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