Removal of the protein-bound solutes indican and p-cresol sulfate by peritoneal dialysis

Twice Weekly versus Thrice Weekly Hemodialysis-A Pilot Cross-Over Equivalence Trial

Key Points

Background

The 2015 Update of the Kidney Disease Outcomes Quality Initiative (KDOQI) Guideline for Hemodialysis Adequacy increased the contribution of residual kidney function in calculating standard Kt/Vurea (stdKt/Vurea). However, no study has assessed the effect of prescribing twice weekly hemodialysis according to this guideline on patients' quality of life or uremic solute levels.

Methods

Twenty six hemodialysis patients with average residual urea clearance (Kru) 4.7±1.8 ml/min and hemodialysis vintage of 12±15 months (range 2 months to 4.9 years) underwent a cross-over trial comparing four weeks of twice weekly hemodialysis and four weeks of thrice weekly hemodialysis. Twice weekly hemodialysis was prescribed to achieve stdKt/Vurea 2.2 incorporating Kru using the 2015 KDOQI Guideline. Thrice weekly hemodialysis was prescribed to achieve spKt/Vurea 1.3 regardless of Kru. Quality of life and plasma levels of secreted uremic solutes and β 2 microglobulin were assessed at the end of each period.

Results

Equivalence testing between twice and thrice weekly hemodialysis based on the Kidney Disease Quality of Life instrument (primary analysis) was inconclusive. Symptoms as assessed by the secondary outcomes Dialysis Symptom Index and Post-Dialysis Recovery Time were not worse with twice weekly hemodialysis. StdKt/Vurea was adequate during twice weekly hemodialysis (2.7±0.5), and ultrafiltration rate and plasma potassium were controlled with minimally longer treatment times (twice weekly: 195±20 versus thrice weekly: 191±17 minutes). Plasma levels of the secreted solutes and β 2 microglobulin were not higher with twice weekly than thrice weekly hemodialysis.

Conclusions

Twice weekly hemodialysis can be prescribed using the higher contribution assigned to Kru by the 2015 KDOQI Guideline. With twice weekly hemodialysis, quality of life was unchanged, and the continuous function of the residual kidneys controlled fluid gain and plasma levels of potassium and uremic solutes without substantially longer treatment times.

Clinical Trial registration number:

NCT03874117.

Exploring the clinical efficacy and mechanism of high-position colon dialysis combined with Traditional Chinese Medicine retention enema in real-world patients with stage 3-5 chronic kidney disease (non-dialysis) based on the theory of the Gut-Kidney axis

Background: With societal and economic development, the annual incidence of chronic kidney disease (CKD) is increasing. Current treatments for CKD are limited, and once patients progress to the uraemic stage, it places a significant economic burden on families and society. Based on the "gut-kidney axis" theory and real-world research, this study aims to evaluate the clinical efficacy, safety, and potential mechanism of high-position colon dialysis combined with traditional Chinese medicine (TCM) retention enema in treating stage 3-5 chronic kidney disease (non-dialysis). Additionally, it seeks to identify new therapeutic targets and approaches for CKD treatment. Methods: The TCM decoction was analyzed using Ultra-Performance Liquid Chromatography-Quadrupole-Orbitrap-High Resolution Mass Spectrometry (UPLC-Q-Orbitrap-HRMS). Participants meeting the inclusion criteria were divided into a control group (n = 153) and a treatment group (n = 159) based on their preferences and physicians' recommendations. Both groups adhered to a high-quality low-protein, low-salt, low-phosphorus, and low-fat diet supplemented with essential amino acids, and were monitored for blood pressure, blood glucose, and blood lipids. The treatment group received high-position colon dialysis combined with TCM retention enemas (administered at least 12 times every other day). Results: Thirteen compounds were identified from the herbs by UPLC-Q-Orbitrap-HRMS. The CKD3-5 treatment group exhibited improvements in blood biochemistry and other laboratory indices, with significant enhancements in renal function-related indices for CKD4 and CKD5 stages (p < 0.05). Following treatment, indoxyl sulfate (IS), endotoxin, and D-lactic acid levels decreased to a certain extent in both groups, with a statistically significant difference observed within the treatment group (p < 0.05). The treatment group displayed a significant reduction in aerobic bacterial colonies, an increase in anaerobic bacterial colonies, a decrease in Escherichia coli colonies, and an increase in Bifidobacterium and Lactobacillus colonies (p < 0.05). No significant changes in colony numbers were observed in the control group. Conclusion: High-position colon dialysis combined with TCM retention enema may serve as an adjuvant treatment for CKD4-5 (non-dialysis), and its mechanism may be related to the reduction of uraemic toxins, improvement of intestinal mucosal barrier function, and regulation of intestinal microecology. Clinical Trial Registration: https://www.chictr.org.cn/, identifier ChiCTR2200062852.

The Removal of Uremic Solutes by Peritoneal Dialysis

ABSTRACT

Peritoneal dialysis (PD) is now commonly prescribed to achieve target clearances for urea or creatinine. The International Society for Peritoneal Dialysis has proposed however that such targets should no longer be imposed. The Society's new guidelines suggest rather that the PD prescription should be adjusted to achieve well-being in individual patients. The relaxation of treatment targets could allow increased use of PD. Measurement of solute levels in patients receiving dialysis individualized to relieve uremic symptoms could also help us identify the solutes responsible for those symptoms and then devise new means to limit their accumulation. This possibility has prompted us to review the extent to which different uremic solutes are removed by PD.

Derivation and elimination of uremic toxins from kidney-gut axis

Uremic toxins are chemicals, organic or inorganic, that accumulate in the body fluids of individuals with acute or chronic kidney disease and impaired renal function. More than 130 uremic solutions are included in the most comprehensive reviews to date by the European Uremic Toxins Work Group, and novel investigations are ongoing to increase this number. Although approaches to remove uremic toxins have emerged, recalcitrant toxins that injure the human body remain a difficult problem. Herein, we review the derivation and elimination of uremic toxins, outline kidney-gut axis function and relative toxin removal methods, and elucidate promising approaches to effectively remove toxins.

In-Situ Synchrotron Imaging, Experimental, and Computational Investigations on the Efficiency of Trametes versicolor Laccase on Detoxification of P-Cresyl Sulfate (PCS) Protein Bound Uremic Toxin (PBUT)

Protein bound uremic toxins (PBUTs) are small substances binding to larger proteins, mostly human serum albumin (HSA), and are challenging to remove by hemodialysis (HD). Among different classes of PBUTs, p-cresyl sulfate (PCS) is the most widely used marker molecule and major toxin, as 95% is bound to HSA. PCS has a pro-inflammatory effect and increases both the uremia symptom score and multiple pathophysiological activities. High-flux HD to clear PCS leads to serious loss of HSA, which results in a high mortality rate. The goal of the present study is to investigate the efficacy of PCS detoxification in serum of HD patients using a biocompatible laccase enzyme from Trametes versicolor. Molecular docking was used to gain an in-depth understanding of the interactions between PCS and the laccase to identify the functional group(s) responsible for ligand-protein receptor interactions. UV-Vis spectroscopy and gas chromatography-mass spectrometry (GC-MS) were used to assess the detoxification of PCS. GC-MS was used to identify the detoxification byproducts and their toxicity was assessed using docking commutations. In situ synchrotron radiation micro-computed tomography (SR-µCT) imaging available at the Canadian Light Source (CLS) was conducted to assess HSA binding with PCS before and after detoxification with laccase and undertake the corresponding quantitative analysis. GC-MS analyses confirmed the detoxification of PCS with laccase at a concentration of 500mg/L. The potential pathway of PCS detoxification in the presence of the laccase was identified. Increasing laccase concentration led to the formation of m-cresol, as indicated by the corresponding absorption in the UV-Vis spectra and a sharp peak on the GC-MS spectra. Our analysis provides insight into the general features of PCS binding on Sudlow site II, as well as insights into PCS detoxification product interactions. The average affinity energy for detoxification products was lower than that of PCS. Even though some byproducts showed potential toxicity, the level was lower than for PCS based on toxicity indexes (e.g., LD50/LC50, carcinogenicity, neurotoxicity, mutagenicity). In addition, these small compounds can also be more easily removed by HD compared to PCS. SR-µCT quantitative analysis showed adhesion of the HSA to a significant reduced extent in the presence of the laccase enzyme in bottom sections of the polyarylethersulfone (PAES) clinical HD membrane tested. Overall, this study opens new frontiers for PCS detoxification.

