Accumulation of furancarboxylic acids in uremic serum as inhibitors of drug binding

Structural Basis of the Change in the Interaction Between Mycophenolic Acid and Subdomain IIA of Human Serum Albumin During Renal Failure

Mycophenolic acid (MP) is an active metabolite of mycophenolate mofetil, a widely used immunosuppressive drug. MP normally exhibits high plasma protein binding (97-99%), but its binding rate is decreased in patients with renal insufficiency. This decreased protein binding is thought to be associated with leukopenia, a side effect of MP. In this study, we characterized the change in protein binding of MP in renal failure patients. Our findings indicate that MP binds strongly to subdomain IIA of human serum albumin. X-ray crystallographic data indicated that the isobenzofuran group of MP forms a stacking interaction with Trp214, and the carboxyl group of MP is located at a position that allows the formation of hydrogen bonds with Tyr150, His242, or Arg257. Due to the specific binding of MP to subdomain IIA, MP is thought to be displaced by uremic toxin (3-carboxy-4-methyl-5-propyl-2-furan-propionic acid) and fatty acids (oleate or myristate) that can bind to subdomain IIA, resulting in the decreased plasma protein binding of MP in renal failure.

Gut Microbiota-Derived Metabolites and Cardiovascular Disease Risk: A Systematic Review of Prospective Cohort Studies

Gut microbiota-derived metabolites have recently attracted considerable attention due to their role in host-microbial crosstalk and their link with cardiovascular health. The MEDLINE-PubMed and Elsevier’s Scopus databases were searched up to June 2022 for studies evaluating the association of baseline circulating levels of trimethylamine N-oxide (TMAO), secondary bile acids, short-chain fatty acids (SCFAs), branched-chain amino acids (BCAAs), tryptophan and indole derivatives, with risk of cardiovascular disease (CVD). A total of twenty-one studies were included in the systematic review after evaluating 1210 non-duplicate records. There were nineteen of the twenty-one studies that were cohort studies and two studies had a nested case–control design. All of the included studies were of high quality according to the “Newcastle–Ottawa Scale”. TMAO was positively associated with adverse cardiovascular events and CVD/all-cause mortality in some, but not all of the included studies. Bile acids were associated with atrial fibrillation and CVD/all-cause mortality, but not with CVD. Positive associations were found between BCAAs and CVD, and between indole derivatives and major adverse cardiovascular events, while a negative association was reported between tryptophan and all-cause mortality. No studies examining the relationship between SCFAs and CVD risk were identified. Evidence from prospective studies included in the systematic review supports a role of microbial metabolites in CVD.

Interaction of Human Serum Albumin with Uremic Toxins: The Need of New Strategies Aiming at Uremic Toxins Removal

Chronic kidney disease (CKD) is acknowledged worldwide to be a grave threat to public health, with the number of US end-stage kidney disease (ESKD) patients increasing steeply from 10,000 in 1973 to 703,243 in 2015. Protein-bound uremic toxins (PBUTs) are excreted by renal tubular secretion in healthy humans, but hardly removed by traditional haemodialysis (HD) in ESKD patients. The accumulation of these toxins is a major contributor to these sufferers’ morbidity and mortality. As a result, some improvements to dialytic removal have been proposed, each with their own upsides and drawbacks. Longer dialysis sessions and hemodiafiltration, though, have not performed especially well, while larger dialyzers, coupled with a higher dialysate flow, proved to have some efficiency in indoxyl sulfate (IS) clearance, but with reduced impact on patients’ quality of life. More efficient in removing PBUTs was fractionated plasma separation and adsorption, but the risk of occlusive thrombosis was worryingly high. A promising technique for the removal of PBUTs is binding competition, which holds great hopes for future HD. This short review starts by presenting the PBUTs chemistry with emphasis on the chemical interactions with the transport protein, human serum albumin (HSA). Recent membrane-based strategies targeting PBUTs removal are also presented, and their efficiency is discussed.

