Polymeric adsorbent for removing toxic proteins from blood of patients with kidney failure

A novel recyclable hemoperfusion adsorbent based on TiO2 nanotube arrays for the selective removal of β2-microglobulin

Prolonged and excessive accumulation of β2-microglobulin (β2m) in the blood can lead to various kidney-related and other diseases. Currently, the most effective method of removing β2m from the blood is hemoperfusion. Although some traditional hemoperfusion adsorbents such as cellulose and polystyrene microspheres have been used for the removal of β2m, their selectivity still needs improvement. Immunosorbents have been developed to address this issue, but high cost and limited application are concerns. TiO2 nanotube arrays (TNTAs) have shown great potential in adsorption-related biomedical applications. In this study, we designed and developed a novel TNTA-based hemoperfusion adsorbent for the removal of β2m, which has demonstrated good biocompatibility, selectivity, and reusability. We investigated the β2m adsorption capacities of TNTAs with different pore sizes. The results indicate that TNTAs with a pore size matching the size of β2m exhibit higher adsorption capacity while also having lower adsorption capacity for albumin, showing the importance of pore size on the selectivity of adsorbents. Additionally, green regeneration of TNTAs is achieved via the photocatalytic activity originating from TiO2. Even after five cycles, the adsorption capacity of TNTAs remained above 70%. Our work demonstrates that inorganic materials with ordered pores are capable to be candidates for hemoperfusion, possessing advantages over traditional organic materials such as high stability, security, and low cost.

Bilirubin Removal by Polymeric Adsorbents for Hyperbilirubinemia Therapy

Hyperbilirubinemia, presenting as jaundice, is a life-threatening critical illness in newborn babies and acute severe hepatic failure patients. Over the past few decades, extracorporeal hemoadsorption by adsorbent therapy has been widely applied in the treatment of hyperbilirubinemia. The capability of hemoadsorption depends on the adsorbents. Most of the clinically used bilirubin adsorbents are made up of styrene/divinylbenzene copolymer and quaternary ammonium salt, which usually have poor biocompatibility and weak mechanical strength. To overcome the drawbacks of commercial polymer adsorbents, advanced synthetic and natural polymers with/without nanomaterials have been designed, and novel adsorbent fabrication technologies have also been developed. In this review, the adsorption mechanism of bilirubin adsorbents has been summarized, which is the basic criterion in adsorbent development. Furthermore, the preparation method, adsorption mechanism, relative merits and practicability of the emerging bilirubin adsorbents have been evaluated. Based on the existing studies, this work highlights the future direction of the efforts on how to design and develop bilirubin adsorbents with good overall clinical performance. Perhaps this study can change traditional perspectives and propose new strategies for bilirubin clearance from the aspects of pathogenic mechanisms, metabolic pathways, and material-based innovation.

Influence of Functional Group Concentration on Hypercrosslinking of Poly(vinylbenzyl chloride) PolyHIPEs: Upgrading Macroporosity with Nanoporosity

With the aim to study the influence of monomer ratio in poly(high internal phase emulsions) (polyHIPEs) on the polymer network architecture and morphology of poly(vinylbenzyl chloride-co-divinylbenzene-co-styrene) after hypercrosslinking via the internal Friedel–Crafts process, polyHIPEs with 80% overall porosity were prepared at three different initial crosslinking degrees, namely 2, 5, and 10 mol.%. All had typical interconnected cellular morphology, which was not affected by the hypercrosslinking process. Nitrogen adsorption and desorption experiments with BET and t-plot modelling were used for the evaluation of the newly introduced nanoporosity and in combination with elemental analysis for the evaluation of the extent of the hypercrosslinking. It was found that, for all three initial crosslinking degrees, the minimum amount of functional monomer, 4-vinylbenzyl chloride, was approximately 30 mol.%. Hypercrosslinking of polymers with lower concentrations of functional monomer did not result in induction of nanoporosity while the initial crosslinking degree had a much lower impact on the formation of nanoporosity.

