Valproic Acid in Treating Young Patients With Recurrent or Refractory Solid Tumors or CNS Tumors

DISEASE CHARACTERISTICS: Histologically confirmed* malignant solid tumor, including CNS tumors, at original diagnosis or relapse Recurrent or refractory disease NOTE: *Histologic confirmation not required for intrinsic brain stem or optic pathway tumors Measurable or evaluable disease, defined by 1 of the following criteria: Any unidimensionally measurable lesion ≥ 10 mm by standard MRI or CT scan for either solid or CNS tumors At least 1 nonmeasurable lesion that is evaluable by nuclear medicine, immunocytochemistry, tumor markers, cerebrospinal fluid cytology, or other reliable measures No known curative therapy exists No documented tumor involvement in the bone marrow PATIENT CHARACTERISTICS: Age 2 to 21 Performance status* Lansky 50-100% (for patients ≤ 10 years of age) Karnofsky 50-100% (for patients > 10 years of age) Life expectancy Not specified Hematopoietic Absolute neutrophil count ≥ 1,000/mm^3 Platelet count ≥ 100,000/mm^3 (transfusion independent) Hemoglobin ≥ 8.0 g/dL (transfusions allowed) Hepatic Bilirubin ≤ 1.5 times upper limit of normal (ULN) ALT ≤ 110 (ULN for this study is 45 U/L) Albumin ≥ 2 g/dL Renal Creatinine clearance or radioisotope glomerular filtration rate ≥ 70 mL/min OR Creatinine based on age as follows: No greater than 0.8 mg/dL (for patients ≤ 5 years of age) No greater than 1.0 mg/dL (for patients 6 to 10 years of age) No greater than 1.2 mg/dL (for patients 11 to 15 years of age) No greater than 1.5 mg/dL (for patients over 15 years of age) Other Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception Neurologic deficits in patients with CNS tumors must be stable for ≥ 1 week before study entry No uncontrolled infection No known urea cycle disorders or other metabolic disorders No other condition that would preclude study compliance PRIOR CONCURRENT THERAPY: Biologic therapy Recovered from prior immunotherapy At least 7 days since prior hematopoietic growth factors that support platelet or WBC number or function At least 7 days since prior antineoplastic biologic agents At least 3 months since prior stem cell transplantation or rescue without total body irradiation No evidence of active graft vs host disease No other concurrent anticancer biologic therapy or immunotherapy Chemotherapy More than 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosoureas) and recovered No other concurrent anticancer chemotherapy Endocrine therapy Patients with CNS tumors must be on a stable or decreasing dose of dexamethasone for the past 7 days Radiotherapy See Biologic therapy Recovered from prior radiotherapy At least 6 months since prior total body irradiation, craniospinal radiotherapy, or radiotherapy to ≥ 50% of the pelvis At least 6 weeks since other prior substantial bone marrow radiotherapy At least 2 weeks since prior local palliative small port radiotherapy No concurrent anticancer radiotherapy Surgery Not specified Other No other concurrent investigational agents No other concurrent anticancer agents No other concurrent anticonvulsants Patients receiving valproic acid (VPA) before study entry must have a total trough VPA concentration < 100 mcg/mL within the past 7 days