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

Indoxyl Sulfate-Induced Macrophage Toxicity and Therapeutic Strategies in Uremic Atherosclerosis

Cardiovascular disease (CVD) frequently occurs in patients with chronic kidney disease (CKD), particularly those undergoing dialysis. The mechanisms behind this may be related to traditional risk factors and CKD-specific factors that accelerate atherosclerosis and vascular calcification in CKD patients. The accumulation of uremic toxins is a significant factor in CKD-related systemic disorders. Basic research suggests that indoxyl sulfate (IS), a small protein-bound uremic toxin, is associated with macrophage dysfunctions, including increased oxidative stress, exacerbation of chronic inflammation, and abnormalities in lipid metabolism. Strategies to mitigate the toxicity of IS include optimizing gut microbiota, intervening against the abnormality of intracellular signal transduction, and using blood purification therapy with higher efficiency. Further research is needed to examine whether lowering protein-bound uremic toxins through intervention leads to a reduction in CVD in patients with CKD.

Metagenome-wide analysis uncovers gut microbial signatures and implicates taxon-specific functions in end-stage renal disease

BACKGROUND

The gut microbiota plays a crucial role in regulating host metabolism and producing uremic toxins in patients with end-stage renal disease (ESRD). Our objective is to advance toward a holistic understanding of the gut ecosystem and its functional capacity in such patients, which is still lacking.

RESULTS

Herein, we explore the gut microbiome of 378 hemodialytic ESRD patients and 290 healthy volunteers from two independent cohorts via deep metagenomic sequencing and metagenome-assembled-genome-based characterization of their feces. Our findings reveal fundamental alterations in the ESRD microbiome, characterized by a panel of 348 differentially abundant species, including ESRD-elevated representatives of Blautia spp., Dorea spp., and Eggerthellaceae, and ESRD-depleted Prevotella and Roseburia species. Through functional annotation of the ESRD-associated species, we uncover various taxon-specific functions linked to the disease, such as antimicrobial resistance, aromatic compound degradation, and biosynthesis of small bioactive molecules. Additionally, we show that the gut microbial composition can be utilized to predict serum uremic toxin concentrations, and based on this, we identify the key toxin-contributing species. Furthermore, our investigation extended to 47 additional non-dialyzed chronic kidney disease (CKD) patients, revealing a significant correlation between the abundance of ESRD-associated microbial signatures and CKD progression.

CONCLUSION

This study delineates the taxonomic and functional landscapes and biomarkers of the ESRD microbiome. Understanding the role of gut microbiota in ESRD could open new avenues for therapeutic interventions and personalized treatment approaches in patients with this condition.

Interventions for fatigue in people with kidney failure requiring dialysis

BACKGROUND

Fatigue is a common and debilitating symptom in people receiving dialysis that is associated with an increased risk of death, cardiovascular disease and depression. Fatigue can also impair quality of life (QoL) and the ability to participate in daily activities. Fatigue has been established by patients, caregivers and health professionals as a core outcome for haemodialysis (HD).

OBJECTIVES

We aimed to evaluate the effects of pharmacological and non-pharmacological interventions on fatigue in people with kidney failure receiving dialysis, including HD and peritoneal dialysis (PD), including any setting and frequency of the dialysis treatment.

SEARCH METHODS

We searched the Cochrane Kidney and Transplant Register of Studies up to 18 October 2022 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

Studies evaluating pharmacological and non-pharmacological interventions affecting levels of fatigue or fatigue-related outcomes in people receiving dialysis were included. Studies were eligible if fatigue or fatigue-related outcomes were reported as a primary or secondary outcome. Any mode, frequency, prescription, and duration of therapy were considered.

DATA COLLECTION AND ANALYSIS

Three authors independently extracted data and assessed the risk of bias. Treatment estimates were summarised using random effects meta-analysis and expressed as a risk ratio (RR) or mean difference (MD), with a corresponding 95% confidence interval (CI) or standardised MD (SMD) if different scales were used. Confidence in the evidence was assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach.

MAIN RESULTS

Ninety-four studies involving 8191 randomised participants were eligible. Pharmacological and non-pharmacological interventions were compared either to placebo or control, or to another pharmacological or non-pharmacological intervention. In the majority of domains, risks of bias in the included studies were unclear or high. In low certainty evidence, when compared to control, exercise may improve fatigue (4 studies, 217 participants (Iowa Fatigue Scale, Modified Fatigue Impact Scale, Piper Fatigue Scale (PFS), or Haemodialysis-Related Fatigue scale score): SMD -1.18, 95% CI -2.04 to -0.31; I2 = 87%) in HD. In low certainty evidence, when compared to placebo or standard care, aromatherapy may improve fatigue (7 studies, 542 participants (Fatigue Severity Scale (FSS), Rhoten Fatigue Scale (RFS), PFS or Brief Fatigue Inventory score): SMD -1.23, 95% CI -1.96 to -0.50; I2 = 93%) in HD. In low certainty evidence, when compared to no intervention, massage may improve fatigue (7 studies, 657 participants (FSS, RFS, PFS or Visual Analogue Scale (VAS) score): SMD -1.06, 95% CI -1.47, -0.65; I2 = 81%) and increase energy (2 studies, 152 participants (VAS score): MD 4.87, 95% CI 1.69 to 8.06, I2 = 59%) in HD. In low certainty evidence, when compared to placebo or control, acupressure may reduce fatigue (6 studies, 459 participants (PFS score, revised PFS, or Fatigue Index): SMD -0.64, 95% CI -1.03 to -0.25; I2 = 75%) in HD. A wide range of heterogenous interventions and fatigue-related outcomes were reported for exercise, aromatherapy, massage and acupressure, preventing our capability to pool and analyse the data. Due to the paucity of studies, the effects of pharmacological and other non-pharmacological interventions on fatigue or fatigue-related outcomes, including non-physiological neutral amino acid, relaxation with or without music therapy, meditation, exercise with nandrolone, nutritional supplementation, cognitive-behavioural therapy, ESAs, frequent HD sections, home blood pressure monitoring, blood flow rate reduction, serotonin reuptake inhibitor, beta-blockers, anabolic steroids, glucose-enriched dialysate, or light therapy, were very uncertain. The effects of pharmacological and non-pharmacological treatments on death, cardiovascular diseases, vascular access, QoL, depression, anxiety, hypertension or diabetes were sparse. No studies assessed tiredness, exhaustion or asthenia. Adverse events were rarely and inconsistently reported.

AUTHORS' CONCLUSIONS

Exercise, aromatherapy, massage and acupressure may improve fatigue compared to placebo, standard care or no intervention. Pharmacological and other non-pharmacological interventions had uncertain effects on fatigue or fatigue-related outcomes in people receiving dialysis. Future adequately powered, high-quality studies are likely to change the estimated effects of interventions for fatigue and fatigue-related outcomes in people receiving dialysis.