A New Polymethylmethacrylate Membrane for Hemodialysis

High molecular weight (MW) solutes are not removed during conventional hemodialysis (HD), and their accumulation is thought to play a role in some long-term HD complications (anemia, bone and joint pain, neuropathy, itching). The present trial was conducted to evaluate the removal capacity during in vivo HD of a new polymethylmethacrylate (PMMA) membrane (Filtryzer BK-F, 1.3 m2) compared to conventional PMMA (BK-P, 1.6 m2) and to cellulose acetate (CA, 1.3 m2). BK-F dialyzers, with a pore size of 100 A° and 62% porosity, are designed to remove high MW substances. Ten stable anuric RDT patients (53 ± 13 years) were treated for one week with each membrane in a randomized sequence. Plasma concentrations of creatinine, BUN and beta2-microglobulin (beta2-M) were measured before (b) and after (a) HD to determine the reduction rate for these substances (%). Beta2-M concentration after HD was corrected for changes in distribution volume. Samples of spent dialysate were collected after 3 minutes, 120 minutes and at the end of HD sessions, and appropriately treated and concentrated for HPLC analysis. The reduction rate for BUN and creatinine was similar for the 3 membranes. BK-F showed a higher beta2-M reduction rate than BK-P (p<0.005) or CA (p<0.0001). HPLC analysis of dialysate showed prevalent peaks < 4 kilodaltons (kDa) throughout HD for BK-P and CA. Solutes > 10 kDa were infrequently detected. Peak profile during HD with BK-F was quite different, showing a predominant peak > 50 kDa which also included albumin. However, albumin loss significantly decreased after 120 minutes and at the end of dialysis compared with the 3-minute values, and was lower than that reported in CAPD patients. With BK-F a peak of MW > 500 kDa was also detected which previous studies indicated as a range characterized by the presence of erythropoiesis inhibitors. Use of the BK-F membrane in HD could afford satisfactory removal of high MW substances, thereby preventing or controlling some long-term HD complications such as anemia or beta2-M amyloid formation.

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

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

Nontargeted metabolite profiling discriminates diet-specific biomarkers for consumption of whole grains, fatty fish, and bilberries in a randomized controlled trial

BACKGROUND

Nontargeted metabolite profiling allows for concomitant examination of a wide range of metabolite species, elucidating the metabolic alterations caused by dietary interventions.

OBJECTIVE

The aim of the current study was to investigate the effects of dietary modifications on the basis of increasing consumption of whole grains, fatty fish, and bilberries on plasma metabolite profiles to identify applicable biomarkers for dietary intake and endogenous metabolism.

METHODS

Metabolite profiling analysis was performed on fasting plasma samples collected in a 12-wk parallel-group intervention with 106 participants with features of metabolic syndrome who were randomly assigned to 3 dietary interventions: 1) whole-grain products, fatty fish, and bilberries [healthy diet (HD)]; 2) a whole-grain-enriched diet with the same grain products as in the HD intervention but with no change in fish or berry consumption; and 3) refined-wheat breads and restrictions on fish and berries (control diet). In addition, correlation analyses were conducted with the food intake data to define the food items correlating with the biomarker candidates.

RESULTS

Nontargeted metabolite profiling showed marked differences in fasting plasma after the intervention diets compared with the control diet. In both intervention groups, a significant increase was observed in 2 signals identified as glucuronidated alk(en)-ylresorcinols [corrected P value (Pcorr) < 0.05], which correlated strongly with the intake of whole-grain products (r = 0.63, P < 0.001). In addition, the HD intervention increased the signals for furan fatty acids [3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF)], hippuric acid, and various lipid species incorporating polyunsaturated fatty acids (Pcorr < 0.05). In particular, plasma CMPF correlated strongly with the intake of fish (r = 0.47, P < 0.001) but not with intakes of any other foods.