Applicability of modified carbon sorbent for removing potentially toxic biologically active molecules of aromatic structure from blood plasma

The modification of the mesoporous carbon sorbent with 3-phenylpropanoic acid was carried out in order to create preparations of complex, prolonged action, exhibiting detoxifying, antibacterial, and antifungal properties due to the applied modifier, which is capable of migrating into the solution and exhibiting its own biospecific properties. A technique was developed for fixing 3-phenylpropionic acid (PhPA) on a carbon support by its adsorption from solution. Three types of sorbents with various content of the modifier (PhPA) and the sorbent without modifier were studied. The sorption activity of new sorbents was studied using liquid-liquid extraction and gas chromatography-mass spectrometry methods on model experiments with plasma and aqueous additives of hydroxylated phenyl-containing acids (PhCAs) in various concentrations. The specific surface area was significantly changed for sorbent, modified with 1 × 10^-3 mol/L of PhPA solution, and was 25% less than the area of unmodified sorbent. Potentially toxic biologically active hydroxylated PhCAs were used to create model solutions. The degrees of sorption of these compounds were close to 100%, except phenyllactic acid (over 80%). The sorbent without modifier and two sorbents with the lowest content of the modifier are considered to be more effective for the purification of the plasma from the hydroxylated PhCAs than the sorbent with the highest concentration of the modifier. Simultaneous adsorption of toxic metabolites from the bloodstream and desorption of beneficial ones can be used for a more subtle correction of the patient's condition.

MOF-Based Antibiofouling Hemoadsorbent for Highly Efficient Removal of Protein-Bound Bilirubin

A metal-organic framework (MOF)-based antibiofouling hemoadsorbent (PCB-MIL101) was developed through a facile encapsulation of MIL-101(Cr) in zwitterionic poly carboxybetaine (PCB) hydrogel. PCB-MIL101 possessed strong mechanical strength and superior hemocompatibility, ensuring its safety in hemoperfusion applications. In addition, it showed efficient and effective adsorption toward bilirubin (BR), and its maximum adsorption capacity was ∼583 mg g^-1. Moreover, due to the protection of antibiofouling PCB hydrogel, PCB-MIL101 showed ability to resist protein adsorption, thus working effectively to remove BR molecules from their binding albumin in biological solutions. The finding in this study provides a novel insight into developing MOF-based hemoadsorbents for the improvement of hemoperfusion therapies.

Nanobody-Based high-performance immunosorbent for selective beta 2-microglobulin purification from blood

Removing β2-microglobulin (β2M) from blood circulation is considered to be the most effective method to delay the occurrence of dialysis-related amyloidosis (DRA). The ideal extracorporeal β2M removal system should be cost-effective, highly specific and having a high capacity. However, the traditional technologies based on size exclusion do not have an adequate specificity, and alternative immunosorbents have limited applications due to low capacity and their high cost. Nanobodies (Nbs), the smallest functional recombinant antibody fragments, offer several advantages to overcome these obstacles. In this study, an anti-β2M Nb with a C-terminal thiol-tag was successfully prepared from E. coli for site-directed and oriented immobilization and usage as capture ligand in a β2M-selective immunosorbent. The prepared immunosorbent showed a high binding capacity of up to 7 mg β2M per mL resin, which is 17 times higher than that of previous studies using single-chain variable antibody fragments (scFv). Furthermore, an exceptional high specificity has been demonstrated as other human serum proteins were not adsorbed during dynamic adsorption experiments. About 80% of the original binding capacity of the immunosorbent was restored after four consecutive easy regenerations, whereas 90% of the original capacity was retained after 1-month storage of the resin. Moreover, the mathematical model fitted very well the in vitro perfusion. The results with this pioneering immunosorbent confirm its possible clinical application and is expected to reach the required clinical effect of immunoadsorption therapy. STATEMENT OF SIGNIFICANCE: Dialysis-related amyloidosis (DRA), associated with the accumulation of β2-microglobulin (β2M), is a serious complication of end-stage kidney disease. Removing β2M from blood circulation by extracorporeal blood purification is considered to be the most effective method to delay the occurrence of DRA. However, the existing methods are incapable to eliminate sufficient quantities of β2M from circulation, either because of lack of specificity, high cost or for low capacity. In this manuscript, we provide a practical and economic immunosorbent based on anti-β2M nanobody for DRA. The prepared immunosorbent was reusable and storable, and demonstrated high specificity and realized a high binding capacity of up to 7 mg β2M per mL resin, which is 17 times higher than that of the previous studies.

