Possibilities of selective and unselective adsorbent development in blood purification

Dieter Falkenhagen (1942–2015): A Multifaceted Scientist

Dieter Falkenhagen was born in 1942 in Dresden, Germany and died in 2015. He specialized in internal medicine and nephrology. Focusing on artificial organ research, he investigated various aspects of the efficacy and safety of hemodialysis and adsorption technologies, including biocompatibility issues related to blood versus surface interactions and the adverse effects of endotoxin contamination. He studied various mathematical models to analyze efficacy and safety, and animal models to help clarify uncertainty issues. Through his studies, adsorbents were developed, resulting in Prometheus, an artificial liver support device. Anticoagulation models, including citrate perfusion, were improved and made safer by his work. He also stepped into bioreactor research to increase efficacy of liver support devices.

Pore Size – a Key Property for Selective Toxin Removal in Blood Purification

Purpose

Extracorporeal blood purification systems based on combined membrane/adsorption technologies are used in acute liver failure to replace detoxification as well as to remove inflammatory mediators in sepsis patients. In addition to coating and chemical modification of the surface, pore size significantly controls the selectivity of adsorption materials.

Methods

This study addresses the adsorption of albumin bound liver toxins, cytokines, and representative plasma compounds on three adsorbents which differ only in pore size distribution. All three adsorbents are based on hydrophobic poly(styrene-divinylbenzene) copolymer matrices and have mean pore sizes of 15, 30, and 100 nm.

Results

The pores of adsorbents act as molecular sieves and prevent the entry of molecules that are larger than their molecular cut-off. The results of this study reveal that adsorbents based on styrene-divinylbenzene polymers with 15 nm pores are suitable for cytokine removal, and the same adsorbents with 30-40 nm pores are the best choice for the removal of albumin-bound toxins in the case of liver failure. Adsorbents with very large pores lack selectivity which leads to uncontrolled adsorption of all plasma proteins. Therefore, hydrophobic adsorbents with large pores offer inadequate plasma compatibility and do not fulfill the requirements for blood purification.

Conclusions

Biocompatibility and efficiency of adsorbents used for blood purification can improved by fine tuning of adsorbent surface pore distributions.

Fluidized Bed Adsorbent Systems for Extracorporeal Liver Support

Acute liver failure based on acute-on-chronic liver failure (AoCLF) or on acute severe damage of the liver caused by different etiologies includes complex mechanisms resulting in severe disturbances of principle liver functions. In order to compensate the liver's function of detoxification as efficiently as possible, fluidized bed absorbent systems have been designed. In these systems, small particles with specific adsorption properties for toxins related to acute liver failure are applied. A special technology based on adsorbents in suspension has been developed under the guidance of our group and is prepared for clinical application during the coming year. This technology is called microspheres-based detoxification system (MDS) and is based on microadsorbents with a diameter of 1-10 microm which are recirculated in suspension. The safety of the MDS is guaranteed by the use of fluorescently labeled magnetic microparticles, which in case of a membrane-leakage are detected in the blood circuit by an optical system equipped with a magnetic trap. In vitro tests with two kinds of microadsorbents (a combination of a hydrophobic neutral resin and an anion exchange resin) showed excellent efficiency of the system with respect to adsorption capacity as well as to the kinetics of elimination of albumin-bound substances (e.g. unconjugated bilirubin or cholic acid) and of non-protein-bound substances (e.g. phenol). Moreover, using a plasma filter or the Albuflow filter as membrane filters in the blood circuit, the MDS technology offers the possibility to remove inflammatory mediators such as tumor necrosis factor-alpha (TNF) by additional use of specific adsorbents.

Apheresis of plasma compounds as a therapeutic principle in severe sepsis and multiorgan dysfunction syndrome

During sepsis there is an increase in the plasma content of several compounds, e.g., bacterial toxins, cytokines, cell debris, free hemoglobin and myoglobin. In blood, these compounds activate various cascade systems, which in large amounts or in more vulnerable patients lead to a disseminated intra-vascular coagulopathy (DIC) with multiorgan dysfunction syndrome (MODS) and death, despite conventional intensive care unit therapy. Therapeutic attempts to reverse these conditions have so far been of limited benefit. These effects have mainly been focused on lowering the blood concentration of single substances such as tumor necrosis factor. By the use of low-and high-flux hemodialysis filters, usually only small amounts of these substances are removed. By the use of plasmapheresis or plasma exchange, the extent of removal is considerably increased. The efficacy varies between the techniques (centrifugation vs. filtration or adsorption) and has also different influences on e.g. the complement system. This report describes these techniques and the therapeutical possibilities given by them. In small trials, blood or plasma exchange has been used as rescue therapy in critically ill patients with a progressive MODS and DIC. A survival of about 80% of the patients has been reported in these studies and the use of combined therapy will be discussed. Controlled trials are required in this field.

