Enzymatic detoxification using lipophilic hollow-fiber membranes: I. Glucuronidation reactions

Enzymatic Detoxification using Lipophilic Hollow-Fiber Membranes: IV. Glutathione Conjugation Reactions

A hollow-fiber technique was used in the enzymatic glutathione conjugation of lipophilic toxins. Native enzyme was circulated on the external side of a lipophilic hollow-fiber membrane while the toxin-containing media (blood, plasma or aqueous solution) were circulated inside the fiber. Glutathione conjugation reactions were catalyzed by rat liver cytosol, with a specific glutathione transferase activity of 40 nmol/min/mg protein (acceptor: 1,2-epoxy-3-(p-nitrophenoxy)propane). Clearance rates of 1,2-epoxy-3-(p-nitrophenoxy)propane, phenylglycidether, styrene oxide, cis-9, 10-epoxystearic acid, cis-9, 10-epoxystearic acid methyl ester, 5a, 6a-cholesterol oxide, 16-, 17a-pregnenolone oxide, and p-nitrobenzylchloride were 10.44, 13.37, 32.25, 7.60, 7.31, 3.92, 4.20 and 29.24 nmol/mg protein/h/cm2 hollow-fiber surface respectively. This technique makes possible glutathione conjugation reactions with crude enzyme preparations over long periods without loss of activity from covalent immobilization and without loss of cofactor from (auto)oxidation. The lipophilic membrane ensures the absence of hemolysis, immunological hazards and hormone loss, while elimination of the toxin is not impaired.

Liver support systems today

A májelégtelenség – akár korábbi májbetegség fennállása nélkül alakult ki (akut májelégtelenség), akár krónikus májbetegség akut dekompenzációja („akut a krónikuson” májelégtelenség) következménye – magas halálozással jár. A végállapotú májbetegségek következtében kialakult májelégtelenség egyetlen kuratív megoldása ma a májtranszplantáció. Ennek fő gátját a rendelkezésre álló donorszervek hiánya képezi, emiatt sok, várólistán szereplő beteg exitál. A transzplantáció korlátai tették szükségessé olyan májtámogató rendszerek kifejlesztését, amelyek alkalmasak a beteg életben tartására a szervátültetésig vagy a máj regenerációjáig. A korai próbálkozások (hemodialízis, hemoperfúzió, cseretranszfúzió, kereszthemodialízis, keresztkeringés, plazmaferézis stb.) elégtelennek bizonyultak. Napjainkban a májpótló kezelésnek két fő iránya alakult ki: a sejtalapú, úgynevezett bioarteficiális és a nem sejtalapú, úgynevezett arteficiális rendszerek. A bioarteficiális rendszerek élő állati májsejteket vagy emberi májtumorsejteket tartalmaznak. Jellegzetességük, hogy a beteg vérét vagy szeparált plazmáját a májsejteket tartalmazó bioreaktoron áramoltatják át. Elviekben a májműködést ezek a metodikák modellezik a legtökéletesebben, mert a máj szintetizáló- és detoxikálófunkcióját egyaránt pótolják. Jelenlegi formájukban azonban még távol állnak az ideális megoldástól, alkalmazásuk számos immunológiai, infektológiai, onkológiai és financiális problémát vet fel, ezért egyelőre csak kísérleti célra állnak rendelkezésre. Az arteficiális rendszerek a klinikum számára már elérhetőek, bár széles körben még nem terjedtek el. Csak a máj detoxikálófunkcióját pótolják, a szintetikus funkció részben a hiányzó anyagok (plazmaproteinek, alvadási faktorok) szubsztitúciójával pótolható. Idetartozik a hemodiabszorpció, amely az Amerikai Egyesült Államokban terjedt el (liver dialysis unit), valamint a főleg Európában használatos albumindialízis és a legújabban kifejlesztett frakcionált plazmaszeparáció és -adszorpció (FPSA). Az albumindialízis egyszerű módszere a „single pass albumin dialysis” (SPAD), ennek továbbfejlesztett változata a „molecular adsorbent recirculating system” (MARS). Az FPSA high-flux hemodialízissel kiegészített változata a Prometheus-rendszer. Bár a felsorolt módszerek hatásosságát számos kísérleti és klinikai tanulmány támasztja alá, a konzervatív kezeléssel szemben a túlélésre kifejtett előnyös hatásuk bizonyítására még nagy esetszámot felölelő, randomizált, kontrollált vizsgálatok elvégzésére van szükség.