Traditional Chinese Medicine: An Exogenous Regulator of Crosstalk between the Gut Microbial Ecosystem and CKD

Chronic kidney disease (CKD) is often accompanied by an imbalance in the gut microbial ecosystem. Notably, the imbalanced gut microbiota and impaired intestinal barrier are the keys to the crosstalk between the gut microbial ecosystem and CKD, which was the central point of previous studies. Traditional Chinese medicine (TCM) has shown considerable efficacy in the treatment of CKD. However, the therapeutic mechanisms have not been fully elucidated. In this review, we explored therapeutic mechanisms by which TCM improved CKD via the gut microbial ecosystem. In particular, we focused on the restored gut microbiota (i.e., short-chain fatty acid- and uremic toxin-producing bacteria), improved gut-derived metabolites (i.e., short-chain fatty acid, indoxyl sulfate, p-Cresyl sulfate, and trimethylamine-N-oxide), and intestinal barrier (i.e., permeability and microbial translocation) as therapeutic mechanisms. The results found that the metabolic pattern of gut microbiota and the intestinal barrier were improved through TCM treatment. Moreover, the microbiota-transfer study confirmed that part of the protective effect of TCM was dependent on gut microbiota, especially SCFA-producing bacteria. In conclusion, TCM may be an important exogenous regulator of crosstalk between the gut microbial ecosystem and CKD, which was partly attributable to the mediation of microbiota-targeted intervention.

Altered gut microbiota and gut-derived p-cresyl sulfate serum levels in peritoneal dialysis patients

Peritoneal dialysis (PD) is a renal replacement therapy for end-stage renal disease. Gut microbiota-derived uremic solutes, indoxyl sulfate (IS), p-cresyl sulfate (PCS), and trimethylamine-N-oxide (TMAO) accumulate in PD patients. The objective was to explore the gut microbiota and their influence on uremic toxins in PD patients and healthy controls (HC). Fecal samples were collected from PD patients (n = 105) and HC (n = 102). 16S rRNA gene regions were sequenced for gut microbiota analysis. IS, PCS, and TMAO levels were measured using HPLC-MS. PD patients exhibited lower alpha diversity and altered gut microbiota composition compared to HC. At the genus level, PD patients showed increased abundance of opportunistic pathogenic bacteria, and decreased abundance of beneficial bacteria. Three Operational Taxonomic Units discriminated PD patients from HC. Phenylalanine metabolism increased in PD, whereas tryptophan metabolism was unaltered. Low serum PCS did not necessarily mean healthier due to the loss of alpha diversity, increased Proteobacteria and opportunistic pathogenic bacteria. High serum PCS was mainly caused by elevated p-cresol-producing bacteria, enriched amino acid related enzymes, and enhanced sulfur metabolism, rather than declined residual renal function. In patients with different urine volumes, the gut microbiota alpha diversity and composition were unaltered, but serum IS and TMAO were significantly elevated in anuric patients. In conclusion, the gut microbiota abundance, composition, and function were altered in PD patients, which increased the PCS levels. We provided a better understanding of the microbiota-metabolite-kidney axis in PD patients. Targeting certain bacteria could decrease the PCS levels, whereas preserving the residual renal function could reduce the IS and TMAO levels.

Improving Solute Clearances by Hemodialysis

The adequacy of hemodialysis is now assessed by measuring the removal of the single-solute urea. The urea clearance provided by contemporary dialysis is a large fraction of the blood flow through the dialyzer and therefore cannot be increased much further. Other solutes however likely contribute more than urea to the residual uremic illness suffered by hemodialysis patients. We here review methods which could be employed to increase the clearance of nonurea solutes. We will separately consider the clearances of free low-molecular-mass solutes, free larger solutes, and protein-bound solutes. New clinical studies will be required to test the extent to which increasing the clearance on nonurea solutes with these various characteristics can improve patients' health.

Association between Cardiac Outcomes and Indoxyl Sulfate Levels in Hemodialysis Patients: A Cross-sectional Study

Objective Indoxyl sulfate (IS) is a protein-bound uremic toxin that is associated with cardiovascular events and mortality in hemodialysis (HD) patients. However, the factors affecting the levels of IS are currently unclear. This study aimed to investigate the factors influencing serum IS concentrations in HD patients. Methods We included 100 HD patients from Guangdong Provincial People's Hospital. Baseline characteristics, including sex, age, clinical features, duration of HD, echocardiography findings, electrocardiogram results, and biochemical indicators, were collected and analyzed in relation to serum total-form IS levels. Results Among all 100 patients, serum IS levels were significantly higher in patients aged ≥ 60 years, males, and patients with mitral regurgitation and inadequate dialysis. Among patients aged < 60 years, IS levels were significantly higher among patients with mitral regurgitation compared with those without. Furthermore, multiple linear regression analysis identified sex, age, ventricular septal thickness, and mitral regurgitation as factors independently associated with serum IS (STDβ = -0.475, 0.162, -0.153, 0.142, and 0.136, respectively; all P < 0.05) adjusted for body mass index, smoking, and fasting plasma glucose. Conclusions Male sex, age ≥ 60 years, ventricular septal thickness, and mitral regurgitation are factors associated with high total serum IS concentrations in Chinese HD patients. Elevated IS levels may play a role in the process of mitral regurgitation in HD patients < 60 years old.

Polyethyleneimine-anchored liposomes as scavengers for improving the efficiency of protein-bound uremic toxin clearance during dialysis

Protein-bound uremic toxins (PBUTs) are significant toxins that are closely related to the prognosis of chronic kidney disease. They cannot be effectively removed by conventional dialysis therapies due to their high albumin binding affinity. Our previous research revealed that cationic liposomes (i.e., polyethyleneimine [PEI]-decorated liposomes) could enhance the clearance of PBUTs via electrostatic interactions. However, the poor biocompatibility (hemolysis) restricted their applications in clinical dialysis treatment. Herein, we produced PEI-anchored, linoleic acid-decorated liposomes (CP-LA liposomes) via the conjugation of PEI to cholesterol chloroformate (Chol-PEI, CP), and linoleic acid (LA) was added to provide liposomal colloidal stability. The CP-LA liposomes outperformed the plain liposomes, demonstrating significantly higher PBUT binding rates and removal rates. In addition, in vitro dialysis simulation verified that the CP-LA liposomes had a better capacity for PBUT clearance than the plain liposomes, especially for PBUTs with a strong negative net charge. Hemolysis and cytotoxicity tests revealed that the biocompatibility of the CP-LA liposomes was better than that of the physically-decorated PEI-liposome. CP-LA liposomes possess great potential for PBUT clearance in clinical dialysis therapy.

