Estimated glomerular filtration rate is a poor predictor of concentration for a broad range of uremic toxins

Assessment of Within- and Inter-Patient Variability of Uremic Toxin Concentrations in Children with CKD

To promote improved trial design in upcoming randomized clinical trials in childhood chronic kidney disease (CKD), insight in the within- and inter-patient variability of uremic toxins with its nutritional, treatment- and patient-related confounding factors is of utmost importance. In this study, the within- and inter-patient variability of a selection of uremic toxins in a longitudinal cohort of children diagnosed with CKD was assessed, using the intraclass correlation coefficient (ICC) and the within-patient coefficient of variation (CV). Subsequently, the contribution of anthropometry, estimated glomerular filtration rate (eGFR), dietary fiber and protein, and use of (prophylactic) antibiotics to uremic toxin variability was evaluated. Based on 403 observations from 62 children (median seven visits per patient; 9.4 ± 5.3 years; 68% males; eGFR 38.5 [23.1; 64.0] mL/min/1.73 m2) collected over a maximum of 2 years, we found that the within-patient variability is high for especially protein-bound uremic toxins (PBUTs) (ICC < 0.7; within-patient CV 37-67%). Moreover, eGFR was identified as a predominant contributor to the within- and inter-patient variability for the majority of solutes, while the impact of the child's anthropometry, fiber and protein intake, and antibiotics on the variability of uremic toxin concentrations was limited. Based on these findings, we would recommend future intervention studies that attempt to decrease uremic toxin levels to select a (non-dialysis) CKD study population with a narrow eGFR range. As the expected effect of the selected intervention should exceed the inter-patient variability of the selected uremic toxins, a narrow eGFR range might aid in improving the trial design.

Metabolites Associated With Uremic Symptoms in Patients With CKD: Findings From the Chronic Renal Insufficiency Cohort (CRIC) Study

RATIONALE & OBJECTIVE

The toxins that contribute to uremic symptoms in patients with chronic kidney disease (CKD) are unknown. We sought to apply complementary statistical modeling approaches to data from untargeted plasma metabolomic profiling to identify solutes associated with uremic symptoms in patients with CKD.

STUDY DESIGN

Cross-sectional.

SETTING & PARTICIPANTS

1,761 Chronic Renal Insufficiency Cohort (CRIC) participants with CKD not treated with dialysis.

PREDICTORS

Measurement of 448 known plasma metabolites.

OUTCOMES

The uremic symptoms of fatigue, anorexia, pruritus, nausea, paresthesia, and pain were assessed by single items on the Kidney Disease Quality of Life-36 instrument.

ANALYTICAL APPROACH

Multivariable adjusted linear regression, least absolute shrinkage and selection operator linear regression, and random forest models were used to identify metabolites associated with symptom severity. After adjustment for multiple comparisons, metabolites selected in at least 2 of the 3 modeling approaches were deemed "overall significant."

RESULTS

Participant mean estimated glomerular filtration rate was 43mL/min/1.73m2, with 44% self-identifying as female and 41% as non-Hispanic Black. The prevalence of uremic symptoms ranged from 22% to 55%. We identified 17 metabolites for which a higher level was associated with greater severity of at least one uremic symptom and 9 metabolites inversely associated with uremic symptom severity. Many of these metabolites exhibited at least a moderate correlation with estimated glomerular filtration rate (Pearson's r≥0.5), and some were also associated with the risk of developing kidney failure or death in multivariable adjusted Cox regression models.

LIMITATIONS

Lack of a second independent cohort for external validation of our findings.

CONCLUSIONS

Metabolomic profiling was used to identify multiple solutes associated with uremic symptoms in adults with CKD, but future validation and mechanistic studies are needed.

PLAIN-LANGUAGE SUMMARY

Individuals living with chronic kidney disease (CKD) often experience symptoms related to CKD, traditionally called uremic symptoms. It is likely that CKD results in alterations in the levels of numerous circulating substances that, in turn, cause uremic symptoms; however, the identity of these solutes is not known. In this study, we used metabolomic profiling in patients with CKD to gain insights into the pathophysiology of uremic symptoms. We identified 26 metabolites whose levels were significantly associated with at least one of the symptoms of fatigue, anorexia, itchiness, nausea, paresthesia, and pain. The results of this study lay the groundwork for future research into the biological causes of symptoms in patients with CKD.

Estimated glomerular filtration rate in observational and interventional studies in chronic kidney disease

Estimated glomerular filtration rate is considered the principal measure of kidney function and, together with albuminuria, is a relevant prognostic factor for the development of end-stage kidney disease. Due to the strong association between estimated glomerular filtration rate and clinical events, such as commencement of dialysis, cardiovascular outcomes and all-cause death, estimated glomerular filtration rate is crucial for clinical decision-making in terms of scheduling follow-up and pharmacological interventions, and planning renal replacement therapies in advanced chronic kidney disease. In this review we discuss the available methods for measuring glomerular filtration rate and for estimating it through mathematical equations developed over the last few decades. We summarize the prognostic association of different percentages of estimated glomerular filtration rate decline and the main clinical outcomes, and how treatments modify estimated glomerular filtration rate decline and the risk of future endpoints. We also examine the role of pre-clinical trial slope and that of estimated glomerular filtration rate as a useful biomarker when evaluating patients for inclusion into both observational and interventional studies.

Predictive value of protein-bound uremic toxins for heart failure in patients with chronic kidney disease

AIMS

This retrospective cohort study aimed to be the first to evaluate the association between plasma protein-bound uremic toxins (PBUTs) concentrations, echocardiographic parameters of heart failure (HF), and incident HF events in patients with chronic kidney disease (CKD) not on dialysis.

METHODS AND RESULTS

Retrospective, single-centre, cohort study at the Ghent University Hospital, Belgium. Adults with CKD stages G1-G5, not on dialysis, could be included. Exclusion criteria were ongoing pregnancy, age <18 years, active acute infection, active malignancy, history of transplantation, or a cardiovascular event within 3 months prior to inclusion. Free and total concentrations of five PBUTs were quantified at baseline: indoxyl sulfate (IxS), p-cresyl sulfate (pCS), p-cresyl glucuronide (pCG), indole-3 acetic acid (IAA), and hippuric acid (HA). Patients were grouped into three echocardiographic categories: normal left ventricular ejection fraction (LVEF) and normal left ventricular end-diastolic pressure (LVEDP), normal LVEF and increased LVEDP, and reduced LVEF, based on available echocardiographic data in a time interval of ±6 months around the plasma sample collection. A total of 523 patients were included between January 2011 and January 2014. Echocardiographic data within the predefined timeframe were available for 210 patients (40% of patients). Levels of pCG and pCS were significantly higher in patients with reduced (<50%) versus normal LVEF (P < 0.05). After a median follow-up 5.5 years, 43 (8.4%) patients reached the composite endpoint of hospitalization or mortality due to HF. Free fractions of IxS, pCS, and pCG showed the strongest association with clinical outcome: free IxS: HR 1.71 (95% CI 1.11-2.63; P = 0.015), free pCS: HR 1.82 (95% CI 1.11-3.01; P = 0.019), and free pCG: HR 1.67 (95% CI 1.08-2.58; P = 0.020), and these results were independent of age, gender, body mass index, diabetes, and systolic blood pressure. In models that were also adjusted for serum creatinine, the free fractions of these PBUTs remained significant.

CONCLUSIONS

Elevated free concentrations of IxS, pCG, and pCS were independently associated with an increased risk of HF events in non-dialysed CKD patients. Further research is necessary to confirm these findings and investigate the potential impact of PBUT-lowering interventions on HF events in this patient group.