Hemocompatible hemoadsorbent for effective removal of protein-bound toxin in serum

Resin hemoperfusion is a life-saving treatment for drug intoxication or hepatic failure of patients. However, current resin adsorbents exhibit a limited hemocompatibility or low adsorption efficiency, representing a major roadblock to successful clinical applications. In this work, we developed a hemocompatible and effective hemoadsorbent based on polystyrene resin (H103) microparticles encapsulated in anti-biofouling zwitterionic poly(carboxybetaine) (PCB) hydrogels. Apart from a strong mechanical stability, this PCB-based adsorbent (PCB-H103) exhibited excellent hemocompatibility (hemolysis ratio was ∼0.64%), which was attributed to the anti-biofouling property of PCB hydrogel. In addition, it can efficiently adsorb both small and middle molecular weight molecules in phosphate-buffered saline, and the efficiencies were significantly higher than poly(ethylene glycol) methacrylate-based and poly(2-hydroxyethyl methacrylate)-based adsorbent counterparts, indicating the favorable permeability of PCB hydrogel coating. More importantly, PCB-H103 could effectively remove protein-bound toxins including phenol red and bilirubin in bovine serum albumin solution or even in 100% fetal bovine serum (FBS). In 100% FBS, the adsorption capacity of PCB-H103 towards bilirubin was 8.3 times higher than that of pristine clinical-scale resin beads. Findings in this work may provide a new strategy for the development of modern resin hemoperfusion technology.

One-step Preparation of a VHH-based Immunoadsorbent for the Extracorporeal Removal of β2-microglobulin

Dialysis-related amyloidosis (DRA), which has been widely recognized to be associated with the accumulation of β2-microglobulin (β2-m) in blood, is one of the most common complications in patients receiving long-term dialysis treatment. The most significant side-effect of existing hemodialysis sorbents for the removal of β2-m from blood is the loss of vital proteins due to non-specific adsorptions. Although the traditional antibodies have the capability to specifically remove β2-m from blood, high cost limits their applications in clinics. Single domain antibodies derived from the Camelidae species serve as a superior choice in the preparation of immunoadsorbents due to their small size, high stability, amenability, simplicity of expression in microbes, and high affinity to recognize and interact with β2-m. In this study, we modified the anti-β2-m VHH by the formylglycine-generating enzyme (FGE), and then directly immobilized the aldehyde-modified VHH to the amino-activated beads. Notably, the fabrication is cost- and time-effective, since all the preparation steps were performed in the crude cell extract without rigorous purification. The accordingly prepared immunoadsorbent with VHHs as ligands exhibited the high capacity of β2-m (0.75 mg/mL). In conclusion, the VHH antibodies were successfully used as affinity ligands in the preparation of novel immunoadsorbents by the site-specific immobilization, and effectively adsorbed β2-m from blood, therefore opening a new avenue for efficient hemodialysis.

A double-lyophilization method for the preparation of CS/GO-COOH scaffold and its application in blood detoxification

The accumulation of uremic toxins in blood might induce chronic renal failure (CRF). The incidence of CRF was as high as 10%. The traditional therapy for CRF was hemodialysis, which was more effective to remove small molecules, such as urea and creatinine. However, this detoxification method ignored the tissue functional adaption due to the retention of macromolecule uremic toxins. To solve this problem, this paper developed a new kind of chitosan/carboxyl graphite oxide (CS/GO-COOH) scaffold via a double-lyophilization method. Then, this synthetic scaffold was characterized by Fourier transform infrared spectroscopy, scanning electron microscope, hydrophilic test, mechanical property, and in vitro detoxification test. Covalent bonding and hydrogen bonding were formed, indicating the strong interactions between CS and GO-COOH. There were interconnected networks in the synthesized scaffold. The mechanical test suggested that the GO-2500 scaffold had excellent mechanical strength, which was 7.41 ± 0.82 MPa with 25% shrink. What is more, GO-2500 could totally rebound within 1s, after compressed to 90% shrink. The rates of GO-2500 were 1587 ± 60 and 246 ± 10% according to the water uptake and retention data, respectively. Furthermore, the detoxification of GO-2500 to urea, creatinine, VB₁₂, and β₂-m were 67.59 ± 2.31, 39.67 ± 2.95, 31.51 ± 2.62, and 83.82 ± 7.76 mg/g, respectively. The resulting CS/GO-COOH scaffold held great potential for the detoxification of uremic toxins.