CONCLUSIONS

Novel biomarkers of the intake of health-beneficial food items included in the Nordic diet were identified by the metabolite profiling of fasting plasma and confirmed by the correlation analyses with dietary records. The one with the most potential was CMPF, which was shown to be a highly specific biomarker for fatty fish intake. This trial was registered at clinicaltrials.gov as NCT00573781.

Phenotypic mapping of metabolic profiles using self-organizing maps of high-dimensional mass spectrometry data

A metabolic system is composed of inherently interconnected metabolic precursors, intermediates, and products. The analysis of untargeted metabolomics data has conventionally been performed through the use of comparative statistics or multivariate statistical analysis-based approaches; however, each falls short in representing the related nature of metabolic perturbations. Herein, we describe a complementary method for the analysis of large metabolite inventories using a data-driven approach based upon a self-organizing map algorithm. This workflow allows for the unsupervised clustering, and subsequent prioritization of, correlated features through Gestalt comparisons of metabolic heat maps. We describe this methodology in detail, including a comparison to conventional metabolomics approaches, and demonstrate the application of this method to the analysis of the metabolic repercussions of prolonged cocaine exposure in rat sera profiles.

The furan fatty acid metabolite CMPF is elevated in diabetes and induces β cell dysfunction

Gestational diabetes (GDM) results from failure of the β cells to adapt to increased metabolic demands; however, the cause of GDM and the extremely high rate of progression to type 2 diabetes (T2D) remains unknown. Using metabolomics, we show that the furan fatty acid metabolite 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) is elevated in the plasma of humans with GDM, as well as impaired glucose-tolerant and T2D patients. In mice, diabetic levels of plasma CMPF induced glucose intolerance, impaired glucose-stimulated insulin secretion, and decreased glucose utilization. Mechanistically, we show that CMPF acts directly on the β cell, causing impaired mitochondrial function, decreasing glucose-induced ATP accumulation, and inducing oxidative stress, resulting in dysregulation of key transcription factors and ultimately reduced insulin biosynthesis. Importantly, specifically blocking its transport through OAT3 or antioxidant treatment could prevent CMPF-induced β cell dysfunction. Thus, CMPF provides a link between β cell dysfunction and GDM/T2D that could be targeted therapeutically.

Hepatic clearance, but not gut availability, of erythromycin is altered in patients with end-stage renal disease

Nonrenal clearance of drugs can be significantly lower in patients with end-stage renal disease (ESRD) than in those with normal renal function. Using erythromycin (ER) as a probe compound, we investigated whether this decrease in nonrenal clearance is due to reduced hepatic clearance (CL(H)) and/or gut metabolism. We also examined the potential effects of the uremic toxins 3-carboxy-4-methyl-5-propyl-2-furan propanoic acid (CMPF) and indoxyl sulfate (Indox) on ER disposition. Route-randomized, two-way crossover pharmacokinetic studies of ER were conducted in 12 ESRD patients and 12 healthy controls after oral (250 mg) and intravenous (125 mg) dosing with ER. In patients with ESRD, CL(H) decreased 31% relative to baseline values (0.35 +/- 0.14 l/h/kg vs. 0.51 +/- 0.13 l/h/kg, P = 0.01), with no change in steady-state volume of distribution. With oral dosing, the bioavailability of ER increased 36% in patients with ESRD, and this increase was not related to changes in gut availability. As expected, plasma levels of CMPF and Indox were significantly higher in the patients than in the healthy controls. However, no correlation was observed between CL(H) of ER and the levels of uremic toxins.