Assembly of chemically modified protein nanocages into 3D materials for the adsorption of uremic toxins

Hemodialysis fails to remove protein-bound uremic toxins that are attributed with high cardiovascular risk. Application of adsorption materials is a viable strategy, but suitable biocompatible adsorbents are still not available. Here, we demonstrate that adsorbents based on the bottom-up assembly of the intrinsically biocompatible protein cage ferritin are applicable for toxin adsorption. Due to the size-exclusion effect of its pores, only small molecules such as uremic toxins can enter the protein cage. Protein redesign techniques that target selectively the inner surface were used to introduce anchor sites for chemical modification. Porous crystalline adsorbents were fabricated by bottom-up assembly of the protein cage. Linkage of up to 96 phenylic or aliphatic molecules per container was verified by ESI-MS. Materials based on unmodified ferritin cages can already adsorb the uremic toxins. The adsorption capacity could be increased by about 50% through functionalization with hydrophobic molecules reaching 458 μg g^-1 for indoxyl sulfate. The biohybrid materials show no contamination with endotoxins and do not activate blood platelets. These findings demonstrate the great potential of protein-based adsorbents for the clearance of uremic toxins: modifications enhance toxin adsorption without diminishing the biocompatibility of the final protein-based material.

Cerebrovascular Disease, Cardiovascular Disease, and Chronic Kidney Disease: Interplays and Influences

PURPOSE OF REVIEW

We reviewed reasons for the high cardiovascular risk (CVD) of patients with chronic kidney disease (CKD), and explored alternatives to treatment of traditional risk factors to reduce CVD in CKD.

RECENT FINDINGS

Besides traditional risk factors, patients with CKD are exposed to uremic toxins of two kinds: systemically derived toxins include asymmetric dimethylarginine (ADMA), total homocysteine (tHcy), thiocyanate, tumor necrosis factor alpha, and interleukin 6. Gut-derived uremic toxins (GDUT), products of the intestinal microbiome, include hippuric acid, indoxyl sulfate, p-cresyl sulfate, p-cresyl glucuronide, phenylacetylglutamine, and trimethylamine N-oxide (TMAO). Cyanocobalamin is toxic in patients with CKD. Approaches to reducing plasma levels of these uremic toxins would include diet to reduce GDUT, kidney transplantation, more intensive dialysis, and vitamin therapy to lower tHcy with methylcobalamin rather than cyanocobalamin. The high CVD risk in CKD requires consideration of therapies beyond treatment of traditional risk factors.

Markers of Kidney Tubular Secretion and Risk of Adverse Events in SPRINT Participants with CKD

BACKGROUND

Kidney tubular secretion is an essential mechanism for clearing many common antihypertensive drugs and other metabolites and toxins. It is unknown whether novel measures of tubular secretion are associated with adverse events (AEs) during hypertension treatment.

METHODS

Among 2089 SPRINT (Systolic Blood Pressure Intervention Trial) participants with baseline eGFR <60 ml/min per 1.73 m2, we created a summary secretion score by averaging across the standardized spot urine-to-plasma ratios of ten novel endogenous tubular secretion measures, with lower urine-to-plasma ratios reflecting worse tubular secretion. Multivariable Cox proportional hazards models were used to evaluate associations between the secretion score and risk of a composite of prespecified serious AEs (hypotension, syncope, bradycardia, AKI, electrolyte abnormalities, and injurious falls). The follow-up protocol for SPRINT routinely assessed two laboratory monitoring AEs (hyperkalemia and hypokalemia).

RESULTS

Overall, 30% of participants experienced at least one AE during a median follow-up of 3.0 years. In multivariable models adjusted for eGFR and albuminuria, lower (worse) secretion scores at baseline were associated with greater risk of the composite AE outcome (hazard ratio per 1-SD lower secretion score, 1.16; 95% confidence interval, 1.04 to 1.27). In analyses of the individual AEs, lower secretion score was associated with significantly greater risk of AKI, serious electrolyte abnormalities, and ambulatory hyperkalemia. Associations were similar across randomized treatment assignment groups.

CONCLUSION

Among SPRINT participants with CKD, worse tubular secretion was associated with greater risk of AEs, independent of eGFR and albuminuria.

Research progress on the relationship between IS and kidney disease and its complications

Indoxyl sulphate (IS) a representative uraemic toxin in the blood of patients with chronic kidney disease (CKD). Its accumulation may be closely related to CKD and the increasing morbidity and mortality of the disease's related complications. Timely and effective detection of the IS level and efficient clearance of IS may effectively prevent the progression of CKD and its related complications. Therefore, this article summarizes the research progress of IS related, including IS in CKD and its associated complications including chronic kidney disease, chronic kidney disease with cardiovascular disease, renal anemia, bone mineral metabolic disease and neuropsychiatric disorders, looking for IS accurate rapid detection methods, and explore the efficient treatment to reduce blood levels of indole phenol sulphate.

A pilot study comparing the efficiency of a novel asymmetric cellulose triacetate (ATA) dialyser membrane (Solacea-190H) to a standard high flux polysulfone dialyser membrane (FX-80) in the setting of extended hours haemodialysis

Aim

To compare small, middle and large‐middle molecule clearance; and expression of markers of inflammation, between Solacea‐190H (asymmetric cellulose triacetate [ATA]) and FX‐80 dialysers in long‐hour haemodialysis patients.

Methods

This pilot, randomized cross‐over trial recruited 10 home haemodialysis patients. The total study duration was 8 weeks, using each dialyser for 4 weeks. Removal of small (urea, phosphate, creatinine and indoxyl sulfate [IS]), middle and large‐middle molecules (beta‐2 microglobulin [β2M], albumin), markers of inflammation (interleukin‐6 [IL‐6], malondialdehyde‐modified low density lipoprotein [MDA‐LDL] and alpha‐1 microglobulin [α1M]), was evaluated in serum and dialysate samples.

Results

Reduction ratios [RR] were calculated for variables at the fourth week of each dialyzer sequence and results expressed as difference in mean RR between dialyzers. There was no difference in clearance of small molecules, with difference in mean RR for urea −2.43 (95% CI ‐6.44, 1.57; p = .19), creatinine −1.82 (95% CI ‐5.50, 1.85; p = .28) and phosphate −2.61 (95% CI −12.45, 7.23; p = .55); clearance of middle and large‐middle molecules with difference in mean RR (range) for β2M 2.2 (95% CI −3.2, 7.7; p = .35), IS 1.8 (95% CI −9.5, 13; p = .72) and albumin −0.6 (95% CI −5.5, 4.2; p = .77). There was lack of induction of markers of inflammation, including IL‐6 15.2 (95% CI −31.9, 62.2; p = .47), MDA‐LDL −8.1 (95% CI ‐22.1, 5.8; p = .21) and α1M −3.50 (95% CI −29.2, 22.2; p = .76). Dialysate removal results were concurrent.

