Trastuzumab and Irinotecan in Treating Patients With HER2/Neu Positive Metastatic Breast Cancer

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