LC-MS/MS method for simultaneous determination of serump-cresyl sulfate and indoxyl sulfate in patients undergoing peritoneal dialysis

A high-throughput liquid chromatography-tandem mass spectrometry assay for the simultaneous quantification of p-cresol sulfate, p-cresol glucuronide, indoxyl sulfate, and indoxyl glucuronide in HepaRG culture medium and the demonstration of mefenamic acid as a potent and selective detoxifying agent

BACKGROUND

p-cresol and indole are uremic compounds which undergo sulfonation to generate the highly toxic p-cresol sulfate (pCS) and indoxyl sulfate (IxS). They are also subjected to glucuronidation to produce the less toxic p-cresol glucuronide (pCG) and indoxyl glucuronide (IG). We developed and validated an assay to quantify these metabolites in HepaRG cells. We also tested the effects of mefenamic acid on their in-situ formations in relation to the development of cellular necrosis.

RESEARCH DESIGN AND METHODS

HepaRG cells were exposed to p-cresol or indole (0-1 mM) with mefenamic acid (0-3000 nM) for 24 hours to generate uremic metabolites. Cells were also exposed to 0.5 mM p-cresol or indole with/without 30 nM mefenamic acid to characterize lactate dehydrogenase (LDH) release.

RESULTS

The assay exhibited high sensitivity and wide calibration ranges covering human concentrations. HepaRG cells also generated physiologically-relevant concentrations of each metabolite. Mefenamic acid inhibited pCS formation in a concentration-dependent manner without affecting pCG, IxS, or IG. Mefenamic acid also reduced LDH release from p-cresol (by 50.12±5.86%) or indole (56.26±3.58%).

CONCLUSIONS

This novel assay is capable of quantifying these metabolites in HepaRG cells. Our novel findings suggest that mefenamic acid can be potentially utilized therapeutically to attenuate pCS-associated toxicities.

Western diet exacerbates a murine model of Balkan nephropathy

Aristolochic acid (AA) ingestion causes Balkan nephropathy, characterized by tubular injury and progression to chronic kidney disease (CKD). AA is taken up by proximal tubule cells via organic anion transport and induces p21-mediated DNA damage response, but little is known about dietary modulating factors. Western diet (WD) is rich in saturated fats and sugars and can promote metabolic disorders and CKD progression. Here, we determined the impact of WD on AA-induced kidney injury. Five-week-old male C57BL/6J mice were fed WD or normal chow (NC) for 8 wk, followed by administration of AA every 3 days for 3 wk. Measurements were performed after the last injection and following a 3-wk recovery. Independent of dosing AA by body weight (3 mg/kg/day) or same dose/mouse (0.1125 mg/day), the AA-induced increase in plasma creatinine and reduction of hematocrit were greater in WD versus NC. This was associated with increased kidney gene expression in WD vs. NC of markers of DNA damage (p21), injury (Kim1 and Ngal), and inflammation (Tnfa) and kidney fibrosis staining. WD alone increased fractional excretion of indoxyl sulfate by 7.5-fold, indicating enhanced kidney organic anion transport. Kidney proteomics identified further WD-induced changes that could increase kidney sensitivity to AA and contribute to the altered response to AA including weakening of energy metabolism, potentiation of immune and infection pathways, and disruption in RNA regulation. In conclusion, WD can increase the susceptibility of mice to Balkan nephropathy, possibly in part through facilitating kidney uptake of the organic anion AA.NEW & NOTEWORTHY This study shows that a Western diet (WD) aggravates a murine model of Balkan nephropathy induced by the application of the organic anion and nephrotoxin aristolochic acid (AA). Mechanistically, this may involve WD-induced kidney organic anion secretion, which can facilitate the AA uptake into proximal tubular cells and thereby contribute to the injury. Kidney proteomics identified further changes induced by feeding a WD that could have increased the sensitivity of the kidney to stress and injury.

Analysis of indoxyl sulfate in biological fluids with emphasis on sample preparation techniques: A comprehensive analytical review

The uremic toxin indoxyl sulfate (IS) has been related to the development of various medical conditions notably chronic kidney disease (CKD). Hence, quantification of this biomarker in biological fluids may be a diagnostic tool to evaluate renal system functionality. Numerous analytical methods including liquid chromatography, gas chromatography, spectroscopy, and electrochemical techniques have since been used to analyze IS in different biological fluids. The current review highlights the relevant studies that assessed IS with a special focus on sample preparation, which is essential to reduce or eliminate the effect of endogenous components from the matrix in bioanalysis.