Conclusion

This study showed no difference in clearance of small, middle and large‐middle molecules, nor expression of markers of inflammation between dialysers.

This pilot cross‐over randomized controlled trial compares small, middle and large‐middle molecule clearance between Solacea‐190H (asymmetric cellulose triacetate [ATA]) and FX‐80 dialysers in 10 patients using long‐hour haemodialysis over an 8‐week period, and showed no difference in clearance between the two dialysers.

Does delivering more dialysis improve clinical outcomes? What randomized controlled trials have shown

Some randomized controlled trials (RCTs) have sought to determine whether different dialysis techniques, dialysis doses and frequencies of treatment are able to improve clinical outcomes in end-stage kidney disease (ESKD). Virtually all of these RCTs were enacted on the premise that 'more' haemodialysis might improve clinical outcomes compared to 'conventional' haemodialysis. Aim of the present narrative review was to analyse these landmark RCTs by posing the following question: were their intervention strategies (i.e., earlier dialysis start, higher haemodialysis dose, intensive haemodialysis, increase in convective transport, starting haemodialysis with three sessions per week) able to improve clinical outcomes? The answer is no. There are at least two main reasons why many RCTs have failed to demonstrate the expected benefits thus far: (1) in general, RCTs included relatively small cohorts and short follow-ups, thus producing low event rates and limited statistical power; (2) the designs of these studies did not take into account that ESKD does not result from a single disease entity: it is a collection of different diseases and subtypes of kidney dysfunction. Patients with advanced kidney failure requiring dialysis treatment differ on a multitude of levels including residual kidney function, biochemical parameters (e.g., acid base balance, serum electrolytes, mineral and bone disorder), and volume overload. In conclusion, the different intervention strategies of the RCTs herein reviewed were not able to improve clinical outcomes of ESKD patients. Higher quality studies are needed to guide patients and clinicians in the decision-making process. Future RCTs should account for the heterogeneity of patients when considering inclusion/exclusion criteria and study design, and should a priori consider subgroup analyses to highlight specific subgroups that can benefit most from a particular intervention.

Uremic Toxins and Protein-Bound Therapeutics in AKI and CKD: Up-to-Date Evidence

Uremic toxins are defined as harmful metabolites that accumulate in the human body of patients whose renal function declines, especially chronic kidney disease (CKD) patients. Growing evidence demonstrates the deteriorating effect of uremic toxins on CKD progression and CKD-related complications, and removing uremic toxins in CKD has become the conventional treatment in the clinic. However, studies rarely pay attention to uremic toxin clearance in the early stage of acute kidney injury (AKI) to prevent progression to CKD despite increasing reports demonstrating that uremic toxins are correlated with the severity of injury or mortality. This review highlights the current evidence of uremic toxin accumulation in AKI and the therapeutic value to prevent CKD progression specific to protein-bound uremic toxins (PBUTs).

The spectrum of kidney dysfunction requiring chronic dialysis therapy: Implications for clinical practice and future clinical trials

Staging to capture kidney function and pathophysiologic processes according to severity is widely used in chronic kidney disease or acute kidney injury not requiring dialysis. Yet the diagnosis of "end-stage kidney disease" (ESKD) considers patients as a single homogeneous group, with negligible kidney function, in need of kidney replacement therapy. Herein, we review the evidence behind the heterogeneous nature of ESKD and discuss potential benefits of recasting the terminology used to describe advanced kidney dysfunction from a monolithic entity to a disease with stages of ascending severity. We consider kidney assistance therapy in lieu of kidney replacement therapy to better reconcile all available types of therapy for advanced kidney failure including dietary intervention, kidney transplantation, and dialysis therapy at varied schedules. The lexicon "kidney dysfunction requiring dialysis" (KDRD) with stages of ascending severity based on levels of residual kidney function (RKF)-that is, renal urea clearance-and manifestations related to uremia, fluid status, and other abnormalities is discussed. Subtyping KDRD by levels of RKF could advance dialysis therapy as a form of kidney assistance therapy adjusted based on RKF and clinical symptoms. We focus on intermittent hemodialysis and underscore the need to personalize dialysis treatments and improve characterization of patients included in clinical trials.

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.

Effects of the L-tyrosine-derived bacterial metabolite p-cresol on colonic and peripheral cells

Specific families of bacteria present within the intestinal luminal content produce p-cresol from L-tyrosine. Although the hosts do not synthesize p-cresol, they can metabolize this compound within their colonic mucosa and liver leading to the production of co-metabolites including p-cresyl sulfate (p-CS) and p-cresyl glucuronide (p-CG). p-Cresol and its co-metabolites are recovered in the circulation mainly conjugated to albumin, but also in their free forms that are excreted in the urine. An increased dietary protein intake raises the amount of p-cresol recovered in the feces and urine, while fecal excretion of p-cresol is diminished by a diet containing undigestible polysaccharides. p-Cresol in excess is genotoxic for colonocytes. In addition, in these cells, this bacterial metabolite decreases mitochondrial oxygen consumption, while increasing the anion superoxide production. In chronic kidney disease (CKD), marked accumulation of p-cresol and p-CS in plasma is measured, and in renal tubular cells, p-cresol and p-CS increase oxidative stress, affect mitochondrial function, and lead to cell death, strongly suggesting that these 2 compounds act as uremic toxins that aggravate CKD progression. p-Cresol and p-CS are also suspected to play a role in the CKD-associated adverse cardiovascular events, since they affect endothelial cell proliferation and migration, decrease the capacity of endothelial wound repair, and increase the senescence of endothelial cells. Finally, the fact that concentration of p-cresol is transiently increased in young autistic children biological fluids, and that intraperitoneal injection of p-cresol in animal models induces some behavioral characteristics observed in the autism spectrum disorders (ASD), raise the view that p-cresol may possibly represent one of the components involved in ASD etiology. Further pre-clinical and clinical studies are obviously needed to determine if the lowering of p-cresol and/or p-CS circulating concentrations, by dietary and/or pharmacological means, would allow, by itself or in combination with other interventions, to improve CKD progression and associated cardiovascular outcomes, as well as some neurological outcomes in children with an early diagnosis of autism.

