Haemodiafiltration at increased plasma ionic strength for improved protein-bound toxin removal

Low dialysate sodium levels for chronic haemodialysis

BACKGROUND

Cardiovascular (CV) disease is the leading cause of death in dialysis patients and is strongly associated with fluid overload and hypertension. It is plausible that low dialysate sodium ion concentration [Na+] may decrease total body sodium content, thereby reducing fluid overload and hypertension and ultimately reducing CV morbidity and death. This is an update of a review first published in 2019.

OBJECTIVES

This review evaluated the harms and benefits of using a low (< 138 mM) dialysate [Na+] for maintenance haemodialysis (HD) patients.

SEARCH METHODS

We searched the Cochrane Kidney and Transplant Register of Studies up to 1 October 2024 through contact with the Information Specialist using search terms relevant to this review. Studies in the Register are identified through searches of CENTRAL, MEDLINE, and EMBASE, conference proceedings, the International Clinical Trials Registry Platform (ICTRP) Search Portal and ClinicalTrials.gov.

SELECTION CRITERIA

Randomised controlled trials (RCTs), both parallel and cross-over, of low (< 138 mM) versus neutral (138 to 140 mM) or high (> 140 mM) dialysate [Na+] for maintenance HD patients were included.

DATA COLLECTION AND ANALYSIS

Two authors independently screened studies for inclusion and extracted data. Statistical analyses were performed using the random-effects model, and results expressed as risk ratios (RR) for dichotomous outcomes, and mean differences (MD) or standardised MD (SMD) for continuous outcomes, with 95% confidence intervals (CI). Confidence in the evidence was assessed using Grades of Recommendation, Assessment, Development and Evaluation (GRADE).

MAIN RESULTS

We included 17 studies randomising 509 patients, with data available for 452 patients after dropouts. All but three studies evaluated a fixed concentration of low dialysate [Na+], with one using profiled dialysate [Na+] and two using individualised dialysate [Na+]. Five were parallel group studies, and 12 were cross-over studies. Of the latter, only six used a washout between intervention and control periods. Most studies were short-term with a median (interquartile range) follow-up of 4 (4 to 16) weeks. Two were of a single HD session and two of a single week's HD. Seven studies were conducted prior to 2000, and six reported the use of obsolete HD practices. Other than for indirectness arising from older studies, risks of bias in the included studies were generally low. Compared to neutral or high dialysate [Na+] (≥ 138 mM), low dialysate [Na+] (< 138 mM) reduces interdialytic weight gain (14 studies, 515 participants: MD -0.36 kg, 95% CI -0.50 to -0.22; high certainty evidence) and antihypertensive medication use (5 studies, 241 participants: SMD -0.37, 95% CI -0.64 to -0.1; high certainty evidence), and probably reduces left ventricular mass index (2 studies, 143 participants: MD -7.65 g/m2, 95% CI -14.48 to -0.83; moderate certainty evidence), predialysis mean arterial pressure (MAP) (5 studies, 232 participants: MD -3.39 mm Hg, 95% CI -5.17 to -1.61; moderate certainty evidence), postdialysis MAP (5 studies, 226 participants: MD -3.17 mm Hg, 95% CI -4.68 to 1.67; moderate certainty evidence), predialysis serum [Na+] (11 studies, 435 participants: MD -1.26 mM, 95% CI -1.81 to -0.72; moderate certainty evidence) and postdialysis serum [Na+] (6 studies, 188 participants: MD -3.09 mM, 95% CI -4.29 to -1.88; moderate certainty evidence). Compared to neutral or high dialysate [Na+], low dialysate [Na+] probably increases intradialytic hypotension events (13 studies, 15,764 HD sessions: RR 1.58, 95% 1.25 to 2.01; moderate certainty evidence) and intradialytic cramps (10 studies, 14,559 HD sessions: RR 1.84, 95% 1.29 to 2.64; moderate certainty evidence). Effect size for important outcomes were generally greater with low dialysate [Na+] compared to high compared with neutral dialysate [Na+], although formal hypothesis testing identifies that the difference was only certain for postdialysis serum [Na+]. Compared to neutral or high dialysate [Na+], it is uncertain whether low dialysate [Na+] affects intradialytic or interdialytic MAP, and dietary salt intake. It is also uncertain whether low dialysate [Na+] changed extracellular fluid status, venous tone, arterial vascular resistance, left ventricular volumes, or fatigue. Studies did not examine CV or all-cause death, CV events, or hospitalisation.

