Activation of the kynurenine pathway identified in individuals with covert hepatic encephalopathy

HE is a neuropsychiatric complication of liver disease characterized by systemic elevation in ammonia and proinflammatory cytokines. These neurotoxins cross the blood-brain barrier and cause neuroinflammation, which can activate the kynurenine pathway (KP). This results in dysregulated production of neuroactive KP metabolites, such as quinolinic acid, which is known to cause astrocyte and neuronal death. Our aim was to compare KP activity between patients with covert HE (CHE), patients without encephalopathic cirrhosis (NHE), and healthy controls (HCs).

This was a single-center prospective cohort study conducted between 2018 and 2021 at St Vincent's Hospital, Sydney. Overall, 13 patients with CHE, 10 patients with NHE, and 12 with HC were recruited. Patients with cirrhosis were diagnosed with CHE if they scored ≤-4 on the Psychometric Hepatic Encephalopathy Score. KP metabolite levels were quantified on plasma samples via HPLC and gas chromatography/mass spectrometry. One-way Kruskal-Wallis test was used to compare the expression levels of KP enzymes.

KP was highly activated in patients with cirrhosis, demonstrated by higher levels of activity in the rate-limiting enzymes, indoleamine 2,3-dioxygenase, and tryptophan-2,3-dioxygenase in both CHE (65.04±20.72, p=0.003) and patients with NHE (64.85±22.10, p=0.015) compared to HC (40.95±7.301). Higher quinolinic acid concentrations were demonstrated in CHE (3726 nM±3385, p<0.001) and patients with NHE (1788 nM±632.3, p=0.032) compared to HC (624 nM±457). KP activation was positively correlated with inflammatory marker C-reactive protein in patients with CHE (Rs=0.721, p≤0.01).

KP is highly activated in patients with CHE, resulting in heightened production of neurotoxic metabolites. Dysregulation of the pathway is demonstrable in patients who do not yet show clinical signs of neurocognitive impairment. Therapeutic agents that modulate KP activity may be able to alleviate symptoms of patients with CHE.