Metabolomics analysis reveals large effects of gut microflora on mammalian blood metabolites

Although it has long been recognized that the enteric community of bacteria that inhabit the human distal intestinal track broadly impacts human health, the biochemical details that underlie these effects remain largely undefined. Here, we report a broad MS-based metabolomics study that demonstrates a surprisingly large effect of the gut "microbiome" on mammalian blood metabolites. Plasma extracts from germ-free mice were compared with samples from conventional (conv) animals by using various MS-based methods. Hundreds of features were detected in only 1 sample set, with the majority of these being unique to the conv animals, whereas approximately 10% of all features observed in both sample sets showed significant changes in their relative signal intensity. Amino acid metabolites were particularly affected. For example, the bacterial-mediated production of bioactive indole-containing metabolites derived from tryptophan such as indoxyl sulfate and the antioxidant indole-3-propionic acid (IPA) was impacted. Production of IPA was shown to be completely dependent on the presence of gut microflora and could be established by colonization with the bacterium Clostridium sporogenes. Multiple organic acids containing phenyl groups were also greatly increased in the presence of gut microbes. A broad, drug-like phase II metabolic response of the host to metabolites generated by the microbiome was observed, suggesting that the gut microflora has a direct impact on the drug metabolism capacity of the host. Together, these results suggest a significant interplay between bacterial and mammalian metabolism.

Recent progress in the analysis of uremic toxins by mass spectrometry

Mass spectrometry (MS) has been successfully applied for the identification and quantification of uremic toxins and uremia-associated modified proteins. This review focuses on recent progress in the analysis of uremic toxins by using MS. Uremic toxins include low-molecular-weight compounds (e.g., indoxyl sulfate, p-cresol sulfate, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid, asymmetric dimethylarginine), middle-molecular-weight peptides, and proteins modified with advanced glycation and oxidation. These uremic toxins are considered to be involved in a variety of symptoms which may appear in patients with stage 5 chronic kidney disease. Based on MS analysis of these uremic toxins, the pathogenesis of the uremic symptoms will be elucidated to prevent and manage the symptoms.

Furan fatty acids: occurrence, synthesis, and reactions. Are furan fatty acids responsible for the cardioprotective effects of a fish diet?

Furan FA (F-acids) are tri- or tetrasubstituted furan derivatives characterized by either a propyl or pentyl side chain in one of the alpha-positions; the other is substituted by a straight long-chain saturated acid with a carboxylic group at its end. F-acids are generated in large amounts in algae, but they are also produced by plants and microorganisms. Fish and other marine organisms as well as mammals consume F-acids in their food and incorporate them into phospholipids and cholesterol esters. F-acids are catabolized to dibasic urofuran acids, which are excreted in the urine. The biogenetic precursor of the most abundant F-acid, F6, is linoleic acid. Methyl groups in the beta-position are derived from adenosylmethionine. Owing to the different alkyl substituents, synthesis of F-acids requires multistep reactions. F-acids react readily with peroxyl radicals to generate dioxoenes. The radical-scavenging ability of F-acids may contribute to the protective properties of fish and fish oil diets against mortality from heart disease.

Uremic toxins: a new focus on an old subject

The uremic syndrome is characterized by an accumulation of uremic toxins due to inadequate kidney function. The European Uremic Toxin (EUTox) Work Group has listed 90 compounds considered to be uremic toxins. Sixty-eight have a molecular weight less than 500 Da, 12 exceed 12,000 Da, and 10 have a molecular weight between 500 and 12,000 Da. Twenty-five solutes (28%) are protein bound. The kinetics of urea removal is not representative of other molecules such as protein-bound solutes or the middle molecules, making Kt/V misleading. Clearances of urea, even in well-dialyzed patients, amount to only one-sixth of physiological clearance. In contrast to native kidney function, the removal of uremic toxins in dialysis is achieved by a one-step membrane-based process and is intermittent. The resulting sawtooth plasma concentrations of uremic toxins contrast with the continuous function of native kidneys, which provides constant solute clearances and mass removal rates. Our increasing knowledge of uremic toxins will help guide future treatment strategies to remove them.