AUTHORS' CONCLUSIONS

Low dialysate [Na+] reduces intradialytic weight gain and probably blood pressure, which are effects directionally associated with improved outcomes. However, the intervention probably increases intradialytic hypotension and probably reduces serum [Na+], effects that are associated with an increased risk of death. The effect of the intervention on overall patient health and well-being is unknown. Further evidence is needed in the form of longer-term studies in contemporary settings, evaluating end-organ effects in small-scale mechanistic studies using optimal methods, and clinical outcomes in large-scale multicentre RCTs.

An overview of circulating and urinary biomarkers capable of predicting the transition of acute kidney injury to chronic kidney disease

INTRODUCTION

Acute kidney injury (AKI) defined by a substantial decrease in kidney function within hours to days and is often irreversible with higher risk to chronic kidney disease (CKD) transition.

AREAS COVERED

The authors discuss the diagnostic and predictive utilities of serum and urinary biomarkers on AKI and on the risk of AKI-to-CKD progression. The authors focus on the relevant literature covering evidence of circulating and urinary biomarkers' capability to predict the transition of AKI to CKD.

EXPERT OPINION

Based on the different modalities of serum and urinary biomarkers, multiple biomarker panel seems to be potentially useful to distinguish between various types of AKI, to detect the severity and the risk of AKI progression, to predict the clinical outcome and evaluate response to the therapy. Serum/urinary neutrophil gelatinase-associated lipocalin (NGAL), serum/urinary uromodulin, serum extracellular high mobility group box-1 (HMGB-1), serum cystatin C and urinary liver-type fatty acid-binding protein (L-FABP) were the most effective in the prediction of AKI-to-CKD transition regardless of etiology and the presence of critical state in patients. The current clinical evidence on the risk assessments of AKI progression is mainly based on the utility of combination of functional, injury and stress biomarkers, mainly NGAL, L-FABP, HMGB-1 and cystatin C.

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.

Increasing the Clearance of Protein-Bound Solutes by Recirculating Dialysate through Activated Carbon

Key Points

Conventional hemodialysis provides limited clearance of uremic solutes that bind to plasma proteins. No studies have yet tested whether increasing the clearance of bound solutes provides clinical benefit. Practical means to increase the dialytic clearance of bound solutes are required to perform such studies.

Background

Conventional hemodialysis provides limited clearance of uremic solutes that bind to plasma proteins. However, no studies have tested whether increasing the clearance of bound solutes provides clinical benefit. Practical means to increase the dialytic clearance of bound solutes are required to perform such studies.

Methods

Artificial plasma was dialyzed using two dialysis systems in series. In the first recirculating system, a fixed small volume of dialysate flowed rapidly through an activated carbon block before passing through two large dialyzers. In a second conventional system, a lower flow of fresh dialysate was passed through a single dialyzer. Chemical measurements tested the ability of the recirculating system to increase the clearance of selected solutes. Mathematical modeling predicted the dependence of solute clearances on the extent to which solutes were taken up by the carbon block and were bound to plasma proteins.

Results

By itself, the conventional system provided clearances of the tightly bound solutes p-cresol sulfate and indoxyl sulfate of only 18±10 and 19±11 ml/min, respectively (mean±SD). Because these solutes were effectively adsorbed by the carbon block, the recirculating system by itself provided p-cresol sulfate and indoxyl sulfate clearances of 45±11 and 53±16 ml/min. It further raised their clearances to 54±12 and 61±17 ml/min when operating in series with the conventional system (P < 0.002 versus conventional clearance both solutes). Modeling predicted that the recirculating system would increase the clearances of bound solute even if their uptake by the carbon block was incomplete.

Conclusions

When added to a conventional dialysis system, a recirculating system using a carbon block sorbent, a single pump, and standard dialyzers can greatly increase the clearance of protein-bound uremic solutes.

Clinical performance comparison of two medium cut-off dialyzers

INTRODUCTION

Medium-cut-off (MCO) dialyzers may beneficially impact outcomes in patients on hemodialysis.

METHODS

In a randomized, controlled trial in maintenance hemodialysis patients, the new Nipro ELISIO-17HX MCO dialyzer was compared to the Baxter Theranova 400 filter regarding middle molecule removal. Furthermore, the suitability of two assays for free lambda-light chain (λFLC) detection (Freelite vs. N-Latex) was verified.

RESULTS

ELISIO-HX achieved slightly lower reduction ratios for β₂ -microglobulin (71.8 ± 6.0 vs. 75.3 ± 5.8%; p = 0.001), myoglobin (54.7 ± 8.6 vs. 64.9 ± 8.7%; p < 0.001), and kappa-FLC (62.1 ± 8.8 vs. 56.3 ± 7.7%; p = 0.021). λFLC reduction ratios were more conclusive with the Freelite assay and not different between ELISIO-HX and Theranova (28.4 ± 3.9 vs. 38.7 ± 13.4%; p = 0.069). The albumin loss of Theranova was considerably higher (2.14 ± 0.45 vs. 0.77 ± 0.25 g; p = 0.001) and the Global Removal Score_{Loss alb} largely inferior (30.6 ± 7.4 vs. 82.4 ± 29.2%/g; p = 0.006) to ELISIO-HX.