Uremic Toxins and Frailty in Patients with Chronic Kidney Disease: A Molecular Insight

The accumulation of uremic toxins (UTs) is a prototypical manifestation of uremic milieu that follows renal function decline (chronic kidney disease, CKD). Frailty as a potential outcome-relevant indicator is also prevalent in CKD. The intertwined relationship between uremic toxins, including small/large solutes (phosphate, asymmetric dimethylarginine) and protein-bound ones like indoxyl sulfate (IS) and p-cresyl sulfate (pCS), and frailty pathogenesis has been documented recently. Uremic toxins were shown in vitro and in vivo to induce noxious effects on many organ systems and likely influenced frailty development through their effects on multiple preceding events and companions of frailty, such as sarcopenia/muscle wasting, cognitive impairment/cognitive frailty, osteoporosis/osteodystrophy, vascular calcification, and cardiopulmonary deconditioning. These organ-specific effects may be mediated through different molecular mechanisms or signal pathways such as peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α), mitogen-activated protein kinase (MAPK) signaling, aryl hydrocarbon receptor (AhR)/nuclear factor-κB (NF-κB), nuclear factor erythroid 2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1), Runt-related transcription factor 2 (RUNX2), bone morphogenic protein 2 (BMP2), osterix, Notch signaling, autophagy effectors, microRNAs, and reactive oxygen species induction. Anecdotal clinical studies also suggest that frailty may further accelerate renal function decline, thereby augmenting the accumulation of UTs in affected individuals. Judging from these threads of evidence, management strategies aiming for uremic toxin reduction may be a promising approach for frailty amelioration in patients with CKD. Uremic toxin lowering strategies may bear the potential of improving patients’ outcomes and restoring their quality of life, through frailty attenuation. Pathogenic molecule-targeted therapeutics potentially disconnect the association between uremic toxins and frailty, additionally serving as an outcome-modifying approach in the future.

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.

Protein-bound uremic toxins (PBUTs) in chronic kidney disease (CKD) patients: Production pathway, challenges and recent advances in renal PBUTs clearance

Uremic toxins, a group of uremic retention solutes with high concentration which their accumulation on the body makes several biological problems, have recently gained a large interest. The importance of this issue more targets patients with compromised kidney function since the presence of these toxins in their bodies contributes to serious illness and death. It is reported that around 14% of people are subjected of CKD's problems. Among different classifications of uremic toxins, protein bound uremic toxins are poorly removed from the body as they tightly bind to proteins like serum albumin. A deeper and closer understanding of methods for removing protein bound uremic toxins and their efficiency is of paramount importance. This article discussed the most critical protein bound uremic toxins from different points of view including their chemistry, binding sites, interactions, and their biological impacts. Concerning the toxicity and high concentration, p-cresyl sulfate (PCS), Indoxyl sulfate (IS), 3-Carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF), and Indole- 3-acetic acid (IAA) was chosen to study in this article. Results offered that the functional groups of mentioned PBUTs and the way that they interact with the adsorbent play an important role in finding substances for removal of them. Furthermore, the development of nanoparticle (NPs) for promising biomedical purposes has been explored. However, there is still a need for further investigation to find biocompatible substances focusing on the removal of PBUTs. PBUTs are a unique class of uremic toxins whose renal clearance mechanisms and role in uremic pathophysiology are still unclear. This review outlines the biochemical aspects of PBUT/protein binding in a view to explaining their renal formation to elimination mechanisms; some examples are drawn from routes involving albumin-binding with indoxyl sulphate, p-cresyl sulfate, p-cresyl glucuronide and hippuric acid. We have also highlighted the kinetic behaviors during dialytic removal of PBUTs to address future concerns regarding dialytic therapy.

Contribution of 'clinically negligible' residual kidney function to clearance of uremic solutes

BACKGROUND

Residual kidney function (RKF) is thought to exert beneficial effects through clearance of uremic toxins. However, the level of native kidney function where clearance becomes negligible is not known.

METHODS

We aimed to assess whether levels of nonurea solutes differed among patients with 'clinically negligible' RKF compared with those with no RKF. The hemodialysis study excluded patients with urinary urea clearance >1.5 mL/min, below which RKF was considered to be 'clinically negligible'. We measured eight nonurea solutes from 1280 patients participating in this study and calculated the relative difference in solute levels among patients with and without RKF based on measured urinary urea clearance.

RESULTS

The mean age of the participants was 57 years and 57% were female. At baseline, 34% of the included participants had clinically negligible RKF (mean 0.7 ± 0.4 mL/min) and 66% had no RKF. Seven of the eight nonurea solute levels measured were significantly lower in patients with RKF than in those without RKF, ranging from -24% [95% confidence interval (CI) -31 to -16] for hippurate, -7% (-14 to -1) for trimethylamine-N-oxide and -4% (-6 to -1) for asymmetric dimethylarginine. The effect of RKF on plasma levels was comparable or more pronounced than that achieved with a 31% higher dialysis dose (spKt/Vurea 1.7 versus 1.3). Preserved RKF at 1-year follow-up was associated with a lower risk of cardiac death and first cardiovascular event.

CONCLUSIONS

Even at very low levels, RKF is not 'negligible', as it continues to provide nonurea solute clearance. Management of patients with RKF should consider these differences.

Predictors of Change in Left-Ventricular Structure and Function in a Trial of Extended Hours Hemodialysis

BACKGROUND

Myocardial pathology is common in patients undergoing hemodialysis. To explore the effects of differing aspects of dialysis treatment on its evolution, we examined the impact of change in markers of volume status, hemodynamics and solute clearance on left ventricular (LV) parameters in a randomized trial of extended hours dialysis.

METHODS AND RESULTS

A Clinical Trial of IntensiVE (ACTIVE) Dialysis randomized 200 patients undergoing hemodialysis to extended dialysis hours (≥ 24 hours/week) or standard hours (12-18 hours/week) for 12 months. In a prespecified substudy, 95 participants underwent cardiac magnetic resonance imaging (CMR) at baseline and at the study's end. Generalized linear regression was used to model the relationship between changes in LV parameters and markers of volume status (normalized ultrafiltration rate and total weekly interdialytic weight gain), hemodynamic changes (systolic and diastolic blood pressure) and solute control (urea clearance, dialysis hours and phosphate). Randomization to extended hours dialysis was not associated with change in any CMR parameter. Reduction in ultrafiltration rate was associated with reduction in LV mass index (P = 0.049) and improved ejection fraction (P = 0.024); reduction in systolic blood pressure was also associated with improvement in ejection fraction (P = 0.045); reduction in interdialytic weight gain was associated with reduced stroke volume (P = 0.038). There were no associations between change in urea clearance, phosphate or total hours per week and CMR parameters.

CONCLUSIONS

Reduction in ultrafiltration rate and blood pressure are associated with improved myocardial parameters in hemodialysis recipients independently of solute clearance or dialysis time. These findings underscore the importance of fluid status and related parameters as potential treatment targets in this population.

Kidney Clearance of Secretory Solutes Is Associated with Progression of CKD: The CRIC Study

BACKGROUND

The secretion of organic solutes by the proximal tubules is an essential intrinsic kidney function. However, the clinical significance of the kidney's clearance of tubular secretory solutes is uncertain.