Organic anion transporter (Slc22a) family members as mediators of toxicity

Exposure of the body to toxic organic anions is unavoidable and occurs from both intentional and unintentional sources. Many hormones, neurotransmitters, and waste products of cellular metabolism, or their metabolites, are organic anions. The same is true for a wide variety of medications, herbicides, pesticides, plant and animal toxins, and industrial chemicals and solvents. Rapid and efficient elimination of these substances is often the body's best defense for limiting both systemic exposure and the duration of their pharmacological or toxicological effects. For organic anions, active transepithelial transport across the renal proximal tubule followed by elimination via the urine is a major pathway in this detoxification process. Accordingly, a large number of organic anion transport proteins belonging to several different gene families have been identified and found to be expressed in the proximal nephron. The function of these transporters, in combination with the high volume of renal blood flow, predisposes the kidney to increased toxic susceptibility. Understanding how the kidney mediates the transport of organic anions is integral to achieving desired therapeutic outcomes in response to drug interactions and chemical exposures, to understanding the progression of some disease states, and to predicting the influence of genetic variation upon these processes. This review will focus on the organic anion transporter (OAT) family and discuss the known members, their mechanisms of action, subcellular localization, and current evidence implicating their function as a determinant of the toxicity of certain endogenous and xenobiotic agents.

Pharmacokinetics of panipenem/betamipron in patients with end-stage renal disease

Panipenem/betamipron (Carbenin), a parenteral carbapenem antibiotic, is used for the treatment of severe and intractable bacterial infections caused by gram-positive and gram-negative bacteria. Because 30% of panipenem and most of the betamipron are excreted in the urine in an unchanged form, renal function is the important determinant of the dosage regimen of panipenem/betamipron. In this study, the pharmacokinetics of panipenem/betamipron were investigated in patients with end-stage renal disease (ESRD) undergoing hemodialysis treatment to establish an appropriate dose regimen. We further attempted to predict the in vivo clearance in patients undergoing hemodialysis based on the in vitro dializability. The pharmacokinetics of panipenem/betamipron were investigated in eight patients after a 1-h intravenous infusion of panipenem/betamipron (500 mg/500 mg). The in vitro extraction ratios of panipenem/betamipron through a high-flux dialyzer were obtained, and compared with those obtained in vivo. The clearances of panipenem in patients were 9.53 +/- 1.26 l/h with hemodialysis, and 2.92 +/- 0.238 l/h without hemodialysis. In contrast, those of betamipron were 4.18 +/- 0.643 l/h and 0.615 +/- 0.511 l/h, respectively. The clearance of panipenem with hemodialysis were predicted well from in vitro extraction ratios, while that of betamipron was overestimated about 1.4-fold, probably due to high plasma protein binding and the binding difference between patients and healthy subjects. After comparing the pharmacokinetic behavior of panipenem in patients with ESRD and that of a surrogate marker of efficacy, we recommend that these patients be treated with 500 mg/500 mg of panipenem/betamipron once daily, which gives a similar clinical result in a patient with normal renal function.

Protein-bound uremic retention solutes

Protein-bound uremic retention solutes are molecules with low molecular weight (MW) but should be considered middle or high MW substances. This article describes the best known substances of this group, which include p-cresol, indoxyl sulfate, hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furan-propionic acid (CMPF), and homocysteine. At concentrations encountered during uremia, p-cresol inhibits phagocyte function and decreases leukocyte adhesion to cytokine-stimulated endothelial cells. CMPF has been implicated in anemia and neurologic abnormalities of uremia. CMPF could alter the metabolism of drugs of inhibiting their binding to albumin and their tubular excretion. Indoxyl sulfate administrated to uremic rats increases the rate of progression of renal failure. Hippuric acid inhibits glucose utilization in the muscle, and its serum concentration is correlated with neurologic symptoms of uremia. Homocysteine predisposes uremic patients to cardiovascular disease through impairment of endothelial and smooth muscle cell functions. The removal of protein-bound compounds by conventional hemodialysis is low. Other strategies to decrease their concentrations include increase in dialyze pore size, daily hemodialysis, peritoneal dialysis, reduction of production or acceleration of degradation, and preservation of residual renal function.