Precision Medicine in Diabetic Kidney Disease: A Narrative Review Framed by Lived Experience

Purpose of review

Diabetic kidney disease (DKD) is a leading cause of chronic kidney disease (CKD) for which many treatments exist that have been shown to prevent CKD progression and kidney failure. However, DKD is a complex and heterogeneous etiology of CKD with a spectrum of phenotypes and disease trajectories. In this narrative review, we discuss precision medicine approaches to DKD, including genomics, metabolomics, proteomics, and their potential role in the management of diabetes mellitus and DKD. A patient and caregivers of patients with lived experience with CKD were involved in this review.

Sources of information

Original research articles were identified from MEDLINE and Google Scholar using the search terms "diabetes," "diabetic kidney disease," "diabetic nephropathy," "chronic kidney disease," "kidney failure," "dialysis," "nephrology," "genomics," "metabolomics," and "proteomics."

Methods

A focused review and critical appraisal of existing literature regarding the precision medicine approaches to the diagnosis, prognosis, and treatment of diabetes and DKD framed by a patient partner's/caregiver's lived experience.

Key findings

Distinguishing diabetic nephropathy from CKD due to other types of DKD and non-DKD is challenging and typically requires a kidney biopsy for a diagnosis. Biomarkers have been identified to assist with the prediction of the onset and progression of DKD, but they have yet to be incorporated and evaluated relative to clinical standard of care CKD and kidney failure risk prediction tools. Genomics has identified multiple causal genetic variants for neonatal diabetes mellitus and monogenic diabetes of the young that can be used for diagnostic purposes and to specify antiglycemic therapy. Genome-wide-associated studies have identified genes implicated in DKD pathophysiology in the setting of type 1 and 2 diabetes but their translational benefits are lagging beyond polygenetic risk scores. Metabolomics and proteomics have been shown to improve diagnostic accuracy in DKD, have been used to identify novel pathways involved in DKD pathogenesis, and can be used to improve the prediction of CKD progression and kidney failure as well as predict response to DKD therapy.

Limitations

There are a limited number of large, high-quality prospective observational studies and no randomized controlled trials that support the use of precision medicine based approaches to improve clinical outcomes in adults with or at risk of diabetes and DKD. It is unclear which patients may benefit from the clinical use of genomics, metabolomics and proteomics along the spectrum of DKD trajectory.

Implications

Additional research is needed to evaluate the role of the use of precision medicine for DKD management, including diagnosis, differentiation of diabetic nephropathy from other etiologies of DKD and CKD, short-term and long-term risk prognostication kidney outcomes, and the prediction of response to and safety of disease-modifying therapies.

Analysis of Serum Advanced Glycation Endproducts Reveals Methylglyoxal-Derived Advanced Glycation MG-H1 Free Adduct Is a Risk Marker in Non-Diabetic and Diabetic Chronic Kidney Disease

Accumulation of advanced glycation endproducts (AGEs) is linked to decline in renal function, particularly in patients with diabetes. Major forms of AGEs in serum are protein-bound AGEs and AGE free adducts. In this study, we assessed levels of AGEs in subjects with and without diabetes, with normal renal function and stages 2 to 4 chronic kidney disease (CKD), to identify which AGE has the greatest progressive change with decline in renal function and change in diabetes. We performed a cross-sectional study of patients with stages 2-4 CKD, with and without diabetes, and healthy controls (n = 135). Nine protein-bound and free adduct AGEs were quantified in serum. Most protein-bound AGEs increased moderately through stages 2-4 CKD whereas AGE free adducts increased markedly. Methylglyoxal-derived hydroimidazolone MG-H1 free adduct was the AGE most responsive to CKD status, increasing 8-fold and 30-fold in stage 4 CKD in patients without and with diabetes, respectively. MG-H1 Glomerular filtration flux was increased 5-fold in diabetes, likely reflecting increased methylglyoxal glycation status. We conclude that serum MG-H1 free adduct concentration was strongly related to stage of CKD and increased in diabetes status. Serum MG-H1 free adduct is a candidate AGE risk marker of non-diabetic and diabetic CKD.

Structural brain abnormalities in children and young adults with severe chronic kidney disease

BACKGROUND

The pathophysiology of neurological dysfunction in severe chronic kidney disease (CKD) in children and young adults is largely unknown. We aimed to investigate brain volumes and white matter integrity in this population and explore brain structure under different treatment modalities.

METHODS

This cross-sectional study includes 24 patients with severe CKD (eGFR < 30) aged 8-30 years (median = 18.5, range = 9.1-30.5) on different therapy modalities (pre-dialysis, n = 7; dialysis, n = 7; transplanted, n = 10) and 21 healthy controls matched for age, sex, and parental educational level. Neuroimaging targeted brain volume using volumetric analysis on T1 scans and white matter integrity with tract-based spatial statistics and voxel-wise regression on diffusion tensor imaging (DTI) data.

RESULTS

CKD patients had lower white matter integrity in a widespread cluster of primarily distal white matter tracts compared to healthy controls. Furthermore, CKD patients had smaller volume of the nucleus accumbens relative to healthy controls, while no evidence was found for abnormal volumes of gray and white matter or other subcortical structures. Longer time since successful transplantation was related to lower white matter integrity. Exploratory analyses comparing treatment subgroups suggest lower white matter integrity and smaller volume of the nucleus accumbens in dialysis and transplanted patients relative to healthy controls.

CONCLUSIONS

Young CKD patients seem at risk for widespread disruption of white matter integrity and to some extent smaller subcortical volume (i.e., nucleus accumbens). Especially patients on dialysis therapy and patients who received a kidney transplant may be at risk for disruption of white matter integrity and smaller volume of the nucleus accumbens.

Nuclear receptor PXR targets AKR1B7 to protect mitochondrial metabolism and renal function in AKI

Acute kidney injury (AKI) is a worldwide public health problem with no specific and satisfactory therapies in clinic. The nuclear pregnane X receptor (PXR) is involved in the progression of multiple diseases, including metabolic diseases, atherosclerosis, hypertension, liver injury, etc. However, its role in kidney injury remains to be understood. In this study, we have investigated the role of PXR in AKI and underlying mechanism(s) involved in its function. PXR was robustly down-regulated and negatively correlated with renal dysfunction in human and animal kidneys with AKI. Silencing PXR in rats enhanced cisplatin-induced AKI and induced severe mitochondrial abnormalities, whereas activating PXR protected against AKI. Using luciferase reporter assays, genomic manipulation, and proteomics data analysis on the kidneys of PXR^-/- rats, we determined that PXR targeted Aldo-keto reductase family 1, member B7 (AKR1B7) to improve mitochondrial function, thereby ameliorating AKI. We confirmed the protective role of PXR against kidney injury using genomic and pharmacologic approaches in an ischemia/reperfusion model of AKI. These findings demonstrate that disabling the PXR/AKR1B7/mitochondrial metabolism axis is an important factor that can contribute to AKI, whereas reestablishing this axis can be useful for treating AKI.

Uraemic toxins impair skeletal muscle regeneration by inhibiting myoblast proliferation, reducing myogenic differentiation, and promoting muscular fibrosis

Uraemic toxins increase in serum parallel to a decline in the glomerular filtration rate and the development of sarcopenia in patients with chronic kidney disease (CKD). This study analyses the role of uraemic toxins in sarcopenia at different stages of CKD, evaluating changes in the muscular regeneration process. Cultured C₂C₁₂ cells were incubated with a combination of indoxyl sulphate and p-cresol at high doses (100 µg/mL) or low doses (25 µg/mL and 10 µg/mL) resembling late or early CKD stages, respectively. Cell proliferation (analysed by scratch assays and flow cytometry) was inhibited only by high doses of uraemic toxins, which inactivated the cdc2-cyclin B complex, inhibiting mitosis and inducing apoptosis (analysed by annexin V staining). By contrast, low doses of uraemic toxins did not affect proliferation, but reduced myogenic differentiation, primed with 2% horse serum, by inhibiting myogenin expression and promoting fibro-adipogenic differentiation. Finally, to assess the in vivo relevance of these results, studies were performed in gastrocnemii from uraemic rats, which showed higher collagen expression and lower myosin heavy chain expression than those from healthy rats. In conclusion, uraemic toxins impair the skeletal muscular regeneration process, even at low concentrations, suggesting that sarcopenia can progress from the early stages of CKD.