Liquid Chromatography-Mass Spectrometry Analytical Methods for the Quantitation of p-Cresol Sulfate and Indoxyl Sulfate in Human Matrices: Biological Applications and Diagnostic Potentials

Indoxyl sulfate (IxS) and p-cresyl sulfate (pCS) are toxic uremic compounds with documented pathological outcomes. This review critically and comprehensively analyzes the available liquid chromatography-mass spectrometry methods quantifying IxS and pCS in human matrices and the biological applications of these validated assays. Embase, Medline, PubMed, Scopus, and Web of Science were searched until December 2023 to identify assays with complete analytical and validation data (N = 23). Subsequently, citation analysis with PubMed and Scopus was utilized to identify the biological applications for these assays (N = 45). The extraction methods, mobile phase compositions, chromatography, and ionization methods were evaluated with respect to overall assay performance (e.g., sensitivity, separation, interference). Most of the assays focused on human serum/plasma, utilizing acetonitrile or methanol (with ammonium acetate/formate or formic/acetic acid), liquid-liquid extraction, reverse phase (e.g., C18) chromatography, and gradient elution for analyte separation. Mass spectrometry conditions were also consistent in the identified papers, with negative electrospray ionization, select multiple reaction monitoring transitions and deuterated internal standards being the most common approaches. The validated biological applications indicated IxS and/or pCS were correlated with renal disease progression and cardiovascular outcomes, with limited data on central nervous system disorders. Methods for reducing IxS and/or pCS concentrations were also identified (e.g., drugs, natural products, diet, dialysis, transplantation) where inconsistent findings have been reported. The clinical monitoring of IxS and pCS is gaining significant interest, and this review will serve as a useful compendium for scientists and clinicians.

Effect of High Sodium Intake on Gut Tight Junctions' Structure and Permeability to Bacterial Toxins in a Rat Model of Chronic Kidney Disease

INTRODUCTION

Uremic toxicity changes the gut structure and permeability, allowing bacterial toxins to translocate from the lumen to the blood during chronic kidney failure (CKD). Clinical fluid overload and tissue edema without uremia have similar effects but have not been adequately demonstrated and analyzed in CKD.

AIMS

To investigate the effect of sodium intake on the plasma concentration of gut-derived uremic toxins, indoxyl sulfate (IS), and p-cresyl sulfate (pCS) and the expression of genes and proteins of epithelial gut tight junctions in a rat model of CKD.

METHODS

Sham-operated (control group, CG) and five-sixths nephrectomized (5/6Nx) Sprague-Dawley rats were randomly assigned to low (LNa), normal (NNa), or high sodium (HNa) diets., Animals were then sacrificed at 8 and 12 weeks and analyzed for IS and pCS plasma concentrations, as well as for gene and protein expression of thigh junction proteins, and transmission electron microscopy (TEM) in colon fragments.

RESULTS

The HNa 5/6Nx groups had higher concentrations of IS and pCS than CG, NNa, and LNa at eight and twelve weeks. Furthermore, HNa 5/6Nx groups had reduced expression of the claudin-4 gene and protein than CG, NNa, and LNa. HNa had reduced occludin gene expression compared to CG. Occludin protein expression was more reduced in HNa than in CG, NNa, and LNa. The gut epithelial tight junctions appear dilated in HNa compared to NNa and LNa in TEM.

CONCLUSION

Dietary sodium intake and fluid overload have a significant role in gut epithelial permeability in the CKD model.

Determination of indoxyl sulfate by spectrofluorimetric method in human plasma through extraction with deep eutectic solvent

A rapid and efficient analytical method was established to quantify indoxyl sulfate (IS) in plasma through extraction technique with a deep eutectic solvent (DES) and spectrofluorimetric method. DES (choline chloride: urea) was mixed with plasma samples for the extraction of IS, followed by the addition of dipotassium hydrogen phosphate (K2HPO4) solution to form an aqueous two-phase system. The fluorescence intensity of IS which was first extracted to the DES-rich-phase and then back-extracted into the salt-rich-phase, was measured by spectrofluorimetric method. Some key factors such as pH, centrifugation speed and time, the volume ratio of DES/salt, and salt concentration were optimized. Under the optimized conditions, the suggested method had a dynamic range between 20 and 160 µg/mL with a coefficient of determination (R2) of 0.99. Precision (relative standard deviation) was less than 15% and accuracy (% relative recovery) was ± 15% at the nominal concentration level. In addition, results showed that IS levels in real samples were higher than 40 µg/mL which was compatible with reported IS levels in end-stage renal disease (ESRD) patients. Overall, all the results reflect the fact that the presented analytical method can potentially be used for the determination of IS in real plasma samples.

Trimethylamine-N-oxide (TMAO) and predicted risk of cardiovascular events after partial nephrectomy

INTRODUCTION

Emerging evidence suggests that uremic toxins, in particular trimethylamine-N-oxide(TMAO), indoxyl-sulfate(IS), and p-cresyl-sulfate(PCS), may associate with increased risk of cardiovascular events(CVe). However, whether uremic toxins increase after partial nephrectomy(PN) and their correlation with risk for CVe remains unknown.

METHODS

100 patients managed with PN were retrospectively reviewed. TMAO/IS/PCS levels were examined by liquid chromatography-mass-spectrometry. Renal-parenchymal-volume-preservation(RPVP) was estimated from CT scans. Predicted risks for CVe were obtained using the Framingham score. Linear regression assessed association between uremic toxins, GFR and risk of CVe. Logistic regression evaluated factors associated with post-PN TMAO.