Controlling the Integration of Polyvinylpyrrolidone onto Substrate by Quartz Crystal Microbalance with Dissipation To Achieve Excellent Protein Resistance and Detoxification

Blood purification systems, in which the adsorbent removes exogenous and endogenous toxins from the blood, are widely used in clinical practice. To improve the protein resistance of and detoxification by the adsorbent, researchers can modify the adsorbent with functional molecules, such as polyvinylpyrrolidone (PVP). However, achieving precise control of the functional molecular density, which is crucial to the activity of the adsorbent, remains a significant challenge. In the present study, we prepared a model system for blood purification adsorbents in which we controlled the integration density of PVP molecules of different molecular weights on an Au substrate by quartz crystal microbalance with dissipation (QCM-D). We characterized the samples with atomic force microscopy, X-ray photoelectron spectroscopy, and QCM-D and found that the molecular density and the chain length of the PVP molecules played important roles in determining the properties of the sample. At the optimal condition, the modified sample demonstrated strong resistance to plasma proteins, decreasing the adsorption of human serum albumin (HSA) and fibrinogen (Fg) by 92.5% and 79.2%, respectively. In addition, the modified sample exhibited excellent detoxification, and the adsorption of bilirubin increased 2.6-fold. Interestingly, subsequent atomistic molecular dynamics simulations indicated that the favorable interactions between PVP and bilirubin were dominated by hydrophobic interactions. An in vitro platelet adhesion assay showed that the adhesion of platelets on the sample decreased and that the platelets were maintained in an inactivated state. The CCK-8 assay indicated that the modified sample exhibited negligible cytotoxicity to L929 cells. These results demonstrated that our method holds great potential for the modification of adsorbents in blood purification systems.

Polymeric Sorbents for Virucide Agents in Blood Treatments

Hydrophilic polystyrene (PS) type resins displaying excellent adsorption capacity for aromatic molecules in water were obtained by chemical modification of mesoporous cross-linked PS-type resins with a variety of hydrophilic reagents. The vinyl groups available on the surface of the resin beads were utilized as reactive sites for free radical grafting of hydrophilic moieties. Qualitative and semi-quantitative assessments of the functionalization degree were performed by Fourier transformed infrared (FTIR) spectroscopic analysis. Wetting and water uptake tests confirmed the occurrence of functionalization. The removal of an acridine virucide agent using the modified PS resins was tested under physiological conditions at pH 7.4. The virucide uptake capacity of the modified resins was in some cases almost quantitative. Antipyrogen treatment at 180°C for 8 h did not significantly affect the resin hydrophilicity, although a slight decrease in the adsorption capacity was detected. These modified resins were evaluated in hemofiltration procedures as virucide sorbents during the antiviral stage.

Absence of NF-κB Activation by a New Polystyrene-Type Adsorbent Designed for Hemoperfusion

AIM

The aim of the study was to evaluate biocompatibility of anew polystyrene-type adsorbent (BetaSorb) designed for hemoperfusion, using second-level biomolecular analyses. The device has recently been developed to enhance beta2-microglobulin removal during hemodialysis. Molecular structure and chemical modifications of the surface beads of this cartridge should prevent exposure of dense hydrophobic surface sites to proteins, and avoid the major drawbacks of previous polystyrene-type adsorbent materials.