Association of Plasma Uremic Solute Levels with Residual Kidney Function in Children on Peritoneal Dialysis

BACKGROUND AND OBJECTIVES

Residual native kidney function confers health benefits in patients on dialysis. It can facilitate control of extracellular volume and inorganic ion concentrations. Residual kidney function can also limit the accumulation of uremic solutes. This study assessed whether lower plasma concentrations of uremic solutes were associated with residual kidney function in pediatric patients on peritoneal dialysis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Samples were analyzed from 29 pediatric patients on peritoneal dialysis, including 13 without residual kidney function and ten with residual kidney function. Metabolomic analysis by untargeted mass spectrometry compared plasma solute levels in patients with and without residual kidney function. Dialytic and residual clearances of selected solutes were also measured by assays using chemical standards.

RESULTS

Metabolomic analysis showed that plasma levels of 256 uremic solutes in patients with residual kidney function averaged 64% (interquartile range, 51%-81%) of the values in patients without residual kidney function who had similar total Kt/V_urea. The plasma levels were significantly lower for 59 of the 256 solutes in the patients with residual kidney function and significantly higher for none. Assays using chemical standards showed that residual kidney function provides a higher portion of the total clearance for nonurea solutes than it does for urea.

CONCLUSIONS

Concentrations of many uremic solutes are lower in patients on peritoneal dialysis with residual kidney function than in those without residual kidney function receiving similar treatment as assessed by Kt/V_urea.

Why Is the GFR So High?: Implications for the Treatment of Kidney Failure

The high GFR in vertebrates obligates large energy expenditure. Homer Smith's teleologic argument that this high GFR was needed to excrete water as vertebrates evolved in dilute seas is outdated. The GFR is proportional to the metabolic rate among vertebrate species and higher in warm-blooded mammals and birds than in cold-blooded fish, amphibians, and reptiles. The kidney clearance of some solutes is raised above the GFR by tubular secretion, and we presume secretion evolved to eliminate particularly toxic compounds. In this regard, high GFRs may provide a fluid stream into which toxic solutes can be readily secreted. Alternatively, the high GFR may be required to clear solutes that are too large or too varied to be secreted, especially bioactive small proteins and peptides. These considerations have potentially important implications for the understanding and treatment of kidney failure.

Improving Clearance for Renal Replacement Therapy

The adequacy of hemodialysis is now assessed by measuring the removal of a single solute, urea. The urea clearance provided by current dialysis methods is a large fraction of the blood flow through the dialyzer, and, therefore, cannot be increased much further. However, other solutes, which are less effectively cleared than urea, may contribute more to the residual uremic illness suffered by patients on hemodialysis. Here, we review a variety of methods that could be used to increase the clearance of such nonurea solutes. New clinical studies will be required to test the extent to which increasing solute clearances improves patients' health.

Review of the structural and functional brain changes associated with chronic kidney disease

Chronic kidney disease (CKD) leads to profound metabolic and hemodynamic changes, which damage other organs, such as heart and brain. The brain abnormalities and cognitive deficit progress with the severity of the CKD and are mostly expressed among hemodialysis patients. They have great socio-economic impact. In this review, we present the current knowledge of involved mechanisms.

Targeting Uremic Toxins to Prevent Peripheral Vascular Complications in Chronic Kidney Disease

Chronic kidney disease (CKD) exhibits progressive kidney dysfunction and leads to disturbed homeostasis, including accumulation of uremic toxins, activated renin-angiotensin system, and increased oxidative stress and proinflammatory cytokines. Patients with CKD are prone to developing the peripheral vascular disease (PVD), leading to poorer outcomes than those without CKD. Cumulative evidence has showed that the synergy of uremic milieu and PVD could exaggerate vascular complications such as limb ischemia, amputation, stenosis, or thrombosis of a dialysis vascular access, and increase mortality risk. The role of uremic toxins in the pathogenesis of vascular dysfunction in CKD has been investigated. Moreover, growing evidence has shown the promising role of uremic toxins as a therapeutic target for PVD in CKD. This review focused on uremic toxins in the pathophysiology, in vitro and animal models, and current novel clinical approaches in reducing the uremic toxin to prevent peripheral vascular complications in CKD patients.

Removal of Protein-Bound Uremic Toxins by Liposome-Supported Peritoneal Dialysis

Background:Protein-bound uremic toxins (PBUTs) are poorly cleared by peritoneal dialysis (PD). This study aimed to enhance PBUT removal in PD by adding a binder to the peritoneal dialysate and to evaluate the feasibility and efficacy of liposome-supported PD (LSPD) to increase the removal of PBUTs compared with albumin PD.Methods:Removal of p-cresyl sulfate (PCS), indoxyl sulfate (IS), and indole-3-acetic acid (3-IAA) was first evaluated in an in vitro PD model using artificial plasma preloaded with test solutes. Male Sprague-Dawley rats (n = 24) were then subjected to 5/6 nephrectomy and fed for 16 weeks to establish end-stage renal failure, after which they were treated with either conventional glucose-based PD, albumin-based PD, or liposome-based PD. Removal of PBUTs and small water-soluble solutes was determined during a 6-hour PD dwell.Results:In vitro experiments showed that adding albumin as a toxin binder to the dialysate markedly increased the removal of PCS, IS, and 3-IAA compared with the control. The uptake capacity of liposomes was comparable with that of albumin for PCS and 3-IAA, though slightly inferior for IS. In vivo PD in uremic rats demonstrated that LSPD resulted in higher intraperitoneal concentrations and more total mass removal for PBUTs than the conventional glucose-based PD, which was comparable with albumin PD.Conclusions:Supplementing conventional glucose-based PD solutions with a binder could efficiently increase the removal of PBUTs. This preliminary study suggested that LSPD may be a promising alternative to albumin PD for increasing PBUT removal in the development of next-generation PD solutions for PD patients.

Association of Tubular Solute Clearance with Symptom Burden in Incident Peritoneal Dialysis

BACKGROUND AND OBJECTIVES

Residual kidney function is important to the health and wellbeing of patients with ESKD. We tested whether the kidney clearances of proximal tubular secretory solutes are associated with burden of uremic and heart failure symptoms among patients on peritoneal dialysis with residual kidney function.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We enrolled 29 patients on incident peritoneal dialysis with residual urine output >250 ml daily. We used targeted liquid chromatography-mass spectrometry to quantify plasma, 24-hour urine, and peritoneal dialysate concentrations of ten tubular secretory solutes. We calculated the kidney and peritoneal dialysis clearances of each secretory solute, creatinine, and urea, and we estimated a composite kidney and peritoneal secretion score. We assessed for uremic symptoms using the Dialysis Symptom Index and heart failure-related symptoms using the Kansas City Cardiomyopathy Questionnaire. We used linear regression to determine associations of composite secretory solute clearances and GFR_urea+Cr with Dialysis Symptom Index symptom score and Kansas City Cardiomyopathy Questionnaire summary score.

