Inclusion Criteria

DISEASE CHARACTERISTICS: Histologically confirmed recurrent or progressive brain tumors
including the following: Anaplastic astrocytoma and glioblastoma multiforme Brain stem
glioma Medulloblastoma or primitive neuroectodermal tumors present in supratentorial or
posterior fossa locations Ependymoma Low grade gliomas Other Measurable or evaluable
disease by CT or MRI OR Histologically confirmed previously untreated glial tumors
including: Brain stem glioma Glioblastoma multiforme Measurable disease after surgery

PATIENT CHARACTERISTICS: Age: 2 to 21 Performance status: Karnofsky 50-100% (over 10 years
of age) Lansky 50-100% (under 10 years of age) Life expectancy: At least 8 weeks
Hematopoietic: Absolute neutrophil count greater than 1,000/mm3 Platelet count greater
than 50,000/mm3 Hemoglobin greater than 7.0 g/dL Transfusion support allowed after bone
marrow transplantation or extensive radiation Hepatic: Bilirubin less than 1.5 mg/dL SGPT
less than 2 times normal Renal: Creatinine no greater than 1.5 mg/dL OR Creatinine
clearance greater than 60 mL/min Other: Not pregnant or nursing Fertile patients must use
effective contraception during and for 6 months after study No significant systemic
illness including infections No amino acidurias or organic acidemias

PRIOR CONCURRENT THERAPY: Biologic therapy: Recovered from all prior immunotherapy No
concurrent prophylactic hematopoietic growth factors Chemotherapy: At least 3 weeks since
prior myelosuppressive chemotherapy (6 weeks for nitrosourea) and recovered No other
concurrent cancer chemotherapy Endocrine therapy: Stable or decreasing dosage of
dexamethasone for intracranial pressure within 2 weeks of study entry No concurrent
dexamethasone used as an antiemetic Radiotherapy: At least 8 weeks since prior
radiotherapy to evaluable lesions and recovered Surgery: At least 4 months since prior
radiosurgery Other: No other concurrent investigational agents