Association of tubular solute clearances with the glomerular filtration rate and complications of chronic kidney disease: the Chronic Renal Insufficiency Cohort study

BACKGROUND

The secretion of organic solutes by the proximal tubules is an essential intrinsic kidney function. The degree to which secretory solute clearance corresponds with the glomerular filtration rate (GFR) and potential metabolic implications of net secretory clearance are largely unknown.

METHODS

We evaluated 1240 participants with chronic kidney disease (CKD) from the multicenter Chronic Renal Insufficiency Cohort (CRIC) Study. We used targeted mass-spectrometry to quantify candidate secretory solutes in paired 24-h urine and plasma samples. CRIC study personnel measured GFR using 125I-iothalamate clearance (iGFR). We used correlation and linear regression to determine cross-sectional associations of secretory clearances with iGFR and common metabolic complications of CKD.

RESULTS

Correlations between iGFR and secretory solute clearances ranged from ρ  = +0.30 for hippurate to ρ = +0.58 for kynurenic acid. Lower net clearances of most secretory solutes were associated with higher serum concentrations of parathyroid hormone (PTH), triglycerides and uric acid. Each 50% lower kynurenic acid clearance was associated with a 21% higher serum PTH concentration [95% confidence interval (CI) 15-26%] and a 10% higher serum triglyceride concentration (95% CI 5-16%) after adjustment for iGFR, albuminuria and other potential confounders. Secretory solute clearances were not associated with statistically or clinically meaningful differences in serum calcium, phosphate, hemoglobin or bicarbonate concentrations.

CONCLUSIONS

Tubular secretory clearances are modestly correlated with measured GFR among adult patients with CKD. Lower net secretory clearances are associated with selected metabolic complications independent of GFR and albuminuria, suggesting potential clinical and biological relevance.

Elevated suPAR Is an Independent Risk Marker for Incident Kidney Disease in Acute Medical Patients

Introduction

Identifying patients at high risk of developing kidney disease could lead to early clinical interventions that prevent or slow disease progression. Soluble urokinase plasminogen activator receptor (suPAR) is an inflammatory biomarker thought to be involved in the pathogenesis and development of kidney disease. We aimed to determine whether elevated plasma suPAR measured at hospital admission is associated with incident kidney disease in patients presenting to the emergency department.

Materials and Methods

This was a retrospective registry-based cohort study performed at the Emergency Department of Copenhagen University Hospital Amager and Hvidovre, Hvidovre, Denmark. Patients were included in the study from November 2013 to March 2017 and followed until June 2017. Patients were excluded if they were diagnosed with kidney disease or died prior to index discharge. Plasma suPAR was measured at hospital admission, and the main outcome was time to incident kidney disease, defined by ICD-10 diagnosis codes for both chronic and acute kidney conditions. Association between suPAR and time to incident kidney disease was assessed by Cox proportional hazard regression analysis.

Results

In total, 25,497 patients (median age 58.1 years; 52.5% female) were admitted to the emergency department and followed for development of kidney disease. In multivariable Cox regression analysis adjusting for age, sex, eGFR, CRP, cardiovascular disease, hypertension, and diabetes, each doubling in suPAR at hospital admission was associated with a hazard ratio of 1.57 (95% CI: 1.38–1.78, P < 0.001) for developing a chronic kidney condition and 2.51 (95% CI: 2.09–3.01, P < 0.001) for developing an acute kidney condition.

Discussion

In a large cohort of acutely hospitalized medical patients, elevated suPAR was independently associated with incident chronic and acute kidney conditions. This highlights the potential for using suPAR in risk classification models to identify high-risk patients who could benefit from early clinical interventions. The main limitation of this study is its reliance on accurate reporting of ICD-10 codes for kidney disease.

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.

An Equation Based on Fuzzy Mathematics to Assess the Timing of Haemodialysis Initiation

In order to develop an equation that integrates multiple clinical factors including signs and symptoms associated with uraemia to assess the initiation of dialysis, we conducted a retrospective cohort study including 25 haemodialysis centres in Mainland China. Patients with ESRD (n = 1281) who commenced haemodialysis from 2008 to 2011 were enrolled in the development cohort, whereas 504 patients who began haemodialysis between 2012 and 2013 were enrolled in the validation cohort comprised. An artificial neural network model was used to select variables, and a fuzzy neural network model was then constructed using factors affecting haemodialysis initiation as input variables and 3-year survival as the output variable. A logistic model was set up using the same variables. The equation’s performance was compared with that of the logistic model and conventional eGFR-based assessment. The area under the bootstrap-corrected receiver-operating characteristic curve of the equation was 0.70, and that of two conventional eGFR-based assessments were 0.57 and 0.54. In conclusion, the new equation based on Fuzzy mathematics, covering laboratory and clinical variables, is more suitable for assessing the timing of dialysis initiation in a Chinese ESRD population than eGFR, and may be a helpful tool to quantitatively evaluate the initiation of haemodialysis.

Evolution of protein-bound uremic toxins indoxyl sulphate and p-cresyl sulphate in acute kidney injury

BACKGROUND

There is a gradual increase in serum concentrations of protein-bound colon-derived uremic toxins indoxyl sulphate (IxS) and p-cresyl sulphate (pCS) as chronic kidney disease (CKD) progresses. In acute kidney injury (AKI), up till now, the retention pattern has not been studied.

METHODS

In this study, 194 adult patients admitted with sepsis to the intensive care unit were included. IxS, pCS and serum creatinine (sCrea) were quantified at inclusion (D0) and at day 4, unless follow-up ended earlier (D_end).

RESULTS

Serum levels of sCrea (P < 0.001), IxS (P < 0.001) and pCS (P < 0.05) were higher in patients with AKI according to RIFLE classification at D0. In contrast with sCrea, IxS and pCS levels only increased from stage I (IxS) and F (pCS) on. When grouped according to evolution in RIFLE class from D0 to D_end, all solute concentrations were higher (P < 0.001) in the group with unfavourable evolution. In this group, there was a marked rise in sCrea (P < 0.001), a moderate one for pCS (P < 0.05), but no change for IxS (P = 0.112). There was a decrease (P < 0.001) of all solute concentrations in the group with favourable evolution. Comparing AKI with CKD patients matched for sCrea, total levels of both IxS and pCS were higher (P < 0.01) in patients with CKD.

CONCLUSIONS

Although concentrations of IxS and pCS both tend to rise in sepsis patients with AKI, their evolution does not conform with that of sCrea. For the same level of sCrea, IxS and pCS concentrations are lower in AKI compared with CKD.

Pharmacokinetic drug evaluation of letermovir prophylaxis for cytomegalovirus in hematopoietic stem cell transplantation

INTRODUCTION

Letermovir is a new antiviral approved to prevent cytomegalovirus infection in hematopoietic stem cell transplant recipients. It has a distinct mechanism of action as it acts as a terminase complex inhibitor, and shows some advantages compared to the current treatment options for cytomegalovirus infection. Areas covered: This review focuses on the efficacy, safety, pharmacokinetics, pharmacodynamics, and drug-drug interactions of letermovir. Expert opinion: Letermovir is a new antiviral to prevent cytomegalovirus infection. Unlike the currently used polymerase inhibitors, it has a distinct mechanism of action with better safety, limited resistance, and no cross-resistance. Although a lot of research on pharmacokinetics and drug-drug interactions has already been performed, it might be useful to clarify the effect of letermovir on voriconazole exposure, the drug-drug interaction between caspofungine and letermovir and the effect of statins on letermovir exposure. Also, the lack of an exposure-response relationship should be confirmed in large real-life post-marketing studies in order to be able to lower the intravenous dose of letermovir.