RESULTS

Mean residual kidney clearances of creatinine and urea were 8±5 and 9±6 ml/min per 1.73 m², respectively, and mean GFR_urea+Cr was 8±5 ml/min per 1.73 m². The residual kidney clearances of most secretory solutes were considerably higher than creatinine and urea clearance, and also, they were higher than their respective peritoneal dialysis clearances. After adjustments for age and sex, each SD higher composite kidney secretion score was associated with an 11-point lower Dialysis Symptom Index score (95% confidence interval, -20 to -1; P=0.03) and a 12-point higher Kansas City Cardiomyopathy Questionnaire score (95% confidence interval, 0.5- to 23-point higher score; P=0.04). Composite peritoneal dialysis secretion score was not associated with either symptom assessment.

CONCLUSIONS

Residual kidney clearances of secretory solutes are higher than peritoneal dialysis clearances. Kidney clearances of secretory solutes are associated with patient-reported uremic and heart failure-related symptoms.

Protein-bound uremic toxins are associated with cognitive function among patients undergoing maintenance hemodialysis

Patients with chronic kidney disease have a greater risk of cognitive impairment. Cerebral uremic solute accumulation causes uremic encephalopathy; however, the association of protein-bound uremic toxins on cognitive function remains unclear. The present study aimed to investigate the association of two protein-bound uremic toxins, namely indoxyl sulfate (IS) and p-cresyl sulfate (PCS), on cognitive function in patients receiving hemodialysis (HD) for at least 90 days. Circulating free form IS and PCS were quantified by liquid chromatography/mass spectrometry. Mini-Mental State Examination (MMSE) and Cognitive Abilities Screening Instrument (CASI) were used to evaluate cognitive function. In total, 260 HD patients were recruited with a mean age of 58.1 ± 11.3 years, of which, 53.8% were men, 40% had diabetes, and 75.4% had hypertension. The analysis revealed that both free IS and free PCS were negatively associated with the CASI score and MMSE. After controlling for confounders, circulating free IS levels persisted to be negatively associated with MMSE scores [β = −0.62, 95% confidence interval (CI): −1.16 to −0.08] and CASI scores (β = −1.97, 95% CI: −3.78 to −0.16), mainly in the CASI domains of long-term memory, mental manipulation, language ability, and spatial construction. However, there was no correlation between free PCS and total MMSE or total CASI scores after controlling for confounders. In conclusion, circulating free form IS, but not PCS is associated with lower cognitive function test scores in HD patients. Thus, a further study is needed to evaluate whether a decrease in free IS levels can slow down cognitive decline in HD patients.

Bisphenol A analogs in patients with chronic kidney disease and dialysis therapy

Bisphenol A (BPA) accumulates in patients with chronic kidney disease (CKD), and hemodialysis filters may contribute to bisphenol burden in patients on hemodialysis (HD). The serum levels of BPA and three BPA analogs, namely, bisphenol B (BPB), bisphenol S (BPS), and bisphenol F (BPF), in 58 patients with CKD, 66 patients on dialysis therapy and 30 healthy control were investigated. The content of four bisphenols (BPs) was also examined in three types of dialysis filters, followed by an in vitro elution experiment to test the release of BPs from the dialysis filters. The serum levels of BPA (r = -0.746, p < 0.05) and BPS (r = -0.433, p < 0.05) in 58 CKD patients and 30 healthy control were correlated with the decrease in estimated glomerular filtration rate. The serum levels of BPs in the HD patients were higher than those in the peritoneal dialysis patients (p < 0.05). In the in vitro study on the BP contents in dialysis filters, BPA was the main form of the BPs in the polysulfone membrane (20.86 ± 1.18 ng/mg) and in the polyamide membrane (18.70 ± 2.88 ng/mg), and a modicum of BPS (0.01 ± 0.01 ng/mg) was detected in the polyethersulfone membrane. The results of the elution experiment were in accordance with the results of BPs content in the dialysis filters. Insufficient renal function may lead to BPs accumulation in patients with CKD, and BPs in dialysis products may cause BPs burden in patients on HD.

Haemodiafiltration does not lower protein-bound uraemic toxin levels compared with haemodialysis in a paediatric population

Background

Haemodiafiltration (HDF) is accepted to effectively lower plasma levels of middle molecules in the long term, while data are conflicting with respect to the additive effect of convection on lowering protein-bound uraemic toxins (PBUTs). Here we compared pre-dialysis β2-microglobulin (β2M) and PBUT levels and the percentage of protein binding (%PB) in children on post-dilution HDF versus conventional high- (hf) or low-flux (lf) haemodialysis (HD) over 12 months of treatment.

Methods

In a prospective multicentre, non-randomized parallel-arm intervention study, pre-dialysis levels of six PBUTs and β2M were measured in children (5–20 years) on post-HDF (n = 37), hf-HD (n = 42) and lf-HD (n = 18) at baseline and after 12 months. Analysis of variance was used to compare levels and %PB in post-HDF versus conventional hf-HD and lf-HD cross-sectionally at 12 months and longitudinal from baseline to 12 months.

Results

For none of the PBUTs, no difference was found in either total and free plasma levels or %PB between post-HDF versus the hf-HD and lf-HD groups. Children treated with post-HDF had lower pre-dialysis β2M levels [median 23.2 (21.5; 26.6) mg/dL] after 12 months versus children on hf-HD [P&lt;0.01; 35.2 (29.3; 41.2) mg/dL] and children on lf-HD [P&lt;0.001; 47.2 (34.3; 53.0) mg/dL]. While β2M levels remained steady in the hf-HD and lf-HD group, a decrease in β2M was demonstrated for children on post-HDF (P&lt;0.01).

Conclusions

While post-HDF successfully decreased β2M, no additive effect on PBUT over 12 months of treatment was found. PBUT removal is complex and hampered by several factors. In children, these factors might be different from adults and should be explored in future research.

Simplified colonic dialysis with hemodialysis solutions delays the progression of chronic kidney disease

BACKGROUND

The colon plays a vital role in the disposal of nitrogenous waste products. Therefore, the colon may provide a therapeutic target for managing chronic kidney disease (CKD).

AIM

To evaluate the efficacy of a simplified model of colonic dialysis with hemodialysis solutions (SCD) to delay the progression of stages 3-5 CKD.

DESIGN

Retrospective study.

METHODS

We retrospectively analyzed 178 stages 3-5 CKD patients who did or did not receive SCD (SCD group, n = 88; control group, n = 90). The follow-up was 36 months. The outcome of CKD progression was defined as a decrease in 50% or more in estimated glomerular filtration rate, starting hemodialysis or peritoneal dialysis or undergoing transplantation. The Kaplan-Meier analysis was used to compare CKD progression between SCD and control groups as well as between subgroups at different CKD stages. Cox proportional hazard models adjusted for patients' characteristics were used to examine the association between SCD and the outcome.

RESULTS

For all patients, the outcome was significantly better in SCD group compared to control group (P < 0.05). The results were similar in the subgroups of patients at stage 4 (P = 0.001) and stage 5 (P = 0.000), but not in the subgroup of patients at stage 3 (P = 0.121). For all patients, SCD was associated with a lower risk of CKD progression after adjusted for patients' characteristics (adjusted hazard ratio, 0.373; 95% confidence interval, 0.201-0.694; P 0.002).

CONCLUSION

SCD is an effective supplementary therapy to delay the progression of stages 4-5 CKD.

