DIPG-48. PRECISION GUIDED THERAPY PROVIDES CLINICAL BENEFIT IN H3K27-ALTERED DIFFUSE MIDLINE GLIOMA

BACKGROUND The utility of precision-guided therapeutic approaches to H3K27-altered diffused midline glioma (DMG) remains uncertain. The Australian Zero Childhood Cancer (ZERO) program combines molecular profiling (whole-genome sequencing (WGS), whole-transcriptome sequencing (RNAseq), and DNA methylation profiling), with in vitro high-throughput drug screening (HTS) and patient-derived xenograft (PDX) drug-efficacy testing in patients with high-risk cancer. METHODS We report on the cohort of patients enrolled with a DMG in the ZERO clinical trial between 2017 and 2023. RESULTS 68 patients were enrolled, predominantly at diagnosis (87%), with pontine lesions (66%) at a mean age of 8.7 years. 59 (87%) patients had WGS + RNAseq successfully performed. Most tumors harbored an H3F3A mutation (n=50), followed by HIST1H3B (n=11), HIST1H3C (n=2), HIST2H3C (n=1) and EZHIP overexpression (n=4). 37 samples were suitable for cell culture, with HTS successfully performed in 25 cases, and PDXs established in 7. Precision-guided therapy (PGT) was recommended by the ZERO multidisciplinary tumor board for 53 patients (78%), including 5 recommendations guided by HTS. PI3K/mTOR inhibitors and MEK inhibitors were the most recommended PGTs. 21 patients (31%), receiving 24 PGTs, were eligible for survival analysis. The clinical benefit rate (per RAPNO) was 52% for patients receiving PGT, with 5 patients exhibiting PR and 6 patients SD >6 months. Notably, the 5 PR included: a PR in a patient with BRAFV600E mutation to dabrafenib/trametinib, a near-CR in a patient with germline BRCA2 mutation to olaparib/durvalumab, a sustained PR in a patient with FGFR1 mutation to trametinib/bevacizumab and 2 patients with PIK3CA mutations who received PGT in the context of recent radiotherapy. Median overall survival of the PGT group was 21.2 months versus 11.8 months for the rest of the cohort (hazard ratio 0.67 [95%CI 0.39-1.1]). CONCLUSIONS These results, for the first time, highlight the potential clinical benefit of PGT in H3K27-altered DMG.