Abstract PR014: Robust disease monitoring using CSF liquid biopsies collected from children undergoing cellular therapy for malignant central nervous system tumors

Abstract Background: Central nervous system (CNS) tumors remain the leading cause of cancer-related mortality in children, necessitating more effective treatment options. Chimeric Antigen Receptor T-cells (CAR-T) have emerged as a promising strategy to treat CNS tumors, with B7H3 (CD276) representing an attractive target due to its high expression across a broad range of entities. B7H3-CAR-T therapy is currently being evaluated in a St. Jude Children’s Research Hospital Phase I clinical trial (NCT05835687) for the treatment of relapse/refractory pediatric CNS tumors. Monitoring disease progression after CAR-T infusion by magnetic resonance imaging (MRI) remains challenging, emphasizing the need for more sensitive biomarkers of treatment response. Detection of circulating tumor DNA (ctDNA) in cerebrospinal fluid (CSF) liquid biopsies holds tremendous potential for minimal residual disease (MRD) monitoring in the clinical setting. Here, we leverage CSF liquid biopsies for longitudinal disease profiling in pediatric CNS tumor patients receiving B7H3-CAR-T immunotherapy. Methods: CSF samples were collected from Ommaya reservoirs of 9 pediatric patients with relapse/refractory CNS tumors undergoing B7H3-CAR-T therapy. CSF was sampled at study enrollment, and weekly intervals pre- and post-infusion. Cell-free DNA (cfDNA) was extracted from 115 longitudinally collected CSF samples and subjected to enzymatic methylation sequencing (EM-seq). Implementation of a custom computational workflow enabled detection of tumor-derived copy number variations (CNVs), estimation of malignant vs. non-malignant fractions, and molecular classification of tumor entity. Results: Tumor-derived CNVs were detected in 7/9 (78%) CSF samples collected at enrollment, prior to immunotherapy. Liquid biopsy-inferred CNV profiles and entity classifications were highly concordant with matched tumor tissue profiles. Chromosomal alterations that were restricted to CSF profiles indicated clonal evolution. Increased ctDNA burden was repeatedly observed after B7H3-CAR-T infusions, suggesting subclinical antitumor activity. Transient subclonal alterations emerging in response to treatment were also identified in a subset of patients. Conclusions: EM-seq is a feasible method for serial tracking of ctDNA burden in patients receiving cellular immunotherapy. CSF liquid biopsies capture the global landscape of tumor heterogeneity and genetic alterations that arise during therapy, providing a sensitive platform for detecting clinical response and disclosing potential mechanisms of treatment resistance. Citation Format: Anna Kostecka, Kyle S Smith, Katie Han, Hong Lin, Deanna Langfitt, Tara Walhart, Stephen Gottschalk, Giedre Krenciute, Christopher DeRenzo, Kelsey Bertrand, Paul A Northcott. Robust disease monitoring using CSF liquid biopsies collected from children undergoing cellular therapy for malignant central nervous system tumors [abstract]. In: Proceedings of the AACR Special Conference: Liquid Biopsy: From Discovery to Clinical Implementation; 2024 Nov 13-16; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2024;30(21_Suppl):Abstract nr PR014.