RESULTS

TMAO, IS and PCS increased from 1.7, 3.7 and 3.5 μmol/L before PN to 3.6, 5.4 and 7.4 μmol/L at latest follow-up, respectively, while GFR declined from 102 to 93 ml/min/1.73 m2 (all p<0.001). TMAO, IS and PCS levels all negatively correlated with GFR(all p<0.001). Predicted 10-year risk of CVe increased from 1.1% pre-PN to 1.7% post-PN(p<0.001), primarily due to increased age(p<0.001), blood pressure(p = 0.002) and total cholesterol(p = 0.003). TMAO(β = 0.038) and GFR (β = -0.02) were independent predictors for predicted 10-year CVe risk on multivariable-analysis. Increased TMAO was an early and sustained finding maintained through 5 years, unlike IS, PCS and eGFR. On multivariable analysis, increased pre-PN TMAO(OR = 2.79) and decreased RPVP(OR = 3.23) were identified as independent risk factors for higher post-PN TMAO, while ischemia type/duration failed to correlate.

CONCLUSION

Uremic toxin levels increased after PN correlating with reduced GFR. Higher TMAO independently associated with greater predicted 10-year CVe risk. Parenchymal mass preserved rather than ischemia time or type associated with increased TMAO.

Gut-Derived Uremic Toxins in CKD: An Improved Approach for the Evaluation of Serum Indoxyl Sulfate in Clinical Practice

In the past years, indoxyl sulfate has been strongly implicated in kidney disease progression and contributed to cardiovascular morbidity. Moreover, as a result of its elevated albumin affinity rate, indoxyl sulfate is not adequately cleared by extracorporeal therapies. Within this scenario, although LC-MS/MS represents the conventional approach for IS quantification, it requires dedicated equipment and expert skills and does not allow real-time analysis. In this pilot study, we implemented a fast and simple technology designed to determine serum indoxyl sulfate levels that can be integrated into clinical practice. Indoxyl sulfate was detected at the time of enrollment by Tandem MS from 25 HD patients and 20 healthy volunteers. Next, we used a derivatization reaction to transform the serum indoxyl sulfate into Indigo blue. Thanks to the spectral shift to blue, its quantity was measured by the colorimetric assay at a wavelength of 420-450 nm. The spectrophotometric analysis was able to discriminate the levels of IS between healthy subjects and HD patients corresponding to the LC-MS/MS. In addition, we found a strong linear relationship between indoxyl sulfate levels and Indigo levels between the two methods (Tandem MS and spectrophotometry). This innovative method in the assessment of gut-derived indoxyl sulfate could represent a valid tool for clinicians to monitor CKD progression and dialysis efficacy.

Assessment of uremic toxins in advanced chronic kidney disease patients on maintenance hemodialysis by LC-ESI-MS/MS

INTRODUCTION

In the advanced stage of chronic kidney disease (CKD), electrolytes, fluids, and metabolic wastes including various uremic toxins, accumulate at high concentrations in the patients' blood. Hemodialysis (HD) is the conventional procedure used worldwide to remove metabolic wastes. The creatinine and urea levels have been routinely monitored to estimate kidney function and effectiveness of the HD process. This study, first from in Indian perspective, aimed at the identification and quantification of major uremic toxins in CKD patients on maintenance HD (PRE-HD), and compared with the healthy controls (HC) as well as after HD (POST-HD).

OBJECTIVES

The study mainly focused on the identification of major uremic toxins in Indian perspective and the quantitative analysis of indoxyl sulfate and p-cresol sulfate (routinely targeted uremic toxins), and phenyl sulfate, catechol sulfate, and guaiacol sulfate (targeted for the first time), apart from creatinine and urea in PRE-HD, POST-HD, and HC groups.

METHODS

Blood samples were collected from 90 HD patients (both PRE-HD and POST-HD), and 74 HCs. The plasma samples were subjected to direct ESI-HRMS and LC/HRMS for untargeted metabolomics and LC-MS/MS for quantitative analysis.

RESULTS

Various known uremic toxins, and a few new and unknown peaks were detected in PRE-HD patients. The p-cresol sulfate and indoxyl sulfate were dominant in PRE-HD, the concentrations of phenyl sulfate, catechol sulfate, and guaiacol sulfate were about 50% of that of indoxyl sulfate. Statistical evaluation on the levels of targeted uremic toxins in PRE-HD, POST-HD, and HC groups showed a significant difference among the three groups. The dialytic clearance of indoxyl sulfate and p-cresol sulfate was found to be < 35%, while that of the other three sulfates was 50-58%.

CONCLUSION

LC-MS/MS method was developed and validated to evaluate five major uremic toxins in CKD patients on HD. The levels of the targeted uremic toxins could be used to assess kidney function and the effectiveness of HD.

