From non-targeted metabolomics to quantitative analysis of uremic compounds in chronic kidney disease

Chronic kidney disease (CKD) is common in Australia and specifically in Tasmania, and is characterised by an accumulation of potentially-harmful compounds in the blood (“Uraemia”). The presented work is a pilot study to assess the feasibility of a larger cohort study, and concentrates on uraemic compounds derived from tryptophan. Disruptions in tryptophan metabolism, mainly via the Tryptophan-Kynurenine pathway were associated with various neurological and psychological disorders as well as premature cardiovascular disease which often accompany CKD. In a preliminary untargeted metabolomics study (RPLC - Thermo LTQ-Orbitrap) we conducted paired comparisons (Wilcoxon test) between serum samples from 12 patients pre- and post-dialysis, which varied considerably from 6 healthy controls (Mann-Whitney; PLS/DA). Determination of tryptophan metabolites and their hemodialysis clearance rates led to compound selection for quantitative analysis, including markers for inflammation and HPA axis activity which trigger enzymatic alterations in tryptophan metabolism. The second stage was a cross-sectional study in 30 adults with CKD (stage 4 at recruitment two years earlier) who were psychologically-assessed and their serum analysed (Waters Xevo QqQ). We hypothesised that (1) the markers neopterin and/or cortisol are related to tryptophan metabolism; (2) tryptophan metabolites relate to the severity of psychological and cognitive symptoms. Our major findings include high associations between the inflammation marker neopterin and 6 tryptophan metabolites (Pearson R 0.46-0.68, all p ≤ 0.01), which increase with decline in kidney function, as measured by eGFR (all p < 0.05). Tryptophan breakdown index (kynurenine / tryptophan) correlated with both neopterin and cortisol (R=0.45 and R=0.37, respectively, both p < 0.05), supporting background of inflammation and to less degree of stress. A few significant associations between metabolites and psychological/cognitive symptoms were found, including between kynurenic acid and cognitive functioning (Spearman R = -0.38, p < 0.05). A novel finding in this study is that indole-3 acetic acid showed the strongest associations with anxiety and depression (R = 0.57, p = 0.001; R = 0.38, p = 0.04 respectively). Further more detailed investigation of these associations is required in a larger prospective cohort, and this may provide a novel direction in improving well-being in this patient population.