Ruxolitinib concentrations in cerebrospinal fluid in patients with hemophagocytic lymphohistiocytosis and potential significance of central nervous system involvement

Abstract Hemophagocytic lymphohistiocytosis (HLH) constitutes a life-threatening hyperinflammatory state characterized by immune dysregulation, cytokine storm and multi-organ failure. Central nervous system involvement (CNS-HLH) represents a severe complication with heterogeneous neurological manifestations and dismal prognosis. Intrathecal chemotherapy remains a routine therapeutic approach to CNS-HLH, with suboptimal outcomes nevertheless. Ruxolitinib, a potent selective JAK1/2 inhibitor, demonstrates significant therapeutic potential in HLH by suppressing pathological inflammatory signaling cascades. Despite preclinical evidence of ruxolitinib in possible blood-brain-barrier (BBB) penetration owing to its small molecular weight (MW 306.34 g/mol), there is paucity of data regardingi ts distribution into human cerebrospinal fluid (CSF) in clinical pharmacokinetics. A priori, we conducted an analysis of paired plasma and CSF samples collected from 10 consecutive HLH patients treated with stable-dose ruxolitinib at Hematology Intensive Care Unit of Ruijin Hospital (Shanghai Jiao Tong University School of Medicine) from August 2024 through July 2025. Ruxolitinib concentrations were quantified via validated liquid chromatography, tandem mass spectrometry (LC-MS/MS). Within this cohort, active CNS involvement developed in 3 patients. Pharmacokinetic analysis revealed median plasma concentrations of 28.57 ng/ml [19.06, 31.61 ng/ml] in CNS-HLH patients, with corresponding median CSF concentrations of 4.01 ng/ml [3.02, 5.05 ng/ml], yielding a median BBB penetration ratio (CSF/plasma × 100%) of 15.84% [14.04%, 15.98%]. Among the 7 non-CNS-HLH patients, median plasma concentration was 32.23 ng/ml [18.73, 54.31 ng/ml], median CSF concentration was 3.55 ng/ml [1.71, 9.82 ng/ml], and median penetration ratio was 11.59% [6.94%, 16.94%], respectively. Notably, following ruxolitinib-containing therapy, all 3 CNS-HLH patients exhibited significant clinical improvement in neurological symptoms and corresponding resolution of CSF abnormalities. This study provides the first clinical evidence of detectable ruxolitinib concentrations within the CSF compartment of HLH patients, validating significant BBB penetration. Critically, the approximate CSF concentrations observed in CNS-HLH patients (4.01 ng/ml) was associated with JAK/STAT pathway inhibition in vitro, suggesting potential therapeutic relevance. These findings pave a crucial pharmacological avenue to ruxolitinib application in CNS-involved HLH, suggesting the therapeutic mechanism of ruxolitinib may be involved in direct CNS immunomodulation.