DIPG-11. PRECLINICAL EFFICACY OF B7-H3 CAR T CELLS IN COMBINATION WITH ONC206

Diffuse intrinsic pontine glioma (DIPG) is a fatal pediatric tumor of the pons affecting over 300 children in the U.S. each year. Chimeric antigen receptor (CAR) T cells are a targeted immunotherapy with remarkable clinical success against hematological malignancies and now are being investigated against central nervous system (CNS) tumors. B7-H3 is a surface antigen specifically expressed on nearly all DIPG but not on normal brain providing an attractive therapeutic window. Therefore, Seattle Children’s opened BrainChild-03 (NCT04185038), a first-in-human phase 1 trial of repeatedly dosed intracranial B7-H3 CAR T cells for children with DIPG. We found the highest planned dose (10x10⁷ cells) to be tolerable and observed preliminary signals of efficacy. ONC206 is a small molecule ClpP-agonist and DRD2-antagonist derivative of ONC201 that is currently being evaluated on PNOC-023 (NCT04732065). Here, we set out to evaluate the combinatorial potential of these modalities. ONC206 effectively reduced DIPG tumor cell viability (IC₅₀ = ~300 nM compared to ONC201 ~1400 nM) against four treatment-naïve patient-deriving DIPG models representing a range of molecular subtypes. Using multiple orthogonal systems, apoptosis of DIPG cells was confirmed. We found that ONC206 drives tumor cell mitochondrial dysfunction and upregulation of DR5, a cell-death receptor involved in the TRAIL pathway, whereas levels of B7-H3, DRD2, and ClpP remain constant. We previously published the efficacy of B7-H3 CAR T cells against DIPG, but here - using low effector:tumor (E:T) ratios (e.g. 0.5:1) of activated B7-H3 CAR T cells and low-dose ONC206 (e.g. 200 nM) - we found a significant in vitro combinatorial benefit against DIPG (p<0.01). Supportive biologic studies suggest this could be an additive benefit and in vivo studies are currently ongoing. Based on the clinical translation of B7-H3 CAR T cells and ONC206, we aim to further validate this combinatorial benefit to help future patients.