DISEASE CHARACTERISTICS:

• Histologically or cytologically confirmed breast cancer

  • Primary and/or metastatic disease

• HER2 overexpression 3+ by immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH)

  • Tumors with HER2 2+ overexpression by IHC allowed if confirmed by FISH
• Progressive disease during or within 12 months of receiving prior regimen containing trastuzumab (Herceptin)

• Unidimensionally measurable disease

  • At least 20 mm by conventional techniques OR at least 10 mm by spiral CT scan

  • The following are not considered measurable:

    • Bone metastases
    • Pleural or peritoneal effusion
    • Ascites
    • Leptomeningeal disease
    • Lymphangitic disease
    • Inflammatory breast cancer
    • Cystic lesions
    • CNS lesions

• CNS metastases allowed if all of the following conditions are met:

  • Asymptomatic
  • At least 3 months since prior surgery and/or cranial irradiation
  • At least 3 weeks since prior steroids

• Hormone receptor status:

  • Not specified

PATIENT CHARACTERISTICS:

Age:

• Not specified

Sex:

• Male or female

Menopausal status:

• Not specified

Performance status:

• ECOG 0-2

Life expectancy:

• Not specified

Hematopoietic:

• Granulocyte count at least 1,000/mm^3
• Platelet count at least 100,000/mm^3

Hepatic:

• Bilirubin no greater than 1.5 times upper limit of normal (ULN)
• SGOT and SGPT no greater than 2 times ULN (5 times ULN for liver metastases)
• Alkaline phosphatase no greater than 2 times ULN (5 times ULN for liver metastases)

Renal:

• Creatinine no greater than 1.5 times ULN

Cardiovascular:

• LVEF at least lower limit of normal by MUGA or echocardiogram
• No congestive heart failure or active ischemic heart disease

Other:

• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception
• No psychiatric illness, medical condition, or uncontrolled infection that would preclude study
• No underlying immunodeficiency (e.g., HIV or autoimmune disease)
• No other prior malignancy within the past 5 years except nonmelanoma skin cancer or carcinoma in situ of the cervix

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• See Disease Characteristics

Chemotherapy:

• Prior cumulative doxorubicin dose no greater than 360 mg/m^2
• At least 3 weeks since prior chemotherapy
• No more than 2 prior chemotherapy regimens for metastatic disease
• No concurrent chemotherapy

Endocrine therapy:

• See Disease Characteristics
• At least 3 weeks since prior endocrine therapy
• No concurrent corticosteroids or dexamethasone
• Concurrent hormones allowed for conditions unrelated to disease (e.g., insulin for diabetes)

Radiotherapy:

• See Disease Characteristics
• At least 3 weeks since prior radiotherapy
• No prior radiotherapy to study lesion, unless evidence of disease progression
• No concurrent palliative radiotherapy

Surgery:

• See Disease Characteristics
• At least 4 weeks since prior major surgery

Other:

• No concurrent immunosuppressive drugs