IMMU-31. PNOC007: H3.3K27M SPECIFIC PEPTIDE VACCINE COMBINED WITH POLY-ICLC FOR THE TREATMENT OF NEWLY DIAGNOSED HLA-A2+ H3.3K27M DIFFUSE MIDLINE GLIOMAS (DMG)

OBJECTIVE: To assess safety and efficacy within a multi-center trial the H3.3K27M specific peptide vaccine with poly-ICLC in HLA-A02.01(+) patients diagnosed with H3.3K27M(+) DMGs. METHODS: After focal radiation therapy, participants 3–21 years of age were enrolled into two strata. Stratum A: newly diagnosed diffuse intrinsic pontine glioma (DIPG); Stratum B: other DMGs. H3.3K27M vaccine was administered with poly-ICLC IM every 3 weeks for 8 doses followed by every 6 weeks for a total of 96 weeks. Immuno-monitoring of peripheral blood mononuclear cell (PBMC) and imaging occurred every 3 months. Modified iRANO criteria were applied. PBMC samples were evaluated by mass cytometry. RESULTS: 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 (7 grade 3; 0 grade 4 related toxicities). Median number of vaccines per participant 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. Among the 19 subjects with longitudinal immune cell assessments, 7 exhibited an expansion of K27M-reactive CD8+ effector memory T-cells correlating with prolonged survival (p=0.028). CONCLUSION: H3.3K27M specific vaccine in combination with poly-ICLC is well tolerated. CyTOF-based immune monitoring of PBMCs facilitates sensitive high-throughput analysis. Further investigation is warranted to determine if this may be predictive of clinical outcomes.