DISEASE CHARACTERISTICS: Histologically proven progressive or recurrent primary malignant glioma Phase I (excluding group A patients): No more than 2 prior chemotherapy regimens, including 1 prior adjuvant therapy and 1 prior regimen for recurrent or progressive tumor, or 2 prior regimens for progressive tumor Phase II and/or group A patients: No more than 1 prior chemotherapy regimen, either as adjuvant or for recurrent disease Measurable disease by MRI or CT scan

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 60-100% Life expectancy: Not specified Hematopoietic: Neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 1.5 mg/dL SGOT no greater than 3 times upper limit of normal Renal: Creatinine no greater than 1.5 mg/dL Cardiovascular: No uncontrolled hypertension No unstable angina No symptomatic congestive heart failure No myocardial infarction within 6 months No serious uncontrolled cardiac arrhythmia Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception No severe nonmalignant systemic disease or active infection No concurrent alcoholism or drug abuse No psychosis HIV negative

PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy No concurrent sargramostim (GM-CSF) Chemotherapy: See Disease Characteristics At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas, mitomycin, or suramin) No prior irinotecan, topotecan, or other topotecan 1 inhibitors No other concurrent chemotherapy Endocrine therapy: Stable or decreasing dosage of corticosteroids within 72 hours of study entry (phase II only) No other concurrent immunosuppressive agents No concurrent hormonal therapy Radiotherapy: At least 4 weeks since prior radiotherapy Patients with prior interstitial brachytherapy or stereotactic radiosurgery must have confirmation of progressive disease No concurrent radiotherapy Surgery: At least 3 weeks since prior resection Other: Acute toxic effects (excluding neurotoxicity or alopecia) of any prior therapy must be resolved No concurrent valproic acid as a single agent Concurrent enzyme-inducing antiepileptic drugs (EIAED) with or without steroids are allowed No concurrent investigational drugs