METHODS

In this prospective cohort study, we evaluated 3416 participants with CKD from the Chronic Renal Insufficiency Cohort (CRIC) study. We measured plasma and 24-hour urine concentrations of endogenous candidate secretory solutes at baseline, using targeted liquid chromatography-tandem mass spectrometry. The study defined CKD progression by a ≥50% decline in the eGFR, initiation of maintenance dialysis, or kidney transplantation. We used Cox proportional hazards regression to test associations of secretory-solute clearances with CKD progression and mortality, adjusting for eGFR, albuminuria, and other confounding characteristics.

RESULTS

Participants in this ancillary study had a mean age of 58 years and 41% were black; the median eGFR was 43 ml/min per 1.73 m2. After adjustment, lower kidney clearances of six solutes-kynurenic acid, pyridoxic acid, indoxyl sulfate, xanthosine, isovalerylglycine, and cinnamoylglycine-were associated with significantly greater risks of CKD progression, with clearance of kynurenic acid, a highly protein-bound solute, having the strongest association. Lower clearances of isovalerylglycine, tiglylglycine, hippurate, and trimethyluric acid were significantly associated with all-cause mortality after adjustment.

CONCLUSIONS

We found lower kidney clearances of endogenous secretory solutes to be associated with CKD progression and all-cause mortality, independent of eGFR and albuminuria. This suggests that tubular clearance of secretory solutes provides additional information about kidney health beyond measurements of glomerular function alone.

Benefits and risks of frequent or longer haemodialysis: weighing the evidence

Although the ability of individuals with end-stage renal disease to maintain body homoeostasis is equally impaired during all weekdays, conventional haemodialysis (HD) treatment is scheduled thrice weekly, containing two short and one long interdialytic interval. This intermittent nature of HD and the consequent fluctuations in volume, metabolic parameters and electrolytes have long been hypothesized to predispose to complications. Large observational studies link the first weekday with an increased risk of cardiovascular morbidity and mortality. Several schemes of frequent and/or longer, home or in-centre HD have been introduced, aiming to alleviate the above risks by both increasing total dialysis duration and reducing the duration of interdialytic intervals. Observational studies in this field have non-uniform results, showing that enhanced frequency in home (but not in-centre) HD is associated with reduced mortality. Evidence from the randomized Daily and Nocturnal Trials of the Frequent HD Network suggest the opposite, showing mortality benefits with in-centre daily but not with home nocturnal dialysis. Secondary analyses of these trials indicate that daily and nocturnal schedules do not have equal effects on intermediate outcomes. Alternative schemes, such as thrice weekly in-centre nocturnal HD or every-other-day HD, seem to also offer improvements in several intermediate endpoints, but need further testing with randomized trials. This review summarizes the effects of frequent and/or longer HD methods on hard and intermediate outcomes, attempting to provide a balanced overview of the field.

Metabolic Activation of Flavin Monooxygenase-mediated Trimethylamine-N-Oxide Formation in Experimental Kidney Disease

Cardiovascular disease (CVD) remains the leading cause of death in chronic kidney disease (CKD) patients despite treatment of traditional risk factors, suggesting that non-traditional CVD risk factors are involved. Trimethylamine-N-oxide (TMAO) correlates with atherosclerosis burden in CKD patients and may be a non-traditional CVD risk factor. Serum TMAO concentrations are significantly increased in CKD patients, which may be due in part to increased hepatic flavin monooxygenase (FMO)-mediated TMAO formation. The objective of this work was to elucidate the mechanism of increased FMO activity in CKD. In this study, FMO enzyme activity experiments were conducted in vitro with liver microsomes isolated from experimental CKD and control rats. Trimethylamine was used as a probe substrate to assess FMO activity. The FMO activator octylamine and human uremic serum were evaluated. FMO gene and protein expression were also determined. FMO-mediated TMAO formation was increased in CKD versus control. Although gene and protein expression of FMO were not changed, metabolic activation elicited by octylamine and human uremic serum increased FMO-mediated TMAO formation. The findings suggest that metabolic activation of FMO-mediated TMAO formation is a novel mechanism that contributes to increased TMAO formation in CKD and represents a therapeutic target to reduce TMAO exposure and CVD.

Protein carbamylation in end stage renal disease: is there a mortality effect?

PURPOSE OF REVIEW

Protein carbamylation is a posttranslational protein modification caused, in part, by exposure to urea's dissociation product cyanate. Additional modulators of protein carbamylation include circulating free amino acid levels, inflammation, diet, smoking, and environmental pollution exposures. Carbamylation reactions can modify protein charge, structure, and function, leading to adverse molecular and cellular responses. These changes have been linked to several pathologic biochemical pathways relevant to patients with end stage renal disease (ESRD) such as accelerated atherosclerosis and dysfunctional erythropoiesis, among others. This review examines the consequences of human protein carbamylation and the clinical impact this is thought to have in patients with ESRD.

RECENT FINDINGS

Recent well-conducted studies across diverse cohorts of patients have independently associated elevations in protein carbamylation to mortality and morbidity in patients with ESRD. Studies are now examining the best strategies to reduce carbamylation load, including interventions aimed at lowering urea levels and restoring amino acid balance. Whether such carbamylation lowering strategies yield clinical improvements remain to be determined.

SUMMARY

Numerous fundamental studies provide plausible mechanisms for the observed association between protein carbamylation burden and adverse clinical outcomes in ESRD. Studies employing nutritional and dialytic interventions to lower carbamylation may mitigate this risk but the net clinical benefit has not been established.

Untargeted metabolomics reveals N, N, N-trimethyl-L-alanyl-L-proline betaine (TMAP) as a novel biomarker of kidney function

The diagnosis and prognosis of chronic kidney disease (CKD) currently relies on very few circulating small molecules, which can vary by factors unrelated to kidney function. In end-stage renal disease (ESRD), these same small molecules are used to determine dialysis dose and dialytic clearance. Therefore, we aimed to identify novel plasma biomarkers to estimate kidney function in CKD and dialytic clearance in ESRD. Untargeted metabolomics was performed on plasma samples from patients with a single kidney, non-dialysis CKD, ESRD and healthy controls. For ESRD patients, pre- and post-dialysis plasma samples were obtained from several dialysis modalities. Metabolomics analysis revealed over 400 significantly different features in non-dialysis CKD and ESRD plasma compared to controls while less than 35 features were significantly altered in patients with a single kidney. N,N,N-trimethyl-L-alanyl-L-proline betaine (TMAP, AUROC = 0.815) and pyrocatechol sulfate (AUROC = 0.888) outperformed creatinine (AUROC = 0.745) in accurately identifying patients with a single kidney. Several metabolites accurately predicted ESRD; however, when comparing pre-and post-hemodialysis, TMAP was the most robust biomarker of dialytic clearance for all modalities (AUROC = 0.993). This study describes TMAP as a novel potential biomarker of kidney function and dialytic clearance across several hemodialysis modalities.

