LAUSR

To assess within a multi-center trial the safety of H3.3K27M specific peptide vaccine in combination with poly-ICLC in HLA-A02.01+ patients diagnosed with H3.3K27M+ diffuse midline gliomas. Subjects 3–21 years of age with CLIA confirmed H3.3K27 mutation status were eligible. Patients were enrolled after completion of focal radiation therapy. The trial enrolled two strata: Stratum A included newly diagnosed patients with diffuse intrinsic pontine glioma (DIPG) (n=19); Stratum B included non-DIPG midline glioma patients including spinal cord tumors as an exploratory strata (n=10). The H3.3K27M vaccine was administered in combination with poly-ICLC every 3 weeks for a total of 8 doses followed by every 6 weeks for a maximum of 96 weeks of therapy. Immuno-monitoring and imaging occurred every 3 months. Modified iRANO criteria are used for ongoing assessment of disease status. Patients’ peripheral blood mononuclear cell (PBMC) samples were evaluated by mass cytometry. From November 2016 until March 2019, 19 eligible patients (median age 11, range 5–17 yrs; 53 % female) were enrolled in Stratum A and 10 eligible patients (median age 13, range 7–18 yrs; 60 % female) in Stratum B. Treatment was well tolerated with 7 grade 3 and 0 grade 4 treatment related toxicities. The most common related side effect included injection site reaction. Median number of vaccine administrations per patient was 6 (range 1–11). Overall survival at 12 months was 40% (95% CI 22–73%) for Stratum A and 39% (95% CI 16–93%) for Stratum B. A mass cytometry-based analysis of patient PBMCs (n = 16) revealed a trend towards an inverse correlation between polymorphonuclear myeloid-derived suppressor cells and the expansion of H3.3K27M-specific CD8+ T-cells (p-value = 0.154). H3.3K27M specific vaccine in combination with poly-ICLC is well tolerated. Further evaluation will include a combination with checkpoint inhibitor nivolumab.