Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically or cytologically confirmed metastatic breast carcinoma

- Received 1-3 prior chemotherapy regimens for metastatic disease

- Documented progressive disease

- Repeated courses of the same chemotherapy agent alone or in combination are
considered a single regimen

- Prior trastuzumab (Herceptin®) alone or with chemotherapy allowed

- Other biologic agents are not considered a chemotherapy regimen

- Measurable disease

- Patients with bone-only disease who are evaluable by tumor markers (e.g.,
CA15-3, CEA, or CA27.29) are eligible

- Patients must have prior evidence of correlation of disease activity with
changes in tumor marker level

- Confirmation of HER2/neu status by a positive test for gene amplification by
fluorescence in situ hybridization or 3+ by immunohistochemistry

- Brain metastases allowed if the following criteria are met:

- Brain metastases were previously treated and are currently stable as documented
by head CT scan with contrast or MRI within 4 weeks of study entry

- Patients with existing brain metastases should have stability documented by
prior imaging ≥ 8 weeks before the baseline scan

- Hormone-receptor status not specified

PATIENT CHARACTERISTICS:

- Menopausal status not specified

- ECOG performance status ≤ 2

- Absolute neutrophil count ≥ 1,000/mm^3

- Platelet count ≥ 100,000/mm^3

- Life expectancy ≥ 12 weeks

- No history of congestive heart failure

- Documented ejection fraction ≥ 45% by MUGA scan or echocardiogram within 1 month of
study entry

- Total bilirubin < 3 times upper limit of normal (ULN)

- AST < 3 times ULN (5 times ULN if due to liver involvement)

- Creatinine < 1.5 times ULN

- No history of serious adverse events related to trastuzumab

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective contraception

- No severe, concurrent illness that would prevent compliance with study protocol

- No chronic severe diarrheal illness

- No history of Gilbert's disease or known deficiency in glucuronidation

- No recent or current history of alcoholism or acute viral hepatitis

PRIOR CONCURRENT THERAPY:

- See Disease Characteristics

- No chemotherapy or hormonal therapy within the past 2 weeks

- Prior or concurrent bisphosphonates allowed

- No prior irinotecan (other camptothecins allowed)

- No concurrent radiotherapy

- No ongoing treatment with any other investigational agent