METHODS

Whole blood of healthy donors was incubated in sterile minicolumns packed with BetaSorb Cuprophan, Hemophan, polysulfone and cellulose acetate. In parallel experiments, whole blood was recirculated for 180 min in a sham dialysis circuit equipped with the study sorbent or Hemophan or polysulfone. Biocompatibility was assessed by means of new biomolecular approaches focused on nuclear factor kappaB (NF-kappaB) activation (assessed by electrophoretic mobility shift assay), TNF-alpha and IL-1beta gene expression (evaluated by real-time PCR), TNF-alpha and IL-1beta production (measured by Western blot assay and ELISA), nitric oxide (NO) generation (detected by electron paramagnetic resonance), free oxygen radical production (by chemiluminescence in a biological assay) and the generation of the complement breakdown product C3d.

RESULTS

In coincubation experiments, 5-min contact with any dialysis device, but BetaSorb, was enough to induce activation of NF-kappaB. The amount of TNF-alpha precursor form was found to increase after 5 min of exposure to each tested polymer, but no traces of mature forms of TNF-alpha or IL-1beta were detected in in vitro experimental conditions using healthy blood. NO and free oxygen radical generation were significantly lower in blood samples exposed to BetaSorb than in control dialysis devices. C3d levels were found to be increased with Hemophan, unaffected by polysulfone, and remarkably decreased with the BetaSorb device. In the sham hemodialysis experiments, NF-kappaB activation and C3d and NO profiles were similar to direct incubation experiments. Compared to basal levels, quantitation of TNF-alpha and IL-1beta mRNA revealed a 15- and 9-fold increase, respectively, in samples exposed to Hemophan for 180 min.

CONCLUSIONS

The new BetaSorb device not only appears to be highly biocompatible, but shares properties that make it probably able to interfere with the activation of the inflammatory state.

Extracorporeal strategies for the removal of middle molecules

Uremic toxins with a molecular weight of less than 500 Da are classified as small nitrogenous waste products. They are highly water soluble, relatively homogeneous, and have no protein binding. Other uremic retention toxins differ significantly from the small nitrogenous metabolite class in molecular weight, heterogeneity, protein binding, and hydrophobicity. The European Uremic Toxin Work Group subdivided molecules into two categories: protein-bound solutes and middle molecules. Middle molecules were defined as toxins in the molecular weight range of 500-60,000 Da, which exceeds the molecular weight of 2000 Da defined in the original middle molecule hypothesis. Under this new proposed definition, most of these middle molecules are low molecular weight peptides and proteins (LMWPs). This concise review focuses on LMWPs. The metabolism of LMWPs is described, including molecular weight, physical conformation, and charge. Factors influencing dialytic removal of LMWPs such as membrane characteristics, protein-membrane interactions, and solute removal mechanisms, as well as strategies to enhance clearance of these compounds are discussed.

Adsorption Mechanism at the Molecular Level between Polymers and Uremic Octapeptide by the 2D1H NMR Technique

To remove uremic octapeptide from the blood stream of uremic patients, various modified polyacylamide cross-linked absorbents were prepared. Adsorption experiments showed these absorbents have significant differences in adsorption capacity to the target peptide. In this paper, two-dimension proton nuclear magnetic resonance (2D 1H NMR) spectroscopy was used to investigate the interaction mechanism between the peptide and the adsorbents. Because of the insolubility of the absorbent, some soluble linear polymers with the same functional groups as the absorbents were employed as the model adsorbents in 2D 1H NMR. The preferred binding site for the peptide and polymers was identified to be at the C-terminal carboxyl group of the octapeptide via chemical shift perturbation effects. In this study, we found that hydrogen bonding, electrostatic, and hydrophobic interactions all play a role in the interaction force but had different contributions. Especially, the great chemical shift changes of the aromatic amino acid residues (Trp) during the interaction between butyl-modified polyacrylamide and octapeptide suggested the hydrophobic interaction, incorporated with the electrostatic force, played an important role in the binding reaction in aqueous solutions. This information not only rationally explained the results of the adsorption experiments, but also identified the effective binding site and mechanism, and shall provide a structural basis for designing better affinity-type adsorbents for the target peptide.

