DISEASE CHARACTERISTICS: Histologically proven recurrent anaplastic oligodendroglioma or anaplastic mixed oligoastrocytoma following primary surgery and radiation therapy Tumors for anaplastic mixed oligoastrocytoma must contain at least 25% oligodendroglial elements Tumors must be clinically aggressive for patients with only 1 anaplastic feature Prior low grade oligodendrogliomas or oligoastrocytomas undergoing repeat biopsy following clinical or radiological progression are eligible Bidimensionally measurable and progressive lesions by CT or MRI

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-3 Life expectancy: Not specified Hematopoietic: Absolute granulocyte count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hepatic: Not specified Renal: Creatinine normal Other: Not pregnant or nursing Fertile patients must use effective contraception No prior malignancies except curatively treated carcinoma in situ of the cervix or nonmelanoma skin cancer No other serious illness or medical condition No active uncontrolled infection No history of neurologic or psychiatric disorder

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: At least 6 weeks since chemotherapy No more than 1 prior chemotherapy regimen No prior camptothecin derivatives Endocrine therapy: At least 2 weeks on stable steroid therapy, if necessary Radiotherapy: At least 2 months since prior radiotherapy No prior radiation therapy for recurrent disease Surgery: At least 6 weeks since prior surgery (except biopsy only) Prior surgery for recurrent disease allowed (including stereotactic biopsy or partial resection) Other: No concurrent experimental drugs or anticancer therapy