GENE-43. LIQUID BIOPSY FOR MONITORING OF TUMOR RESPONSE IN CHILDREN WITH DIFFUSE MIDLINE GLIOMA

Children diagnosed with diffuse midline histone 3 K27M mutant (H3K27M) glioma continue to have a dismal prognosis with a median survival time of about nine months post-diagnosis. MRI, the standard for assessing tumor growth, is often not sensitive for detecting early tumor regression/progression. We evaluate the utility of biofluids from patients and their xenograft models, for detection of circulating tumor DNA (ctDNA). The goal is to develop a comprehensive panel for ctDNA detection and quantification, capable of assessing response to treatment. Twenty two CSF samples were collected at various stages of disease (upfront, recurrence, and postmortem) from pediatric high grade midline gliomas. Upfront and longitudinal plasma samples (n=68) were collected with each MRI, from 19 diffuse intrinsic pontine glioma (DIPG) patients enrolled in a Phase I clinical trial. For monitoring ctDNA, we developed a liquid biopsy assay using a sensitive and robust digital droplet PCR system on CSF and plasma samples. We successfully detected major DIPG driver mutations including H3F3A, HIST1H3B, and ACVR1. Specifically, we detected H3K27M in 80% of CSF and upfront plasma samples. Our data indicate a concordance of H3K27M in tumor and CSF for 70% of patients (n=13). In accordance with published studies in adult CNS cancers, we detected higher levels of ctDNA in CSF compared to plasma. Longitudinal analysis of plasma showed a drastic decrease in ctDNA following radiotherapy in 86% of patients (n=14), and patients were categorized as responders, where dynamic changes in ctDNA correlated with changes in MRI tumor measurements. Furthermore, we have detected H3K27M in ctDNA of plasma samples in xenograft models of DIPG. We conclude that biofluid is a suitable medium for detection and quantification of ctDNA. Liquid biopsy has the potential to complement, or in some cases eliminate the need for biopsies, and may inform recurrence and tumor response to treatment.