A swan song for Kt/Vurea

Dialyzer clearance of urea multiplied by dialysis time and normalized for urea distribution volume (Kt/V_urea or simply Kt/V) has been used as an index of dialysis adequacy since more than 30 years. This article reviews the flaws of Kt/V, starting with a lack of proof of concept in three randomized controlled hard outcome trials (RCTs), and continuing with a long list of conditions where the concept of Kt/V was shown to be flawed. This information leaves little room for any conclusion other than that Kt/V, as an indicator of dialysis adequacy, is obsolete. The dialysis patient might benefit more if, instead, the nephrology community concentrates in the future on pursuing the optimal dialysis dose that conforms with adequate quality of life and on factors that are likely to affect outcomes more than Kt/V. These include residual renal function, volume status, dialysis length, ultrafiltration rate, the number of intra-dialytic hypotensive episodes, interdialytic blood pressure, serum potassium and phosphate, serum albumin, and C reactive protein.

Indoxyl Sulfate Induces Renal Fibroblast Activation through a Targetable Heat Shock Protein 90-Dependent Pathway

Indoxyl sulfate (IS) accumulation occurs early during chronic kidney disease (CKD) progression and contributes to renal dysfunction by inducing fibrosis, inflammation, oxidative stress, and tissue remodeling. Renal toxicity of high IS concentrations (250 μM) has been widely explored, particularly in resident tubular and glomerular cells, while the effect of a moderate IS increase on kidneys is still mostly unknown. To define the effects of IS accumulation on renal fibroblasts, we first analyzed kidneys of C57BL/6 mice receiving IS (0.1%) in drinking water for 12 weeks. As a next step, we treated renal fibroblasts (NRK-49F) with IS (20 μM) with or without the HSP90 inhibitor 17-AAG (1 μM). In mouse kidneys, IS increased the collagen deposition and HSP90 and α-SMA expression (immunohistochemistry) in interstitial fibroblasts and caused tubular necrosis (histological H&E and picrosirius red staining). In NRK-49F cells, IS induced MCP1, TGF-β, collagen I, α-SMA, and HSP90 gene/protein expression and Smad2/3 pathway activation. IS had no effects on fibroblast proliferation and ROS production. 17-AAG counteracted IS-induced MCP1, TGF-β, collagen I, and α-SMA expression and Smad2/3 phosphorylation. Our study demonstrates that the IS increase promotes renal fibroblast activation by a HSP90-dependent pathway and indicates HSP90 inhibition as a potential strategy to restrain IS-induced kidney inflammation and fibrosis in CKD.

Dialysis Reimbursement: What Impact Do Different Models Have on Clinical Choices?

Allowing patients to live for decades without the function of a vital organ is a medical miracle, but one that is not without cost both in terms of morbidity and quality of life and in economic terms. Renal replacement therapy (RRT) consumes between 2% and 5% of the overall health care expenditure in countries where dialysis is available without restrictions. While transplantation is the preferred treatment in patients without contraindications, old age and comorbidity limit its indications, and low organ availability may result in long waiting times. As a consequence, 30–70% of the patients depend on dialysis, which remains the main determinant of the cost of RRT. Costs of dialysis are differently defined, and its reimbursement follows different rules. There are three main ways of establishing dialysis reimbursement. The first involves dividing dialysis into a series of elements and reimbursing each one separately (dialysis itself, medications, drugs, transportation, hospitalisation, etc.). The second, known as the capitation system, consists of merging these elements in a per capita reimbursement, while the third, usually called the bundle system, entails identifying a core of procedures intrinsically linked to treatment (e.g., dialysis sessions, tests, intradialyitc drugs). Each one has advantages and drawbacks, and impacts differently on the organization and delivery of care: payment per session may favour fragmentation and make a global appraisal difficult; a correct capitation system needs a careful correction for comorbidity, and may exacerbate competition between public and private settings, the latter aiming at selecting the least complex cases; a bundle system, in which the main elements linked to the dialysis sessions are considered together, may be a good compromise but risks penalising complex patients, and requires a rapid adaptation to treatment changes. Retarding dialysis is a clinical and economical goal, but the incentives for predialysis care are not established and its development may be unfavourable for the provider. A closer cooperation between policymakers, economists and nephrologists is needed to ensure a high quality of dialysis care.

In silico comparison of protein-bound uremic toxin removal by hemodialysis, hemodiafiltration, membrane adsorption, and binding competition

Protein-bound uremic toxins (PBUTs) are poorly removed during hemodialysis (HD) due to their low free (dialyzable) plasma concentration. We compared PBUT removal between HD, hemodiafiltration (HDF), membrane adsorption, and PBUT displacement in HD. The latter involves infusing a binding competitor pre-dialyzer, which competes with PBUTs for their albumin binding sites and increases their free fraction. We used a mathematical model of PBUT/displacer kinetics in dialysis comprising a three-compartment patient model, an arterial/venous tube segment model, and a dialyzer model. Compared to HD, improvements in removal of prototypical PBUTs indoxyl sulfate (initial concentration 100 µM, 7% free) and p-cresyl sulfate (150 µM, 5% free) were: 5.5% and 6.4%, respectively, for pre-dilution HDF with 20 L replacement fluid; 8.1% and 9.1% for post-dilution HDF 20 L; 15.6% and 18.3% for pre-dilution HDF 60 L; 19.4% and 22.2% for complete membrane adsorption; 35.0% and 41.9% for displacement with tryptophan (2000 mg in 500 mL saline); 26.7% and 32.4% for displacement with ibuprofen (800 mg in 200 mL saline). Prolonged (one-month) use of tryptophan reduces the IS and pCS time-averaged concentration by 28.1% and 29.9%, respectively, compared to conventional HD. We conclude that competitive binding can be a pragmatic approach for improving PBUT removal.

IHDIP: a controlled randomized trial to assess the security and effectiveness of the incremental hemodialysis in incident patients

BACKGROUND

Most people who make the transition to renal replacement therapy (RRT) are treated with a fixed dose thrice-weekly hemodialysis réegimen, without considering their residual kidney function (RKF). Recent papers inform us that incremental hemodialysis is associated with preservation of RKF, whenever compared with conventional hemodialysis. The objective of the present controlled randomized trial (RCT) is to determine if start HD with one sessions per week (1-Wk/HD), it is associated with better patient survival and other safety parameters.

