The kynurenine pathway in pediatric "mild-to-moderate" traumatic brain injury: translational insights from a prospective human study and a large-animal model

Elucidating the biochemical pathways affected by pediatric traumatic brain injury (TBI) is essential for identifying informative blood-based biomarkers that may support future precision medicine and clinical trials. The kynurenine pathway (KP)-the primary route for tryptophan (Trp) degradation-represents a promising candidate due to its established link to (neuro)inflammation and TBI. The current study used liquid chromatography with tandem mass spectrometry to investigate KP metabolites in serum from 54 human patients with pediatric mild TBI (pmTBI; age 8-18 years) at ∼ 7 days and ∼ 4 months post-injury and 38 age- and sex-matched healthy controls (HC). The early temporal trajectories of KP metabolites were examined in more detail in serum samples collected from 33 juvenile swine with mild-to-moderate traumatic brain injury (mmTBI) at pre-injury baseline, and at 5 min, 35 min, 2.5 h, 24 h, and 7 days post-injury. Data from 10 sham animals were collected at equivalent time points. Interleukin 1 receptor antagonist (IL-1RA), IL-1β, IL-6, IL-10 and tumor necrosis factor (TNF) α were examined as measures of inflammation. In human pmTBI, significantly lower concentrations of Trp, 3-hydroxykynurenine (3HK), 3-hydroxyanthranilic acid (3HA), xanthurenic acid (XA) and picolinic acid (PA) were observed relative to HC, with stronger effects at 4 months relative to 7 days post-injury. Lower concentrations of Trp, 3HA, and XA at 4 months were associated with persistent post-concussive symptoms (PCS). As predicted, findings for inflammatory markers were null at these time points. In the large-animal model, an increased response of the anti-inflammatory IL-1RA was found at 2.5 h post-injury in mmTBI relative to sham animals, without any group differences in KP metabolites or other inflammatory markers. Both animal groups showed prominent temporal metabolite changes, including increased Trp at 2.5 h and decreased PA up to 24 h post-injury, likely reflecting cumulative effects of isoflurane anesthesia and associated dampening of pro-inflammatory responses. Altogether, our findings indicate long-lasting effects of pmTBI on the KP in humans. Disparate profiles were observed for human and large-animal injuries, which highlights the importance of incorporating clinically relevant biomarkers in preclinical studies to improve the translation of preclinical findings into successful future clinical trials.