Adsorption of the uremic toxin p-cresol onto hemodialysis membranes and microporous adsorbent zeolite silicalite

Para-cresol CH3C6H4OH is a protein-bound solute which is not eliminated efficiently by hemodialysis systems. In this study, we present adsorption of p-cresol as a complementary process to hemodialysis. The kinetics and isotherms of adsorption onto cellulose-based membranes (cellulose diacetate and triacetate), synthetic membranes (polyamide, polysulfone, polyacrylonitrile and polymethylmethacrylate) and microporous zeolite silicalite (MFI), have been evaluated in static conditions. The results indicate that p-cresol has a low affinity to all membranes but polysulfone and polyamide and that the times to reach equilibrium conditions are slow. In contrast, equilibration time on silicalite is fast (2 min to eliminate 90%) while adsorption levels are high (maximum adsorption about 106 mg g(-1)). Adsorption onto microporous adsorbents could be a novel way to eliminate uremic toxins from blood.

Assessment of the Stability of an Immunoadsorbent for the Extracorporeal Removal of Beta-2-Microglobulin from Blood

BACKGROUND

Dialysis-related amyloidosis (DRA) is a devastating and costly condition that affects patients with end stage kidney disease. A key feature of DRA is the formation of amyloid fibrils, consisting primarily of beta2-microglobulin. Except for kidney transplantation, conventional kidney replacement therapies, which are based on nonspecific mechanisms, do not adequately address beta2-microglobulin removal. An antihuman beta2-microglobulin single-chain variable region antibody fragment (scFv) was developed to confer specificity to beta2-microglobulin removal during hemodialysis.

METHODS

The scFv was immobilized onto agarose and characterized for beta2m binding capacity, thermal stability at 37 degrees C, regeneration capacity, storage conditions, and sterility.

RESULTS

The beta2-microglobulin binding capacity was 1.3 mg/ml scFv gel. The immunoadsorbent is thermally stable, can be regenerated, stored short-term in 20% ethanol, lyophilized for long-term storage, and withstand process conditions similar to that of a patient's hemodialysis therapy.

CONCLUSIONS

The results support further investigation of immobilized scFvs as a novel tool to remove beta2-microglobulin from blood.

Preparation of novel mesoporous carbons for the adsorption of an inflammatory cytokine (IL-1β)

Mesoporous carbons derived from two types of sulphonated styrene divinylbenzene copolymers (Macronet MN500HS and CT275, Purolite International Ltd) were produced and their adsorptive capacity for the proinflammatory cytokine IL-1 beta (MW 14.4 kDa) determined. The carbons produced had surface areas from 400 to 1200 m(2)g(-1) and pore volume between 0.2 and 1.4 cm(3)g(-1). The mechanical strength of the carbon beads with surface area values up to 800 m(2)g(-1) were robust. The highest adsorption value of IL-1 beta was 150 ng g(-1) for a mesoporous carbon with surface area around 900 m(2)g(-1) and pore volume around 1.3 cm(3)g(-1). However, there was a trade-off between adsorptive capacity and mechanical strength. When used in conjunction with existing treatment modalities, the materials produced have the potential to enhance the removal of uraemic toxins.

Single-chain antibody fragment-based adsorbent for the extracorporeal removal of β2-microglobulin

BACKGROUND

Dialysis-related amyloidosis (DRA) is a frequent complication of end-stage renal disease (ESRD) that has been associated with the accumulation of beta2-microglobulin (beta2-m). Removal of beta2-m results in the loss of important proteins due to the nonspecific nature of current therapies. Although whole antibodies can potentially be used to confer specificity to beta2-m removal from blood, single-chain variable region (scFv) antibody fragments could potentially offer several advantages as immunoadsorption ligands due to their size, genetic definition, ability to be expressed by microbes, and amenability for in vitro evolution.

METHODS

An antihuman beta2-m scFv was constructed from the BBM.1 hybridoma and expressed by a yeast display vector. The binding affinity of the wild-type scFv fragment was quantified by flow cytometry analysis. Soluble scFv was expressed by a yeast secretion vector, purified, and immobilized onto agarose beads. The binding capacity of the immunoadsorbent was measured by equilibrating samples with saturating quantities of fluorescent beta2-m in serum.