Shen-Shuai-Ning granule decreased serum concentrations of indoxyl sulphate in uremic patients undergoing peritoneal dialysis

Clearance of protein-bound uremic toxins (PBUTs) by dialysis is a challenge in the treatment of uremic patients. Shen-Shuai-Ning (SSN), a traditional Chinese medicine formulation, has been used commonly in China to retard kidney disease progression and decrease uremic toxins in chronic kidney disease (CKD) patients, but the effects of SSN on serum PBUTs in dialysis patients were not investigated. We conducted a randomized controlled trial in patients on peritoneal dialysis (PD) at dialysis center of Changzheng Hospital to evaluate the effects of SSN on serum PBUTs. Participants with SSN intervention took 5 g SSN granule three times daily for 12 weeks, while the baseline medications and dialysis prescriptions remained during the study in all patients. The serum concentrations of indoxyl sulphate (IS) and p-cresol sulphate (PCS) were determined by HPLC/MS/MS and biochemical parameters were assessed during the study. Sixty PD patients were enrolled and randomly allocated into SSN group and control group. Total IS level was significantly lower in SSN group than in control group at week 4, 8, and 12 (27.28 ± 18.19, 29.73 ± 19.10, and 29.41 ± 17.61 mg/l compared with 39.25 ± 20.23, 44.86 ± 23.91, and 45.34 ± 33.52 mg/l, respectively). However, there were no statistical difference of total PCS, free forms of IS and PCS concentrations between SSN group and control group during 12 weeks follow-up. Administration of SSN granule orally decreased serum total IS level effectively in uremic patients on PD with good tolerance. Benefits of PD patients’ outcomes from IS reduction by SSN awaits further large size and long duration clinical trials to verify.

Anti-ageing effect of red ginseng revealed by urinary metabonomics using RRLC-Q-TOF-MS

INTRODUCTION

Red ginseng (RG) is one of the main processed products of the roots and rhizomes of Panax ginseng C.A. Meyer and is used for anti-ageing. But how metabonomic influences of RG on the progress of ageing are less researched.

OBJECTIVE

A metabonomic method was developed to study the characters of the ageing process and the effects of total ginsenosides of red ginseng (TGRG) on the progress of ageing.

METHODS

Urine samples from four different ages (4, 12, 18 and 24 months old) of rats and interference after TGRG were analysed by rapid resolution liquid chromatography coupled with quadruple-time-of-flight mass spectrometry (RRLC-Q-TOF-MS) and multivariate statistical analysis were performed for the pattern recognition and characteristic metabolites identification.

RESULTS

Fourteen potential biomarkers were found and identified by MS/MS analysis by referring to authentic chemicals. The analysis of metabolic pathways indicated that the reduced energy and lipid metabolism, the decline of kidney function and amino acids metabolism disorders were the main features of ageing. After TGRG administration, lipid and amino acids metabolism of 18 and 24 month-old rats were adjusted to restore a younger level, and nine related biomarkers in the ageing process reset to a younger level were recognised.

CONCLUSION

These changes showed that TGRG may produce an anti-ageing effect by intervening in the lipid metabolism and correcting the amino acid metabolism disorders in ageing rats.

Uremic Toxin Clearance and Cardiovascular Toxicities

Uremic solutes contribute to cardiovascular disease in renal insufficiency. In this review we describe the clearance of selected uremic solutes, which have been associated with cardiovascular disease. These solutes—indoxyl sulfate (IS), p-cresol sulfate (PCS), phenylacetylglutamine (PAG), trimethylamine-n-oxide (TMAO), and kynurenine—exemplify different mechanisms of clearance. IS and PCS are protein-bound solutes efficiently cleared by the native kidney through tubular secretion. PAG and TMAO are not protein-bound but are also cleared by the native kidney through tubular secretion, while kynurenine is not normally cleared by the kidney. Increases in the plasma levels of the normally secreted solutes IS, PCS, TMAO, and PAG in chronic kidney disease (CKD) are attributable to a reduction in their renal clearances. Levels of each of these potential toxins are even higher in patients on dialysis than in those with advanced chronic kidney disease, which can be accounted for in part by a low ratio of dialytic to native kidney clearance. The rise in plasma kynurenine in CKD and dialysis patients, by contrast, remains to be explained. Our ability to detect lower levels of the potential uremic cardiovascular toxins with renal replacement therapy may be limited by the intermittency of treatment, by increases in solute production, and by the presence of non-renal clearance. Reduction in the levels of uremic cardiovascular toxins may in the future be achieved more effectively by inhibiting their production.

Uremic Toxin Clearance and Cardiovascular Toxicities

Uremic solutes contribute to cardiovascular disease in renal insufficiency. In this review we describe the clearance of selected uremic solutes, which have been associated with cardiovascular disease. These solutes-indoxyl sulfate (IS), p-cresol sulfate (PCS), phenylacetylglutamine (PAG), trimethylamine-n-oxide (TMAO), and kynurenine-exemplify different mechanisms of clearance. IS and PCS are protein-bound solutes efficiently cleared by the native kidney through tubular secretion. PAG and TMAO are not protein-bound but are also cleared by the native kidney through tubular secretion, while kynurenine is not normally cleared by the kidney. Increases in the plasma levels of the normally secreted solutes IS, PCS, TMAO, and PAG in chronic kidney disease (CKD) are attributable to a reduction in their renal clearances. Levels of each of these potential toxins are even higher in patients on dialysis than in those with advanced chronic kidney disease, which can be accounted for in part by a low ratio of dialytic to native kidney clearance. The rise in plasma kynurenine in CKD and dialysis patients, by contrast, remains to be explained. Our ability to detect lower levels of the potential uremic cardiovascular toxins with renal replacement therapy may be limited by the intermittency of treatment, by increases in solute production, and by the presence of non-renal clearance. Reduction in the levels of uremic cardiovascular toxins may in the future be achieved more effectively by inhibiting their production.

Dihydroartemisinin inhibits indoxyl sulfate (IS)-promoted cell cycle progression in mesangial cells by targeting COX-2/mPGES-1/PGE2 cascade

Dihydroartemisinin (DHA) is a semisynthetic derivative of artemisinin and has been used as an antimalarial drug. Recently, roles of artemisinin and its derivatives in treating diseases besides antimalarial effect were documented. Thus, this study was undertaken to investigate the role of DHA in indoxyl sulfate (IS)-promoted cell cycle progression in glomerular mesangial cells, as well as the potential mechanisms. Under the basal condition, DHA significantly retarded the cell cycle progression as shown by decreased cell percentage in S phase and increased cell percentage in G1/G0 phases in line with reduced cell cycle proteins cyclin A2 and cyclin D1. Interestingly, DHA also inactivated the COX-2/mPGES-1/PGE₂ cascade which has been shown to play a critical role in promoting the mesangial cell cycle progression by our previous studies. Next, we investigated the role of DHA in IS-triggered cell cycle progression in this mesangial cell line. As expected, DHA treatment significantly retarded IS-induced cell cycle progression and inhibited the activation of COX-2/mPGES-1/PGE₂ cascade induced by IS. In summary, these data indicated that DHA inhibited the cell cycle progression in glomerular mesangial cells under normal condition or IS challenge possibly through the inhibition of COX-2/mPGES-1/PGE₂ cascade, suggesting a potential of DHA in treating glomerular diseases with mesangial cell proliferation.

Application of automated peritoneal dialysis in urgent-start peritoneal dialysis patients during the break-in period

Objective

Whether automated peritoneal dialysis (APD) is a feasible strategy for urgent-start peritoneal dialysis (PD) therapy during the break-in period remains unclear. This study was conducted to compare the efficacy as well as complications among three PD modes during the break-in period.