Fractionated plasma separation and adsorption system: a novel system for blood purification to remove albumin bound substances

The removal of albumin bound substances has gained increasing interest in different diseases, especially in acute and chronic liver disease. Therefore, a new system, the fractionated plasma separation and adsorption (FPSA) system, was developed based on combined membrane and adsorbent blood purification techniques. The most important contribution to the FPSA system was the development of a new polysulfone hollow-fiber filter, which is characterized by a sieving coefficient of 0.89 for human serum albumin (HSA) but only of 0.17 for fibrinogen, and 0 (zero) for IgM immunoglobulins. Using a closed filtrate circuit connected to the new polysulfone filter which integrates 1 or 2 adsorption columns and also a high flux dialyzer adapted to a dialysis machine, the FPSA system opens excellent possibilities for the relatively specific removal of albumin bound substances from the blood such as albumin bound bilirubin or even tryptophan. In comparison to other systems (for example, the Molecular Adsorbent Recirculating System [MARS] and albumin dialysis systems), the FPSA system enables much higher elimination of strongly bound albumin substances. The first clinical investigations have recently started based on a modified dialysis machine designed with all necessary safety measures.

Extracorporeal adsorbent-based strategies in sepsis

Gram-negative bacterial sepsis remains a challenging diagnostic and therapeutic dilemma to the practicing clinician. Bacterial-derived products (eg, gram-negative bacterial lipopolysaccharide or endotoxin) and host inflammatory mediators (eg, tumor necrosis factor-alpha and interleukin-1) are believed to play a pivotal role in the pathogenesis of sepsis and septic shock. Despite the many advances in the treatment of sepsis, mortality rates in septic patients remain high. Indeed, numerous clinical trials using biologically engineered immunotherapies targeting specific inflammatory mediators have proven unsuccessful. This lack of success has led to a renewed interest in blood purification techniques using extracorporeal therapies. During sepsis, circulating bacterial-derived products as well as inflammatory mediators can be reduced and/or eliminated by various extracorporeal adjunctive therapies such as plasma exchange, continuous renal replacement, and adsorbent-based therapies. Adsorbents have commonly been used orally for gastrointestinal removal of toxins or drugs. However, their potential use in sepsis has received little attention. The incorporation of adsorbents in hemoperfusion columns has allowed their use for the removal of toxic compounds from the circulatory system. Adsorbents developed for use in sepsis can bind toxins in a nonselective (eg, charcoal), selective (eg, polymyxin B-immobilized polystyrene-derivative fiber), or specific (eg, antibody-coated microsphere-based detoxification system) way. However, despite an explosive development in the experimental use of these promising therapies, randomized clinical trials are currently lacking. In summary, a multi-disciplinary complex therapeutic approach remains a prerequisite to the successful treatment of sepsis.

Extracorporeal endotoxin removal by immobilized polyethylenimine

The neutralization of bacterial endotoxins (ET) is still an unsolved problem in therapeutic medicine. The efficacy of anti-endotoxin antibodies or receptor antagonists and other substances interfering with the endotoxin-induced pathomechanisms is dependent on an intact cellular degradation system of the host. However, the phagocytosis function of that system seems to be impaired regularly in patients with intense or long-lasting endotoxemia or septic shock and in patients undergoing hemodialysis. Extracorporeal adsorption of ET might well be an effective support in the anti-ET therapy by lowering the amount of circulating ET and thus relieving the defense system of the body. In this work a new ET-adsorbent based on macroporous cellulosic beads with immobilized polyethylenimine (PEI) was tested for its ET-removal capacity in vitro. A test solution with 100 ng/ml ET from Escherichia coli 055:B5 was recirculated in a system containing the adsorbent beads. Polymyxin B immobilized to the same carrier was used for comparison. PEI as well as polymyxin B showed complete removal of ET from plasma and water as was measured by the Limulus Amebocyte Lysate (LAL) test (Chromogenix). The biocompatibility of the PEI absorber was superior to that of polymyxin B. The results indicate that the PEI absorber is of high efficacy and possibly of interest for the treatment of endotoxemia.