CONCLUSIONS

The new ELISIO-HX expands the choice of dialyzers for MCO hemodialysis.

Improving the In Vitro Removal of Indoxyl Sulfate and p-Cresyl Sulfate by Coating Diatomaceous Earth (DE) and Poly-vinyl-pyrrolidone-co-styrene (PVP-co-S) with Polydopamine

Polydopamine (PDA) is a synthetic eumelanin polymer mimicking the biopolymer secreted by mussels to attach to surfaces with a high binding strength. It exhibits unique adhesive properties and has recently attracted considerable interest as a multifunctional thin film coating. In this study, we demonstrate that a PDA coating on silica- and polymer-based materials improves the entrapment and retention of uremic toxins produced in specific diseases. The low-cost natural nanotextured fossil diatomaceous earth (DE), an abundant source of mesoporous silica, and polyvinylpyrrolidone-co-Styrene (PVP-co-S), a commercial absorbent comprising polymeric particles, were easily coated with a PDA layer by oxidative polymerization of dopamine at mild basic aqueous conditions. An in-depth chemical-physical investigation of both the resulting PDA-coated materials was performed by SEM, AFM, UV-visible, Raman spectroscopy and spectroscopic ellipsometry. Finally, the obtained hybrid systems were successfully tested for the removal of two uremic toxins (indoxyl sulfate and p-cresyl sulfate) directly from patients' sera.

The Dual Roles of Protein-Bound Solutes as Toxins and Signaling Molecules in Uremia

In patients with severe kidney disease, renal clearance is compromised, resulting in the accumulation of a plethora of endogenous waste molecules that cannot be removed by current dialysis techniques, the most often applied treatment. These uremic retention solutes, also named uremic toxins, are a heterogeneous group of organic compounds of which many are too large to be filtered and/or are protein-bound. Their renal excretion depends largely on renal tubular secretion, by which the binding is shifted towards the free fraction that can be eliminated. To facilitate this process, kidney proximal tubule cells are equipped with a range of transport proteins that cooperate in cellular uptake and urinary excretion. In recent years, innovations in dialysis techniques to advance uremic toxin removal, as well as treatments with drugs and/or dietary supplements that limit uremic toxin production, have provided some clinical improvements or are still in progress. This review gives an overview of these developments. Furthermore, the role protein-bound uremic toxins play in inter-organ communication, in particular between the gut (the side where toxins are produced) and the kidney (the side of their removal), is discussed.

Recent advances in dialysis membranes

PURPOSE OF REVIEW

Improvement in hemodialysis treatment and membrane technology are focused on two aims: the first one is to achieve a better control of circulating uremic solutes by enhancing removal capacity and by broadening molecular weight spectrum of solutes cleared; the second one is to prevent inflammation by improving hemocompatibility of the global dialysis system.

RECENT FINDINGS

Despite impressive progresses in polymers chemistry few hazards are still remaining associated with leaching or sensitization to polymer additives. Research has focused on developing more stable polymers by means of additives or processes aiming to minimize such risks. Membrane engineering manufacturing with support of nanocontrolled spinning technology has opened up membrane to middle and large molecular weight substances, while preserving albumin losses. Combination of diffusive and enhanced convective fluxes in the same hemodialyzer module, namely hemodiafiltration, provides today the highest solute removal capacity over a broad spectrum of solutes.

SUMMARY

Dialysis membrane is a crucial component of the hemodialysis system to optimize solute removal efficacy and to minimize blood membrane biological reactions. Hemodialyzer is much more than a membrane. Dialysis membrane and hemodialyzer choice are parts of a treatment chain that should be operated in optimized conditions and adjusted to patient needs and tolerance, to improve patient outcomes.

Reduction of protein-bound uraemic toxins in plasma of chronic renal failure patients: A systematic review

BACKGROUND

Protein-bound uraemic toxins (PBUTs) accumulate in patients with chronic kidney disease and impose detrimental effects on the vascular system. However, a unanimous consensus on the most optimum approach for the reduction of plasma PBUTs is still lacking.

METHODS

In this systematic review, we aimed to identify the most efficient clinically available plasma PBUT reduction method reported in the literature between 1980 and 2020. The literature was screened for clinical studies describing approaches to reduce the plasma concentration of known uraemic toxins. There were no limits on the number of patients studied or on the duration or design of the studies.

