DISEASE CHARACTERISTICS: Histologically confirmed supratentorial anaplastic oligodendroglioma or anaplastic oligoastrocytoma not requiring immediate radiotherapy Newly diagnosed, progressive, or recurrent disease Bidimensionally measurable disease At least 1.5 cm2 by MRI

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 70-100% Life expectancy: More than 12 weeks Hematopoietic: Neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hemoglobin at least 10 g/dL Hepatic: Bilirubin less than 1.5 times upper limit of normal (ULN) SGOT/SGPT less than 2.5 times ULN Alkaline phosphatase less than 2 times ULN Renal: BUN less than 1.5 times ULN Creatinine less than 1.5 times ULN Other: Neurologically stable No nonmalignant systemic disease No acute infection treated with intravenous antibiotics No frequent vomiting No medical condition (e.g., partial bowel obstruction) that would interfere with oral medication intake No other prior or concurrent malignancy except surgically cured carcinoma in situ of the cervix or basal cell or squamous cell carcinoma of the skin No AIDS-related illness HIV negative Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception

PRIOR CONCURRENT THERAPY: Biologic therapy: No more than 1 prior biologic therapy No concurrent biologic therapy No concurrent growth factors (e.g., epoetin alfa) Chemotherapy: No more than 1 prior chemotherapy regimen No other concurrent chemotherapy Endocrine therapy: Must be on stable dose of steroids for at least 1 week prior to study Concurrent steroids allowed Radiotherapy: See Disease Characteristics No concurrent radiotherapy Surgery: At least 2 weeks since prior surgical resection (newly diagnosed patients must be enrolled within 28 days of surgery or biopsy) Recovered from prior major surgery Other: No other concurrent investigational drugs