METHODS/DESIGN

IHDIP is a multicenter RCT experimental open trial. It is randomized in a 1:1 ratio and controlled through usual clinical practice, with a low intervention level and non-commercial. It includes 152 incident patients older than 18 years, with a RRF of ≥4 ml/min/1.73 m2, measured by renal clearance of urea (KrU). The intervention group includes 76 patients who will start with incremental HD (1-Wk/HD). The control group includes 76 patients who will start with thrice-weekly hemodialysis régimen. The primary outcome is assessing the survival rate, while the secondary outcomes are the morbidity rate, the clinical parameters, the quality of life and the efficiency.

DISCUSSION

This study will enable to know the number of sessions a patient should receive when starting HD, depending on his RRF. The potentially important clinical and financial implications of incremental hemodialysis warrant this RCT.

TRIAL REGISTRATION

U.S. National Institutes of Health, ClinicalTrials.gov . Number: NCT03239808 , completed 13/04/2017.

SPONSOR

Foundation for Training and Research of Health Professionals of Extremadura.

Individualized Hemodialysis Treatment: A Perspective on Residual Kidney Function and Precision Medicine in Nephrology

Background: Residual kidney function (RKF) is often expected to inevitably and rapidly decline among hemodialysis patients and, hence, has been inadvertently ignored in clinical practice. The importance of RKF has been revisited in some recent studies. Given that patients with end-stage renal disease now tend to initiate maintenance hemodialysis therapy with higher RKF levels, there seem to be important opportunities for incremental hemo­dialysis by individualizing the dose and frequency according to their RKF levels. This approach is realigned with precision medicine and patient-centeredness. Summary: In this article, we first review the available methods to estimate RKF among hemodialysis patients. We then discuss the importance of maintaining and monitoring RKF levels based on a variety of clinical aspects, including volume overload, blood pressure control, mineral and bone metabolism, nutrition, and patient survival. We also review several potential measures to protect RKF: the use of high-flux and biocompatible membranes, the use of ultrapure dialysate, the incorporation of hemodiafiltration, incremental hemodialysis, and a low-protein diet, as well as general care such as avoiding nephrotoxic events, maintaining appropriate blood pressure, and better control of mineral and bone disorder parameters. Key Message: Individualized hemodialysis regimens may maintain RKF, lead to a better quality of life without compromising long-term survival, and ensure precision medicine and patient-centeredness in nephrology practice.

Population-Level Analysis to Determine Parameters That Drive Variation in the Plasma Metabolite Profiles

The plasma metabolome is associated with multiple phenotypes and diseases. However, a systematic study investigating clinical determinants that control the metabolome has not yet been conducted. In the present study, therefore, we aimed to identify the major determinants of the plasma metabolite profile. We used ultra-high performance liquid chromatography (UHPLC) coupled to quadrupole time of flight mass spectrometry (QTOF-MS) to determine 106 metabolites in plasma samples from 2503 subjects in a cross-sectional study. We investigated the correlation structure of the metabolite profiles and generated uncorrelated metabolite factors using principal component analysis (PCA) and varimax rotation. Finally, we investigated associations between these factors and 34 clinical covariates. Our results suggest that liver function, followed by kidney function and insulin resistance show the strongest associations with the plasma metabolite profile. The association of specific phenotypes with several components may suggest multiple independent metabolic mechanisms, which is further supported by the composition of the associated factors.

Rationale and design of DiPPI: A randomized controlled trial to evaluate the safety and effectiveness of progressive hemodialysis in incident patients

INTRODUCTION

Progressive haemodialysis (HD) is a starting regime for renal replacement therapy (RRT) adapted to each patient's necessities. It is mainly conditioned by the residual renal function (RRF). The frequency of sessions with which patients start HD (one or two sessions per week), is lower than that for conventional HD (three times per week). Such frequency is increased (from one to two sessions, and from two to three sessions) as the RRF declines.

METHODOLOGY/DESIGN

IHDIP is a multicentre randomised experimental open trial. It is randomised in a 1:1 ratio and controlled through usual clinical practice, with a low intervention level and non-commercial. It includes 152 patients older than 18 years with chronic renal disease stage 5 and start HD as RRT, with an RRF of ≥4ml/min/1.73m², measured by renal clearance of urea (KrU). The intervention group includes 76 patients who will start with one session of HD per week (progressive HD). The control group includes 76 patients who will start with three sessions per week (conventional HD). The primary purpose is assessing the survival rate, while the secondary purposes are the morbidity rate (hospital admissions), the clinical parameters, the quality of life and the efficiency.

DISCUSSION

This study will enable us to know, with the highest level of scientific evidence, the number of sessions a patient should receive when starting the HD treatment, depending on his/her RRF.

TRIAL REGISTRATION

Registered at the U.S. National Institutes of Health, ClinicalTrials.gov under the number NCT03239808.

Metabolomics in chronic kidney disease: Strategies for extended metabolome coverage

Chronic kidney disease (CKD) is becoming a major public health issue as prevalence is increasing worldwide. It also represents a major challenge for the identification of new early biomarkers, understanding of biochemical mechanisms, patient monitoring and prognosis. Each metabolite contained in a biofluid or tissue may play a role as a signal or as a driver in the development or progression of the pathology. Therefore, metabolomics is a highly valuable approach in this clinical context. It aims to provide a representative picture of a biological system, making exhaustive metabolite coverage crucial. Two aspects can be considered: analytical and biological coverage. From an analytical point of view, monitoring all metabolites within one run is currently impossible. Multiple analytical techniques providing orthogonal information should be carried out in parallel for coverage improvement. The biological aspect of metabolome coverage can be enhanced by using multiple biofluids or tissues for in-depth biological investigation, as the analysis of a single sample type is generally insufficient for whole organism extrapolation. Hence, recording of signals from multiple sample types and different analytical platforms generates massive and complex datasets so that chemometric tools, including data fusion approaches and multi-block analysis, are key tools for extracting biological information and for discovery of relevant biomarkers. This review presents the recent developments in the field of metabolomic analysis, from sampling and analytical strategies to chemometric tools, dedicated to the generation and handling of multiple complementary metabolomic datasets enabling extended metabolite coverage to improve our biological knowledge of CKD.

Protein Carbamylation in Chronic Kidney Disease and Dialysis

Protein carbamylation is a nonenzymatic posttranslational protein modification that can be driven, in part, by exposure to urea's dissociation product, cyanate. In humans, when kidney function is impaired and urea accumulates, systemic protein carbamylation levels increase. Additional mediators of protein carbamylation have been identified including inflammation, diet, smoking, circulating free amino acid levels, and environmental exposures. Carbamylation reactions on proteins are capable of irreversibly changing protein charge, structure, and function, resulting in pathologic molecular and cellular responses. Carbamylation has been mechanistically linked to the biochemical pathways implicated in atherosclerosis, dysfunctional erythropoiesis, kidney fibrosis, autoimmunity, and other pathological domains highly relevant to patients with chronic kidney disease. In this review, we describe the biochemical impact of carbamylation on human proteins, the mechanistic role carbamylation can have on clinical outcomes in kidney disease, the clinical association studies of carbamylation in chronic kidney disease, including patients on dialysis, and the promise of therapies aimed at reducing carbamylation burden in this vulnerable patient population.