Sample preparation and chromatographic methods for the determination of protein-bound uremic retention solutes in human biological samples: An overview

Protein-bound uremic retention solutes, such as indole-3-acetic acid, indoxyl sulfate, p-cresol and p-cresol sulfate, are associated with the development of several pathologies, namely renal, cardiovascular, and bone toxicities, due to their potential accumulation in the human body, thus requiring analytical methods for monitoring and evaluation. The present review addresses conventional and advanced sample treatment procedures for sample handling and the chromatographic analytical methods developed for quantification of these compounds in different biological fluids, with particular focus on plasma, serum, and urine. The sample preparation and chromatographic methods coupled to different detection systems are critically discussed, focusing on the different steps involved for sample treatment, namely elimination of interfering compounds present in the sample matrix, and the evaluation of their environmental impact through the AGREEprep tool. There is a clear trend for the application of liquid-chromatography coupled to tandem mass spectrometry, which requires protein precipitation, solid-phase extraction and/or dilution prior to analysis of biological samples. Furthermore, from a sustainability point of view, miniaturized methods resorting to microplate devices are highly recommended.

A robust, accurate, sensitive LC-MS/MS method to measure indoxyl sulfate, validated for plasma and kidney cells

Proximal tubular damage is an important prognostic determinant in various chronic kidney diseases (CKDs). Currently available diagnostic methods do not allow for early disease detection and are neither efficient. Indoxyl sulfate (IS) is an endogenous metabolite and protein-bound uremic toxin that is eliminated via renal secretion, but accumulates in plasma during tubular dysfunction. Therefore, it may be suitable as a tubular function marker. To evaluate this, a fast bioanalytical method was developed and validated for IS in various species and a kidney cell line using LC-MS/MS. An isotope-labeled IS potassium salt as an internal standard and acetonitrile (ACN) as a protein precipitant were used for sample pretreatment. The analyte was separated on a Polaris 3 C18-A column by gradient elution using 0.1% formic acid in water and ACN, and detected by negative electrospray ionization in selected reaction monitoring mode. The within-day (≤ 4.0%) and between-day (≤ 4.3%) precisions and accuracies (97.7 to 107.3%) were within the acceptable range. The analyte showed sufficient stability at all conditions investigated. Finally, applying this assay, significantly higher plasma and lower urine concentrations of IS were observed in mice with diabetic nephropathy with tubular damage, which encourages validation toward its use as a biomarker.

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.

Clostridioides difficile Infection Dysregulates Brain Dopamine Metabolism

Gastrointestinal illnesses and dysbiosis are among the most common comorbidities reported in patients with neurodevelopmental disorders. The manuscript reports that C. difficile infection (CDI), predisposed by antibiotic-induced gut dysbiosis, causes significant alterations in dopamine metabolism in major dopaminergic brain regions in mice (P < 0.05). In addition, C. difficile infected mice exhibited significantly reduced dopamine beta-hydroxylase (DBH) activity compared to controls (P < 0.01). Moreover, a significantly increased serum concentration of p-cresol, a DBH inhibiting gut metabolite produced by C. difficile, was also observed in C. difficile infected mice (P < 0.05). Therefore, this study suggests a potential mechanistic link between CDI and alterations in the brain dopaminergic axis. Such alterations may plausibly influence the precipitation and aggravation of dopamine dysmetabolism-associated neurologic diseases in infected patients.

IMPORTANCE The gut-brain axis is thought to play a significant role in the development and manifestation of neurologic diseases. This study reports significant alterations in the brain dopamine metabolism in mice infected with C. difficile, an important pathogen that overgrows in the gut after prolonged antibiotic therapy. Such alterations in specific brain regions may have an effect on the precipitation or manifestation of neurodevelopmental disorders in humans.

Highlighting Levels of Indoxyl Sulphate among Critically Ill Patients with Acute Nephrotoxicity; Correlations Between Indoxyl Sulphate Levels and Patients' Characteristics

Background

Many animal studies suggested that the uremic toxin indoxyl sulphate can add to renal damage following induced nephrotoxicity and this effect has not been proved in patients with such complication.

Methods

This is a prospective, case-control, and an observational study conducted on 74 critically ill patients with acute nephrotoxicity. It was designed to measure serum levels of indoxyl sulphate on the day of enrollment and over the course of their illness using high performance liquid chromatography (HPLC-UV) and to test the correlation between these levels and patient's demographics, clinical characteristics, physiological variables, and their outcomes.

Results

Critically ill patients with acute nephrotoxicity had significantly higher total (tIS) and free (fIS) indoxyl sulphate than healthy controls and significantly lower than patients with end-stage renal disease (ESRD). Although, no correlation was found between tIS or fIS and mortality, among survivors, tIS, fIS, creatinine and eGFR were independently associated with no renal recovery.

Conclusion

Serum indoxyl sulphate levels were elevated in critically ill patients with acute nephrotoxicity. There is an association between high levels of indoxyl sulphate and no renal-recovery outcome among survivors of acute nephrotoxicity. Early removal of indoxyl sulphate from patients' blood may improve their outcomes.

Association between serum indoxyl sulfate levels with carotid-femoral pulse wave velocity in patients with chronic kidney disease

BACKGROUND

The role of indoxyl sulfate (IS), an important protein-bound uremic toxin, in arterial stiffness (AS) in patients with chronic kidney disease (CKD) is unclear.

MATERIALS AND METHODS

We investigated the association between serum IS levels and AS in a cross-sectional study of 155 patients with CKD. Patients in the AS group was defined as carotid-femoral pulse wave velocity (cfPWV) value >10 m/s measured by a validated tonometry system (SphygmoCor), while values ≤10 m/s were regarded as without AS group Serum IS was measured by liquid chromatography-mass spectrometry analysis.