RESULTS

Out of 1274 identified publications, 101 studies describing therapeutic options aiming at the reduction of PBUTs in CKD patients were included in this review. We stratified the studies by the PBUTs and the duration of the analysis into acute (data from a single procedure) and longitudinal (several treatment interventions) trials. Reduction ratio (RR) was used as the measure of plasma PBUTs lowering efficiency. For indoxyl sulphate and p-cresyl sulphate, the highest RR in the acute studies was demonstrated for fractionated plasma separation, adsorption and dialysis system. In the longitudinal trials, supplementation of haemodialysis patients with AST-120 (Kremezin®) adsorbent showed the highest RR. However, no superior method for the reduction of all types of PBUTs was identified based on the published studies.

CONCLUSIONS

Our study shows that there is presently no technique universally suitable for optimum reduction of all PBUTs. There is a clear need for further research in this field.

Improved Dialysis Removal of Protein-Bound Uraemic Toxins with a Combined Displacement and Adsorption Technique

INTRODUCTION

Protein-bound uraemic toxins (PBUTs) are poorly removed by conventional dialytic techniques, given their high plasma protein binding, and thus low, free (dialysable) plasma concentration. Here, we evaluated and compared PBUTs removal among conventional haemodialysis (HD), adsorption-based HD, displacement-based HD, and their 2 combinations both in vitro and in vivo.

METHODS

The removal of PBUTs, including 3-carboxy-4-methyl-5-propyl-2-furan-propanoic acid (CMPF), p-cresyl sulphate (PCS), indoxyl sulphate (IS), indole-3-acetic acid (3-IAA), and hippuric acid, was first evaluated in an in vitro single-pass HD model. Adsorption consisted of adding 40 g/L bovine serum albumin (Alb) to the dialysate and displacement involved infusing fatty acid (FA) mixtures predialyser. Then, uraemic rats were treated with either conventional HD, Alb-based HD, lipid emulsion infusion-based HD or their combination to calculate the reduction ratio (RR), and the total solute removal (TSR) of solutes after 4 h of therapy.

RESULTS

In vitro dialysis revealed that FAs infusion prefilter increased the removal of PCS, IS, and 3-IAA 3.23-fold, 3.01-fold, and 2.24-fold, respectively, compared with baseline and increased the fractional removal of CMPF from undetectable at baseline to 14.33 ± 0.24%, with a dialysis efficacy markedly superior to Alb dialysis. In vivo dialysis showed that ω-6 soybean oil-based lipid emulsion administration resulted in higher RRs and more TSRs for PCS, IS, and 3-IAA after 4-h HD than the control, and the corresponding TSR values for PCS and IS were also significantly increased compared to that of Alb dialysis. Finally, the highest dialysis efficacy for highly bound solute removal was always observed with their combination both in vitro and in vivo.

CONCLUSIONS

The concept of combined displacement- and adsorption-based dialysis may open up new avenues and possibilities in the field of dialysis to further enhance PBUTs removal in end-stage renal disease.

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.

Challenges of reducing protein-bound uremic toxin levels in chronic kidney disease and end stage renal disease

The prevalence of chronic kidney disease (CKD) in the worldwide population is currently estimated between 11% and 13%. Adequate renal clearance is compromised in these patients and the accumulation of a large number of uremic retention solutes results in an irreversible worsening of renal function which can lead to end stage renal disease (ESRD). Approximately three million ESRD patients currently receive renal replacement therapies (RRTs), such as hemodialysis, which only partially restore kidney function, as they are only efficient in removing mainly small, unbound solutes from the circulation while leaving larger and protein-bound uremic toxins (PBUTs) untouched. The accumulation of PBUTs in patients highly increases the risk of cardiovascular events and is associated with higher mortality and morbidity in CKD and ESRD. In this review, we address several strategies currently being explored toward reducing PBUT concentrations, including clinical and medical approaches, therapeutic techniques, and recent developments in RRT technology. These include preservation of renal function, limitation of colon derived PBUTs, oral sorbents, adsorbent RRT technology, and use of albumin displacers. Despite the promising results of the different approaches to promote enhanced removal of a small percentage of the more than 30 identified PBUTs, on their own, none of them provide a treatment with the required efficiency, safety and cost-effectiveness to prevent CKD-related complications and decrease mortality and morbidity in ESRD.

