DISEASE CHARACTERISTICS:

• Histologically confirmed* diagnosis of 1 of the following:

  • Recurrent or refractory solid tumor or lymphoma (for patients in group 1)

    • Measurable or evaluable disease

  • Recurrent or refractory leukemia (for patients in group 2)

    • Greater than 25% blasts in the bone marrow (i.e., M3 bone marrow)
    • Active extramedullary disease allowed except leptomeningeal disease

  • Recurrent or refractory CNS tumor of 1 of the following types (for patients in group 3):

    • Neuroblastoma
    • Medulloblastoma/CNS primitive neuroectodermal tumor
    • Atypical teratoid rhabdoid tumor NOTE: *Histologic confirmation not required for intrinsic brain stem tumors
• No known curative therapy or therapy proven to prolong survival with an acceptable quality of life exists
• No bone marrow involvement by disease (for patients in groups 1 and 3)
• No active CNS leukemia

PATIENT CHARACTERISTICS:

Age

• 1 to 21

Performance status

• Lansky 50-100% (for patients ≤ 10 years of age)
• Karnofsky 60-100% (for patients > 10 years of age)

Life expectancy

• Not specified

Hematopoietic

• Absolute neutrophil count ≥ 1,000/mm^3 (for solid tumor patients)
• Platelet count ≥ 100,000/mm^3* (for solid tumor patients) (20,000/mm^3** for leukemia patients)
• Hemoglobin ≥ 8.0 g/dL (RBC transfusion allowed) (for solid tumor and leukemia patients)
• Triglycerides < 300 mg/dL (for patients in group 3) NOTE: *Transfusion independent

NOTE: **Transfusion allowed

Hepatic

• Bilirubin ≤ 1.5 times upper limit of normal (ULN)
• ALT ≤ 5 times ULN
• 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)
• Negative dipstick for protein OR < 1,000 mg protein/24 hour urine collection (for patients in group 3)
• No evidence of gross hematuria (for patients in group 3)

Other

• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception
• Body surface area ≥ 0.5 m^2
• Neurologic deficits in patients with CNS tumors must be stable for ≥ 1 week before study entry
• Able to swallow whole capsules
• No uncontrolled infection
• Skin toxicity < grade 1 (for patients in group 3)

PRIOR CONCURRENT THERAPY:

Biologic therapy

• Recovered from prior immunotherapy
• At least 7 days since prior hematopoietic growth factors
• At least 7 days since prior antineoplastic biologic agents

• At least 2 months since prior stem cell transplantation or rescue

  • No evidence of active graft-versus-host disease
• No other concurrent biologic therapy or immunotherapy

Chemotherapy

• More than 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosoureas) and recovered
• No concurrent chemotherapy

Endocrine therapy

• Patients with CNS tumors must be on a stable or decreasing dose of dexamethasone for ≥ 7 days prior to study entry
• No concurrent dexamethasone for antinausea or antiemetic therapy

Radiotherapy

• Recovered from prior radiotherapy
• At least 2 weeks since prior local, palliative, small-port radiotherapy
• At least 3 months since prior total-body irradiation, radiotherapy to the craniospinal area, or radiotherapy to ≥ 50% of the pelvis
• At least 6 weeks since other prior substantial radiotherapy to the bone marrow
• No concurrent radiotherapy

Surgery

• Not specified

Other

• At least 2 weeks since prior valproic acid
• No other concurrent investigational agents
• No other concurrent anticancer therapy
• No concurrent enzyme-inducing anticonvulsants