Dipg-11. Preclinical Efficacy of B7-H3 Car T Cells in Combination With Onc206

Abstract 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 (p0.01). 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.