Indoxyl Sulphate is Associated with Atrial Fibrillation Recurrence after Catheter Ablation

Renal dysfunction results in the accumulation of various uremic toxins, including indoxyl sulphate (IS), and is a major risk factor for atrial fibrillation (AF). Experimental studies have demonstrated that IS exacerbates atrial remodelling via oxidative stress, inflammation, and fibrosis. However, its clinical impact on AF-promoting cardiac remodelling has not been described. Therefore, the purpose of this study was to clarify the relationship between basal IS levels and the 1-year outcomes after catheter ablation for the treatment of AF. Our prospective observational study included data from 125 patients with AF who underwent catheter ablation. Over a 1-year follow-up period, AF recurrence was identified in 21 patients. The 1-year AF-free survival was significantly lower in patients with high serum IS levels (≥0.65 μg/mL) than in those with low IS levels (60.1 ± 10.4% versus 85.2 ± 3.9%, P = 0.007). Univariable analysis identified that an IS concentration ≥ 0.65 μg/mL was associated with AF recurrence (hazard ratio [HR] = 3.10 [1.26-7.32], P = 0.015), and this association was maintained in multivariate analysis (HR = 3.67 [1.13-11.7], P = 0.031). Thus, in patients undergoing AF ablation, serum IS levels at baseline independently predict the recurrence of arrhythmia.

Association of Uremic Toxins and Inflammatory Markers with Physical Performance in Dialysis Patients

Association of higher serum levels of uremic toxins and inflammatory markers with poorer physical performance is understudied. We measured the six-minute walk test (6MWT), 10 repetition sit-to-stand test (STS-10), handgrip strength (HGS), and Human Activity Profile (HAP) questionnaire score in 90 prevalent hemodialysis patents, with low comorbidity to reduce the potential confounding of concomitant disease. Midweek pre-dialysis serum levels of asymmetric dimethyl-arginine (ADMA), β2-microglobulin (B2M), high-sensitivity C-reactive protein (hs-CRP), indoxyl sulfate (IS), insulin-like growth factor 1 (IGF-1), interleukin 6 (IL-6), myostatin, and urea were analyzed as predictor parameters of physical performance measures in adjusted models. Serum levels of most measured toxins were not significantly related to performance, except for ADMA, which was significantly related to poorer performance in the STS-10 test (B = 0.11 ± 0.03 s, p < 0.01). Higher hs-CRP was associated with poorer results in the 6MWT (B = −2.6 ± 0.97 m, p < 0.01) and a lower HAP score (B = −0.36 ± 0.14, p = 0.01). There were no other significant associations found. We conclude that inflammation may be a more important pathway to physical impediment than uremic toxemia. This suggests that there is a large physical rehabilitation potential in non-inflamed uremic patients.

Metabolic Abnormalities in Diabetes and Kidney Disease: Role of Uremic Toxins

PURPOSE OF REVIEW

Chronic kidney disease (CKD) is characterized by the accumulation of uremic retention solutes (URS) and is associated with perturbations of glucose homeostasis even in absence of diabetes. The underlying mechanisms of insulin resistance, β cell failure, and increase risk of diabetes in CKD, however, remain unclear. Metabolomic studies reported that some metabolites are similar in CKD and diabetic kidney disease (DKD) and contribute to the progression to end-stage renal disease. We attempted to discuss the mechanisms involved in the disruption of carbohydrate metabolism in CKD by focusing on the specific role of URS.

RECENT FINDINGS

Recent clinical data have demonstrated a defect of insulin secretion in CKD. Several studies highlighted the direct role of some URS (urea, trimethylamine N-oxide (TMAO), p-cresyl sulfate, 3-carboxylic acid 4-methyl-5-propyl-2-furan propionic (CMPF)) in glucose homeostasis abnormalities and diabetes incidence. Gut dysbiosis has been identified as a potential contributor to diabetes and to the production of URS. The complex interplay between the gut microbiota, kidney, pancreas β cell, and peripheral insulin target tissues has brought out new hypotheses for the pathogenesis of CKD and DKD. The characterization of intestinal microbiota and its associated metabolites are likely to fill fundamental knowledge gaps leading to innovative research, clinical trials, and new treatments for CKD and DKD.

Gut Microbiota and Cardiovascular Uremic Toxicities

Cardiovascular disease (CVD) remains a major cause of high morbidity and mortality in patients with chronic kidney disease (CKD). Numerous CVD risk factors in CKD patients have been described, but these do not fully explain the high pervasiveness of CVD or increased mortality rates in CKD patients. In CKD the loss of urinary excretory function results in the retention of various substances referred to as "uremic retention solutes". Many of these molecules have been found to exert toxicity on virtually all organ systems of the human body, leading to the clinical syndrome of uremia. In recent years, an increasing body of evidence has been accumulated that suggests that uremic toxins may contribute to an increased cardiovascular disease (CVD) burden associated with CKD. This review examined the evidence from several clinical and experimental studies showing an association between uremic toxins and CVD. Special emphasis is addressed on emerging data linking gut microbiota with the production of uremic toxins and the development of CKD and CVD. The biological toxicity of some uremic toxins on the myocardium and the vasculature and their possible contribution to cardiovascular injury in uremia are also discussed. Finally, various therapeutic interventions that have been applied to effectively reduce uremic toxins in patients with CKD, including dietary modifications, use of prebiotics and/or probiotics, an oral intestinal sorbent that adsorbs uremic toxins and precursors, and innovative dialysis therapies targeting the protein-bound uremic toxins are also highlighted. Future studies are needed to determine whether these novel therapies to reduce or remove uremic toxins will reduce CVD and related cardiovascular events in the long-term in patients with chronic renal failure.

Uremic Toxins and Clinical Outcomes: The Impact of Kidney Transplantation

Non-transplanted and transplanted patients with chronic kidney disease (CKD) differ in terms of mortality and the risk of clinical events. This difference is probably due to the difference of both traditional and non-traditional risk factors. Uremic retention solutes may constitute important non-traditional risk factors in this population. In the present review, we selected a set of uremic toxins that have been associated with harmful effects, and are an appealing target for adjuvant therapy in CKD. For each toxin reviewed here, relevant studies were selected and the relationship with hard clinical outcomes of uremic toxins were compared between non-transplanted CKD patients and transplanted patients taking into account the level of glomerular filtration rate in these two situations.

Accumulation of uraemic toxins is reflected only partially by estimated GFR in paediatric patients with chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) in childhood is characterised by the accumulation of uraemic toxins resulting in a multisystem disorder that has a negative impact on quality of life. Childhood CKD is predominantly defined by a decrease in glomerular filtration rate, estimated (eGFR) by a single serum measurement of endogenous biomarkers, e.g. creatinine. The objective of this study was to evaluate how accurately eGFR predicts the concentration of uraemic toxins in a paediatric CKD cohort.