Renal disposition of a furan dicarboxylic acid and other uremic toxins in the rat

The aim of this study was to understand the mechanisms that underlie the renal elimination of albumin-bound uremic toxins, particularly the highly bound furan acid 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), that accumulate in chronic renal failure. These toxins inhibit the binding of acidic drugs and have various other untoward effects. The pharmacokinetics and tissue distribution of CMPF plus three other such toxins, indoxyl sulfate, indole acetic acid, and hippuric acid, have been examined in the anesthetized rat. The effects of p-aminohippuric (PAH) acid and tetraethylammonium on the uptake of CMPF by rat renal cortical slices in vitro were also investigated to characterize its mechanism of uptake. Plasma and tissue concentrations of the uremic toxins were determined by high-performance liquid chromatography. The rate of elimination of the toxins from plasma was indoxyl sulfate > hippuric acid > indole acetic acid > CMPF. Although the renal clearance of CMPF was low, its main elimination pathway was via urinary excretion with active tubular secretion. In renal cortical slice experiments, mutual inhibition between CMPF and PAH was observed. In addition, alpha-ketoglutarate stimulated the uptake of CMPF by renal cortical slices. The base tetraethylammonium did not inhibit slice uptake of CMPF. The pharmacokinetics of CMPF was characterized by slow plasma clearance and localization in the kidney. Furthermore, the evidence from experiments with renal cortical slices indicates that the uptake of CMPF is mediated by an anion/dicarboxylate exchanger, similar to that for PAH.

Effect of protein-bound uraemic toxins on the thermodynamic characteristics of human albumin

The ability of albumin to bind drugs and other lipophilic organic acids is decreased in chronic renal failure by the accumulation of albumin-bound uraemic toxins such as hippuric acid, indoxyl sulphate and 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF). This furan acid is the most highly bound and is not removed by haemodialysis. The inhibitory effects of these three uraemic toxins on the interaction of three marker ligands sodium octanoate (for medium chain fatty acids), salicylic acid and phenol red (bilirubin site/site I) with albumin have been investigated by differential scanning microcalorimetry and flow microcalorimetry. CMPF was the most potent inhibitor and its binding site coincided with that of bilirubin (site I). Indoxyl sulphate binds to the site for medium-chain fatty acids and tryptophan (site II) and hippuric acid, the weakest inhibitor, inhibited binding to the salicylic acid site.

Decreased bilirubin-binding capacity in uremic serum caused by an accumulation of furan dicarboxylic acid

In chronic renal failure, substances that are effectively excreted in healthy subjects accumulate in serum. These substances, uremic toxins, include a variety of organic acids. It has been reported that a decrease in the bilirubin (BR) binding capacity occurs in the serum of renal failure patients. 3-Carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) has a high affinity for human serum albumin (HSA) and is a potent inhibitor of the serum protein binding of many drugs. We recently reported that CMPF and BR share the binding site for dicarboxylate molecules on the HSA molecule [Pharm Res 1999;16:916-923]. In this study, in order to confirm whether CMPF is involved in the decrease of BR serum binding capacity in chronic renal failure patients, the total concentrations of uremic toxins, CMPF, and indoxyl sulfate (IS) and the free BR concentration in serum from healthy volunteers and renal failure patients were determined. Both total CMPF and IS concentrations correlate with the free BR concentration. However, results from the peroxidase method reveal that IS cannot displace BR under the physiological condition [IS]/[HSA] <1. We, therefore, conclude that CMPF is one of the substances which contribute to the decreased binding capacity of BR in uremic serum.

