LTBK-03. Targeted mass spectrometry of serial CSF and serum specimens from children with diffuse intrinsic pontine glioma treated with intracranial B7-H3 CAR T cells

Abstract Considering the high expression of B7-H3 (CD276) on diffuse intrinsic pontine glioma (DIPG) and preliminary evidence that repeated locoregional delivery of CAR T cells to patients with CNS tumors was feasible and tolerable, we open the phase 1 trial BrainChild-03 (NCT04185038). Children with DIPG in Dose Regimen 1 received 40 intraventricular B7H3CAR doses, with the two patients enrolling post-progression surviving >400 days from initial CAR T infusion. To evaluate for immune and tumor responses that may correlate with clinical and radiographic evaluations, we performed targeted proteomic analysis on serial CSF and serum biospecimens. MRM-MS is a targeted mass spectrometry that provides sensitive measurement of proteins in cancer tissues and fluids. In 2 patients with longitudinal biospecimens, we identified 50 CSF and 59 serum proteins above level of detection. In general, there were fewer serum fluctuations compared to the CSF, supporting that intracranial delivery provides local immune activation. Sharp fluctuations of several immunoregulatory peptides were measured in the CSF at pre and post infusion timepoints, including BCL10, CXCL13, TIM-3, ICOSLG, and PD-L2. Notably, several analytes tracked consistently in the CSF of both patients, including markers of macrophage maturation and immune cell recruitment, including CD14, CD163, CD44, CSF-1, CXCL13 and VCAM-1. B7-H3 was detected in the CSF and serum of both patients. S005, who progressed on protocol therapy, had a sharp increase in B7-H3 in the CSF during Course 2, while S008, who had clinical improvement on protocol therapy, had consistently lower B7-H3 present. Notably, serum B7-H3 steadily declined in both patients over time, with the exception of a transient increase in S005 between Courses 4 and 5. Future work for all enrolled patients with DIPG will explore the correlation of these targeted proteomic measurements with clinical end points for potential use as markers of efficacy of therapy or adverse events.