IMMU-08. SALVAGE IMMUNOTHERAPIES FOR REPLICATION REPAIR DEFICIENT (RRD) HIGH-GRADE GLIOMA FAILING ANTI-PD1 MONOTHERAPY: AN IRRDC REPORT

BACKGROUND: Although the initial response to immune-checkpoint inhibition (ICI) for patients with DNA replication-repair deficient high-grade glioma (RRD-HGG) is encouraging, the role of immune-based salvage approaches for those progressing on anti-PD1 monotherapy is unknown. METHODS: We performed an international registry study of patients managed using central molecular, genomic, radiological review and treatment recommendations between 2015-2021. Post-progression treatment included re-irradiation where feasible, and continuation of anti-PD1 with anti-CTLA4 (ipilimumab), or a MEK-inhibitor (MEKi). Outcomes included radiological response (iRANO), toxicity, second progression-free (PFS2) and overall survival (OS2). Companion biomarkers were performed centrally. RESULTS: Among 75 patients with RRD-HGG receiving PD-1 blockade, 20 are progression-free at a median follow-up of 44.6-months. For 55 patients with relapsed/progressive tumors, continuation of ICI (n=38) resulted in median OS2 of 11.6-months (51% alive) versus 1.2-months when ICI was discontinued (n=17; no survivors, p<0.001). The combination of ipilimumab/nivolumab (n=24) resulted in response/stable disease in 75% with median OS2 of 12.1-months but high autoimmune toxicities (54%). The addition of MEKi led to response in 3/5 patients with prolonged survival. The addition of re-irradiation improved median OS2, especially in tumors with lower mutation burden (p=0.002), and those who received ipilimumab (median OS2=33-months). Several biological insights were gained. Early radiological ‘flare’ in 33% of RRD-HGG on combined immunotherapy and radiation were associated with immunogenic radiation induced signatures. MEKi responses were associated with reinvigoration of peripheral immune response. Finally, delayed, sustained responses were observed in ultra-hypermutant RRD-HGG exhibiting changes in somatic mutational spectra. CONCLUSION: These data suggest that the continuous mutagenesis renders hypermutant RRD-HGG susceptible to checkpoint inhibitors beyond initial progression. The combination with re-irradiation and additional immune/targeted agents can maximize survival in these children and young adults. Future research should focus on biology-driven rational immunotherapy combinations that also result in lower toxicity to maximize patient benefit.