A history of uraemic toxicity and of the European Uraemic Toxin Work Group (EUTox)

The uraemic syndrome is a complex clinical picture developing in the advanced stages of chronic kidney disease, resulting in a myriad of complications and a high early mortality. This picture is to a significant extent defined by retention of metabolites and peptides that with a preserved kidney function are excreted or degraded by the kidneys. In as far as those solutes have a negative biological/biochemical impact, they are called uraemic toxins. Here, we describe the historical evolution of the scientific knowledge about uraemic toxins and the role played in this process by the European Uraemic Toxin Work Group (EUTox) during the last two decades. The earliest knowledge about a uraemic toxin goes back to the early 17th century when the existence of what would later be named as urea was recognized. It took about two further centuries to better define the role of urea and its link to kidney failure, and one more century to identify the relevance of post-translational modifications caused by urea such as carbamoylation. The knowledge progressively extended, especially from 1980 on, by the identification of more and more toxins and their adverse biological/biochemical impact. Progress of knowledge was paralleled and impacted by evolution of dialysis strategies. The last two decades, when insights grew exponentially, coincide with the foundation and activity of EUTox. In the final section, we summarize the role and accomplishments of EUTox and the part it is likely to play in future action, which should be organized around focus points like biomarker and potential target identification, intestinal generation, toxicity mechanisms and their correction, kidney and extracorporeal removal, patient-oriented outcomes and toxin characteristics in acute kidney injury and transplantation.

Cardiorenal syndrome: Multi-organ dysfunction involving the heart, kidney and vasculature

Cardiorenal syndrome (CRS) is a multi-organ disease, encompassing heart, kidney and vascular system dysfunction. CRS is a worldwide problem, with high morbidity, mortality, and inflicts a significant burden on the health care system. The pathophysiology is complex, involving interactions between neurohormones, inflammatory processes, oxidative stress and metabolic derangements. Therapies remain inadequate, mainly comprising symptomatic care with minimal prospect of full recovery. Challenges include limiting the contradictory effects of multi-organ targeted drug prescriptions and continuous monitoring of volume overload. Novel strategies such as multi-organ transplantation and innovative dialysis modalities have been considered but lack evidence in the CRS context. The adjunct use of pharmaceuticals targeting alternative pathways showing positive results in preclinical models also warrants further validation in the clinic. In recent years, studies have identified the involvement of gut dysbiosis, uraemic toxin accumulation, sphingolipid imbalance and other unconventional contributors, which has encouraged a shift in the paradigm of CRS therapy.

Improved dialytic removal of protein-bound uremic toxins by intravenous lipid emulsion in chronic kidney disease rats

BACKGROUND

Protein-bound uremic toxins (PBUTs) have received extensive attention, as their accumulation leads to pleiotropic toxic biological effects, while the removal of these solutes by conventional dialysis therapies is severely hampered. This study aimed to examine whether increased removal of PBUTs could be achieved with intravenous lipid emulsion (ILE).

METHODS

PBUTs such as 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF), p-cresyl sulfate (PCS) and indoxyl sulfate (IS) were spiked with human serum albumin (HSA) solution and the inhibitory effects of free fatty acid (FFA) on the binding of CMPF, PCS and IS to HSA were examined separately in vitro by ultrafiltration. In vitro dialysis of albumin solution was then performed to investigate the effects of fatty acid (FAs) mixtures infusion on the fractional removal of PBUTs. Finally, the inhibitory effect of FFA on the binding of PBUTs to albumin was examined in uremic rats, and blood purification therapy was conducted to calculate the reduction ratio (RR) and the total solute removal (TSR) of solutes.

RESULTS

The percentage protein binding of CMPF, PCS and IS decreased significantly with increasing FFAs concentrations, and the inhibitory effect was more remarkable with the addition of oleic acid or linoleic acid than that of eicosapentaenoic acid and docosahexaenoic acid. In vitro infusion of FAs increased the fractional removal of CMPF to 14.40 ± 2.38%. PCS, IS and indole-3-acetic acid removal increased from 8.00 ± 2.43%, 11.68 ± 1.54% and 15.38 ± 3.97%, respectively, at baseline to 28.21 ± 5.99%, 35.42 ± 5.27% and 40.18 ± 5.05%, respectively, when FAs were present. In vivo, rat serum concentrations of free PBUTs were significantly higher in the ILE group than in the control group, and administration of ILE resulted in higher RRs and more TSR for PBUTs after 3 h of hemodialysis (HD) therapy compared with the control group.

CONCLUSIONS

Administration of ILE effectively increased the dialytic removal of PBUTs. This method could be applied to current HD therapy.

