PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent biologic therapy Chemotherapy: No prior nitrosourea or temozolomide No more than 1 prior chemotherapy regimen allowed for patients with glioblastoma At least 6 weeks since mitomycin or procarbazine and recovered At least 4 weeks since other prior chemotherapy and recovered No other concurrent chemotherapy Endocrine therapy: If receiving steroids, must be on a stable steroid dose for at least 72 hours prior to study No other concurrent endocrine therapy Radiotherapy: At least 6 weeks since radiotherapy No greater than 10-20% of marrow irradiated in prior radiotherapy No other concurrent radiotherapy Surgery: Surgery allowed