BIOM-47. LONGITUDINAL MONITORING OF PLASMA H3K27M IN DIFFUSE MIDLINE GLIOMA PATIENTS TREATED WITH ONC201

Abstract

Abstract Diffuse midline glioma, H3 K27M-mutant (DMG) is a 2016 WHO Grade IV glioma that has no established treatment beyond first-line radiation. ONC201 is an investigational small molecule that has been shown to be clinically active in recurrent DMG clinical trials. While biopsies of DMG are sometimes feasible, many patients defer secondary to complication risk. MR scans have many limitations in monitoring DMG progression, including distinguishing pseudoprogression and pseudoresponse and measuring diffuse lesions that often do not contrast enhance. Digital droplet PCR (ddPCR) is capable of sensitively detecting and quantifying the allelic frequency of circulating-tumor DNA (ctDNA) fragments against a backdrop of non-tumor DNA. Using sequence-specific probes for H3F3A (H3.3 K27M) and HIST1H3B (H3.1 K27M) ddPCR detects very low frequency variants and provides an assessment of mutational burden. A pilot cohort of 5 patients treated with ONC201 who had a range of outcomes were assessed with serial ctDNA analyses. Two patients with immediately progressive disease had a concordant H3 K27M ctDNA increase that precedes radiographic detection by 4 weeks. Two patients with >50% tumor regressions while on ONC201 had concordant H3 K27M ctDNA burden at the onset of response and subsequent radiographic progression was preceded by increases in ctDNA 8–16 weeks prior. One patient who had prolonged stable disease had decreased H3 K27M ctDNA burden over time. Upon radiographic progression, the addition of bevacizumab with ONC201 caused a radiographic pseudoresponse, however H3 K27M ctDNA remained stable. These pilot results suggest H3 K27M ctDNA may be a sensitive and accurate biomarker of disease burden. Longitudinal evaluation of H3 K27M ctDNA in a cohort of 34 recurrent contrast-enhancing H3 K27M-mutant glioma patients while on ONC201 will be reported. Primary tumor locations range across the thalamus, cerebellum, basal ganglia, temporal lobe, and midbrain; median age is 31 years old (range 20–70).