Brief Summary: Phase I trial to study the safety of combining O6-benzylguanine with temozolomide in treating children who have recurrent or refractory brain tumors. Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. O6-benzylguanine may increase the effectiveness of temozolomide by making tumor cells more sensitive to the drug.

Detailed Description: PRIMARY OBJECTIVES: I. To determine the maximum tolerated dose of temozolomide (Temodar) when administered with O6-benzylguanine (O6-BG) with and without G-CSF support to pediatric patients with refractory brain tumors stratified by previous radiotherapy. SECONDARY OBJECTIVES: I. To characterize the pharmacokinetics of temozolomide and O6-BG when used in combination. II. To characterize toxicities associated with the combination of O6-BG and temozolomide with and without G-CSF support. III. To document antitumor response in patients when treated with O6-BG and temozolomide. IV. To determine the levels of MGMT enzyme and mismatch repair (MMR) proteins in tumor tissue, investigating a possible correlation with patient outcome. OUTLINE: This is a dose-escalation study of temozolomide with and without filgrastim (G-CSF). Patients are stratified according to prior radiotherapy (RT)/myeloablative therapy (no RT or focal RT vs craniospinal RT or myeloablative therapy). Patients receive O6-benzylguanine IV continuously on days 1 and 2 and oral temozolomide on day 1. Treatment repeats every 28 days for up to 12 courses in the absence of disease progression or unacceptable toxicity. Cohorts of 2-6 patients in each stratum receive escalating doses of temozolomide until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose at which 25% of patients experience DLT. Once the MTD is determined, additional patients are treated at that dose level for a total of 12 patients treated at the MTD. For courses 1-12, patients experiencing neutropenia may also receive G-CSF subcutaneously or IV daily beginning on day 3 and continuing until blood counts recover. If neutropenia is the dose-limiting toxicity (DLT) for the first 2 strata, patients are further stratified according to concurrent G-CSF support (yes vs no).Cohorts of 3-6 patients in each stratum receive escalating doses of temozolomide with G-CSF until the MTD is determined. The MTD is defined as the dose preceding that at which at least 2 of 3 or 2 of 6 patients experience DLT. Once the MTD is determined, 6 additional patients are treated at that dose. Patients are followed for resolution of all adverse events occurring while on treatment and/or within 30 days of the last administration of study drug. Patients will be followed for the shortest of 1) three months after the last protocol based treatment, or 2) the date other therapy is initiated.