DDDR-76. Adaptive evolution of drug responses to monotherapy ONC201 in PDOX models of pediatric brain tumors

Abstract BACKGROUND Pediatric brain tumors are leading cause of cancer-related deaths in children. ONC201, a selective dopamine receptor D2 (DRD2) antagonist and ClpP agonist, is in phase I-III clinical trials for various cancers, notably H3K27 mutant gliomas. However, resistance has been reported and its mechanism remains unclear. This study was to assess therapeutic efficacy and adaptive drug responses of ONC201 longitudinally across multiple patient -derived orthotopic mouse models (PDOX). Additionally, two patient samples with ONC201 treatment failure were obtained post-mortem. METHODS DRD2 mRNA were profiled by RNA-seq. ONC201 cytotoxicity was evaluated in vitro in PDOX-derived tumor cells. Total of 11 PDOX, 6 glioblastoma (GBM), 3 medulloblastoma (MB), and 2 ependymoma (EPN) were treated with ONC201 (125mg/kg) 2-3 weeks after tumor implantation and monitored for survival times. Immunohistochemistry was used to examine phenotypical changes. Single-cell RNA sequencing was performed to uncover the trajectory of drug responsiveness during the course of ONC 201 treatment and identify treatment-responsive and resistant cell subpopulations. RESULTS DRD2 expression varied across tumors with highest in MBs (RNA seq FPKM=4), moderate in GBMs (FPKM=1.5-2.0), and low in EPNs (FPKM=0.4-0.5). ONC201 inhibited tumor cell proliferation in 8 of 11 models in vitro (IC50 = 0.26–0.52 µM) and significantly extended animal survival by 10–21% (P < 0.05) in 50% (3/6) GBMs whereas no benefits in MB or EPNs. Single-cell analysis revealed the trajectory of drug responses, identified drug-responsive and -resistant cell populations and potential resistance-associated genes. CONCLUSION DRD2 expression and in vitro cytotoxicity did not reliably predict in vivo efficacy. ONC201 demonstrated anti-tumor efficacy in pediatric GBMs, extending beyond H3K27 mutant tumors, but resistance emerged with enriched population of therapy-resistant cells. These findings highlight Inclusion criteria other than DRD2 expression is needed for ONC201 therapy and suggest combination therapies to prevent tumor recurrence.