Effects of uremic toxins and fatty acids on serum protein binding of furosemide: possible mechanism of the binding defect in uremia

To elucidate the mechanism of impaired serum binding of furosemide observed in patients with renal dysfunction, we examined in vitro the serum protein binding of furosemide in the absence and presence of uremic toxins that are endogenously retained solutes in uremic serum and act as inhibitors of drug binding. Analysis of the binding data of furosemide at its therapeutic concentration (6.6 mg/L) indicated that, among the four uremic toxins studied, 3-carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF) showed the greatest inhibitory potency for the binding of furosemide to serum; moreover, the inhibition was competitive. CMPF thus most likely represents the primary determinant for the serum binding defect of furosemide in uremia. However, CMPF and oleate appear to exert a synergistic effect on the inhibition of furosemide serum binding—perhaps through a cascade effect on furosemide-binding inhibition in the oleate–CMPF–furosemide system, in which the binding of oleate to its low-affinity sites indirectly displaces furosemide from albumin and thus increases the transiently liberated CMPF molecules. Similar cascade effects on furosemide binding in the presence of CMPF were also originated by other long-chain (C18) fatty acids, linoleate and stearate, although to a lesser extent. Because CMPF is not effectively removed by ordinary hemodialysis treatment, the combined direct and cascade effects of CMPF and fatty acids appear to contribute to the increase in the free fraction of furosemide during hemodialysis.

Mass spectrometry in the search for uremic toxins

This article reviews the literature on the mass spectrometry (MS) that has been used in the research of uremic toxins. Gas chromatography/mass spectrometry (GC/MS) has been most often used for the analysis of low-molecular-weight compounds in uremic blood such as organic acids, phenols, and polyols. However, it cannot be used for the analysis of middle- to high-molecular-weight substances or for involatile compounds. The development of fast atom bombardment (FAB) and liquid secondary ion mass spectrometry (LSIMS) has made possible the analysis of middle-molecules and involatile low-molecular-weight substances such as peptides and nucleosides. The development of atmospheric pressure chemical ionization (APCI) has also lead to the analysis of involatile low-molecular-weight substances. The recent advances in ionization methods, such as electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI), have permitted the MS analysis of high-molecular-weight substances such as beta 2-microglobulin, a major component of dialysis amyloid. Liquid chromatography/mass spectrometry (LC/MS), using ESI, APCI, or FAB as an ionization method, is currently the preferred method for the analysis of low- to high-molecular-weight substances in uremic blood. ESI-LC/MS and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOFMS) are useful for elucidating the structure of post-translationally modified proteins obtained from the blood and tissues of uremic patients. Post-translational modification such as the formation of advanced glycation end-products and carbamoylation is enhanced in uremic patients, and is considered to be responsible for some uremic symptoms. Laser microprobe MS is unique in its capability for the two-dimensional detection of atoms such as aluminum in a tissue section obtained from uremic patients. This review focuses on the mainstream research for discovering uremic toxins, specific uremic toxins identified or quantified using MS, and the MS analysis of post-translationally modified proteins in uremia. These studies have provided ample evidence that MS has played an important role in the search for uremic toxins.

Synthesis and physicochemical properties of the furan dicarboxylic acid, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, an inhibitor of plasma protein binding in uraemia

The furan dicarboxylic acid, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (5-propyl FPA) accumulates in the plasma of patients with chronic renal failure and is a major contributor to the drug binding defect of uraemic plasma. This acid has also been implicated in several other aspects of the uraemic syndrome: anaemia, irregularities of thyroid function, neurological symptoms and inhibition of active tubular secretion. The acid is not commercially available and its synthesis, starting with Meldrum's acid and methyl succinyl chloride, is described. The pKa values were measured by titration and values of 3.2 and 3.6 respectively were assigned to the carboxylic acid groups attached directly to the ring at position 3 and at position 2 (on the side-chain). The partition coefficient (log P) between hydrochloric acid and octanol was 1.2 and the distribution coefficient (log D; octanol-phosphate buffer pH 7.4) was -0.59. The pKa values and the degree of hydrophobic character of 5-propyl FPA are consistent with those of other protein-bound acids which undergo active tubular secretion by the kidney and this substance may serve as an endogenous marker for the effects of drugs and disease on this process.

