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