Uromodulin and Tryptophan Metabolite Clearance in Hemodialyzed Patients

Background: Certain metabolites of the tryptophan-kynurenine (Trp-KYN) pathway, which are primarily cleared via tubular transport, have been linked to end-stage kidney disease (ESKD). Uromodulin-a protein expressed exclusively in the kidneys-is a key regulator of renal structure and function, as well as a direct marker of tubular health. This preliminary study explores the hypothesis that serum uromodulin correlates with Trp-KYN metabolites, potentially revealing new pathophysiological pathways in patients undergoing kidney replacement therapy (KRT). Given the link between serum uromodulin, Trp-KYN metabolites, and tubular function, we examined their correlation in KRT patients. Furthermore, we assessed how various clinical and dialysis parameters influence serum uromodulin levels. Methods: A total of 64 stable patients from a single dialysis center receiving hemodialysis (HD) or hemodiafiltration (HDF) were enrolled. Pre- and post-dialysis concentrations of uromodulin, Trp, KYN, kynurenic acid (KYNA), 3-hydroxykynurenine (3-OHKYN), and their reduction ratios (RRs) were established. High-performance liquid chromatography (HPLC) was used to estimate the KYN pathway metabolite levels, whereas uromodulin concentration was measured using an immunoenzymatic assay. Results: Detectable serum uromodulin was found in only 30 patients. This group was predominantly male (p < 0.001) and characterized by shorter dialysis vintage (p < 0.001), a higher prevalence of residual kidney function (RKF) (p = 0.001) and diabetes mellitus (p = 0.028), higher pre-dialysis serum phosphorus levels (p = 0.015), and more frequent use of loop diuretics (p = 0.004). Furthermore, univariate analysis revealed significantly higher pre-dialysis (p = 0.004) and post-dialysis (p = 0.025) serum Trp concentrations in the uromodulin-positive group. Pre-dialysis serum uromodulin concentration correlated positively with pre-dialysis Trp level (p < 0.001) and negatively with the pre-dialysis KYN/Trp ratio (p = 0.008), but not with other metabolites that are also subject to tubular transport mechanisms. Post-dialysis uromodulin levels correlated positively only with post-dialysis Trp level (p = 0.005). Patients treated with HDF had significantly higher RR for uromodulin than those treated with HD (p = 0.01). Conclusions: The presented data indicate that serum uromodulin levels are correlated with RKF. Additionally, the presence of detectable serum uromodulin may indicate reduced immunological activation, leading to diminished activity within the Trp-KYN pathway.