Background: Cytomegalovirus (CMV) antigens have been reported in over 90% of GBM tumors. CD4⁺and CD8⁺T cells are most frequently directed against the gB and pp65 antigens, respectively, and are immunogenic targets in a CMV-based GBM vaccine. Methods: We have initiated a phase I/IIa clinical trial for patients with recurrent GBM using gB/pp65 enveloped virus-like particles (eVLPs) formulated with GM-CSF and administered intradermally. Subjects are vaccinated monthly until tumor progression, with immunomonitoring performed 2 weeks after each vaccination and MRI exams every 6 weeks. In phase I, eligible patients were age 18-70 with Karnofsky Performance Status at least 70, normal end-organ function, on stable or decreasing corticosteroids of at most 4mg dexamethasone (or equivalent), with recurrent GBM following any standard initial therapy and any number of recurrences. The primary endpoint was safety/tolerability and secondarily to assess immunogenicity. Three vaccine doses (0.4µg, 2µg, and 10µg pp65) were evaluated with 6 subjects in each cohort and DSMB safety review of the first 3 subjects in each cohort prior to enrolling additional subjects. Results: The DSMB identified no DLTs or safety concerns with any of the doses. Grade 2, 3 or 4 AEs occurred in 66%, 22% and 11% of participants, respectively, but were not related to vaccine administration. Twelve men and 6 women were enrolled with a median age 54 (range 39-66). Prior therapies included radiotherapy, temozolomide, and nivolumab. Immunological analyses demonstrate robust boosting of CMV-specific antibody titers and T cell responses against both gB and pp65 antigens in some but not all subjects, across all dose cohorts. Boosting of IFN-gsecreting T cells (measured by ELISPOT) exceeded the assay threshold for several subjects. Stable disease by MRI of 3 months or greater has been observed in 2 subjects in the high dose cohort and 1 subject in the low dose cohort and may correlate with vaccine response. Conclusions: The phase IIa extension phase of the trial planned to begin in Q2 2019 is designed to explore efficacy in an additional 10 subjects that will receive the optimal vaccine dose and includes the additional requirements of unifocal, measurable enhancing tumor 1-3 cm across at first recurrence and no prior immunotherapy. Clinical trial information: NCT03382977