Abstract 6477: Peptide-based tumor-associated antigen vaccine in glioblastoma: Correlative results of the ETAPA Phase I clinical trial

Abstract Introduction/Rationale: Immunotherapy has yet to yield benefit in glioblastoma (GBM), in part due to inadequate T-cell priming and insufficient CD4+ helper support. To enhance coordinated CD4+ and CD8+ activation, the ETAPA phase 1 trial (NCT05283109) evaluated P30-EPS, a peptide vaccine linking three class I tumor-associated antigens (CMV pp65, Survivin, EphA2) to the universal class II tetanus toxoid epitope P30 (TVSFWLRVPKVSASHLE). This design aims to broadly recruit CD4+ T-cell help and augment cytotoxic responses. Methods: Adults with newly diagnosed, MGMT-unmethylated GBM (n = 18) received seven post-radiation intramuscular vaccinations with Hiltonol (20 µg/kg) and either 300 µg (n = 6) or 400 µg (n = 12) of P30-EPS. Peripheral blood mononuclear cells (PBMCs) were analyzed longitudinally by IFN-γ ELISpot, high-parameter spectral flow cytometry, and 5′ scRNA TCR sequencing to quantify antigen-specific cytokine production, helper and cytotoxic phenotypes, lineage distribution, clonal expansion, and repertoire diversity. Paired single-cell V(D)J TCR sequencing of PBMCs was used to resolve CD4+ and CD8+ subsets, define activation and memory-associated transcriptional programs, and map clonotype-specific trajectories. Baseline and on-treatment tumors are being profiled using spatial transcriptomics (ST) and spatial proteomics (SP) to localize vaccine-responsive T-cell populations within the tumor microenvironment. Results: The vaccine was well tolerated in all 18 patients, with all adverse events ≤ grade 2. P30-EPS induced antigen-specific IFN-γ responses in 11 patients, with stronger recall responses at the 400-µg dose. Patients with longer progression-free survival exhibited higher baseline frequencies of naïve CD4+ T cells and fewer terminally differentiated effector-memory (CD45RA+CCR7-) cells, indicating a more plastic helper pool capable of sustaining CD8+ priming. Longitudinal TCR sequencing demonstrated expansion of vaccine-associated clonotypes after boosting, including emergence of new low-frequency clones and persistence of dominant clones, consistent with durable memory formation. Early single-cell transcriptomic and paired TCR analyses revealed coordinated activation signatures in helper and cytotoxic compartments that paralleled peripheral clonotype expansion. Pre- and post-vaccination tumor ST and SP analyses will be presented. Conclusion: ETAPA achieved its primary objectives, demonstrating safety and correlative findings indicating that a universal CD4+ helper epitope can enhance cytotoxic and memory responses in GBM. Integration of PBMC scRNA/TCRseq with tumor ST and SP profiling will define intratumoral correlates of systemic vaccine-induced immunity. These insights now support development of our next epitope-linked personalized vaccine for patients with glioblastoma. Citation Format: Elizabeth Owens, Kelly M. Hotchkiss, Pamela Norberg, Evan Buckley, Katayoun Ayasoufi, Stevie Threatt, Justin T. Low, Madison L. Shoaf, Melody Goldston, Kristen Batich, Margaret O. Johnson, James E. Herndon, Smita K. Nair, Kent Weinhold, Henry S. Friedman, David M. Ashley, Annick Desjardins, John H. Sampson, Mustafa Khasraw. Peptide-based tumor-associated antigen vaccine in glioblastoma: Correlative results of the ETAPA Phase I clinical trial abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6477.