Uremic Retention Solutes Research Articles

Albumin-binding capacity (ABiC) is reduced in patients with chronic kidney disease along with an accumulation of protein-bound uraemic toxins

Albumin is an important transport protein for non-water-soluble protein-bound drugs and uraemic toxins. Its transport capacity is reduced in patients with advanced chronic kidney disease (CKD) and unbound fractions of uraemic toxins are related to complications of CKD. We investigated whether this reduction could be quantified and how it correlated with the stages of CKD. Albumin-binding capacity (ABiC) is a dye-based method that quantifies the remaining binding capacity of one major binding site (site II) of the albumin molecule. Blood samples from 104 CKD patients were incubated with a binding site-specific fluorescent marker and the amount of unbound marker was determined by means of fluorescence detection after filtration. Measurements in a pooled human plasma were used for reference. Glomerular filtration rate and serum indoxyl sulphate (IS) levels were also determined. Impairment of renal function was associated with a reduction in ABiC (mean ± SD: 118 ± 12; 111 ± 11; 99 ± 8 and 79 ± 9% for Stages 1/2, 3, 4 and 5, respectively; P < 0.001) and an increase in IS (3.9 ± 1.1; 6.2 ± 3.2; 16.3 ± 14.9 and 56.1 ± 28.6 μmol/L for Stages 1/2, 3, 4 and 5, respectively; P < 0.001). In dialysis patients, ABiC was lower in those with urine outputs <500 mL/day than in those with preserved urine output (73.7 ± 6.0 versus 83.8 ± 8.5%; P < 0.001). Impaired albumin function in CKD patients can be quantified, is related to severity of kidney disease and is associated with an accumulation of uraemic albumin-bound retention solutes.

Metabolomic profiling of the autosomal dominant polycystic kidney disease rat model

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited systemic disease characterized by renal cyst expansion, resulting in renal failure. With the progression of renal damage, the accumulation of uremic compounds is recently reported to subsequently cause further renal damage and hypertension. Finding uremic toxins and sensitive markers for detecting the early stage of ADPKD is necessary to clarify its pathophysiological process and to prevent its progression. The aim of this study was to analyze the profile of uremic retention solutes of ADPKD by capillary electrophoresis-mass spectrometry (CE-MS) using the Han:SPRD rat model. Two hundred and ninety-seven cations and 190 anions were comprehensively analyzed by CE-MS in Han:SPRD rats and control rats. We found 21 cations and 19 anions that accumulated significantly in the heterozygous (Cy/+) ADPKD rat model compared with control rats. Among the compounds, increases in 5-methyl-2'-deoxycytidine, glucosamine, ectoine, allantoate, α-hydroxybenzoate, phenaceturate and 3-phenylpropionate and decreases in 2-deoxycytidine, decanoate and 10-hydroxydecanoate were newly identified in the ADPKD Cy/+ rats. We identified uremic retention solutes in ADPKD Cy/+ rats. Compounds related to ADPKD could be useful markers for detecting the early stage of ADPKD.

Reduction in Protein-Bound Solutes Unacceptable as Marker of Dialysis Efficacy during Alternate-Night Nocturnal Hemodialysis

Background: The uremic retention solutes indoxyl sulfate and p-cresyl sulfate are linked to cardiovascular disease and overall survival. Dialytic clearances are limited, which is principally attributed to tight protein binding. Extending dialysis duration would be expected to substantially increase protein-bound uremic solute removal. The aim of the current study was to study protein-bound uremic retention solute clearances and kinetics during longer-hours nocturnal hemodialysis. Methods: In a prospective cohort study of 32 maintenance alternate-night nocturnal hemodialysis patients, we followed serum concentrations, solute removals and solute clearances of p-cresyl sulfate and indoxyl sulfate. Spent dialysate sampling was fractionated to compare solute removals between the first 4 h and next 4 h of nocturnal dialysis. Single-compartment variable volume kinetics were calculated. Results: Dialytic clearances of protein-bound uremic retention solutes are maintained during nocturnal (longer-hours) dialysis. Clearances of indoxyl sulfate exceed those of p-cresyl sulfate, presumably due to less tight protein-binding. Apparent distribution volumes increase substantially during nocturnal dialysis, indicative of multi-compartmental behavior of the protein-bound uremic retention solutes indoxyl sulfate and p-cresyl sulfate. Conclusions: During nocturnal hemodialysis, serum concentrations of protein-bound solute concentrations are reduced less than predicted. Reduction ratios are not a valid tool to estimate total solute removal of protein-bound uremic retention solutes.

