Abstract 6400: Preclinical efficacy of combinatorial B7-H3 CAR T cell and ONC206 treatment in DIPG/DMG

B7-H3 CAR T cell treatment in combination with ONC206 aims to assess benefit and significant extension of survival in orthotopic DIPG mouse models. BACKGROUND. Diffuse intrinsic pontine glioma (DIPG) is a fatal pediatric tumor arising in the pons and affecting over 300 children each year in the United States. Chimeric antigen receptor (CAR) T cells, a targeted immunotherapy showing remarkable results in treating hematologic cancers, have recently been translated to children with DIPG. B7-H3 is a protein expressed in nearly all DIPG but present at only low levels in normal brain tissue. Seattle Children’s first-in-human phase 1 clinical trial of repeated intracranial anti-B7-H3 CAR T cells (BrainChild-03 trial, NCT04185038) found the highest planned dose (100 million CAR T cells) to be tolerable. We also demonstrated a survival of 20.1 months for treated patients, but while 3 patients are alive beyond 42 months, response to therapy was not universal. To uncover a beneficial combinatorial strategy, we tested B7-H3 CAR T cells with ONC206, a small molecule imipridone targeting ClpP and DRD2, being evaluated in the PNOC023 clinical trial (NCT04732065) for pediatric patients with CNS tumors. METHODS. The combinatorial strategy was performed against multiple DIPG models (i.e., PBT-22FH, PBT-24FH, PBT-27FH, PBT-29FH, and SU-DIPG-XIII) before being validated in orthotopic xenograft DIPG mouse models. B7-H3 CAR T cells (1 million) were intracranially delivered at day 7, whereas ONC206 (50 mg/kg) was administered twice a week via oral gavage. RESULTS. We initially assessed an ONC206 IC50 at approximately 250 nM and demonstrated a pro-apoptotic effect in various DIPG models. Next, we showed that ONC206 induces mitochondrial dysfunction, which limits tumor cell respiration and activates cell death pathways. After a 120h treatment with ONC206, we observed upregulation of Caspase-3, Caspase-7, ATF4, CHOP, ClpP, and DR5, whereas levels of B7-H3 and DRD2 remained constant.Given that B7-H3 CAR T cells are highly effective as single agents in preclinical settings, we used lower Effector:Tumor (E:T) ratios and low concentrations of ONC206, revealing a significant additive effect in the models studied (p-value < 0.003). Interestingly, the phenotype of CAR T cells was unaffected by ONC206, and multiplex cytokine assays not only showed unaltered levels of TNF-α and IFN-γ, but also detected increased levels of IL2, IL29, VEGFA, IP10, and Granzyme B post-treatment. Preclinical studies confirmed an additive benefit in the aggressive and metastatic SU-DIPG-XIII model, significantly extending the survival of mice up to 94 days (p-value < 0.005), compared to mice treated solely with B7-H3 CAR T cells (median survival = 25 days). CONCLUSIONS. B7-H3 CAR T cells and ONC206 may have a combinatorial benefit. Therefore, future studies will continue to validate these findings, optimize dosing strategies, and assess immunocompetent DIPG models. Citation Format: Andrea Timpanaro, Edward Song, Matthew C. Biery, Ryma Toumi, Davina Lau, Leo Elena-Sanchez, Michael Meechan, Ashmitha Rajendran, Jason Wendler, Joshua Gustafson, Lily Winter, Carl Koschmann, Myron Evans, Siobhan Pattwell, Shannon Oda, Jessica Foster, Matthew D. Dun, Michael C. Jensen, Nicholas A. Vitanza. Preclinical efficacy of combinatorial B7-H3 CAR T cell and ONC206 treatment in DIPG/DMG [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6440.