RESULTS

Of these CKD patients, AS was present in 51 (32.9%) patients, who were older, had a higher rate of diabetes, higher systolic blood pressure (SBP), and higher IS levels compared to those without AS. By multivariable logistic regression analysis, IS (adjusted odds ratio [aOR] 1.436, 95% confidence interval [CI] 1.085-1.901, p = 0.011), age (aOR 1.058, 95% CI 1.021-1.097, p = 0.002), and SBP (aOR 1.019, 95%CI 1.000-1.038, p = 0.049) were independent predictors of AS. By multivariable stepwise linear regression analysis, logarithmically transformed IS, age, DM, and SBP were significantly correlated with cfPWV. The area under the receiver-operating characteristic curve for serum log-IS was 0.677 (95%CI 0.598-0.750, p = 0.0001) to predict the development of AS in patients with CKD.

CONCLUSION

These finding demonstrate that in addition to older and higher SBP, a high serum IS level is a significant biomarker associated with AS in patients with CKD.

Toxin Removal and Inflammatory State Modulation during Online Hemodiafiltration Using Two Different Dialyzers (TRIAD2 Study)

Uremic toxins play a pathological role in atherosclerosis and represent an important risk factor in dialysis patients. Online hemodiafiltration (HDF) has been introduced to improve the clearance of middle- and large-molecular-weight solutes (>500 Da) and has been associated with reduced cardiovascular mortality compared to standard hemodialysis. This non-randomized, open-label observational study will explore the efficacy of two dialyzers currently used for online HDF, a polysulfone-based high-flux membrane, and a cellulose triacetate membrane, in hemodialysis patients with signs of middle-molecule intoxication or intradialytic hypotension. In particular, the two filters will be evaluated for their ability in uremic toxin removal and modulation of inflammatory status. Sixteen subjects in standard chronic bicarbonate hemodialysis requiring a switch to online HDF in view of their clinical status will be enrolled and divided into two treatment arms, according to the previous history of hypersensitivity to polysulfone/polyethersulfone dialysis filters and hypersensitivity to drugs or other allergens. Group A will consist of 16 patients without a previous history of hypersensitivity and will be treated with a polysulfone filter (Helixone FX100), and group B, also consisting of 16 patients, with a previous history of hypersensitivity and will be treated with asymmetric triacetate (ATA; SOLACEA 21-H) dialyzer. Each patient will be followed for a period of 24 months, with monthly assessments of circulating middle-weight toxins and protein-bound toxins, markers of inflammation and oxidative stress, lymphocyte subsets, activated lymphocytes, and monocytes, cell apoptosis, the accumulation of advanced glycation end-products (AGEs), variations in arterial stiffens measured by pulse wave velocity (PWV), and mortality rate. The in vitro effect on endothelial cells of uremic serum collected from patients treated with the two different dialyzers will also be investigated to examine the changes in angiogenesis, cell migration, differentiation, apoptosis and proliferative potential, and gene and protein expression profile. The expected results will be a better awareness of the different effects of polysulfone gold-standard membrane for online HDF and the new ATA membrane on the removal of uremic toxins removal and inflammation due to blood-membrane interaction.

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 Serum Indoxyl Sulfate Levels with Skeletal Muscle Mass and Strength in Chronic Hemodialysis Patients: A 2-year Longitudinal Analysis

Sarcopenia is highly prevalent in patients undergoing chronic hemodialysis (HD). This study investigated the relationship among serum indoxyl sulfate (IS) levels, muscle mass, and strength in HD patients. A total of 108 HD patients were enrolled. Skeletal muscle mass and handgrip strength (HGS) were assessed, using bioimpedance analysis and a hand-held dynamometer, respectively. Skeletal muscle index (SMI) was defined as skeletal muscle mass/height² (kg/m²). Serum IS, p-cresol sulfate (PCS), and trimethylamine N-oxide (TMAO) levels were determined using liquid chromatography-mass spectrometry. Patients were classified into two groups based on median serum IS values. HGS measurement was repeated after 2 years. Patients in the high IS group had longer HD duration and higher serum TMAO levels than those in the low IS group. Log-normalized IS level was negatively correlated with SMI (r = - 0.227; p = 0.018), but PCS and TMAO levels were not. Among 78 patients who completed 2-year follow-up, those in the high IS group (n = 41) showed greater absolute (- 2.48 kg versus - 0.25 kg, p = 0.035) and relative HGS loss (- 9.1% versus 1.4%, p = 0.036) than those in the low IS group, after adjustment for potential confounders. Indoxyl sulfate (IS) may play a significant role in uremic sarcopenia. Further large-scale studies are needed to confirm our preliminary findings.