Protein-Bound Uremic Toxins: New Insight from Clinical Studies

The uremic syndrome is attributed to the progressive retention of a large number of compounds which, under normal conditions, are excreted by healthy kidneys. The compounds are called uremic toxins when they interact negatively with biological functions. The present review focuses on a specific class of molecules, namely the family of protein-bound uremic toxins. Recent experimental studies have shown that protein-bound toxins are involved not only in the progression of chronic kidney disease (CKD), but also in the generation and aggravation of cardiovascular disease. Two protein-bound uremic retention solutes, namely indoxyl sulfate and p-cresyl sulfate, have been shown to play a prominent role. However, although these two molecules belong to the same class of molecules, exert toxic effects on the cardiovascular system in experimental animals, and accumulate in the serum of patients with CKD they may have different clinical impacts in terms of cardiovascular disease and other complications. The principal aim of this review is to evaluate the effect of p-cresyl sulfate and indoxyl sulfate retention on CKD patient outcomes, based on recent clinical studies.

Metabolite profiling of hemodialysate using gas chromatography time-of-flight mass spectrometry

Hemodialysis is an important alternative for renal replacement therapy to remove uremic retention solutes (URS) for the uremic syndrome. The metabolites in the hemodialysate directly reflect the efficiency of URS clearance. Here we report a highly sensitive and reliable metabolomic procedure for the measurement of small molecule metabolites in hemodialysate using gas chromatography coupled with time-of-flight mass spectrometry (GC/TOF/MS). The method was developed and evaluated through orthogonal experimental design using multivariate statistical analysis. The optimized method involves the use of methanol and water in the ratio of 3:1 (v/v) for dissolving the lyophilized solid of the hemodialysate after degradation of urea with urease (no less than 50 U) for 10 min. Validation of the method demonstrated a good linearity with regression coefficients greater than 0.99. Relative standard deviations of precision and stability of proposed method were less than 15%, and recoveries ranged from 71.8 to 115.8%. This method was successfully applied in the metabolite profiling of human hemodialysate samples which was able to differentiate the patients treated with high-flux hemodialysis from those with low-flux hemodialysis. The metabolomic results reveal a higher concentration of URS, and thus, better URS removal, from the patients under high-flux dialysis than those under low-flux dialysis.

Uremic Toxins and Oral Adsorbents

Uremic toxins are associated with various disorders in patients with end-stage renal disease and it is difficult to remove some of these toxins by dialysis. Since some uremic toxins are generated by bacterial metabolites in the colon, oral adsorbents that interfere with the absorption of uremic toxins or their precursors are believed to prevent their accumulation in the body. AST-120 adsorbs various uremic retention solutes in the gastrointestinal system and has potential for providing clinical benefit. Sevelamer hydrochloride binds some harmful compounds in addition to phosphate and seems to have pleiotropic effects that include lowering serum LDL cholesterol levels and reduction of inflammation. The effect of sevelamer hydrochloride on indoxyl sulfate and p-cresol has been shown in an in vitro study; however, in vivo studies in mice or humans did not demonstrate this effect on protein-binding uremic toxins. Oral adsorbents are thus one of the important modalities in the treatment of uremic syndrome.