Proximal Tubular Secretory Clearance: A Neglected Partner of Kidney Function

The secretion of small molecules by the proximal tubules of the kidneys represents a vital homeostatic function for rapidly clearing endogenous solutes and medications from the circulation. After filtration at the glomerulus, renal blood flow is directed through a network of peritubular capillaries, where transporters of the proximal tubules actively secrete putative uremic toxins and hundreds of commonly prescribed drugs into the urine, including protein-bound substances that cannot readily cross the glomerular basement membrane. Despite its central physiologic importance, tubular secretory clearance is rarely measured or even estimated in clinical or research settings. Major barriers to estimating tubular solute clearance include uncertainty regarding optimal endogenous secretory markers and a lack of standardized laboratory assays. The creation of new methods to measure tubular secretion could catalyze advances in kidney disease research and clinical care. Differences in secretory clearance relative to the GFR could help distinguish among the causes of CKD, particularly for disorders that primarily affect the tubulointerstitium. As the primary mechanism by which the kidneys excrete medications, tubular secretory clearance offers promise for improving kidney medication dosing, which is currently exclusively on the basis of filtration. The differing metabolic profiles of retained solutes eliminated by secretion versus glomerular filtration suggest that secretory clearance could uniquely inform uremic toxicity, refine existing measures of residual kidney function, and improve prediction of cardiovascular and kidney disease outcomes. Interdisciplinary research across clinical, translational, and laboratory medicine is needed to bring this often neglected kidney function into the limelight.

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.

Measuring residual renal function for hemodialysis adequacy: Is there an easier option?

Most patients starting hemodialysis (HD) have residual renal function. As such, there has been increased interest in starting patients with less frequent and shorter dialysis session times. However, for this incremental approach to be successful, patients require regular monitoring of residual renal function, so that as residual renal function declines, the amount of HD is appropriately increased. Currently most dialysis centers rely on interdialytic urine collections. However, many patients find these inconvenient and there may be marked intrapatient variability due to compliance issues. Thus, alternative markers of residual renal function are required for routine clinical practice. Currently three middle sized molecules; cystatin C, β2 microglobulin, and βtrace protein have been investigated as potential endogenous markers of glomerular filtration. Although none is ideal, combinations of these markers have been proposed to provide a more accurate estimation of glomerular clearance, and in particular cut offs for minimal residual renal function. However, in patients with low levels of residual renal function it remains unclear as to whether the benefits of residual renal function equally apply to glomerular filtration or tubular function.

Extended Duration Nocturnal Hemodialysis and Changes in Plasma Metabolite Profiles

BACKGROUND AND OBJECTIVES

In-center, extended duration nocturnal hemodialysis has been associated with variable clinical benefits, but the effect of extended duration hemodialysis on many established uremic solutes and other components of the metabolome is unknown. We determined the magnitude of change in metabolite profiles for patients on extended duration nocturnal hemodialysis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In a 52-week prospective, observational study, we followed 33 patients receiving conventional thrice weekly hemodialysis who converted to nocturnal hemodialysis (7-8 hours per session, three times per week). A separate group of 20 patients who remained on conventional hemodialysis (3-4 hours per session, three times per week) served as a control group. For both groups, we applied liquid chromatography-mass spectrometry-based metabolite profiling on stored plasma samples collected from all participants at baseline and after 1 year. We examined longitudinal changes in 164 metabolites among those who remained on conventional hemodialysis and those who converted to nocturnal hemodialysis using Wilcoxon rank sum tests adjusted for multiple comparisons (false discovery rate <0.05).

RESULTS

On average, the nocturnal group had 9.6 hours more dialysis per week than the conventional group. Among 164 metabolites, none changed significantly from baseline to study end in the conventional group. Twenty-nine metabolites changed in the nocturnal group, 21 of which increased from baseline to study end (including all branched-chain amino acids). Eight metabolites decreased after conversion to nocturnal dialysis, including l-carnitine and acetylcarnitine. By contrast, several established uremic retention solutes, including p-cresol sulfate, indoxyl sulfate, and trimethylamine N-oxide, did not change with extended dialysis.

CONCLUSIONS

Across a wide array of metabolites examined, extended duration hemodialysis was associated with modest changes in the plasma metabolome, with most differences relating to metabolite increases, despite increased dialysis time. Few metabolites showed reduction with more dialysis, and no change in several established uremic toxins was observed.

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.

Renal Replacement Therapy and Incremental Hemodialysis for Veterans with Advanced Chronic Kidney Disease

Each year approximately 13,000 Veterans transition to maintenance dialysis, mostly in the traditional form of thrice-weekly hemodialysis from the start. Among >6000 dialysis units nationwide, there are currently approximately 70 Veterans Affairs (VA) dialysis centers. Given this number of VA dialysis centers and their limited capacity, only 10% of all incident dialysis Veterans initiate treatment in a VA center. Evidence suggests that, among Veterans, the receipt of care within the VA system is associated with favorable outcomes, potentially because of the enhanced access to healthcare resources. Data from the United States Renal Data System Special Study Center "Transition-of-Care-in-CKD" suggest that Veterans who receive dialysis in a VA unit exhibit greater survival compared with the non-VA centers. Substantial financial expenditures arise from the high volume of outsourced care and higher dialysis reimbursement paid by the VA than by Medicare to outsourced providers. Given the exceedingly high mortality and abrupt decline in residual kidney function (RKF) in the first dialysis year, it is possible that incremental transition to dialysis through an initial twice-weekly hemodialysis regimen might preserve RKF, prolong vascular access longevity, improve patients' quality of life, and be a more patient-centered approach, more consistent with "personalized" dialysis. Broad implementation of incremental dialysis might also result in more Veterans receiving care within a VA dialysis unit. Controlled trials are needed to examine the safety and efficacy of incremental hemodialysis in Veterans and other populations; the administrative and health care as well as provider structure within the VA system would facilitate the performance of such trials.

Residual Kidney Function: Implications in the Era of Personalized Medicine

The association of residual kidney function (RKF) with improved outcomes in peritoneal dialysis and hemodialysis patients is now widely recognized. RKF provides substantial volume and solute clearance even after dialysis initiation. In particular, RKF provides clearance of nonurea solutes, many of which are potential uremic toxins and not effectively removed by conventional hemodialysis. The presence of RKF provides a distinct advantage to incident dialysis patients and is an opportunity for nephrologists to individualize dialysis treatments tailored to their patients' unique solute, volume, and quality of life needs. The benefits of RKF present the opportunity to personalize the management of uremia.