Protein-Bound Uremic Toxins in Hemodialysis Patients Relate to Residual Kidney Function, Are Not Influenced by Convective Transport, and Do Not Relate to Outcome

Protein-bound uremic toxins (PBUTs) are predominantly excreted by renal tubular secretion and hardly removed by traditional hemodialysis (HD). Accumulation of PBUTs is proposed to contribute to the increased morbidity and mortality of patients with end-stage kidney disease (ESKD). Preserved PBUT excretion in patients with residual kidney function (RKF) and/or increased PBUT clearance with improved dialysis techniques might improve the prognosis of patients with ESKD. The aims of this study are to explore determinants of PBUTs in HD patients, and investigate whether hemodiafiltration (HDF) lowers PBUT plasma concentrations, and whether PBUTs are related to the outcome. Predialysis total plasma concentrations of kynurenine, kynurenic acid, indoxyl sulfate, indole-3-acetic acid, p-cresyl sulfate, p-cresyl glucuronide, and hippuric acid were measured by UHPLC-MS at baseline and after 6 months of follow-up in the first 80 patients participating in the CONvective TRAnsport Study (CONTRAST), a randomized controlled trial that compared the effects of online HDF versus low-flux HD on all-cause mortality and new cardiovascular events. RKF was inversely related to kynurenic acid (p < 0.001), indoxyl sulfate (p = 0.001), indole-3-acetic acid (p = 0.024), p-cresyl glucuronide (p = 0.004) and hippuric acid (p < 0.001) plasma concentrations. Only indoxyl sulfate decreased by 8.0% (−15.3 to 34.6) in patients treated with HDF and increased by 11.9% (−15.4 to 31.9) in HD patients after 6 months of follow-up (HDF vs. HD: p = 0.045). No independent associations were found between PBUT plasma concentrations and either risk of all-cause mortality or new cardiovascular events. In summary, in the current population, RKF is an important determinant of PBUT plasma concentrations in HD patients. The addition of convective transport did not consistently decrease PBUT plasma concentrations and no relation was found between PBUTs and cardiovascular endpoints.

Differences in Dialysis Efficacy Have Limited Effects on Protein-Bound Uremic Toxins Plasma Levels over Time

The protein-bound uremic toxins para-cresyl sulfate (pCS) and indoxyl sulfate (IS) are associated with cardiovascular disease in chronic renal failure, but the effect of different dialysis procedures on their plasma levels over time is poorly studied. The present prospective, randomized, cross-over trial tested dialysis efficacy and monitored pre-treatment pCS and IS concentrations in 15 patients on low-flux and high-flux hemodialysis and high-convective volume postdilution hemodiafiltration over six weeks each. Although hemodiafiltration achieved by far the highest toxin removal, only the mean total IS level was decreased at week three (16.6 ± 12.1 mg/L) compared to baseline (18.9 ± 13.0 mg/L, p = 0.027) and to low-flux dialysis (20.0 ± 12.7 mg/L, p = 0.021). At week six, the total IS concentration in hemodiafiltration reached the initial values again. Concentrations of free IS and free and total pCS remained unaltered. Highest beta₂-microglobulin elimination in hemodiafiltration (p < 0.001) led to a persistent decrease of the plasma levels at week three and six (each p < 0.001). In contrast, absent removal in low-flux dialysis resulted in rising beta₂-microglobulin concentrations (p < 0.001). In conclusion, this trial demonstrated that even large differences in instantaneous protein-bound toxin removal by current extracorporeal dialysis techniques may have only limited impact on IS and pCS plasma levels in the longer term.

Bioengineering Organs for Blood Detoxification

For patients with severe kidney or liver failure the best solution is currently organ transplantation. However, not all patients are eligible for transplantation and due to limited organ availability, most patients are currently treated with therapies using artificial kidney and artificial liver devices. These therapies, despite their relative success in preserving the patients' life, have important limitations since they can only replace part of the natural kidney or liver functions. As blood detoxification (and other functions) in these highly perfused organs is achieved by specialized cells, it seems relevant to review the approaches leading to bioengineered organs fulfilling most of the native organ functions. There, the culture of cells of specific phenotypes on adapted scaffolds that can be perfused takes place. In this review paper, first the functions of kidney and liver organs are briefly described. Then artificial kidney/liver devices, bioartificial kidney devices, and bioartificial liver devices are focused on, as well as biohybrid constructs obtained by decellularization and recellularization of animal organs. For all organs, a thorough overview of the literature is given and the perspectives for their application in the clinic are discussed.

Indoxyl Sulfate Elimination in Renal Replacement Therapy: Influence of Citrate- versus Acetate-Buffering Component during Bicarbonate Dialysis

Indoxyl sulfate has been identified as a major factor in the dysregulation of several genes. It is classified as a poorly dialyzable uremic toxin and thus a leading cause in the poor survival rate of dialysis patients. A monocentric, prospective, open cohort study was performed in 43 male patients undergoing chronic renal replacement therapy in a single hemodialysis center. The aim of the study was to determine the influence of acetate- versus citrate-buffered dialysis fluids in hemodialysis (HD) and postdilution hemodiafiltration (HDF) settings on the elimination of indoxyl sulfate. Also, additional factors potentially influencing the serum concentration of indoxyl sulfate were evaluated. For this purpose, the predialysis and postdialysis concentration ratio of indoxyl sulfate and total protein was determined. The difference was of 1.15 (0.61; 2.10), 0.89 (0.53; 1.66), 0.32 (0.07; 0.63), and 0.44 (0.27; 0.77) μmol/g in acetate HD and HDF and citrate HD and HDF, respectively. Acetate HD and HDF were superior when concerning IS elimination when compared to citrate HD and HDF. Moreover, residual diuresis was determined as the only predictor of lower indoxyl sulfate concentration, suggesting that it should be preserved as long as possible. This trial is registered with EU PAS Register of Studies EUPAS23714.