METHODS

In 65 children (10.8 [5.1; 14.7] years) with CKD (eGFR 44 [20; 64] mL/min/1.73 m²), serum concentrations were determined of small solutes (uric acid [UA], urea, symmetric dimethylarginine [SDMA], asymmetric dimethylarginine [ADMA]), middle molecules (β2-microglobulin [β2M], complement factor D [CfD]) and protein-bound solutes (p-cresylglucuronide [pCG], hippuric acid, indole acetic acid, indoxyl sulphate [IxS], p-cresylsulfate [pCS] and 3-carboxy-4-methyl-5-propyl-furanpropionic acid [CMPF]). Spearman's correlation coefficients (r) were calculated to correlate uraemic toxin concentrations with three different eGFR equations, based on either serum creatinine or β2M.

RESULTS

Updated Schwartz eGFR was correlated reasonably well with concentrations of creatinine (r = -0.98), urea (r_s = -0.84), SDMA (r = -0.82) and middle molecules CfD and β2M (both r_s = -0.90). In contrast, poor correlation coefficients were found for CMPF (r_s = -0.32), UA (r_s = -0.45), ADMA (r_s = -0.47) and pCG (r_s = -0.48). The other toxins, all protein-bound, had r_s between -0.75 and -0.57. Comparable correlations were found between the three evaluated eGFR equations and uraemic toxin concentrations.

CONCLUSIONS

This study demonstrates that eGFR poorly predicts concentrations of protein-bound uraemic toxins, UA and ADMA in childhood CKD. Therefore, eGFR only partially reflects the complexity of the accumulation pattern of uraemic toxins in childhood CKD.

Establishing the presence or absence of chronic kidney disease: Uses and limitations of formulas estimating the glomerular filtration rate

The development of formulas estimating glomerular filtration rate (eGFR) from serum creatinine and cystatin C and accounting for certain variables affecting the production rate of these biomarkers, including ethnicity, gender and age, has led to the current scheme of diagnosing and staging chronic kidney disease (CKD), which is based on eGFR values and albuminuria. This scheme has been applied extensively in various populations and has led to the current estimates of prevalence of CKD. In addition, this scheme is applied in clinical studies evaluating the risks of CKD and the efficacy of various interventions directed towards improving its course. Disagreements between creatinine-based and cystatin-based eGFR values and between eGFR values and measured GFR have been reported in various cohorts. These disagreements are the consequence of variations in the rate of production and in factors, other than GFR, affecting the rate of removal of creatinine and cystatin C. The disagreements create limitations for all eGFR formulas developed so far. The main limitations are low sensitivity in detecting early CKD in several subjects, e.g., those with hyperfiltration, and poor prediction of the course of CKD. Research efforts in CKD are currently directed towards identification of biomarkers that are better indices of GFR than the current biomarkers and, particularly, biomarkers of early renal tissue injury.

Chronic Kidney Disease

The definition and classification of chronic kidney disease (CKD) have evolved over time, but current international guidelines define this condition as decreased kidney function shown by glomerular filtration rate (GFR) of less than 60 mL/min per 1·73 m², or markers of kidney damage, or both, of at least 3 months duration, regardless of the underlying cause. Diabetes and hypertension are the main causes of CKD in all high-income and middle-income countries, and also in many low-income countries. Incidence, prevalence, and progression of CKD also vary within countries by ethnicity and social determinants of health, possibly through epigenetic influence. Many people are asymptomatic or have non-specific symptoms such as lethargy, itch, or loss of appetite. Diagnosis is commonly made after chance findings from screening tests (urinary dipstick or blood tests), or when symptoms become severe. The best available indicator of overall kidney function is GFR, which is measured either via exogenous markers (eg, DTPA, iohexol), or estimated using equations. Presence of proteinuria is associated with increased risk of progression of CKD and death. Kidney biopsy samples can show definitive evidence of CKD, through common changes such as glomerular sclerosis, tubular atrophy, and interstitial fibrosis. Complications include anaemia due to reduced production of erythropoietin by the kidney; reduced red blood cell survival and iron deficiency; and mineral bone disease caused by disturbed vitamin D, calcium, and phosphate metabolism. People with CKD are five to ten times more likely to die prematurely than they are to progress to end stage kidney disease. This increased risk of death rises exponentially as kidney function worsens and is largely attributable to death from cardiovascular disease, although cancer incidence and mortality are also increased. Health-related quality of life is substantially lower for people with CKD than for the general population, and falls as GFR declines. Interventions targeting specific symptoms, or aimed at supporting educational or lifestyle considerations, make a positive difference to people living with CKD. Inequity in access to services for this disease disproportionally affects disadvantaged populations, and health service provision to incentivise early intervention over provision of care only for advanced CKD is still evolving in many countries.

Uraemia: an unrecognized driver of central neurohumoral dysfunction in chronic kidney disease?

Chronic kidney disease (CKD) carries a large cardiovascular burden in part due to hypertension and neurohumoral dysfunction - manifesting as sympathetic overactivity, baroreflex dysfunction and chronically elevated circulating vasopressin. Alterations within the central nervous system (CNS) are necessary for the expression of neurohumoral dysfunction in CKD; however, the underlying mechanisms are poorly defined. Uraemic toxins are a diverse group of compounds that accumulate as a direct result of renal disease and drive dysfunction in multiple organs, including the brain. Intensive haemodialysis improves both sympathetic overactivity and cardiac baroreflex sensitivity in renal failure patients, indicating that uraemic toxins participate in the maintenance of autonomic dysfunction in CKD. In rodents exposed to uraemia, immediate early gene expression analysis suggests upregulated activity of not only pre-sympathetic but also vasopressin-secretory nuclei. We outline several potential mechanisms by which uraemia might drive neurohumoral dysfunction in CKD. These include superoxide-dependent effects on neural activity, depletion of nitric oxide and induction of low-grade systemic inflammation. Recent evidence has highlighted superoxide production as an intermediate for the depolarizing effect of some uraemic toxins on neuronal cells. We provide preliminary data indicating augmented superoxide production within the hypothalamic paraventricular nucleus in the Lewis polycystic kidney rat, which might be important for mediating the neurohumoral dysfunction exhibited in this CKD model. We speculate that the uraemic state might serve to sensitize the central actions of other sympathoexcitatory factors, including renal afferent nerve inputs to the CNS and angiotensin II, by way of recruiting convergent superoxide-dependent and pro-inflammatory pathways.

Uremic Solutes in Chronic Kidney Disease and Their Role in Progression

Background

To date, over 150 possible uremic solutes have been listed, but their role in the progression of CKD is largely unknown. Here, the association between a selected panel of uremic solutes and progression in CKD patients was investigated.

Methods

Patients from the MASTERPLAN study, a randomized controlled trial in CKD patients with a creatinine clearance between 20 and 70 ml/min per 1.73m², were selected based on their rate of eGFR decline during the first five years of follow-up. They were categorized as rapid (decline >5 ml/min per year) or slow progressors. Concentrations of eleven uremic solutes were obtained at baseline and after one year of follow-up. Logistic regression was used to compare the odds for rapid to slow progression by uremic solute concentrations at baseline. Variability in uremic solute levels was assessed using scatter plots, and limits of variability were calculated.

Results

In total, 40 rapidly and 40 slowly progressing patients were included. Uremic solutes were elevated in all patients compared to reference values for healthy persons. The serum levels of uremic solutes were not associated with rapid progression. Moreover, we observed substantial variability in solute levels over time.

Conclusions

Elevated concentrations of uremic solutes measured in this study did not explain differences in rate of eGFR decline in CKD patients, possibly due to lack of power as a result of the small sample size, substantial between patient variability, and variability in solute concentrations over time. The etiology of intra-individual variation in uremic solute levels remains to be elucidated.

