Background: The looser chromatin structure seen following treatment with histone deacetylase (HDAC) inhibitors is hypothesized to render cells more sensitive to drugs targeting DNA. A phase I trial of vorinostat (VOR), an HDAC inhibitor, combined with temozolomide (TEM) was conducted to investigate the maximum tolerated dose (MTD) and dose limiting toxicity (DLT) of this combination. VOR pharmacokinetics (PK), serum MGMT promoter status, and histone acetylation in peripheral blood mononuclear cells were also assessed. Methods: VOR followed by TEM were orally administered, once daily, for 5 consecutive days every 28 days at 3 dose levels using the rolling 6 design. The respective dose levels for VOR and TEM were: (1) 230 mg/m²/day; 150 mg/m²/day, (2) 300 mg/m²/day; 150 mg/m²/day and (3) 300 mg/m²/day; 200 mg/m²/day. Studies of histone accumulation in peripheral blood mononuclear cells were performed on day 1 at 0, 6, and 24 h after vorinostat dosing. Vorinistat pharmacokinetics (PK) and serum MGMT promoter status were also assessed. Results: Nineteen eligible patients, 12 males, median age 8 yrs (range, 2-20 ) with high grade glioma [n=7], ependymomas [n=4], medulloblastoma [n=2], PNET [n=2], ATRT [n=2], carcinoma [n=1], and ganglioglioma [n=1] were enrolled. Seventeen patients were fully evaluable for toxicity. There were no DLTs observed at dose level 1 (n=6) or 2 (n=6). DLT (myelosuppression) occurred in 2 patients at dose level 3: thrombocytopenia [n=2] and neutropenia [n=1]. Non-dose–limiting grade 3 or 4 toxicities related to protocol therapy were hematologic and included neutropenia [n=8], lymphopenia [n=7], thrombocytopenia [n=6], anemia [n=1] and leucopenia [n=6]. No grade 5 drug-related serious adverse events were observed. Results of PK and correlative biology studies will be presented. Conclusions: Five-day courses of VOR 300 mg/m²/day in combination with TEM 150 mg/m²/day are well tolerated in children with recurrent CNS malignancies.