Methods

Ninety-six patients treated with urgent-start PD after catheterization were retrospectively analyzed. Patients were divided into three groups, incremental continuous ambulatory PD (CAPD) group (n = 26); APD group (n = 42); and APD–CAPD group (n = 28). Clinical parameters at the end of the break-in period and 1 month after the initiation of PD treatment were collected and analyzed.

Results

Compared with the traditional incremental CAPD, APD and APD–CAPD were superior as they could effectively remove small-molecule uremic toxins and correct electrolyte imbalance (P < 0.05), while did not increase the incidence of early complications during the break-in period (P > 0.05). However, APD led to a significant decline in albumin and pre-albumin, as compared with APD–CAPD and CAPD (P < 0.05). A PD strategy consisting 6 days of APD and 3 days of CAPD showed a great advantage in preventing excessive protein loss. There were no significant differences in all tested biochemical parameters among the three groups at 1 month after treatment (all P > 0.05).

Conclusion

Application of APD for urgent-start PD during the break-in period is feasible. A combination of APD and CAPD regimens seems to be a more reasonable mode.

Prognostic Value of Residual Urine Volume, GFR by 24-hour Urine Collection, and eGFR in Patients Receiving Dialysis

BACKGROUND AND OBJECTIVES

Residual kidney function can be assessed by simply measuring urine volume, calculating GFR using 24-hour urine collection, or estimating GFR using the proposed equation (eGFR). We aimed to investigate the relative prognostic value of these residual kidney function parameters in patients on dialysis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Using the database from a nationwide prospective cohort study, we compared differential implications of the residual kidney function indices in 1946 patients on dialysis at 36 dialysis centers in Korea between August 1, 2008 and December 31, 2014. Residual GFR calculated using 24-hour urine collection was determined by an average of renal urea and creatinine clearance on the basis of 24-hour urine collection. eGFR-urea, creatinine and eGFR β2-microglobulin were calculated from the equations using serum urea and creatinine and β2-microglobulin, respectively. The primary outcome was all-cause death.

RESULTS

During a mean follow-up of 42 months, 385 (19.8%) patients died. In multivariable Cox analyses, residual urine volume (hazard ratio, 0.96 per 0.1-L/d higher volume; 95% confidence interval, 0.94 to 0.98) and GFR calculated using 24-hour urine collection (hazard ratio, 0.98; 95% confidence interval, 0.95 to 0.99) were independently associated with all-cause mortality. In 1640 patients who had eGFR β2-microglobulin data, eGFR β2-microglobulin (hazard ratio, 0.98; 95% confidence interval, 0.96 to 0.99) was also significantly associated with all-cause mortality as well as residual urine volume (hazard ratio, 0.96 per 0.1-L/d higher volume; 95% confidence interval, 0.94 to 0.98) and GFR calculated using 24-hour urine collection (hazard ratio, 0.97; 95% confidence interval, 0.95 to 0.99). When each residual kidney function index was added to the base model, only urine volume improved the predictability for all-cause mortality (net reclassification index =0.11, P=0.01; integrated discrimination improvement =0.01, P=0.01).

CONCLUSIONS

Higher residual urine volume was significantly associated with a lower risk of death and exhibited a stronger association with mortality than GFR calculated using 24-hour urine collection and eGFR-urea, creatinine. These results suggest that determining residual urine volume may be beneficial to predict patient survival in patients on dialysis.

p-Cresyl Sulfate

If chronic kidney disease (CKD) is associated with an impairment of kidney function, several uremic solutes are retained. Some of these exert toxic effects, which are called uremic toxins. p-Cresyl sulfate (pCS) is a prototype protein-bound uremic toxin to which many biological and biochemical (toxic) effects have been attributed. In addition, increased levels of pCS have been associated with worsening outcomes in CKD patients. pCS finds its origin in the intestine where gut bacteria metabolize aromatic amino acids, such as tyrosine and phenylalanine, leading to phenolic end products, of which pCS is one of the components. In this review we summarize the biological effects of pCS and its metabolic origin in the intestine. It appears that, according to in vitro studies, the intestinal bacteria generating phenolic compounds mainly belong to the families Bacteroidaceae, Bifidobacteriaceae, Clostridiaceae, Enterobacteriaceae, Enterococcaceae, Eubacteriaceae, Fusobacteriaceae, Lachnospiraceae, Lactobacillaceae, Porphyromonadaceae, Staphylococcaceae, Ruminococcaceae, and Veillonellaceae. Since pCS remains difficult to remove by dialysis, the gut microbiota could be a future target to decrease pCS levels and its toxicity, even at earlier stages of CKD, aiming at slowing down the progression of the disease and decreasing the cardiovascular burden.

Limited reduction in uremic solute concentrations with increased dialysis frequency and time in the Frequent Hemodialysis Network Daily Trial

The Frequent Hemodialysis Network Daily Trial compared conventional three-times weekly treatment to more frequent treatment with a longer weekly treatment time in patients receiving in-center hemodialysis. Evaluation at one year showed favorable effects of more intensive treatment on left ventricular mass, blood pressure, and phosphate control, but modest or no effects on physical or cognitive performance. The current study compared plasma concentrations of uremic solutes in stored samples from 53 trial patients who received three-times weekly in-center hemodialysis for an average weekly time of 10.9 hours and 30 trial patients who received six-times weekly in-center hemodialysis for an average of 14.6 hours. Metabolomic analysis revealed that increased treatment frequency and time resulted in an average reduction of only 15 percent in the levels of 107 uremic solutes. Quantitative assays confirmed that increased treatment did not significantly reduce levels of the putative uremic toxins p-cresol sulfate or indoxyl sulfate. Kinetic modeling suggested that our ability to lower solute concentrations by increasing hemodialysis frequency and duration may be limited by the presence of non-dialytic solute clearances and/or changes in solute production. Thus, failure to achieve larger reductions in uremic solute concentrations may account, in part, for the limited benefits observed with increasing frequency and weekly treatment time in Frequent Hemodialysis Daily Trial participants.

Role of anuria in the relationship between indoxyl sulfate and anemia in peritoneal dialysis patients

Background

Experimental evidence suggests that indoxyl sulfate (IS) is associated with chronic kidney disease-related anemia. However, clinical studies are limited, and few have explored the potential confounding effect of anuria. This study, thus, evaluated the association between IS and anemia in both non-anuric and anuric peritoneal dialysis (PD) patients.

Methods

This cross-sectional and observational study included 165 chronic PD patients aged 19–84 years. Their serum IS levels in total and free forms were measured by ultra performance liquid chromatography. Correlations between serum IS and hemoglobin (Hb) were performed in both non-anuric and anuric groups.

Results

Among the study subjects, 90 were non-anuric and 75 were anuric. As a whole, there was no correlation between IS and Hb. Nonetheless, subsequent analysis of the non-anuric patients showed that Hb is negatively correlated with IS levels (r_s=−0.405, P<0.001 for total form and r_s=−0.296, P=0.005 for free form). Factors that significantly affected Hb levels in the stepwise multiple regression analysis include total IS and iron saturation. In contrast for anuric patients, serum ferritin, albumin, iron saturation, use of angiotensin-converting enzyme inhibitor/angiotensin II receptor blocker, but not serum IS, were predictors for anemia in the multiple regression model.

Conclusions

Serum IS is associated with an increased severity of anemia in non-anuric PD patients and not in anuric ones, indicating anuria could be a confounding factor in such association.

