Dysregulation of the Kynurenine Pathway in Relapsing Remitting Multiple Sclerosis and Its Correlations With Progressive Neurodegeneration.

Background and objectives: Despite the absence of acute lesion activity in multiple sclerosis (MS), chronic neurodegeneration continues to progress, and a potential underlying mechanism could be the kynurenine pathway (KP). Prolonged activation of the KP from chronic inflammation is known to exacerbate the progression of neurodegenerative diseases through the production of neurotoxic metabolites. Among the 8 KP metabolites, six of them, namely kynurenine (KYN), 3-hydroxylkynurenine (3HK), anthranilic acid (AA), kynurenic acid (KYNA), and quinolinic acid (QUIN), have been associated with neurodegeneration.

Methods: To gain insights into the links between the KP and neurodegeneration in MS, we investigated the KP metabolomics profile of relapsing remitting MS (RRMS) patients and their correlation with parameters of neurodegeneration in brain and retinal. Outpatients with a clinical diagnosis of RRMS (n = 98) or age-matched and sex-matched healthy controls (n = 39) were included. MS participants undertook yearly evaluation of MRI and optical coherence tomography scan to evaluate neuroaxonal loss. Blood samples were collected at the baseline from all participants for the biochemical analysis of KP metabolites.

Results: We identified increased plasma levels of AA and 3HAA in the MS group, indicating an anti-inflammatory response alongside active neurodegeneration. By contrast, plasma levels of KYNA and 3HK were lower in the MS group than in healthy controls. Our analysis revealed a higher KYN:tryptophan (TRP) and QUIN:KYNA ratios in the MS cohort, suggesting activation of the pathway toward the production of neurotoxic QUIN. Another important finding was that KP metabolites were correlated with measures of axonal degeneration in patients with MS. Notably, central brain atrophy positively correlated with the TRP levels, but negatively correlated with KYN and level KYN:TRP ratio. Finally, the choroid plexus volume was inversely correlated with KYNA plasma levels.

Discussion: These findings highlight changes in the biosynthesis of KP during the progression of RRMS and its correlation with axonal loss. This study underscores the potential of targeting the KP in developing novel treatments for neuroaxonal damage in MS and warrants future research in greater depth.