A rapid and sensitive method for simultaneous determination of eight protein-bound uremic toxins in human serum by UHPLC-MS/MS: application in assessing peritoneal dialysis

A simple, rapid, reliable and sensitive ultra-high performance liquid chromatography tandem spectrometry (UHPLC-MS/MS) method was established for determination of eight serum protein-bound uremic toxins (hippuric acid, indoxyl sulfate, indole-3-acetic acid, kynurenic acid, L-kynurenine, melatonin, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid, 4-hydroxyhippuric acid) in serum from chronic kidney disease (CKD) dialysis patients. The chromatographic separation was achieved on an Atlantis T3 column (3 μm, 2.1 mm × 100 mm) using a gradient elution with acetonitrile (phase B) and 0.1% formic acid and 10 mmol/L ammonium acetate aqueous solution (phase A). The flow rate was 0.3 mL/min with analytical time of 5 min. The pretreatment procedure was developed with a simple protein precipitation and the hydrochlorothiazide was used as internal standard. The calibration ranges were set as 156.250-20000.000 ng/mL for indoxyl sulfate, hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid; 78.125-10000.000 ng/mL for L-kynurenine, indole-3-acetic acid and 4-hydroxyhippuricacid; 1.562-200.000 ng/mL for kynurenic acid; 0.078-10.000 ng/mL for melatonin. The UHPLC-MS/MS method for quantification of eight protein-bound uremic toxins was successfully developed and validated, and its clinical practicability was assessed on 81 serum samples from CKD patients.

The Association between Gut Microbiota and Uremia of Chronic Kidney Disease

Chronic kidney disease (CKD)-associated uremia aggravates-and is aggravated by-gut dysbiosis. However, the correlation between CKD severity and gut microbiota and/or their uremic metabolites is unclear. We enrolled 103 CKD patients with stage 1 to 5 and 46 healthy controls. We analyzed patients' gut microbiota by MiSeq system and measured the serum concentrations of four uremic metabolites (p-cresyl sulfate, indoxyl sulfate, p-cresyl glucuronide, and trimethylamine N-oxide) by liquid chromatography-tandem mass spectrometry. Serum concentrations of the uremic metabolites increased with kidney function deterioration. Gut microbial diversity did not differ among the examined patient and control groups. In moderate or higher stage CKD groups, Oscillibacter showed positive interactions with other microbiota, and the proportions of Oscillibacter were positively correlated with those of the uremic metabolites. The gut microbiota, particularly Oscillibacter, was predicted to contribute to pyruvate metabolism which increased with CKD progression. Relative abundance of Oscillibacter was significantly associated with both serum uremic metabolite levels and kidney function. Predicted functional analysis suggested that kidney-function-associated changes in the contribution of Oscillibacter to pyruvate metabolism in CKD may greatly affect the gut environment according to kidney function, resulting in dysbiosis concomitant with uremic toxin production. The gut microbiota could be associated with uremia progression in CKD. These results may provide basis for further metagenomics analysis of kidney diseases.

Applying mass spectrometry-based assays to explore gut microbial metabolism and associations with disease

The workings of the gut microbiome have gained increasing interest in recent years through the mounting evidence that the microbiota plays an influential role in human health and disease. A principal focus of this research seeks to further understand the production of metabolic by-products produced by bacteria resident in the gut, and the subsequent interaction of these metabolites on host physiology and pathophysiology of disease. Gut bacterial metabolites of interest are predominately formed via metabolic breakdown of dietary compounds including choline and ʟ-carnitine (trimethylamine N-oxide), amino acids (phenol- and indole-containing uremic toxins) and non-digestible dietary fibers (short-chain fatty acids). Investigations have been accelerated through the application of mass spectrometry (MS)-based assays to quantitatively assess the concentration of these metabolites in laboratory- and animal-based experiments, as well as for direct circulating measurements in clinical research populations. This review seeks to explore the impact of these metabolites on disease, as well as to introduce the application of MS for those less accustomed to its use as a clinical tool, highlighting pertinent research related to its use for measurements of gut bacteria-mediated metabolites to further understand their associations with disease.

Association between Serum Indoxyl Sulfate Levels and Endothelial Function in Non-Dialysis Chronic Kidney Disease

Indoxyl sulfate (IS), a product metabolized from tryptophan, is negatively correlated with renal function and cardiovascular diseases in patients with chronic kidney disease (CKD). We investigated the association between serum IS levels and endothelial function in patients with CKD. Fasting blood samples were obtained from 110 patients with stages 3–5 CKD. The endothelial function, represented by vascular reactivity index (VRI), was measured non-invasively using digital thermal monitoring. Serum IS levels were determined using liquid chromatography–mass spectrometry. Twenty-one (19.1%), 36 (32.7%), and 53 (48.2%) patients had poor (VRI < 1.0), intermediate (1.0 ≤ VRI < 2.0), and good (VRI ≥ 2.0) vascular reactivity. By univariate linear regression analysis, a higher prevalence of smoking, advanced age, higher systolic, and diastolic blood pressure (DBP), elevated levels of serum phosphorus, blood urea nitrogen, creatinine, and IS were negatively correlated with VRI values, but estimated glomerular filtration rate negatively associated with VRI values. After being adjusted by using multivariate stepwise linear regression analysis, DBP and IS levels were significantly negatively associated with VRI values in CKD patients. We concluded that IS level associated inversely with VRI values and had a modulating role in endothelial function in patients with stages 3–5 CKD.

