PL01.4. A LATE BREAKING ABSTRACT: A LONG PEPTIDE VACCINE TARGETING THE CLONAL DRIVER MUTATION H3K27M IN ADULT PATIENTS WITH DIFFUSE MIDLINE GLIOMA

Abstract

Abstract BACKGROUND Clonal and mutually exclusive substitution of lysine 27 to methionine in histone H3-3A (H3K27M) defines an aggressive subtype of diffuse glioma. These diffuse midline gliomas (DMG) are aggressive, incurable primary brain tumors that occur predominantly in a midline location in children and young adults. Surgical treatment options remain limited, response to chemoradiation is poor and palliative radiotherapy remains the only intervention with proven benefit, resulting in a median overall survival between 10 and 15 months after initial diagnosis. Previous studies have shown that a H3K27M-specific long peptide vaccine (H3K27M-vac) induces mutation-specific immune responses capable of controlling H3K27M+ tumors in a major histocompatibility complex (MHC)-humanized mouse model. MATERIAL AND METHODS Here we describe the first-in-human treatment with H3K27M-vac of eight adult patients with recurrent H3K27M+ DMG. Induction of mutation-specific immune responses was determined in peripheral blood using Interferon-γ ELISpot assays. Class II HLA-restricted peripheral H3K27M-specific T cells were expanded in vitro to characterize and track unique H3K27M-reactive T cell responses in blood and cerebrospinal fluid (CSF). RESULTS Repeat vaccinations with H3K27M-vac were safe and induced mutation-specific immune responses in five of eight patients after a median of two vaccinations. Median progression free survival after start of vaccination was 6.2 months and median overall survival was 12.8 months. In vitro restimulation of peripheral CD4+ and CD8+ T cells with H3 mutant peptide or wild type peptide revealed that H3K27M specific immune responses are CD4-mediated and can be suppressed by blocking antibodies against MHC II, but not MHC I. Furthermore, proximity ligation assay of formalin-fixed paraffin-embedded primary tumor tissue revealed that H3K27M neoepitope co-localizes with HLA class II-DR. One patient showed radiographic pseudoprogression (PsPD) according to iRANO criteria within six weeks after first detection of mutation-specific peripheral immune response. Following PsPD, three out of the top ten vaccine-induced, K27M-expanded CD4+ TCR clonotypes from peripheral blood were detectable in the CSF of this patient who went into sustained complete remission for > 31 months. CONCLUSION H3K27M-vac is safe and induces H3K27M-specific CD4 T cell responses in patients with H3K27M+ DMG. An ongoing multicenter, phase 1 clinical trial for adult patients with newly diagnosed H3K27M+ DMG will assess the safety and immunogenicity of H3K27M-vac in combination with atezolizumab in standard-of-care radiotherapy (NCT04808245).