RESULTS

The displayed scFv possessed a nanomolar affinity (KD= 0.008 +/- 0.004 mg-beta2-m/L). The immunoadsorbent exhibited an adsorption site density of 0.41 +/- 0.01 mg beta2-m/mL settled gel. Under saturating conditions, the mass ratio of adsorbed beta2-m to immobilized antibody is 70% greater than any previous literature report for whole antibodies. Preliminary specificity experiments suggest that the scFv-based immunoadsorbent is specific toward human beta2-m.

CONCLUSION

Recombinant DNA technology was successfully used to engineer an scFv-based immunoadsorbent. Use of immobilized scFvs during hemodialysis may minimize loss of valuable proteins and facilitate the removal of macromolecules that are significantly larger than the molecular weight cut-off of the membrane.

Middle molecules and small-molecular-weight proteins in ESRD: properties and strategies for their removal

Molecular weight has traditionally been the parameter most commonly used to classify uremic toxins, with a value of approximately 500 Da frequently used as a demarcation point below which the molecular weights of small nitrogenous waste products fall. This toxin group, the most extensively studied from a clinical perspective, is characterized by a high degree of water solubility and the absence of protein binding. However, uremia is mediated by the retention of a plethora of other compounds having characteristics that differ significantly from those of the previously mentioned group. As opposed to the relative homogeneity of the nitrogenous metabolite class, other uremic toxins collectively are a very heterogeneous group, not only with respect to molecular weight but also other characteristics, such as protein binding and hydrophobicity. A recently proposed classification scheme by the European Uraemic Toxin Work Group subdivides the remainder of molecules into 2 categories: protein-bound solutes and middle molecules. For the latter group, the Work Group proposes a molecular weight range (500-60,000 Da) that incorporates many toxins identified since the original middle molecule hypothesis, for which the upper molecular weight limit was approximately 2,000 Da. In fact, low-molecular-weight peptides and proteins (LMWPs) comprise nearly the entire middle molecule category in the new scheme. The purpose of this article is to provide an overview of the middle molecule class of uremic toxins, with the focus on LMWPs. A brief review of LMWP metabolism under conditions of normal (and in a few cases, abnormal) renal function will be presented. The physical characteristics of several LMWPs will also be presented, including molecular weight, conformation, and charge. Specific LMWPs to be covered will include beta 2-microglobulin, complement proteins (C3a and Factor D), leptin, and proinflammatory cytokines. The article will also include a discussion of the treatment-related factors influencing dialytic removal of middle molecules. Once these factors, which include membrane characteristics, protein-membrane interactions, and solute removal mechanisms, are discussed, an overview of the different therapeutic strategies used to enhance clearance of these compounds is provided.

Effect of a novel adsorbent on cytokine responsiveness to uremic plasma

BACKGROUND

Middle molecules such as beta2-microglobulin (beta2M) and advanced glycation end products (AGE)-modified proteins contribute to inflammation in uremia. The BetaSorb column is a new adsorptive device, which contains copolymeric beads, suitable for removal of beta2M and other middle molecules. We assessed the effect of this column on the bioreactivity of uremic plasma, as measured by cytokine responsiveness.

METHODS

Uremic plasma was perfused in vitro through the column (10 mL/min) and samples were collected after 10 to 30 passes. Endotoxin-stimulated tumor necrosis factor-alpha (TNF-alpha) and interleukin-10 (IL-10) production by THP-1-derived monocytes was measured following brief exposure to uremic plasma. beta2M levels were measured. The contribution of AGE-modified proteins to the bioreactivity of uremic plasma was explored.

RESULTS

TNF-alpha and IL-10 production markedly decreased after 30 passes (629 +/- 78 vs. 144 +/- 62 pg/mL; 207 +/- 25 vs. 117 +/- 23 pg/mL; P=0.04). The column removed beta2M efficiently with a marked decline in plasma levels by 99% after 30 passes. Neutralization of AGE receptor (RAGE) resulted in a further reduction in the bioreactivity of uremic plasma. This was observed with nonperfused, as well as perfused, uremic plasma, suggesting that AGE-modified proteins were biologically active and still present after perfusion.