The gut-kidney axis: indoxyl sulfate, p-cresyl sulfate and CKD progression

The fundamental insights into uraemic toxicity have evolved little since publication of the classical monograph by Cushny nearly a century ago [1]. Organic metabolites are still thought to substantially contribute to uremia (albeit urea might not be the culprit), yet evidence unequivocally demonstrating toxicity of any single uraemic constituent is lacking [2]. To advance research in uraemic toxicity, the European Toxin work group (EUTox) developed a classification of uraemic blood constituents according to characteristics that affect their removal during dialysis [3]. Besides small water-soluble molecules (e.g. urea and creatinine) and peptides/proteins (e.g. β2-microglobulin), they identified a group of solutes that circulate in equilibrium between free solute versus bound to carrier proteins. Tight protein binding severely limits solute clearances by dialysis [4]. Intriguingly, a substantial number of these so-called proteinbound uraemic retention solutes originate from protein fermentation in the large intestine, including p-cresyl sulfate and indoxyl sulfate [5]. Recently, several groups demonstrated direct associations between p-cresol, mainly reflecting p-cresyl sulfate, and overall mortality and cardiovascular disease in end-stage renal disease [6,7] and in chronic kidney disease (CKD) [8,9]. Likewise, direct associations between indoxyl sulfate and overall mortality and cardiovascular disease were reported [10]. While indoxyl sulfate and p-cresyl sulfate are frequently thought of as independent uraemic retention solutes, they share common ground. First, as mentioned before, p-cresyl sulfate and indoxyl sulfate both originate from bacterial protein fermentation in the large intestine. Colonic microbiota degrade tryptophan to indole. Further hydroxylation results in 3-hydroxy-indole, the majority of which is sulfonated to indoxyl sulfate. In parallel, fermentation of tyrosine results in p-cresol and ultimately p-cresyl sulfate [11]. Recently, we reported on sulfate conjugation of p-cresol in CKD [12,13]. Second, most p-cresyl sulfate and indoxyl sulfate circulates noncovalently bound to albumin and competes for the same albumin-binding sites (Sudlow site II) [14] (Figure 1). Indoxyl sulfate and p-cresyl sulfate are interchangeable marker molecules to study behaviour of protein-bound solutes during dialysis [15]. In this issue of Nephrology Dialysis Transplantation, Wu et al. demonstrate that serum indoxyl sulfate is associated with progression of CKD, confirming previous findings. Niwa et al. f irst advanced the hypothesis that accumulation of indoxyl sulfate accelerates glomerular sclerosis and progression of kidney disease [16,17]. Animal and small-scale human studies on CKD patients suggested retardation of CKD progression by adsorption of indole in the large intestine [18,19]. Intriguingly, Wu et al. equally demonstrate that p-cresyl sulfate is associated with CKD progression. Does this indicate that indoxyl sulfate and p-cresyl sulfate be considered equally valid markers for CKD progression? This illustrates one of the key problems we are faced with when investigating uremia. One of the hallmarks of uraemic retention solutes is that they all move more or less in the same direction. When glomerular filtration rate falls, concentrations of the uraemic retention solutes we measure, and most likely a host of solutes that we are not aware of, all rise. Indeed, in the current study, Wu et al. observed a moderate correlation between indoxyl sulfate and estimated glomerular filtration rate (eGFR) (r −0.72, P < 0.001), between p-cresyl sulfate and eGFR (r −0.64, P < 0.001) and between indoxyl sulfate and p-cresyl sulfate (r 0.66, P < 0.001). From a statistical point of view, if nominally related measures actually quantify the same phenomenon, then they are redundant, i.e. collinear. This might lead us to conclude that indoxyl sulfate and p-cresyl sulfate are plain markers of kidney function. The strongpoint of the study by Wu et al. is that they went to great length to correct for residual confounding, including by correction for related protein-bound uraemic retention solutes. They thus demonstrate that, while indoxyl sulfate is independently associated with CKD progression, this association is lost after correction for p-

Influence of switching connection ports of double-lumen permanent tunnelled catheters on total solute removal during dialysis