Indoxyl Sulfate Induces Mesangial Cell Proliferation via the Induction of COX-2

Indoxyl sulfate (IS) is one of important uremic toxins and is markedly accumulated in the circulation of end stage renal disease (ESRD) patients, which might contribute to the damage of residual nephrons and progressive loss of residual renal function (RRF). Thus this study was undertaken to investigate the role of IS in modulating mesangial cell (MC) proliferation and the underlying mechanism. The proliferation of MCs induced by IS was determined by cell number counting, DNA synthase rate, and cell cycle phase analysis. COX-2 expression was examined by Western blotting and qRT-PCR, and a specific COX-2 inhibitor NS398 was applied to define its role in IS-induced MC proliferation. Following IS treatment, MCs exhibited increased total cell number, DNA synthesis rate, and number of cells in S and G2 phases paralleled with the upregulation of cyclin A2 and cyclin D1. Next, we found an inducible inflammation-related enzyme COX-2 was remarkably enhanced by IS, and the inhibition of COX-2 by NS398 significantly blocked IS-induced MC proliferation in line with a blockade of PGE2 production. These findings indicated that IS could induce MC proliferation via a COX-2-mediated mechanism, providing new insights into the understanding and therapies of progressive loss of RRF in ESRD.

Effect of a sustained difference in hemodialytic clearance on the plasma levels of p-cresol sulfate and indoxyl sulfate

BACKGROUND

The protein-bound solutes p-cresol sulfate (PCS) and indoxyl sulfate (IS) accumulate to high plasma levels in renal failure and have been associated with adverse events. The clearance of these bound solutes can be altered independently of the urea clearance by changing the dialysate flow and dialyzer size. This study tested whether a sustained difference in clearance would change the plasma levels of PCS and IS.

METHODS

Fourteen patients on thrice-weekly nocturnal hemodialysis completed a crossover study of two periods designed to achieve widely different bound solute clearances. We compared the changes in pre-dialysis plasma PCS and IS levels from baseline over the course of the two periods.

RESULTS

The high-clearance period provided much higher PCS and IS clearances than the low-clearance period (PCS: 23 ± 4 mL/min versus 12 ± 3 mL/min, P < 0.001; IS: 30 ± 5 mL/min versus 17 ± 4 mL/min, P < 0.001). Despite the large difference in clearance, the high-clearance period did not have a different effect on PCS levels than the low-clearance period [from baseline, high: +11% (-5, +37) versus low: -8% (-18, +32), (median, 25th, 75th percentile), P = 0.50]. In contrast, the high-clearance period significantly lowered IS levels compared with the low-clearance period [from baseline, high: -4% (-17, +1) versus low: +22% (+14, +31), P < 0.001). The amount of PCS removed in the dialysate was significantly greater at the end of the high-clearance period [269 (206, 312) versus 199 (111, 232) mg per treatment, P < 0.001], while the amount of IS removed was not different [140 (87, 196) versus 116 (89, 170) mg per treatment, P = 0.15].

CONCLUSIONS

These findings suggest that an increase in PCS generation prevents plasma levels from falling when the dialytic clearance is increased. Suppression of solute generation may be required to reduce plasma PCS levels in dialysis patients.

Kt/Vurea and Nonurea Small Solute Levels in the Hemodialysis Study

The Hemodialysis (HEMO) Study showed that high-dose hemodialysis providing a single-pool Kt/V_urea of 1.71 provided no benefit over a standard treatment providing a single-pool Kt/V_urea of 1.32. Here, we assessed whether the high-dose treatment used lowered plasma levels of small uremic solutes other than urea. Measurements made ≥3 months after randomization in 1281 patients in the HEMO Study showed a range in the effect of high-dose treatment compared with that of standard treatment: from no reduction in the level of p-cresol sulfate or asymmetric dimethylarginine to significant reductions in the levels of trimethylamine oxide (-9%; 95% confidence interval [95% CI], -2% to -15%), indoxyl sulfate (-11%; 95% CI, -6% to -15%), and methylguanidine (-22%; 95% CI, -18% to -27%). Levels of three other small solutes also decreased slightly; the level of urea decreased 9%. All-cause mortality did not significantly relate to the level of any of the solutes measured. Modeling indicated that the intermittency of treatment along with the presence of nondialytic clearance and/or increased solute production accounted for the limited reduction in solute levels with the higher Kt/V_urea In conclusion, failure to achieve greater reductions in solute levels may explain the failure of high Kt/V_urea treatment to improve outcomes in the HEMO Study. Furthermore, levels of the nonurea solutes varied widely among patients in the HEMO Study, and achieved Kt/V_urea accounted for very little of this variation. These results further suggest that an index only on the basis of urea does not provide a sufficient measure of dialysis adequacy.

Future Avenues to Decrease Uremic Toxin Concentration

In this article, we review approaches for decreasing uremic solute concentrations in chronic kidney disease and in particular, in end-stage renal disease (ESRD). The rationale to do so is the straightforward relation between concentration and biological (toxic) effect for most toxins. The first section is devoted to extracorporeal strategies (kidney replacement therapy). In the context of high-flux hemodialysis and hemodiafiltration, we discuss increasing dialyzer blood and dialysate flows, frequent and/or extended dialysis, adsorption, bioartificial kidney, and changing physical conditions within the dialyzer (especially for protein-bound toxins). The next section focuses on the intestinal generation of uremic toxins, which in return is stimulated by uremic conditions. Therapeutic options are probiotics, prebiotics, synbiotics, and intestinal sorbents. Current data are conflicting, and these issues need further study before useful therapeutic concepts are developed. The following section is devoted to preservation of (residual) kidney function. Although many therapeutic options may overlap with therapies provided before ESRD, we focus on specific aspects of ESRD treatment, such as the risks of too-strict blood pressure and glycemic regulation and hemodynamic changes during dialysis. Finally, some recommendations are given on how research might be organized with regard to uremic toxins and their effects, removal, and impact on outcomes of uremic patients.

New methods and technologies for measuring uremic toxins and quantifying dialysis adequacy

This publication reviews the currently available methods to identify uremic retention solutes, to determine their biological relevance and to quantify their removal. The analytical methods for the detection of uremic solutes have improved continuously, allowing the identification of several previously unknown solutes. Progress has been accelerated by the development of comprehensive strategies such as genomics, proteomics and the latest "omics" area, metabolomics. Those methodologies will be further refined in future. Once the concentration of solutes of interest is known based on targeted analysis, their biological relevance can be studied by means of in vitro, ex vivo, or animal models, provided those are representative for the key complications of the uremic syndrome. For this to come to pass, rigid protocols should be applied, e.g., aiming at free solute concentrations conform those found in uremia. Subsequently, the decrease in concentration of relevant solutes should be pursued by nondialysis (e.g., by influencing nutritional intake or intestinal generation, using sorbents, modifying metabolism, or preserving renal function) and dialysis methods. Optimal dialysis strategies can be sought by studying solute kinetics during dialysis. Clinical studies are necessary to assess the correct impact of those optimized strategies on outcomes. Although longitudinal studies of solute concentration and surrogate outcome studies are first steps in suggesting the usefulness of a given approach, ultimately hard outcome randomized controlled trials are needed to endorse evidence-based therapeutic choices. The nonspecificity of dialysis removal is however a handicap limiting the chances to provide proof of concept that a given solute or group of solutes has definite biological impact.

