DISEASE CHARACTERISTICS: Histologically proven advanced breast cancer Must have received anthracycline and taxane based chemotherapy for metastatic disease Measurable disease No CNS metastases or carcinomatous meningitis Hormone receptor status: Not specified

PATIENT CHARACTERISTICS: Age: 18 and over Menopausal status: Not specified Performance status: ECOG 0-2 Life expectancy: At least 12 weeks Hematopoietic: Granulocyte count greater than 1500/mm3 Hemoglobin at least 9.0 g/dL Platelet count greater than 100,000/mm3 Hepatic: Bilirubin no greater than 1.3 mg/dL SGOT no greater than 3 times upper limit of normal (no greater than 5 times ULN if liver involvement) No Gilbert's disease Renal: Creatinine no greater than 2.0 mg/dL Calcium at least 12.0 mg/dL Cardiovascular: No myocardial infarction in the past 6 months No congestive heart failure requiring therapy Other: No active or uncontrolled infection HIV negative No psychiatric disorder that would preclude study No prior 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 seizures No uncontrolled diabetes mellitus (random blood sugar at least 200 mg) No other severe disease that would preclude study Not pregnant or nursing Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: See Disease Characteristics Greater than 4 weeks since prior chemotherapy No prior irinotecan or topotecan Endocrine therapy: Not specified Radiotherapy: Greater than 4 weeks since prior radiotherapy No radiotherapy to greater than 30% of bone marrow Surgery: Not specified Other: No concurrent phenytoin, phenobarbitol, or other antiepileptic prophylaxis