Os04.3.a P30-Eps Peptide Vaccine Is Safe and Immunogenic in Newly Diagnosed Glioblastoma: A Phase 1b Trial

Abstract BACKGROUND The P30-EPS vaccine is a novel multi-epitope peptide immunotherapy targeting class I HLA-A2 epitopes of tumor-associated antigens (EphA2, CMV pp65, survivin) fused to a universal class II helper epitope (P30) to enhance CD4⁺ and CD8⁺ T cell activation. This phase 1b trial evaluated the safety, immunogenicity, and preliminary efficacy of P30-EPS in patients with newly diagnosed MGMT unmethylated glioblastoma. MATERIALS AND METHODS Eighteen patients (median age 60 years; 13 male, 5 female) with KPS ≥70 were enrolled following completion of chemoradiotherapy. Adjuvant temozolomide was omitted, and instead patients received intramuscular P30-EPS, 300 mg (n=6) in the first cohort and 400 mg (n=12) in the second cohort, scheduled in “priming” and “booster” phases to enhance the expansion of vaccine-specific T cells. During the booster phase, patients received biweekly Hiltonol (poly-ICLC). Safety was assessed according to CTCAE v5.0. Serial Peripheral Blood Mononuclear Cells (PBMCs) were collected for assessment of vaccine-specific immune responses via IFN-γ ELISPOT and multiparameter flow cytometry, and to define and track T Cell Receptor (TCR) repertoire changes. Patients with ambiguous MRI findings during treatment underwent biopsy to differentiate progression from pseudoprogression, with plans for single-nuclei RNA sequencing (snRNA-seq) on paired baseline and post-vaccine tumors. RESULTS Accrual of 18 patients was completed in February 2025. To date, adverse events have included grade 1 injection site reactions (n=6) and grade 1 bradycardia (n=1), with no grade 2-5 adverse events reported. Eleven of twelve patients evaluated to date demonstrated increased IFN-γ-producing T cells by ELISPOT. Patients receiving the 400 mg dose exhibited a greater magnitude of IFN-γ⁺ ELISPOT responses during the booster phase (vaccines 6 and 7) and a broader response across vaccine antigens compared to the 300 mg cohort, which peaked earlier and declined. Flow cytometry showed stable T and NK T cell populations across time points, while CD19⁺ B cell frequencies increased prior to vaccine 7, correlating with Hiltonol administration. Vaccine immunogenicity was observed regardless of HLA-A2 status. CONCLUSION P30-EPS administered with Hiltonol demonstrated a favorable safety profile and elicited vaccine-specific immune responses in patients with newly diagnosed glioblastoma. The 400 mg dose was associated with stronger immunogenicity and early signals of clinical activity. We will present clinical outcomes, including survival and MRI responses for all 18 patients, along with longitudinal 5′ scRNA VDJ TCR profiling of PBMCs and paired baseline and on-treatment tumor snRNA-seq. Analysis of TCR diversity dynamics and their association with clinical benefit, particularly in patients lacking HLA-A2 expression, may inform future vaccine development and optimization.