DISEASE CHARACTERISTICS:

• Histologically confirmed recurrent or progressive brain tumor after radiotherapy, chemotherapy, or bone marrow transplantation

  • High grade glioma (anaplastic astrocytoma or glioblastoma multiforme)
  • Brain stem glioma
  • Medulloblastoma or primitive neuroectodermal tumors present in supratentorial or posterior fossa
  • Other
• Histological confirmation waived in brain stem tumors
• Patients previously treated with radiosurgery require a biopsy, PET scan or NMR spectroscopy
• Measurable disease by CT or MRI imaging that clearly demonstrates recurrent or progressive nature of the lesion
• Histologic evidence of bone marrow involvement allowed

PATIENT CHARACTERISTICS:

Age:

• 2 to 21

Performance status:

• Lansky 50-100% (10 and under)
• Karnofsky 50-100% (over 10)

Life expectancy:

• At least 8 weeks

Hematopoietic:

• Absolute neutrophil count at least 1,000/mm^3*
• Platelet count at least 50,000/mm^3*
• Hemoglobin at least 8.0 g/dL* NOTE: *Transfusion allowed

Hepatic:

• Bilirubin no greater than 1.5 mg/dL
• SGPT no greater than 2 times normal

Renal:

• Creatinine normal for age OR
• Creatinine clearance at least 70 mL/min

Other:

• No other concurrent significant systemic illness (e.g., infection)
• No significant electrolyte abnormalities
• No amino acidurias or organic acidemias
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception during and for 6 months after study

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• See Disease Characteristics
• Recovered from toxic effects of prior immunotherapy
• Prior bone marrow transplantation allowed
• No concurrent prophylactic hematopoietic growth factors except for neutropenia or documented infection

Chemotherapy:

• See Disease Characteristics
• At least 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosoureas)
• Recovered from prior chemotherapy
• No other concurrent chemotherapy

Endocrine therapy:

• If receiving dexamethasone must be on stable or decreasing dose for 2 weeks prior to study

Radiotherapy:

• See Disease Characteristics
• Recovered from prior radiotherapy
• More than 8 weeks since prior radiotherapy to evaluable lesion
• More than 4 months since prior radiosurgery to evaluable lesion
• Prior extensive radiotherapy (i.e., craniospinal radiotherapy or field encompassing region greater than hemipelvis) allowed

Surgery:

• See Disease Characteristics
• See Radiotherapy

Other:

• No other concurrent investigational agents
• Concurrent electrolyte supplements to maintain electrolyte levels allowed