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.

From portable dialysis to a bioengineered kidney

INTRODUCTION

Since the advent of peritoneal dialysis (PD) in the 1970s, the principles of dialysis have changed little. In the coming decades, several major breakthroughs are expected.

AREAS COVERED

Novel wearable and portable dialysis devices for both hemodialysis (HD) and PD are expected first. The HD devices could facilitate more frequent and longer dialysis outside of the hospital, while improving patient's mobility and autonomy. The PD devices could enhance blood purification and increase technique survival of PD. Further away from clinical application is the bioartificial kidney, containing renal cells. Initially, the bioartificial kidney could be applied for extracorporeal treatment, to partly replace renal tubular endocrine, metabolic, immunoregulatory and secretory functions. Subsequently, intracorporeal treatment may become possible.

EXPERT COMMENTARY

Key factors for successful implementation of miniature dialysis devices are patient attitudes and cost-effectiveness. A well-functioning and safe extracorporeal blood circuit is required for HD. For PD, a double lumen PD catheter would optimize performance. Future research should focus on further miniaturization of the urea removal strategy. For the bio-artificial kidney (BAK), cost effectiveness should be determined and a general set of functional requirements should be defined for future studies. For intracorporeal application, water reabsorption will become a major challenge.

The effect of isohydric hemodialysis on the binding and removal of uremic retention solutes

Background

There is growing evidence that the accumulation of protein- bound uremic retention solutes, such as indoxyl sulfate, p-cresyl sulfate and kynurenic acid, play a role in the accelerated cardiovascular disease seen in patients undergoing chronic hemodialysis. Protein-binding, presumably to albumin, renders these solutes poor-dialyzable.

We previously observed that the free fraction of indoxyl sulfate was markedly reduced at the end of hemodialysis. We hypothesized that solute binding might be pH-dependent and attributed the changes in free solute concentration to the higher serum pH observed at the end of standard hemodialysis with dialysis buffer bicarbonate concentration greater than 35 mmol/L. We observed that acidification of uremic plasma to pH 6 in vitro greatly increased the proportion of freeIS.

Methods

We tested our hypothesis by reducing the dialysate bicarbonate buffer concentration to 25 mmol/L for the initial half of the hemodialysis treatment (“isohydric dialysis”). Eight stable hemodialysis patients underwent “isohydric dialysis” for 90 minutes and then were switched to standard buffer (bicarbonate = 37mmol/L). A second dialysis, 2 days later, employed standard buffer throughout.

Results

We found a clearcut separation of blood pH and bicarbonate concentrations after 90 minutes of “isohydric dialysis” (pH = 7.37, bicarbonate = 22.4 mmol/L) and standard dialysis (pH = 7.49, bicarbonate = 29.0 mmol/L). Binding affinity varied widely among the 10 uremic retention solutes analyzed. Kynurenic acid (0.05 free), p-cresyl sulfate (0.12 free) and indoxyl sulfate (0.13 free) demonstrated the greatest degree of binding. Three solutes (indoxyl glucuronide, p-cresyl glucuronide, and phenyl glucuronide) were virtually unbound. Analysis of free and bound concentrations of uremic retention solutes confirmed our prediction that binding of solute is affected by pH. However, in a mixed models analysis, we found that the reduction in total uremic solute concentration during dialysis accounted for a greater proportion of the variation in free concentration, presumably an effect of saturation binding to albumin, than did the relatively small change in pH produced by isohydric dialysis. The effect of pH on binding appeared to be restricted to those solutes most highly protein-bound.

The solutes most tightly bound exhibited the lowest dialyzer clearances. An increase in dialyzer clearance during isohydric and standard dialyses was statistically significant only for kynurenic acid.

Conclusion

These findings provide evidence that the binding of uremic retention solutes is influenced by pH. The effect of reducing buffer bicarbonate concentration (“isohydric dialysis:”), though significant, was small but may be taken to suggest that further modification of dialysis technique that would expose blood to a greater decrease in pH would lead to a greater increase the free fraction of solute and enhance the efficacy of hemodialysis in the removal of highly protein-bound uremic retention solutes.