Catheter dysfunction is a concern when using double-lumen catheters in hemodialysis (HD). Reversing the connection mode results in higher blood flows, but also enhanced recirculation. We evaluated total solute removal (TSR) of different uremic retention solutes during a complete HD session, once with reversed (RL) and once with correctly connected lines (CL). Genius dialysis was performed in 22 HD patients at maximum blood flow (QB), once with CL and once with RL. TSR was determined for urea, creatinine, phosphate and ß2-microglobulin (ß2M). Using a kinetic model, we simulated TSR and reduction ratio (RR) for urea for different percentages of access recirculation and different QB during CL vs. RL. RR and TSR of the tested solutes were not different in clinical practice between CL and RL. Mathematically, urea RR did not differ with CL or RL, but TSR decreased by 4.5%-23.3% when changing from CL to RL for a recirculation of 5%-25%, respectively. For a recirculation of 5%-25%, QB in RL should be increased by 6.7% and 52.0%, 8.5% and 72.0%, and 10.0% and 115.2%, respectively, for a blood flow in CL mode of 150, 200 or 250 ml/min. Connecting patients to double-lumen dialysis catheters in RL does not impair TSR in clinical practice. Mathematically, TSR during RL was dependent upon the obtained QB and degree of recirculation. A nomogram indicating the increase in QB needed in RL to obtain equal TSR as in the CL mode, at different degrees of recirculation, is provided.

Cardiovascular Biomarkers in Chronic Kidney Disease

Cardiovascular Biomarkers in Chronic Kidney DiseaseCardiovascular morbidity and mortality are markedly increased in chronic renal failure patients. Although it cannot be regarded as a cardiovascular disease risk equivalent, kidney dysfunction is considered an independent predictor of increased cardiovascular risk that increases with deteriorating kidney function. The association is a very complex one, and the term cardiorenal syndrome is now widely used. Cardiovascular disease in chronic kidney disease patients usually manifests as ischemic heart disease (in the form of angina, acute coronary syndrome or sudden cardiac death), cerebrovascular disease, peripheral vascular disease, and congestive heart failure. Vascular disease includes atherosclerosis and vascular calcifications, and cardiomyopathy comprises left ventricular hypertrophy, cardiac fibrosis and left ventricular systolic and diastolic dysfunction. In addition to the well-established traditional risk factors such as hypertension, hyperlipidemia, insulin resistance and diabetes mellitus, the association is supported by synergistic action of non-traditional risk factors such as excessive calcium-phosphorus load, hyperparathyroidism, anemia, hemodynamic overload, malnutrition, inflammation, hyperhomocysteinemia, altered nitric oxide synthase and increased oxidative stress. This paper summarizes the current understanding of the significance of specific uremic retention solutes, natriuretic peptides, biochemical markers of disorders in calcium-phosphorus homeostasis, systemic inflammation, oxidative stress, and dyslipidemia.

Dialysis modality-dependent changes in serum metabolites: accumulation of inosine and hypoxanthine in patients on haemodialysis

The body metabolism of patients with end-stage renal disease may be altered in response to long-term dialysis treatment. Moreover, the pattern of serum metabolites could change depending on the type of dialysis modality used. However, dialysis modality-dependent changes in serum metabolites are poorly understood. Our aim was to profile comprehensively serum metabolites by exploiting a novel method of (1)H-NMR-based metabonomics and identify the differences in metabolite patterns in subjects receiving haemodialysis (HD) and peritoneal dialysis (PD). Anuric and non-diabetic HD patients were matched to PD patients for age, sex and dialysis duration. Accurate concentrations of serum metabolites were determined using the target-profiling procedure, and differences in the levels of metabolites were compared using multivariate analysis. Principal Components Analysis score plots showed that the metabolic patterns could be discriminated by dialysis modalities. Hypoxanthine and inosine were present only with HD, whereas serum xanthine oxidase activity and uric acid levels were not different. In contrast, PD was associated with higher levels of lactate, glucose, maltose, pyruvate, succinate, alanine, and glutamate linked to glucose metabolism and the tri-carboxylic acid cycle. Maltose appeared only in patients using icodextrin solution for PD. Known uraemic retention solutes such as urea, creatinine, myo-inositol and trimethylamine-N-oxide were increased in both dialysis groups. Metabonomics shows apparent differences in the profiles of serum metabolites between HD and PD, which were influenced by dialysis-related processes. Inosine and hypoxanthine are present only in HD patients, which is likely to represent more hypoxic and oxidative stress.