Plasma clearance in the rat of a furan dicarboxylic acid which accumulates in uremia

The furan dicarboxylic acid 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (5-propyl FPA) accumulates in the plasma of patients with chronic renal failure and has been implicated in several aspects of the uremic syndrome: the defective binding of organic acids in uremic plasma, inhibition of active tubular secretion, anemia and the severity of neurological symptoms. Evidence from experiments with rat kidney slices suggests that 5-propyl FPA undergoes active tubular secretion, and so its clearance after an intravenous bolus dose (5 mg/kg; 21 mumol/kg) was investigated in anaesthetized female Wistar albino rats in vivo. The effects of intravenous bolus doses of p-aminohippuric acid (PAH) and probenecid on the clearance of this dose of 5-propyl FPA were also studied. The mean values (N = 16) for plasma half-life, plasma clearance and apparent volume of distribution of 5-propyl FPA were 3.6 hours, 2.4 ml . min(-1) . kg(-1) and 0.69 liter . kg(-1), respectively. An equimolar dose of PAH did not affect the clearance of 5-propyl FPA, but a tenfold higher molar dose of PAH (40.4 mg/kg) increased the area under the plasma-concentration time curve of 5-propyl FPA, and there was a trend towards a decrease in the clearance and a prolongation of the half-life. Probenecid at a fivefold higher dose than 5-propyl FPA had a similar effect to PAH and increased the AUC of 5-propyl FPA. PAH and probenecid decreased the plasma clearance of 5-propyl FPA, which is evidence that this uremic metabolite undergoes active tubular secretion. It follows that 5-propyl FPA could therefore inhibit the secretion of other organic acids.

A naturally occurring furan fatty acid enhances drug inhibition of thyroxine binding in serum

We studied the thyroxine (T4)-displacing effects of a naturally occurring, highly albumin-bound furanoid acid that accumulates in serum in renal failure to concentrations in excess of 0.2 mmol/L. This substance, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), has been shown to displace acidic drugs from albumin binding. The effects of CMPF on ligand binding were assessed in the following systems: (1) T4 binding to T4-binding globulin (TBG) and transthyretin (TTR), (2) T4 binding in undiluted serum, (3) T4-displacing potency of fenclofenac, furosemide, diflunisal, and aspirin in undiluted serum, (4) serum binding of [14C]-drug preparations, and (5) serum binding of [14C]-oleic acid. CMPF had a minor direct effect on T4 binding to TBG comparable in relative affinity to that of aspirin, ie, almost 7 orders of magnitude less than T4 itself. CMPF alone at a concentration of 0.3 mmol/L, which produced only a 10% to 14% increase in free T4 augmented the T4-displacing effects of high therapeutic concentrations of the various drugs in undiluted serum as follows: furosemide by 180%, fenclofenac by 160%, diflunisal by 130%, and aspirin by 40%. In the presence of fenclofenac, increments of CMPF from 0.075 to 0.3 mmol/L progressively augmented the T4-displacing effect of this drug, associated with a progressive increase in its calculated free concentration. CMPF also inhibited the binding of [14C]-oleic acid, suggesting that in some situations CMPF could also indirectly influence thyroid hormone binding by increasing the unbound concentration of nonesterified fatty acids (NEFA), as previously described.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, an inhibitor of plasma protein binding in uraemia, with human albumin

The furan dicarboxylic acid 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (5-propyl FPA) accumulates in uraemic plasma and is a potent inhibitor of the binding of other anionic ligands to albumin. The interaction of 5-propyl FPA with human albumin has been investigated by equilibrium dialysis at 37 degrees and pH 7.4. Analysis of the binding data on the basis of a two-site model gave binding parameters of n1 = 0.6 and K1 = 4.8 x 10(6) M-1 for the primary binding site. 5-Propyl FPA binding was observed to decrease as the pH was raised from 6.4 to 8.3 which emphasizes the need for pH control of whole plasma or serum. Temperature, however, had little effect on binding as assessed by equilibrium dialysis at 10 degrees, 25 degrees and 37 degrees. The high affinity of 5-propyl FPA for albumin explains its retention in uraemic plasma, its potency as a binding inhibitor and points to active tubular secretion as the mechanism by which it is normally excreted by the kidney.