CONCLUSION

The sorbent beads removed uremic solute(s) that prime monocytes to enhanced cytokine production. Removal of beta2M was efficient, and of native and AGE-modified middle molecules likely.

Hypercross-linked polystyrene and its potentials for liquid chromatography: a mini-review

Hypercross-linked polymeric adsorbing materials are obtained under conditions that (i) their polymeric network is formed in the presence of large amounts of a thermodynamically good solvent (porogen) and (ii) the network is rigid. Hypercross-linked polystyrene is a transparent microporous low-density material with an apparent inner surface area of over 1,000 m2/g and an unprecedented adsorption capacity. To enhance the mass transfer, the adsorbent beads may be provided with large transport pores, in addition to the inherent micropores; these beads are opaque. Hypercross-linked polystyrene sorbents are widely used for large scale adsorption of organic compounds from aqueous and gaseous media and for solid-phase extraction of trace components. Novel perspective application areas of the materials are high-performance liquid chromatography column packings and blood purification. Present mini-review summarises basic principles of obtaining hypercross-linked materials, their structural peculiarities and distinguishing properties, as well as major application areas. Important new unpublished data are also included.

Sorbent hemoperfusion in end-stage renal disease: an in-depth review

The excessive mortality in dialysis patients has rekindled interest in research of adsorbent removal of nontraditional uremic toxins. Middle-molecular-weight substances, predominantly small proteins, have been correlated with specific uremic syndromes and implicated in the uremic state. New developments in polymer technology and carbon pyrrolization techniques have produced sorbents possessing mesopores of sufficient size to trap middle-molecular-weight substances. Clinical application of hemoperfusion devices containing these sorbents is early in its development. Studies related to hemoperfusion in uremia are discussed in detail.

A novel immunoadsorption device for removing beta2-microglobulin from whole blood

BACKGROUND

High plasma levels of beta2-microglobulin (beta2m) have been implicated in the formation of the severely destructive and potentially fatal amyloid deposits that are characteristic of dialysis-related amyloidosis (DRA). Conventional renal replacement technologies remove insufficient quantities of beta2m to normalize plasma levels. This limitation arises because of nonspecific adsorptive qualities and reliance on size exclusion, which can also remove other middle molecular weight proteins. These nonspecific approaches also make it difficult to evaluate the role and contribution of middle molecular weight molecules to the pathology of DRA and other morbidities of end-stage renal disease. A high-affinity and biologically specific approach could target a protein, prevent a significant loss of other important molecules, and improve the apparent adsorption rate within an extracorporeal device.

METHODS

Agarose-immobilized murine anti-human beta2m monoclonal antibodies were used in a Vortex Flow Plasmapheretic Reactor (VFPR) to remove donor baseline and controlled amounts of recombinant beta2m from human blood in vitro. The extracorporeal circuit was hemoperfused at 200 mL/min for two hours.

RESULTS

The immunoadsorptive media had a binding site density of 30 microg beta2m per mL of settled gel. The VFPR cleared baseline quantities of donor beta2m below detectable limits of the assay. The experiments with higher initial beta2m concentrations reached an equilibrium concentration within 20 minutes, corresponding to a 92% clearance. No deleterious hemocompatibility issues were observed (complete blood count, total protein, and plasma free hemoglobin).

CONCLUSIONS

The adsorptive kinetics of the VFPR are optimal for the conditions used and support the use of immunoadsorption for the removal of beta2m.

Modalities for the removal of beta 2-microglobulin from blood

The long-term accumulation of beta(2)-microglobulin (beta(2)M) in patients with kidney failure results in a debilitating condition referred to as dialysis-related amyloidosis (DRA). There have been few methods specifically designed to remove the large quantities of beta(2)M that are produced by the body. This article briefly reviews current modalities and concepts for the removal of beta(2)M from blood. The various approaches are classified according to the mechanism of beta(2)M clearance. The potential application of immunoadsorption, a biologically specific approach to remove macromolecules, in the treatment and understanding of DRA is discussed.