Removal of uraemic retention solutes in standard bicarbonate haemodialysis and long-hour slow-flow bicarbonate haemodialysis

Several studies already stressed the importance of haemodialysis (HD) time in the removal of uraemic toxins. In those studies, however, also the amount of dialysate and/or processed blood was altered. The present study aimed to investigate the isolated effect of the factor time t (by processing the same total blood and dialysate volume in two different time schedules) on the removal and kinetic behaviour of some small, middle and protein-bound molecules. The present study had a crossover design: 11 stable anuric HD patients underwent two bicarbonate HD sessions (~ 4 and ~ 8 h) in a random sequence, at least 1 week apart. The GENIUS single-pass batch dialysis system and the high-flux FX80 dialysers (Fresenius Medical Care, Bad Homburg, Germany) were used. The volume of blood and dialysate processed, volume of ultrafiltration, and dialysate composition were prescribed to be the same. For each patient, blood was sampled from the arterial line at 0, 60, 120, 180 and 240 min (all sessions), and at 360 and 480 min (8-h sessions). Dialysate was sampled at the end of HD from the dialysate tank. The following solutes were investigated: (i) small molecules: urea, creatinine, phosphorus and uric acid; (ii) middle molecule: β(2)M; and (iii) protein-bound molecules: homocysteine, hippuric acid, indole-3-acetic acid and indoxyl sulphate. Total solute removals (solute concentration in the spent dialysate of each analyte × 90 L - the volume of dialysate) (TSR), clearances (TSR of a solute/area under the plasma water concentration time curve of the solute) (K), total cleared volumes (K × dialysis time) (TCV), and dialyser extraction ratios (K/blood flow rate) (ER) were determined. The percent differences of TSR, K, TCV and ER between 4- and 8-h dialyses were calculated. Single-pool Kt/Vurea, and post-dialysis percent rebounds of urea, creatinine and β(2)M were computed. TSR, TCV and ER were statistically significantly larger during prolonged HD for all small and middle molecules (at least, P < 0.01). Specifically, the percent increases of TSR (8 h vs 4 h) were: for urea 22.6.0% (P < 0.003), for creatinine 24.8% (P < 0.002), for phosphorus 26.6% (P < 0.001), and for β(2)M 39.2% (P < 0.005). No statistically significant difference was observed for protein-bound solutes in any of the parameters being studied. Single-pool Kt/Vurea was 1.41 ± 0.19 for the 4-h dialysis sessions and 1.80 ± 0.29 for the 8-h ones. The difference was statistically significant (P < 0.0001). Post-dialysis percent rebounds of urea, creatinine and β(2)M were statistically significantly greater in the 4-h dialysis sessions (at least, P < 0.0002). The present controlled study using a crossover design indicates that small and middle molecules are removed more adequately from the deeper compartments when performing a prolonged HD, even if blood and dialysate volumes are kept constant. Hence, factor time t is very important for these retention solutes. The kinetic behaviour of protein-bound solutes is completely different from that of small and middle molecules, mainly because of the strength of their protein binding.

Serum levels of total p-cresylsulphate are associated with angiographic coronary atherosclerosis severity in stable angina patients with early stage of renal failure

Objective p-Cresylsulphate (PCS), a protein-bound uraemic retention solute, is known to cause endothelial dysfunction and possibly plays a role in coronary atherosclerosis. We aimed to investigate the relationship of total PCS with traditional biomarkers associated with chronic coronary atherosclerosis. In addition, the relationship between serum total PCS levels and the severity of coronary artery stenosis was also explored. Methods and results Serum total PCS concentrations were measured by using the Ultra Performance LC System in 202 consecutive stable angina patients, and their associations with angiographic indexes of the number of diseased vessels and modified Gensini score were estimated. Patients with significant coronary artery stenosis have higher median serum total PCS levels than patients with normal coronary arteries. Statistically significant associations were observed between the serum total PCS levels and the number of diseased vessels ( β = 0.261, p = 0.0002), and modified Gensini score ( β = 0.171, p = 0.016). Using multivariate analysis, serum total PCS level was independently associated with the presence and severity of CAD. Conclusions This study indicates that serum total PCS levels are significantly higher in the presence of CAD and are correlated with the severity of the disease, which suggest that increased serum total PCS may be involved in the pathogenesis of coronary atherosclerosis.