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

Background

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

Methods

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

Results

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

Conclusions

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

Tryptophan metabolism, its relation to inflammation and stress markers and association with psychological and cognitive functioning: Tasmanian Chronic Kidney Disease pilot study

Background

Adults with chronic kidney disease (CKD) exhibit alterations in tryptophan metabolism, mainly via the kynurenine pathway, due to higher enzymatic activity induced mainly by inflammation. Indoles produced by gut-microflora are another group of tryptophan metabolites related to inflammation and conditions accompanying CKD. Disruptions in tryptophan metabolism have been associated with various neurological and psychological disorders. A high proportion of CKD patients self-report symptoms of depression and/or anxiety and decline in cognitive functioning. This pilot study examines tryptophan metabolism in CKD and explores associations with psychological and cognitive functioning.

Methods

Twenty-seven adults with CKD were part of 49 patients recruited to participate in a prospective pilot study, initially with an eGFR of 15–29 mL/min/1.73 m². Only participants with viable blood samples and complete psychological/cognitive data at a 2-year follow-up were included in the reported cross-sectional study. Serum samples were analysed by Liquid Chromatography coupled to Mass Spectrometry, for tryptophan, ten of its metabolites, the inflammation marker neopterin and the hypothalamic–pituitary–adrenal (HPA) axis marker cortisol.

Results

The tryptophan breakdown index (kynurenine / tryptophan) correlated with neopterin (Pearson R = 0.51 P = 0.006) but not with cortisol. Neopterin levels also correlated with indoxyl sulfate (R = 0.68, P < 0.0001) and 5 metabolites of tryptophan (R range 0.5–0.7, all P ≤ 0.01), which were all negatively related to eGFR (P < 0.05). Higher levels of kynurenic acid were associated with lower cognitive functioning (Spearman R = −0.39, P < 0.05), while indole-3 acetic acid (IAA) was correlated with anxiety and depression (R = 0.52 and P = 0.005, R = 0.39 and P < 0.05, respectively).

Conclusions

The results of this preliminary study suggest the involvement of inflammation in tryptophan breakdown via the kynurenine pathway, yet without sparing tryptophan metabolism through the 5-HT (serotonin) pathway in CKD patients. The multiple moderate associations between indole-3 acetic acid and psychological measures were a novel finding. The presented pilot data necessitate further exploration of these associations within a large prospective cohort to assess the broader significance of these findings.

Electronic supplementary material

The online version of this article (doi:10.1186/s12882-016-0387-3) contains supplementary material, which is available to authorized users.

Indoxyl sulphate and kidney disease: Causes, consequences and interventions

In the last decade, chronic kidney disease (CKD), defined as reduced renal function (glomerular filtration rate (GFR) < 60 mL/min per 1.73 m(2) ) and/or evidence of kidney damage (typically manifested as albuminuria) for at least 3 months, has become one of the fastest-growing public health concerns worldwide. CKD is characterized by reduced clearance and increased serum accumulation of metabolic waste products (uremic retention solutes). At least 152 uremic retention solutes have been reported. This review focuses on indoxyl sulphate (IS), a protein-bound, tryptophan-derived metabolite that is generated by intestinal micro-organisms (microbiota). Animal studies have demonstrated an association between IS accumulation and increased fibrosis, and oxidative stress. This has been mirrored by in vitro studies, many of which report cytotoxic effects in kidney proximal tubular cells following IS exposure. Clinical studies have associated IS accumulation with deleterious effects, such as kidney functional decline and adverse cardiovascular events, although causality has not been conclusively established. The aims of this review are to: (i) establish factors associated with increased serum accumulation of IS; (ii) report effects of IS accumulation in clinical studies; (iii) critique the reported effects of IS in the kidney, when administered both in vivo and in vitro; and (iv) summarize both established and hypothetical therapeutic options for reducing serum IS or antagonizing its reported downstream effects in the kidney.

Age-dependent increase in serum levels of indoxyl sulphate and p-cresol sulphate is not related to their precursors: Tryptophan and tyrosine

AIM

Retention of indoxyl sulphate and p-cresol sulphate is associated with many diseases. The aim of the present study was to examine serum levels of indoxyl sulphate and p-cresol sulphate, the dynamics of their changes according to age, and their precursors.

METHODS

The study included 180 healthy individuals aged 20-90 years (n = 180), divided into subgroups by decade (n = 30 in each subgroup) and into subgroups of ≥65 years (n = 42) or <65 years (n = 138). Serum indoxyl sulphate and p-cresol sulphate, tryptophan, and tyrosine were measured using high-performance liquid chromatography-mass spectrometry.

RESULTS

The 70-90 years age group had higher indoxyl sulphate than the 50-59 years age group (P = 0.033). The 70-90 years age group had higher p-cresol sulphate than the 20-29 years (P < 0.001), 30-39 years (P < 0.001), 40-49 years (P = 0.007) and 50-59 years (P = 0.001) age groups; the 60-69 years age group had higher p-cresol sulphate than the 20-29 years (P = 0.043) and 30-39 years (P = 0.011) age groups. Indoxyl sulphate and p-cresol sulphate serum levels were higher in those aged ≥65 years. Indoxyl sulphate and p-cresol sulphate serum levels correlated positively with age, but not with tryptophan and tyrosine, respectively.

CONCLUSIONS

Healthy aging is associated with indoxyl sulphate and p-cresol sulphate serum level increases, which are not linked to tryptophan and tyrosine serum levels. Geriatr Gerontol Int 2017; 17: 1022-1026.

Identification of biomarkers for development of end-stage kidney disease in chronic kidney disease by metabolomic profiling

A critical issue in the management of chronic kidney disease (CKD) is to prevent patients from the progression to end-stage kidney disease (ESKD), however, there is only limited number of biomarkers for the discrimination of the high-risk CKD patients. We aimed to identify the metabolites which possess the ability to predict the earlier kidney deterioration. We performed capillary electrophoresis and liquid chromatography mass spectrometry (CE-MS)-based metabolic profiling in a prospective cohort, which consisted of referred 112 CKD patients with median follow-up period of 4.4 years. The association between the levels of candidate metabolites and the outcomes (progression to ESKD alone or in combination with death before ESKD) were assessed by multivariate Cox proportional hazard models after adjusting for the baseline covariates. A total of 218 metabolites were detected in the plasma of CKD patients. We identified 16 metabolites which have predictive values for the composite outcome: The risk for composite outcome was elevated from 2.0- to 8.0-fold in those with higher levels of 16 plasma metabolites. Our results suggest that the measurement of these metabolites may facilitate CKD management by predicting the risk of progression to ESKD.

Signaling of Serum Amyloid A Through Receptor for Advanced Glycation End Products as a Possible Mechanism for Uremia-Related Atherosclerosis

OBJECTIVE

Cardiovascular disease is the leading cause of death in patients with end-stage renal disease. Serum amyloid A (SAA) is an acute phase protein and a binding partner for the multiligand receptor for advanced glycation end products (RAGE). We investigated the role of the interaction between SAA and RAGE in uremia-related atherogenesis.

APPROACH AND RESULTS

We used a mouse model of uremic vasculopathy, induced by 5 of 6 nephrectomy in the Apoe(-/-) background. Sham-operated mice were used as controls. Primary cultures of Ager(+/+) and Ager(-/-) vascular smooth muscle cells (VSMCs) were stimulated with recombinant SAA, S100B, or vehicle alone. Relevance to human disease was assessed with human VSMCs. The surface area of atherosclerotic lesions at the aortic roots was larger in uremic Apoe(-/-) than in sham-operated Apoe(-/-) mice (P<0.001). Furthermore, atherosclerotic lesions displayed intense immunostaining for RAGE and SAA, with a pattern similar to that of α-SMA. Ager transcript levels in the aorta were 6× higher in uremic animals than in controls (P<0.0001). Serum SAA concentrations were higher in uremic mice, not only after 4 weeks of uremia but also at 8 and 12 weeks of uremia, than in sham-operated animals. We investigated the functional role of RAGE in uremia-induced atherosclerosis further, in animals lacking RAGE. We found that the induction of uremia in Apoe(-/-) Ager(-/-) mice did not accelerate atherosclerosis. In vitro, the stimulation of Ager(+/+) but not of Ager(-/-) VSMCs with SAA or S100B significantly induced the production of reactive oxygen species, the phosphorylation of AKT and mitogen-activated protein kinase-extracellular signal-regulated kinases and cell migration. Reactive oxygen species inhibition with N-acetyl cysteine significantly inhibited both the phosphorylation of AKT and the migration of VSMCs. Similar results were obtained for human VSMCs, except that the phosphorylation of mitogen-activated protein kinase-extracellular signal-regulated kinases, rather than of AKT, was subject to specific redox-regulation by SAA and S100B. Furthermore, human aortic atherosclerotic sections were positively stained for RAGE and SAA.

