CSF proteomics identifies ADGRG1 as a predictive biomarker of intrathecal immune checkpoint inhibitor response in leptomeningeal metastasis

BACKGROUND: Leptomeningeal metastasis (LM) is a fatal complication of advanced cancer with limited therapeutic options and poor prognosis. Immune checkpoint inhibitors (ICIs) have shown promise in systemic disease, but intrathecal ICIs efficacy and immunological impact in LM remain unclear. Cerebrospinal fluid (CSF) proteomics may provide a unique window into the CNS immune microenvironment and enable response prediction. METHODS: We enrolled 62 patients with LM who received intrathecal pemetrexed (InPe) with or without ICIs (InPe+programmed cell death protein-1 (PD-1), InPe+PD-1+cytotoxic T-lymphocyte associated protein 4 (CTLA-4), InPe+PD-1+vascular endothelial growth factor (VEGF)). Matched CSF (pretreatment and post-treatment) and pretreatment serum samples were collected for high-content proteomic profiling using the Olink platform. Differential expression, pathway enrichment, and machine learning-based modeling were applied to identify treatment-induced changes and predictive biomarkers. RESULTS: Nearly half of the patients achieved clinical response. Post-treatment CSF showed enrichment of cytokine and chemokine signaling pathways, with a marked decrease in EGF. Adding PD-1 inhibitor restored immune cell function and upregulated interferon-γ. Compared with serum, CSF proteomic profiles provided superior predictive performance (area under the curve (AUC) 0.884 vs 0.780). A five-protein CSF signature (ADGRG1, CD28, CCL23, DCN, IL-15) achieved robust prediction (AUC 0.968 in InPe+PD-1 training cohort, 0.917 in InPe+PD-1+CTLA-4 validation cohort, and 1 in InPe+PD-1 subsequent validation cohort). ADGRG1 was significantly higher in non-responders at baseline (p=0.031) and decreased after treatment, and specific enrichment in dura-derived LM-associated macrophages, suggesting a macrophage-derived source and potential role in LM progression. CONCLUSIONS: This study provides the first high-content proteomic atlas of intrathecal ICI therapy in LM, identifies intrathecal ICI therapy-specific immune remodeling in LM, and establishes a CSF-based predictive model with high accuracy. ADGRG1 represents a promising biomarker of treatment responsiveness and a potential mechanistic link between macrophage biology and LM progression.