Dinucleoside polyphosphates: newly detected uraemic compounds with an impact on leucocyte oxidative burst

Dinucleoside polyphosphates (Np(n)N) have pathophysiologic roles in cardiovascular disease and are newly detected uraemic retention solutes. They were retrieved in human plasma, tissues and cells. Although their impact on several cell systems involved in vascular damage (endothelium, smooth muscle cells and thrombocytes) has been evaluated, their effect on different types of leucocytes has never been studied. This study evaluates, for the first time, the impact of Np(n)N on monocyte, granulocyte and lymphocyte oxidative burst activity at baseline and after stimulation with N-formyl-methionine-leucine-phenylalanine (fMLP) and phorbol 12-myristate 13-acetate (PMA) in whole blood. Diadenosine triphosphate (Ap(3)A) to diadenosine hexaphosphate (Ap(6)A) were tested to investigate the effect of the number of phosphate groups on reactive oxygen species (ROS) production. The effect of the type of nucleoside was evaluated by comparing adenosine guanosine tetraphosphate, diguanosine tetraphosphate, uridine adenosine tetraphosphate (Up(4)A) and diadenosine tetraphosphate (Ap(4)A). This study demonstrated that lymphocytes are especially susceptible to intracellular diadenosine polyphosphates. Depending on the phosphate chain length, different effects were observed. At baseline and with fMLP, Ap(4)A, Ap(5)A and Ap(6)A enhanced lymphocyted-free radical production. In addition, Ap(3)A, Ap(4)A and Ap(5)A increased PMA-stimulated ROS production in lymphocytes. Monocytes and granulocytes parallel the lymphocyte response albeit with an inhibition of Ap(6)A on granulocytes. Considering Np(n)N with four phosphate groups, Up(4)A showed the most important stimulatory effects on monocytes and Ap(4)A on lymphocytes. Np(n)N mainly have a leucocyte-activating impact, most significant for Ap(4)A, considering phosphate chain length, and for Up(4)A, considering the type of nucleosides. These results suggest that the pro-inflammatory effects of Np(n)N can contribute to the development of atherosclerosis, probably in the early stages of chronic kidney disease, but their chemical composition affects their activity.

HPLC Study of Uremic Fluids Related to Optical Dialysis Adequacy Monitoring

Purpose The aim of this study was to investigate uremia-related high-performance liquid chromatography (HPLC) ultraviolet (UV) absorbance profiles of serum and spent dialysate and to study the removal of uremic retention solutes in connection with optical dialysis adequacy monitoring. Methods 10 uremic patients were investigated using online spectrophotometry at a wavelength of 280 nm over the course of 30 hemodialysis treatments. The dialysate and blood samples were taken and analyzed simultaneously using standard biochemical methods and reversed-phase HPLC. Filters with cutoff at 3 kDa and 70 kDa were used for the pre-treatment of the serum. The chromatographic peaks were detected by a UV detector at wavelengths of 254 and 280 nm. Results This study indicated that the main solute responsible for UV absorbance in the spent dialysate is a low-molecular-weight, water-soluble, non-protein-bound compound uric acid (UA). Three additional uremic retention solutes – creatinine (CR), indoxyl sulphate (IS) and hippuric acid (HA) – were identified from the HPLC profiles. The number of detected HPLC peaks was not significantly different for a serum filtered through the 3 kDa or 70 kDa cutoff filters, and was lower for the spent dialysate, indicating that the molecular weight (MW) of the main UV chromophores in the uremic fluids did not exceed 3 kDa. The reduction ratio (RR) estimated by the total area of HPLC peaks at 254 nm and 280 nm in the serum and by the online UV absorbance at 280 nm was best related to the removal of small water-soluble non-protein bound solutes like urea (UR), CR and UA. Conclusions The present study contributes new information on the removal of uremic retention solutes during hemodialysis and on the origin of the optical dialysis adequacy monitoring signal.