Approaches to uremia

The development of dialysis was a dramatic step forward in medicine, allowing people who would soon have died because of lack of kidney function to remain alive for years. We have since found, however, that the "artificial kidney" does not live up fully to its name. Dialysis keeps patients alive but not well. Part of the residual illness that dialysis patients experience is caused by retained waste solutes that dialysis does not remove as well as native kidney function does. New means are available to identify these toxic solutes, about which we currently know remarkably little, and knowledge of these solutes would help us to improve therapy. This review summarizes our current knowledge of toxic solutes and highlights methods being explored to identify additional toxic solutes and to enhance the clearance of these solutes to improve patient outcomes.

Removal of Different Classes of Uremic Toxins in APD vs CAPD: A Randomized Cross-Over Study

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AIM

In this study, we investigated, and this for the different classes of uremic toxins, whether increasing dialysate volume by shifting from continuous ambulatory peritoneal dialysis (CAPD) to higher volume automated peritoneal dialysis (APD) increases total solute clearance. ♦

METHODS

Patients on peritoneal dialysis were randomized in a cross-over design to one 24-hour session of first a CAPD regimen (3*2 L of Physioneal 1.36% and 1*2 L of icodextrin) or APD (consisting of 5 cycles of 2 L Physioneal 1.36 and 1 cycle of 2 L Extraneal), and the other week the alternate regime, each patient serving as his/her own control. Dialysate, blood and urine samples were collected and frozen for later batch analysis of concentrations of urea, creatinine, phosphorus, uric acid, hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid, indoxyl sulfate, indole acetic acid, and p-cresyl sulfate. For the protein-bound solutes, total and free fractions were determined. Total, peritoneal and renal clearance (K) and mass removal (MR) of each solute were calculated, using validated models. ♦

RESULTS

In 15 patients (11 male, 3 diabetics, 56 ± 16 years, 8 on CAPD, time on peritoneal dialysis 12 ± 14 months, and residual renal function of 9.9 ± 5.4 mL/min) dialysate over plasma ratio for creatinine (D/Pcrea) was 0.62 ± 0.10. Drained volume and obtained ultrafiltration were higher with APD vs CAPD (13.3 ± 0.5 L vs 8.5 ± 0.7 L and 1.3 ± 0.5 L vs 0.5 ± 0.7 L), whereas urine output was lower (1.0 ± 0.5 L vs 1.4 ± 0.6 L). Total clearance and MR tended to be higher for CAPD vs APD for all small and water soluble solutes, but mainly because of higher renal contribution, with no difference in the peritoneal contribution. For the protein-bound solutes, no differences in clearance or mass removal were observed. ♦

CONCLUSION

Although the drained dialysate volume nearly doubled, APD did not result in better peritoneal clearance or solute removal vs classic CAPD. APD resulted in better ultrafiltration, but at the expense of residual urinary output and clearance.

Prominent accumulation in hemodialysis patients of solutes normally cleared by tubular secretion

Dialytic clearance of urea is efficient, but other small solutes normally secreted by the kidney may be cleared less efficiently. This study tested whether the high concentrations of these solutes in hemodialysis patients reflect a failure of passive diffusion methods to duplicate the efficacy of clearance by tubular secretion. We compared the plasma concentrations and clearance rates of four solutes normally cleared by tubular secretion with the plasma concentrations and clearance rates of urea and creatinine in patients receiving maintenance hemodialysis and normal subjects. The predialysis concentrations (relative to normal subjects) of unbound phenylacetylglutamine (122-fold), hippurate (108-fold), indoxyl sulfate (116-fold), and p-cresol sulfate (41-fold) were much greater than the concentrations of urea (5-fold) and creatinine (13-fold). The dialytic clearance rates (relative to normal subjects) of unbound phenylacetylglutamine (0.37-fold), hippurate (0.16-fold), indoxyl sulfate (0.21-fold), and p-cresol sulfate (0.39-fold) were much lower than the rates of urea (4.2-fold) and creatinine (1.3-fold). Mathematical modeling showed that prominent accumulation of the normally secreted solutes in hemodialysis patients could be accounted for by lower dialytic clearance relative to physiologic clearance combined with the intermittency of treatment. Whether or not more efficient removal of normally secreted solutes improves outcomes in dialysis patients remains to be tested.

Serum concentrations of p-cresyl sulfate and indoxyl sulfate, but not inflammatory markers, increase in incident peritoneal dialysis patients in parallel with loss of residual renal function

BACKGROUND

High serum concentrations of the protein-bound uremic retention solutes p-cresyl sulfate (PCS) and indoxyl sulfate (IndS) and inflammation are associated with increased cardiovascular morbidity and mortality in chronic kidney disease. Renal clearance contributes to up to 80% of the total clearance of PCS and IndS in peritoneal dialysis (PD) patients. Cross-sectional studies evaluating the impact of residual renal function (RRF) on serum concentrations of PCS, IndS, and circulating inflammatory markers have yielded conflicting results. ♢

METHODS

To clarify this issue, we carried out a prospective observational cohort study in incident PD patients (n = 35; 19 men; mean age: 55 ± 17 years). Midday blood samples were collected and analyzed for total serum PCS, IndS, C-reactive protein, and high-sensitivity interleukin 6. Peritoneal and renal clearances were calculated from urine and dialysate collections, and RRF was calculated as the mean of renal urea nitrogen and creatinine clearances. Patients were assessed 1, 6, 12, and 24 months after PD start. Differences between time points were analyzed using linear mixed models (LMMs). ♢

RESULTS

Residual renal function declined significantly over time (LMM p < 0.0001). Peritoneal clearances of both toxins tended to increase, but did not compensate for the declining renal clearances. Serum concentrations of PCS and IndS increased significantly over time (LMM p = 0.01; p = 0.0009). In contrast, total mass removal of both toxins remained stable. Circulating inflammatory markers did not change over time. ♢

CONCLUSIONS

Our data indicate that serum concentrations of PCS and IndS, but not inflammatory markers, increase in incident PD patients in parallel with loss of RRF.

Effects of intestinal bacteria-derived p-cresyl sulfate on Th1-type immune response in vivo and in vitro

Protein fermentation by intestinal bacteria generates various compounds that are not synthesized by their hosts. An example is p-cresol, which is produced from tyrosine. Patients with chronic kidney disease (CKD) accumulate high concentrations of intestinal bacteria-derived p-cresyl sulfate (pCS), which is the major metabolite of p-cresol, in their blood, and this accumulation contributes to certain CKD-associated disorders. Immune dysfunction is a CKD-associated disorder that frequently contributes to infectious diseases among CKD patients. Although some studies imply pCS as an etiological factor, the relation between pCS and immune systems is poorly understood. In the present study, we investigated the immunological effects of pCS derived from intestinal bacteria in mice. For this purpose, we fed mice a tyrosine-rich diet that causes the accumulation of pCS in their blood. The mice were shown to exhibit decreased Th1-driven 2, 4-dinitrofluorobenzene-induced contact hypersensitivity response. The concentration of pCS in blood was negatively correlated with the degree of the contact hypersensitivity response. In contrast, the T cell-dependent antibody response was not influenced by the accumulated pCS. We also examined the in vitro cytokine responses by T cells in the presence of pCS. The production of IFN-γ was suppressed by pCS. Further, pCS decreased the percentage of IFN-γ-producing Th1 cells. Our results suggest that intestinal bacteria-derived pCS suppressesTh1-type cellular immune responses.