DISEASE CHARACTERISTICS: Histologically confirmed malignant glioma that is progressive or recurrent following radiation therapy or chemotherapy Anaplastic astrocytoma Glioblastoma multiforme Low grade glioma that has progressed to biopsy proven high grade glioma is eligible if the progression occurred after radiotherapy with or without chemotherapy Measurable progression or recurrence by serial MR or CT imaging

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 60-100% Life expectancy: Greater than 2 months Hematopoietic: Absolute neutrophil count 1,500/mm3 Platelet count at least 100,000/mm3 Hepatic:

Bilirubin no greater than 1.5 mg/dL Transaminases no greater than 4 times upper limit of normal Renal:

Creatinine no greater than 1.7 mg/dL Cardiovascular: No uncontrolled hypertension No angina pectoris No evidence of uncontrolled cardiac dysrhythmia Other: No serious infection or other medical illness No other prior or concurrent malignancy within 5 years except curatively treated basal or squamous cell carcinoma of the skin or carcinoma in situ of the cervix and breast Not pregnant or nursing Adequate contraception required of all fertile patients

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior topoisomerase 1 inhibitor (topotecan, irinotecan, 9- aminocamptothecin) No more that 1 prior chemotherapy regimen At least 6 weeks since nitrosourea and recovered At least 3 weeks since any other chemotherapy and recovered Endocrine therapy: Not specified Radiotherapy: At least 3 months since completion of most recent course of radiotherapy and recovered Surgery: Not specified Other: No concurrent investigational agents At least 14 days since prior valproic acid No concurrent valproic acid