Free p-cresylsulphate is a predictor of mortality in patients at different stages of chronic kidney disease

Uraemic toxins are considered to be emerging mortality risk factors in chronic kidney disease (CKD) patients. p-Cresol (a prototype protein-bound uraemic retention solute) has been shown to exert toxic effects in vitro. Recently, it has been demonstrated that p-cresol is present in plasma as its sulphate conjugate, p-cresylsulphate. The present study evaluated the distribution of free and total p-cresylsulphate and sought to determine whether these parameters were associated with vascular calcification, arterial stiffness and mortality risk in a cohort of CKD patients. One hundred and thirty-nine patients (mean +/- SD age: 67 +/- 12; males: 60%) at different stages of CKD (8% at Stage 2, 26.5% at Stage 3, 26.5% at Stage 4, 7% at Stage 5 and 32% at Stage 5D) were enrolled in this study. Baseline total and free p-cresylsulphate presented an inverse relationship with renal function and were significantly associated with vascular calcification. During the study period (mean follow-up period: 779 +/- 185 days), 38 patients died [including 22 from cardiovascular (CV) causes]. In crude survival analyses, free (but not total) p-cresylsulphate was shown to be a predictor of overall and CV death. Higher free p-cresylsulphate levels (>0.051 mg/100 mL; median) were associated with mortality independently of well-known predictors of survival such as age, vascular calcification, anaemia and inflammation. Serum levels of free and total p-cresylsulphate (the main in vivo circulating metabolites of p-cresol) were elevated in later CKD stages. However, only free p-cresylsulphate seems to be a predictor of survival in CKD.

P-Cresyl Sulfate and Indoxyl Sulfate in Hemodialysis Patients

Indoxyl sulfate and p-cresyl sulfate are important representatives of the protein-bound uremic retention solutes. Serum levels of p-cresyl sulfate and indoxyl sulfate are linked to cardiovascular outcomes and chronic kidney disease progression, respectively. They share important features such as the albumin-binding site, low dialytic clearance, and both originate from protein fermentation. Whether serum concentrations are related is, however, not known. In an observational study in 75 maintenance hemodialysis patients, we studied agreement between indoxyl sulfate and p-cresyl sulfate serum concentrations, dialytic reduction rates, and dialytic clearances. Concentrations were determined by HPLC. Dialytic clearances were determined from total spent dialysate collections. In vitro spiking experiments were performed to explore protein binding characteristics. Indoxyl sulfate and p-cresyl sulfate total serum concentrations were not related (r = 0.02, P = 0.9), whereas free serum concentrations were only moderately related (r = 0.53, P < 0.001). Indoxyl sulfate and p-cresyl sulfate share the same albumin binding site, for which they are competitive binding inhibitors. Intriguingly, indoxyl sulfate and p-cresyl sulfate reduction rates (r = 0.91, P < 0.001) and dialytic clearances (r = 0.97, P < 0.001) correlated tightly. Indoxyl sulfate and p-cresyl sulfate serum concentrations are not associated, suggesting different metabolic pathways. Indoxyl sulfate and p-cresyl sulfate are both valid markers to monitor behavior of protein-bound solutes during dialysis. Finally, they are competitive binding inhibitors for the same albumin binding site.

P-Cresyl sulfate serum concentrations in haemodialysis patients are reduced by the prebiotic oligofructose-enriched inulin

Protein-bound uraemic retention solutes, including p-cresyl sulfate and indoxyl sulfate, contribute substantially to the uraemic syndrome. These and several other uraemic retention solutes originate from intestinal bacterial protein fermentation. We investigated whether the prebiotic oligofructose-enriched inulin reduced serum concentration of p-cresyl sulfate and indoxyl sulfate, through interference with intestinal generation. We performed a single centre, non-randomized, open-label phase I/II study in maintenance HD patients with a 4-week, escalating dose regimen of oligofructose-enriched inulin (ORAFTI Synergy 1, Tienen, Belgium) (www.clinicaltrials.gov NCT00695513). Changes in p-cresyl sulfate and indoxyl sulfate serum concentrations as well as changes in p-cresyl sulfate and indoxyl sulfate generation rates were analysed. Compliance with therapy was excellent. p-Cresyl sulfate serum concentrations at 4 weeks were significantly reduced by 20% (intention to treat, P = 0.01; per protocol, P = 0.03). Also p-cresyl sulfate generation rates were reduced (P = 0.007). In contrast, neither indoxyl sulfate generation rates (P = 0.9) nor serum concentrations (P = 0.4) were significantly changed. The prebiotic oligofructose-inulin significantly reduced p-cresyl sulfate generation rates and serum concentrations in haemodialysis patients. Whether reduction of p-cresyl sulfate serum concentrations, an independent predictor of cardiovascular disease in HD patients, will result in improved cardiovascular outcomes remains to be proven.