Development and validation of a UHPLC-MS/MS method for measurement of a gut-derived uremic toxin panel in human serum: An application in patients with kidney disease

Gut-derived uremic toxins contribute to the uremic syndrome and are gaining attention as potentially modifiable cardiovascular disease risk factors in patients with underlying chronic kidney disease. A simple, rapid, robust, accurate and precise ultra-performance liquid chromatography-tandem mass spectrometry method was developed and validated for the simultaneous determination of a panel of four gut-derived uremic toxins in human serum. The panel was comprised of kynurenic acid, hippuric acid, indoxyl sulfate, and p-cresol sulfate. Serum samples were protein precipitated with acetonitrile containing deuterated internal standards. Chromatographic separation of analytes was accomplished with an Acquity BEH C18 (2.1 × 100 mm, 1.7 μm) column by isocratic elution at a flow rate of 0.3 mL/min with a mobile phase composed of solvent A (10 mM ammonium formate; pH 4.3) and solvent B (acetonitrile) (85:15, v/v). Analytes were detected using heated electrospray ionization and selected reaction monitoring. The total run-time was 4 min. Standard curves were linear and correlation coefficients (r) were ≥0.997 for concentration ranges of 0.01-0.5 μg/mL for kynurenic acid, 0.25-80 μg/mL for p-cresol sulfate, and 0.2-80 μg/mL for hippuric acid and indoxyl sulfate. Intra- and inter-day accuracy and precision were within 19.3% for the LLOQs and ≤10.9% for all other quality controls. Matrix effect from serum was <15% and recovery was ≥81.3% for all analytes. The method utilizes a short run-time, simple/inexpensive sample processing, has passed FDA validation recommendations, and was successfully applied to study patients with kidney disease.

Development of the LC-MS/MS method for determining the p-cresol level in plasma

p-Cresol is a protein-bound uremic retention solute that originates in the intestine through bacterial metabolism and accumulates throughout the body in case of kidney failure. To date, there has been no method to analyze unconjugated p-cresol concentration in the blood with a limit of detection lower than 75 pg. Thus, the aim of this study was to develop and validate a novel liquid chromatography-tandem mass spectrometry method for the determination of unconjugated p-cresol in plasma with a lower detection limit than what has been determined using previously described methods. Sample preparation included derivatization of p-cresol with dansyl chloride (derivatization reagent) showed to be a better approach to analyze the compound. The method optimization involved various pH, time of the reaction, and concentration of derivatization reagent. The validation process was performed according to the procedures prescribed by the European Medicines Agency. All analyzed validation criteria were fulfilled. The novel validated method was applied to compare the level of p-cresol in patients with chronic renal failure before and after dialysis (n = 24). Additionally, the concentration of p-cresol was determined in patients with multiple organ dysfunction syndrome (n = 23). The established method can be used for determination of p-cresol in the plasma in further clinical research.

The fecal microbiome and serum concentrations of indoxyl sulfate and p-cresol sulfate in cats with chronic kidney disease

Background

Intestinal dysbiosis has been documented in humans with chronic kidney disease (CKD) and is thought to contribute to production of the uremic toxins indoxyl sulfate (IS) and p‐cresol sulfate (pCS). Characteristics of the fecal microbiome in cats with CKD and correlation to serum concentrations of uremic toxins are unknown.

Objectives

To characterize the fecal microbiome and measure serum IS and pCS concentrations of cats with CKD in comparison to healthy older cats.

Animals

Thirty client‐owned cats with CKD (International Renal Interest Society stages 2‐4) and 11 older (≥8 years) healthy control cats.

Methods

Prospective, cross‐sectional study. Fecal samples were analyzed by sequencing of 16S rRNA genes and Escherichia coli quantitative PCR (qPCR). Serum concentrations of IS and pCS measured using liquid chromatography tandem mass spectrometry.

Results

Cats with CKD had significantly decreased fecal bacterial diversity and richness. Escherichia coli qPCR showed no significant difference in bacteria count between control and CKD cats. Cats with stage 2 (P = .01) and stages 3 and 4 (P = .0006) CKD had significantly higher serum IS concentrations compared to control cats. No significant difference found between stage 2 and stages 3 and 4 CKD. The pCS concentrations were not significantly different between CKD cats and control cats.

Conclusions and Clinical Importance

Decreased fecal microbiome diversity and richness is associated with CKD in cats. Indoxyl sulfate concentration is significantly increased with CKD, and cats with stage 2 CKD may suffer from a similar uremic toxin burden as do cats with later stage disease.

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

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

Supplementation of Short-Chain Fatty Acid, Sodium Propionate, in Patients on Maintenance Hemodialysis: Beneficial Effects on Inflammatory Parameters and Gut-Derived Uremic Toxins, A Pilot Study (PLAN Study)

BACKGROUND

In end-stage renal disease (ESRD), gut-derived uremic toxins play a crucial role in the systemic inflammation and oxidative stress promoting the excess morbidity and mortality. The biochemical derangement is in part a consequence of an insufficient generation of short-chain fatty acids (SCFA) due to the dysbiosis of the gut and an insufficient consumption of the fermentable complex carbohydrates.

