Background: Bevacizumab is frequently used to treat recurrent GBM, but responses are generally not durable. Panobinostat is a histone deacetylase inhibitor with anti-neoplastic and anti-angiogenic effects in GBM and may work synergistically with bevacizumab. We conducted a multicenter phase I/II trial of panobinostat in combination with bevacizumab in patients with recurrent GBM. Methods: In the phase II trial, patients with recurrent GBM were treated with oral panobinostat 30 mg three times per week, every other week, in combination with bevacizumab 10 mg/kg every other week. The primary endpoint was 6-month progression-free survival (PFS6) and the study was powered to discriminate between a 35% and 55% PFS6 rate (85% power at an alpha level of 0.07). A planned interim analysis specified suspension of accrual and careful data review if 12 or more of the first 21 patients accrued to the study progress within 6 months of initiating treatment. Patients with recurrent GBM enrolled in the phase I study at the maximum tolerated dose (which is the phase II dose) were eligible for inclusion in the interim analysis. Results: Thirteen of the first 21 patients accrued to the GBM arm of the study had progressed within 6 months of initiating study treatment. The study was closed to further accrual and a planned interim analysis was performed. Median age was 53 (range 22-66) and median KPS was 80% (60%-100%). PFS6 rate was 33.9% [95% CI 12.8, 56.5), median was PFS 5 months [95% CI 3 months, NR], and median OS was 342 days [95% CI 203 days, NR]. Five patients (23.8%) achieved partial responses. Conclusions: Although reasonably well-tolerated, this phase I/II study of panobinostat and bevacizumab in recurrent GBM did not meet criteria for continued accrual and the study was closed. Updated outcome and safety data will be presented at the meeting. Study Supported by: Novartis and Genentech Clinical trial information: NCT00859222.