The uremic retention solute p-cresyl sulfate alters NO signal transduction by alteration of the soluble guanylate cyclase redox state

Chronic kidney disease (CKD) is associated with excessive cardiovascular disease and mortality. Nitric oxide (NO) – soluble guanylate cyclase (sGC) signaling is impaired in CKD patients, contributing to a near ubiquitous endothelial dysfunction. Loss of kidney function induces major alterations in the blood concentration of numerous molecules. Several of these so-called uremic retention solutes are known to interfere with NO-sGC signaling. p-Cresol/p-cresyl sulfate (PCS), a prototypic representative of the protein-bound uremic retention solutes, is independently associated with overall mortality and incident cardiovascular disease in hemodialysis patients. The mechanisms underlying this association remain elusive. We hypothesized that PCS interferes with NO-sGC signaling, thereby contributing to CKD-associated endothelial dysfunction and cardiovascular mortality.

The Uremic Retention Solute p-Cresyl Sulfate and Markers of Endothelial Damage

Cardiovascular disease is highly prevalent in patients with chronic kidney disease. In hemodialysis patients, the protein-bound uremic retention solute p-cresol is independently associated with cardiovascular disease. The underlying mechanisms have not been elucidated. (1) Prospective observational study of humans and (2) in vitro study in human umbilical vein endothelial cells. Hemodialysis patients. p-Cresol and its main derivative p-cresyl sulfate. Endothelial dysfunction. We studied: (1) the relation between p-cresol and blood markers of endothelial dysfunction, including soluble P-selectin and endothelial microparticles; and (2) direct effects of p-cresol and p-cresyl sulfate on endothelial cell cultures. (1) In a cohort of 100 maintenance hemodialysis patients, free serum p-cresol concentrations (median, 11.7 micromol/L; interquartile range, 15.2) were directly associated with circulating endothelial microparticles (P = 0.007), but not with soluble P-selectin (mean, 37.7 +/- 14.4 [SD] pg/mL). Other independent determinants of the degree of circulating microparticles were greater serum phosphorus (mean, 4.8 +/- 1.5 mg/dL; P = 0.008) and serum calcium concentrations (mean, 9.3 +/- 0.8 mg/dL; P = 0.03), whereas treatment with active vitamin D (P = 0.008) and vintage (median, 25 months; P = 0.04) were inversely associated. (2) In vitro, p-cresyl sulfate induced a dose-dependent increase in the shedding of endothelial microparticles (P < 0.001) by human umbilical vein endothelial cells. Shedding was reduced, but not completely aborted, in the presence of albumin, whereas the selective Rho kinase inhibitor Y-27632 abrogated the p-cresyl sulfate-induced generation of endothelial microparticles. The relationship between p-cresyl sulfate and shedding of endothelial microparticles in vivo was not mechanistically explored. p-Cresyl sulfate induces shedding of endothelial microparticles in the absence of overt endothelial damage in vitro and is independently associated with the number of endothelial microparticles in hemodialysis patients. These findings suggest that p-cresyl sulfate alters endothelial function in hemodialysis patients.

PROGRESS IN UREMIC TOXIN RESEARCH: Platelet/Leukocyte Activation, Inflammation, and Uremia

Chronic kidney disease (CKD) is a state of chronic, low-grade inflammation which contributes to the accelerated progression of chronic inflammatory disturbances of which atherosclerosis is a major example. Platelet and leukocyte activation and interaction, evoked by the uremic condition, play an important role in this process. The effect of specific uremic retention solutes, progressively retained in uremia, on important platelet and leukocytes functions is discussed and summarized. The main uremic toxins involved are molecules with a molecular weight above 500 Da (the so-called "middle molecules") and/or protein-bound molecules. Classification of the molecules and elucidation of the pathophysiological pathways involved will result in new therapeutic strategies pursuing specific removal or pharmacological neutralization of molecular impact.