Inclusion Criteria

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