CONCLUSIONS

Uremia upregulates SAA and RAGE expression in the aortic wall and in atherosclerotic lesions in mice. Ager(-/-) animals are protected against the uremia-induced acceleration of atherosclerosis. SAA modulates the functions of murine and human VSMCs in vitro in a RAGE-dependent manner. This study, therefore, identifies SAA as a potential new uremic toxin involved in uremia-related atherosclerosis through interaction with RAGE.

The rebirth of interest in renal tubular function

The measurement of glomerular filtration rate by the clearance of inulin or creatinine has evolved over the past 50 years into an estimated value based solely on plasma creatinine concentration. We have examined some of the misconceptions and misunderstandings of the classification of renal disease and its course, which have followed this evolution. Furthermore, renal plasma flow and tubular function, which in the past were estimated by the clearance of the exogenous aryl amine, para-aminohippurate, are no longer measured. Over the past decade, studies in experimental animals with reduced nephron mass and in patients with reduced renal function have identified small gut-derived, protein-bound uremic retention solutes ("uremic toxins") that are poorly filtered but are secreted into the lumen by organic anion transporters (OATs) in the proximal renal tubule. These are not effectively removed by conventional hemodialysis or peritoneal dialysis. Residual renal function, urine produced in patients with advanced renal failure or undergoing dialysis treatment, may represent, at least in part, secretion of fluid and uremic toxins, such as indoxyl sulfate, mediated by proximal tubule OATs and might serve as a useful survival function. In light of this new evidence of the physiological role of proximal tubule OATs, we suggest that measurement of renal tubular function and renal plasma flow may be of considerable value in understanding and managing chronic kidney disease. Data obtained in normal subjects indicate that renal plasma flow and renal tubular function might be measured by the clearance of the endogenous aryl amine, hippurate.

Proteinuria Is an Independent Risk Factor for First Incident Stroke in Adults Under Treatment for Hypertension in China

Background

Conflicting evidence exists regarding whether reduced estimated glomerular filtration rate (eGFR) and proteinuria are independent risk factors for stroke and its subtypes in hypertensive patients. This study investigated the association of these renal measures with first incident stroke in adults under treatment for hypertension in China.

Methods and Results

The study included 19 599 adults aged 45 to 75 years who participated in the China Stroke Primary Prevention Trial. Baseline eGFR was calculated using the Chronic Kidney Disease Epidemiology Collaboration creatinine equation. Proteinuria was assessed by qualitative dipstick urinalysis and in a subset by the quantitative albumin–creatinine ratio method. Cox regression analysis was used to examine the effects of eGFR and proteinuria on the risk of first incident stroke. During a median of 4.5 years of follow‐up, a total of 585 first strokes (472 ischemic strokes) were identified. Compared to participants without proteinuria, participants with proteinuria (trace or more by dipstick) had a 35% increased risk of first stroke: the adjusted hazard ratio (HR) (95% CI) was 1.35 (1.09–1.66, P=0.005). The results were robust in subgroup analyses. In a subset with data on proteinuria measured by quantitative albumin–creatinine ratio, a similar association was found. In both independent and combined analyses with proteinuria, eGFR was not significantly associated with stroke.

Conclusions

In adults under treatment for hypertension in China, baseline proteinuria measured by dipstick or quantitative albumin–creatinine ratio, but not reduced eGFR, was found to be an independent risk factor for first incident stroke and ischemic stroke.

Should dialysis be offered to all elderly patients?

Nephrologist are often faced with the question of the appropriate initiation and withdrawal from dialysis. Many clinicians feel that patient should be offered dialysis when they have ESRD regardless of the potential risks vs. benefits. My position in this debate is that nephrologists have the obligation to order treatments that are indicated and effective for their patients and will provide more benefit that harm. They should not order dialysis in patient that are not likely to benefit from the treatment. Patients have the right to refuse treatments but not the right to demand that a clinician order an ineffective treatment. Shared decision making is the key principle in deciding on the initiation and withdrawal from dialysis. The national guideline; Shared Decision Making: The Appropriate Initiation and Withdrawal from Dialysis supports this approach.

Uremic toxins and their effects on multiple organ systems

Nearly all body organs and systems are affected by the toxicity of uremic compounds retained in the course of renal dysfunction. Knowledge about the origin, chemical structure and composition of the retained endogenous substances responsible for these symptoms is far from complete. Organic retention solutes present a great variety of properties which makes their accurate classification extremely difficult. Their potential toxicity remains to be elucidated with meticulous observation of clearly formulated rules guiding the process. Toxicity assessment is a complex process because not just one but several retained compounds may be simultaneously involved in the same biological and metabolic processes. The search for new uremic compounds and combining them into panels of substances involved in the same pathophysiological processes seems to offer a novel approach to identifying and explaining any so far unexplored specific effects of endogenous compounds on the body organs and systems.

Effect of uraemia on endothelial cell damage is mediated by the integrin linked kinase pathway

KEY POINTS

Patients with chronic kidney disease have a higher risk of developing cardiovascular diseases than the general population. Their vascular endothelium is dysfunctional, among other things, because it is permanently exposed to uraemic toxins, several of which have poor clearance by conventional dialysis. Recent studies have demonstrated the important role of integrin-linked kinase (ILK) in the maintenance of endothelial integrity and in this study we investigate the involvement of ILK in the mechanism underlying vascular endothelial damage that occurs in uraemia. For the first time, we demonstrate the implication of ILK in the protection against endothelial cell damage (inhibition of proliferation, toxicity, oxidative stress and programed cell death) induced by uraemic serum from chronic kidney disease patients and uraemic toxins. This molecular mechanism may have clinical relevance because it highlights the importance of maintaining high levels of ILK activity to help preserve endothelial integrity, at least in early stages of chronic kidney disease.

ABSTRACT

Patients with chronic kidney disease (CKD) have a higher risk of developing cardiovascular diseases. Their vascular endothelium is dysfunctional, among other things, because it is permanently exposed to uraemic toxins, several of which, mostly protein-bound compounds such as indoxyl sulfate (IS) and p-cresyl sulphate, having poor clearance by conventional dialysis, induce endothelial toxicity. However, the molecular mechanism by which uraemic toxins regulate early stages of endothelial dysfunction remains unclear. Recent studies have demonstrated the important role of integrin-linked kinase (ILK) in the maintenance of endothelial integrity. In this study, we investigate the involvement of ILK in the mechanism underlying vascular endothelial damage that occurs in uraemia. First, we show that incubation of EA.hy926 cells with human uraemic serum from CKD patients upregulates ILK activity. This ILK activation also occurs when the cells are exposed to IS (25-100 μg ml(-1)), p-cresol (10-100 μg ml(-1)) or both combined, compared to human serum control. Next, we observed that high doses of both toxins together induce a slight decrease in cell proliferation and increase apoptosis and reactive oxygen species production. Interestingly, these toxic effects displayed a strong increase when the ILK protein is knocked down by small interfering RNA, even at low doses of uraemic toxins. Abrogation of AKT has demonstrated the ILK/AKT signalling pathway involved in these processes. This study has demonstrated the implication of ILK in the protection against endothelial cell damage induced by uraemic toxins, a molecular mechanism that could play a protective role in the early stages of endothelial dysfunction observed in uraemic patients.