Enhancing dialyser clearance-from target to development

Products of metabolism accumulate in kidney failure and potentially have toxic effects. Traditionally these uraemic toxins are classified as small, middle-sized and protein-bound toxins, and clearance during dialysis is affected by diffusion, convection and adsorption. As current dialysis practice effectively clears small solutes, increasing evidence supports a toxic effect for middle-sized and protein-bound toxins. Therefore, newer approaches to standard dialysis practice are required to look beyond urea clearance. Current dialysers have been developed to effectively clear small solutes and secondly to increase middle-sized toxin clearances. However, there is no ideal dialyser which can effectively clear all uraemic toxins. Advances in nanotechnology have led to improvements in manufacturing, with the production of smoother membrane surfaces and uniformity of pore size. The introduction of haemodiafiltration has led to changes in dialyser design to improve convective clearances. Both diffusional and convectional clearances can be increased by changing dialyser designs to alter blood and dialysate flows, and novel dialyser designs using microfluidics offer more efficient solute clearances. Adjusting surface hydrophilicity and charge alter adsorptive properties, and greater clearance of protein-bound toxins can be achieved by adding carbon or other absorptive monoliths into the circuit or by developing composite dialyser membranes. Other strategies to increase protein-bound toxins clearances have centred on disrupting binding and so displacing toxins from proteins. Just as the hollow fibre design replaced the flat plate dialyser, we are now entering a new era of dialyser designs aimed to increase the spectrum of uraemic toxins which can be cleared by dialysis.

Using binding competitors of albumin to promote the removal of protein-bound uremic toxins in hemodialysis: Hope or pipe dream?

Chronic kidney disease is associated with the accumulation of a large range of uremic retention solutes as referred to as uremic toxins. Some of these compounds belong to the group of Protein Bound Uremic Toxins (PBUT) due to their tight interactions with plasma proteins and especially serum albumin. These PBUT therefore exist in the bloodstream into two forms: a major bound (and non-diffusible) fraction and a minor free fraction. As a result, these compounds are poorly removed by most of the renal replacement therapies (such as hemodialysis) and their concentration can hardly be decreased in end-stage renal disease patients. An increase of the free fraction of PBUT could be achieved using chemical displacers that could compete with PBUT for binding to serum albumin. This review summarizes and discusses the interest of chemicals displacers as a valuable option to enhance PBUT removal in CKD patients.

Increasing Haemodialytic Clearances as Residual Renal Function Declines: An Incremental Approach

Many patients with chronic kidney disease start undergoing thrice-weekly haemodialysis (HD), aiming for an HD sessional dialyzer urea clearance target, irrespective of whether they have residual renal function (RRF). While increasing sessional dialyzer urea clearance above a target of 1.2 has not been shown to improve patient survival, it has been shown that the preservation of RRF improves patient self-reported outcomes and survival. Observational studies have suggested that initiating twice-weekly HD schedules leads to greater preservation of RRF. This has led to the concept of following an incremental approach to initiating HD, steadily increasing the amount of weekly dialyzer clearance as RRF decreases. Incremental dialysis practice requires the regular assessment of RRF to prevent inadequate delivery of dialysis treatment. Once RRF is lost, then the dialysis schedule and modality need to be adjusted to try to increase the middle-sized solute clearance and protein-bound toxins.

Disposition and clinical implications of protein-bound uremic toxins

In patients with chronic kidney disease (CKD), adequate renal clearance is compromised, resulting in the accumulation of a plethora of uremic solutes. These uremic retention solutes, also named uremic toxins, are a heterogeneous group of organic compounds with intrinsic biological activities, many of which are too large to be filtered and/or are protein bound. The renal excretion of protein-bound toxins depends largely on active tubular secretion, which shifts the binding and allows for active secretion of the free fraction. To facilitate this process, renal proximal tubule cells are equipped with a range of transporters that co-operate in basolateral uptake and luminal excretion. Many of these transporters have been characterized as mediators of drug disposition, but have recently been recognized for their importance in the proximal renal tubular transport of uremic toxins as well. This also indicates that during uremia, drug disposition may be severely affected as a result of drug–uremic toxin interaction. In addition, CKD patients receive various drugs to treat their complications potentially resulting in drug–drug interactions (DDIs), also for drugs that are non-renally excreted. This review discusses the current knowledge on formation, disposition and removal of protein-bound uremic toxins. Furthermore, implications associated with drug treatment in kidney failure, as well as innovative renal replacement therapies targetting the protein-bound uremic toxins are being discussed. It will become clear that the complex problems associated with uremia warrant a transdisciplinary approach that unites research experts in the area of fundamental biomedical research with their colleagues in clinical nephrology.

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.