AIM OF THE STUDY

The primary end-point was to evaluate the potential efficacy of SCFA (specifically, sodium propionate (SP)) for patients on maintenance hemodialysis (MHD) on systemic inflammation. Secondary end-points included potential attenuation of oxidative stress markers, insulin resistance and production of gut-derived uremic toxins indoxyl sulfate and p-cresol sulfate, as well as health status after SP supplementation.

STUDY DESIGN

We performed a single-center non-randomized pilot study in 20 MHD patients. They received the food additive SP with a daily intake of 2 × 500 mg in the form of capsules for 12 weeks. Pre-dialysis blood samples were taken at the beginning, after six weeks and at the end of the administration period, as well as four weeks after withdrawal of the treatment.

RESULTS

The subjects revealed a significant decline of inflammatory parameters C-reactive protein (-46%), interleukin IL-2 (-27%) and IL-17 (-15%). The inflammatory parameters IL-6 and IFN-gamma showed a mild non-significant reduction and the anti-inflammatory cytokine IL-10 increased significantly (+71%). While the concentration of bacterial endotoxins and TNF-α remained unchanged, the gut-derived uremic toxins, indoxyl sulfate (-30%) and p-cresyl sulfate (-50%), revealed a significant decline. The SP supplementation reduced the parameters of oxidative stress malondialdehyde (-32%) and glutathione peroxidase activity (-28%). The serum insulin levels dropped by 30% and the HOMA-index by 32%. The reduction of inflammatory parameters was associated with a lowering of ferritin and a significant increase in transferrin saturation (TSAT). Four weeks after the end of the treatment phase, all improved parameters deteriorated again. Evaluation of the psycho-physical performance with the short form 36 (SF-36) questionnaire showed an enhancement in the self-reported physical functioning, general health, vitality and mental health. The SP supplementation was well tolerated and without important side effects. No patient had left the study due to intolerance to the medication. The SP supplementation in MHD patients reduced pro-inflammatory parameters and oxidative stress and improved insulin resistance and iron metabolism. Furthermore, SP effectively lowered the important gut-derived uremic toxins indoxyl and p-cresol sulfate. These improvements were associated with a better quality of life. Further controlled studies are required in a larger cohort to evaluate the clinical outcome.

Simultaneous Determination of Twenty-Five Compounds in Rat Plasma Using Ultra-High Performance Liquid Chromatography-Polarity Switching Tandem Mass Spectrometry and Its Application to a Pharmacokinetic Study

An attempt was made to characterize the pharmacokinetic profiles of Qishen Keli (QSKL) that has been widely proved to be effective in clinical practice. A method using ultra-high performance liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS/MS) for the simultaneous determination of 25 analytes in rat plasma was developed and validated. Satisfactory chromatographic separation was achieved on an ACQUITY UPLC HSS T3 column with gradient elution using mobile phase consisting of 0.02% aqueous formic acid (A) and acetonitrile fortified with 0.02% formic acid (B), and analyte detection was carried out using polarity-switching multiple reaction monitoring mode. Method validation assays in terms of selectivity, linearity, inter- and intra-day variations, matrix effect, and recovery demonstrated the newly developed method to be specific, sensitive, accurate, and precise. Following the oral administration of QSKL at a single dose, the qualified method was successfully applied for pharmacokinetic investigations in sham and model rats. Mild differences occurred for the pharmacokinetic patterns of most components between those two groups, whereas significant differences were observed for glycyrrhizic acid and glycyrrhetic acid. The obtained findings could provide meaningful information for the clarification of the effective material basis of QSKL.

Simultaneous quantitative analysis of uremic toxins by LC-MS/MS with a reversed-phase/cation-exchange/anion-exchange tri-modal mixed-mode column

Column choice is crucial to the development of liquid chromatography/tandem mass spectrometry (LC-MS/MS) methods because analyte selectivity is dependent on the nature of the stationary phase. Recently, mixed-mode chromatography, which employs a combination of two or more stationary phases and solvent systems, has emerged as an alternative to multiple, complementary, single-column systems. This report describes the development and validation of a novel analytical method based on LC-MS/MS employing a reversed-phase/cation-exchange/anion-exchange tri-modal column (Scherzo SS-C18; Imtakt) for the simultaneous quantification of various uremic toxins (UTx), including creatinine, 1-methyladenosine, trimethylamine-N-oxide, indoxyl sulfate, p-cresyl sulfate, phenyl sulfate and 4-ethylphenyl sulfate. Stable isotope-labeled compounds were prepared as internal standards (ISs) for each analyte. Mobile phase optimization and appropriate gradient conditions resulted in satisfactory retention and peak resolution that could not have been attained with a single stationary phase LC system. The essential validation parameters, including intra- and inter-assay precision and accuracy, were adequate. The validated method was applied to measure serum levels of the aforementioned compounds in 19 patients with chronic kidney disease. This is the first report detailing the simultaneous quantification of these analytes using stable isotopes as ISs. Our results suggest that Scherzo SS-C18 columns will be considered breakthrough tools in the development of analytical methods for compounds that are difficult to quantify simultaneously in traditional LC systems.