Inclusion Criteria:

- 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

- 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

- Performance status - Lansky 50-100% (for patients ≤ 10 years of age)

- Performance status - Karnofsky 60-100% (for patients > 10 years of age)

- 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)

- Bilirubin ≤ 1.5 times upper limit of normal (ULN)

- ALT ≤ 5 times ULN

- Albumin ≥ 2 g/dL

- Creatinine clearance or radioisotope glomerular filtration rate ≥ 70 mL/min

- 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)

- 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)

- 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

- More than 3 weeks since prior myelosuppressive chemotherapy (6 weeks for
nitrosoureas) and recovered

- No concurrent chemotherapy

- 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

- 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

- At least 2 weeks since prior valproic acid

- No other concurrent investigational agents

- No other concurrent anticancer therapy

- No concurrent enzyme-inducing anticonvulsants