than 3 prior chemotherapy regimens for metastatic disease
• No other concurrent chemotherapy

Endocrine therapy:

• Prior hormonal therapy in the adjuvant or metastatic setting allowed
• Leuprolide initiated prior to study allowed provided therapy continues throughout study
• No other concurrent hormonal therapy

Radiotherapy:

• At least 1 week since prior radiotherapy for metastatic or early-stage breast cancer

• No concurrent radiotherapy

  • Whole brain radiotherapy or stereotactic radiosurgery for brain metastases allowed provided study therapy is held during and for 1 week after radiotherapy

Surgery:

• Not specified

Other:

• Recovered from prior anticancer treatments
• No other concurrent experimental treatments
• Concurrent bisphosphonates initiated before study allowed
• Concurrent bisphosphonates initiated during study allowed provided there has been no evidence of progressive disease and the bone sites do not constitute the only sites of evaluable disease