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

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

L-Arginine and its metabolites in kidney and cardiovascular disease

L-Arginine is a semi essential amino acid synthesised from glutamine, glutamate and proline via the intestinal-renal axis in humans and most mammals. L-Arginine degradation occurs via multiple pathways initiated by arginase, nitric-oxide synthase, Arg: glycine amidinotransferase, and Arg decarboxylase. These pathways produce nitric oxide, polyamines, proline, glutamate, creatine and agmatine with each having enormous biological importance. Several disease are associated to an L-arginine impaired levels and/or to its metabolites: in particular various L-arginine metabolites may participate in pathogenesis of kidney and cardiovascular disease. L-Arginine and its metabolites may constitute both a marker of pathology progression both the rationale for manipulating L-arginine metabolism as a strategy to ameliorate these disease. A large number of studies have been performed in experimental models of kidney disease with sometimes conflicting results, which underlie the complexity of Arg metabolism and our incomplete knowledge of all the mechanisms involved. Moreover several lines of evidence demonstrate the role of L-arg metabolites in cardiovascular disease and that L-arg administration role in reversing endothelial dysfunction, which is the leading cause of cardiovascular diseases, such as hypertension and atherosclerosis. This review will discuss the implication of the mains L-arginine metabolites and L-arginine-derived guanidine compounds in kidney and cardiovascular disease considering the more recent literature in the field.

The cardiovascular effect of the uremic solute indole-3 acetic acid

In CKD, uremic solutes may induce endothelial dysfunction, inflammation, and oxidative stress, leading to increased cardiovascular risk. We investigated whether the uremic solute indole-3 acetic acid (IAA) predicts clinical outcomes in patients with CKD and has prooxidant and proinflammatory effects. We studied 120 patients with CKD. During the median study period of 966 days, 29 patients died and 35 experienced a major cardiovascular event. Kaplan-Meier analysis revealed that mortality and cardiovascular events were significantly higher in the higher IAA group (IAA>3.73 µM) than in the lower IAA group (IAA<3.73 µM). Multivariate Cox regression analysis demonstrated that serum IAA was a significant predictor of mortality and cardiovascular events after adjustments for age and sex; cholesterol, systolic BP, and smoking; C-reactive protein, phosphate, body mass index, and albumin; diastolic BP and history of cardiovascular disease; and uremic toxins p-cresyl sulfate and indoxyl sulfate. Notably, IAA level remained predictive of mortality when adjusted for CKD stage. IAA levels were positively correlated with markers of inflammation and oxidative stress: C-reactive protein and malondialdehyde, respectively. In cultured human endothelial cells, IAA activated an inflammatory nongenomic aryl hydrocarbon receptor (AhR)/p38MAPK/NF-κB pathway that induced the proinflammatory enzyme cyclooxygenase-2. Additionally, IAA increased production of endothelial reactive oxygen species. In conclusion, serum IAA may be an independent predictor of mortality and cardiovascular events in patients with CKD. In vitro, IAA induces endothelial inflammation and oxidative stress and activates an inflammatory AhR/p38MAPK/NF-κB pathway.

Cardiovascular disease relates to intestinal uptake of p-cresol in patients with chronic kidney disease

Background

Serum p-cresyl sulfate (PCS) associates with cardiovascular disease in patients with chronic kidney disease. PCS concentrations are determined by intestinal uptake of p-cresol, human metabolism to PCS and renal clearance. Whether intestinal uptake of p-cresol itself is directly associated with cardiovascular disease in patients with renal dysfunction has not been studied to date.

Methods

We performed a prospective study in patients with chronic kidney disease stage 1 – 5 (NCT00441623). Intestinal uptake of p-cresol, under steady state conditions, was estimated from 24 h urinary excretion of PCS. Primary endpoint was time to first cardiovascular event, i.e., cardiac death, myocardial infarction/ischemia, ventricular arrhythmia, cardiovascular surgery, ischemic stroke or symptomatic peripheral arterial disease. Statistical analysis was done using Kaplan-Meier estimates and Cox proportional hazard analyses.

Results

In a cohort of 200 patients, median 24 h urinary excretion of PCS amounted to 457.47 μmol (IQR 252.68 – 697.17). After a median follow-up of 52 months, 25 patients reached the primary endpoint (tertile 1/2/3: 5/6/14 events, log rank P 0.037). Higher urinary excretion of PCS was directly associated with cardiovascular events (univariate hazard ratio per 100 μmol increase: 1.112, P 0.002). In multivariate analysis, urinary excretion of PCS remained a predictor of cardiovascular events, independent of eGFR (hazard ratio 1.120, P 0.002).

Conclusions

In patients with chronic kidney disease, intestinal uptake of p-cresol associates with cardiovascular disease independent of renal function. The intestinal generation and absorption of p-cresol may be therapeutic targets to reduce cardiovascular disease risk in patients with renal dysfunction.

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

♦

AIM

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

METHODS

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

RESULTS

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

CONCLUSION

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

Dimethylarginines ADMA and SDMA: the real water-soluble small toxins?

Uremia occurs if the kidney loses the ability to eliminate toxic compounds at a sufficient rate into the urine. In 1970, N-N, N-G- and N-G,N׳-G-dimethyl-arginine (asymmetric dimethylarginine [ADMA] and symmetric dimethylarginine) were isolated from human urine. It was anticipated that both substances might be important in the pathophysiology and for the diagnosis of various pathologic states. It took 22 years, however, before this idea materialized when it was found that ADMA, which is increased in hemodialysis patients, inhibits the synthesis of the endothelial-derived relaxing factor, identified as nitric oxide. ADMA correlates with traditional and nontraditional cardiovascular risk factors and is a strong predictor of cardiovascular events and death in both patients with chronic kidney disease and in the general population. It also seems to mediate adverse cardiovascular effects of drugs such as proton pump inhibitors. To date, we have no specific pharmacologic therapy at hand to neutralize the deleterious effects of ADMA, curbing the enthusiasm for this marker and mediator of cardiovascular disease.

Uremic solutes and risk of end-stage renal disease in type 2 diabetes: metabolomic study

Here we studied plasma metabolomic profiles as determinants of progression to ESRD in patients with Type 2 diabetes (T2D). This nested case-control study evaluated 40 cases who progressed to ESRD during 8-12 years of follow-up and 40 controls who remained alive without ESRD from the Joslin Kidney Study cohort. Controls were matched with cases for baseline clinical characteristics; although controls had slightly higher eGFR and lower levels of urinary albumin excretion than T2D cases. Plasma metabolites at baseline were measured by mass spectrometry-based global metabolomic profiling. Of the named metabolites in the library, 262 were detected in at least 80% of the study patients. The metabolomic platform recognized 78 metabolites previously reported to be elevated in ESRD (uremic solutes). Sixteen were already elevated in the baseline plasma of our cases years before ESRD developed. Other uremic solutes were either not different or not commonly detectable. Essential amino acids and their derivatives were significantly depleted in the cases, whereas certain amino acid-derived acylcarnitines were increased. All findings remained statistically significant after adjustment for differences between study groups in albumin excretion rate, eGFR or HbA1c. Uremic solute differences were confirmed by quantitative measurements. Thus, abnormal plasma concentrations of putative uremic solutes and essential amino acids either contribute to progression to ESRD or are a manifestation of an early stage(s) of the disease process that leads to ESRD in T2D.