Artificial and bioartificial liver support: A review of perfusion treatment for hepatic failure patients

Liver transplantation and blood purification therapy, including plasmapheresis, hemodiafiltration, and bioartificial liver support, are the available treatments for patients with severe hepatic failure. Bioartificial liver support, in which living liver tissue is used to support hepatic function, has been anticipated as an effective treatment for hepatic failure. The two mainstream systems developed for bioartificial liver support are extracorporeal whole liver perfusion (ECLP) and bioreactor systems. Comparing various types of bioartificial liver in view of function, safety, and operability, we concluded that the best efficacy can be provided by the ECLP system. Moreover, in our subsequent experiments comparing ECLP and apheresis therapy, ECLP offers more ammonia metabolism than HD and HF. In addition, ECLP can compensate amino acid imbalance and can secret bile. A controversial point with ECLP is the procedure is labor intensive, resulting in high costs. However, ECLP has the potential to reduce elevated serum ammonia levels of hepatic coma patients in a short duration. When these problems are solved, bioartificial liver support, especially ECLP, can be adopted as an option in ordinary clinical therapy to treat patients with hepatic failure.

Development and perspectives of perfusion treatment for liver failure

To treat patients with severe liver failure, liver transplantation and blood purification therapy, including plasmapheresis, hemodiafiltration, and bioartificial liver support, are available. The two mainstream systems developed for bioartificial liver support are extracorporeal whole liver perfusion (ECLP) and the bioreactor system (BIS). We developed a method of cross-plasma perfusion, in which plasma is exchanged between the blood circuit of the patient and that of a hepatic functioning unit, through which immunologically free whole human blood is perfused. From the aspects of efficacy and epidemic safety, the best system of bioartificial liver support for clinical use is considered to be ECLP in cross-plasma perfusion. In opposition, a social antagonist for zoonosis has consistently been raised, with controversy surrounding the use of xenogeneic organs for human treatment, which might be final obstacle. It is possible that the combination therapy of hemodiafiltration and the administration of human serum albumin and anticoagulant factors, which minimizes the economic and medical resource costs through the development of transgenic livestock that secrete human pharmaceuticals systemically, will become a more desirable and practical treatment for patients with severe liver failure.

Diffusion characteristics of chitosan-entrapped microsomal UDP-glucuronyl transferase gel beads

Rabbit hepatic microsomal UDP-glucuronyl transferase (EC 2.4.1.17) was immobilized by entrapment in chitosan which is an ionotropic gelation agent and the resulting preparation was used as a biocatalyst for the glucuronidation of 1-naphthol in order to study the drug metabolism in vitro and obtaining artificial liver support (detoxification). In this study, depending on the porous structure of the chitosan/UDPGT gel beads, mainly the diffusion characteristics of 1-naphthol from chitosan gel beads in well stirred solutions were investigated, and the values of the effective diffusion coefficients (Dc) indicate that UDP-glucuronyl transferase in chitosan beads has no additional diffusion barrier. Using the data, the capacity of the chitosan-gel as an immobilization matrix for microsomal UDP-glucuronyl transferase is discussed from the point of view of diffusion characteristics.

Substrate specificity and the use of chitosan-entrapped rabbit hepatic microsomal UDP-glucuronyl transferase for detoxification

Rabbit hepatic microsomal UDP-Glucuronyl transferase (EC.2.4.1.17) was immobilized in chitosan, which is an ionotropic gelatation agent, and the resulting preparation was used as a biocatalyst for the glucuronidation of toxic substances and drugs such as; 1-naphthol, phenol, 4-nitrophenol, bilirubin, testesterone, estrone, lamotrigine, imipramine and chlorpromazine. For studying the drug metabolism in vitro and obtaining artificial liver support (detoxification) chitosan-entrapped UDPGT towards 1-naphthol was tested by using two model reactor system. The conversion curves of 1-naphthol to its glucuronide indicated that the column reactor system was found to be better and seems suitable for detoxification with a high yield of glucuronide formation.

Preparation and characterization of chitosan-entrapped microsomal UDP-glucuronyl transferase

The hepatic microsomal UDP-Glucuronyl transferase which catalyze the glucuronidation of drugs, pesticides, carcinogens and other xenobiotics, was immobilized by entrappment in chitosan. Chemical and physical characterization were made by using 1-naphthol as substrate. Thermal, storage and operational stabilities of the immobilized enzyme was also searched and found to be better in comparison with the free enzyme. In conclusion, chitosan gel beads appear to have good characteristics for use as UDPGT immobilization support suitable for large scale use and offer the prospect that immobilized UDPGT may become an important form of catalyst in medicine.

Assessment of artificial liver support technology

Despite more than 30 yr of research and development, an artificial liver has still not yet become clinical reality. Although previous attempts using a multiplicity of techniques including hemodialysis, hemoperfusion, plasma exchange, extracorporeal perfusion, and crosshemodialysis have shown minor improvement in patients with acute hepatic failure, limited clinical trials have failed to demonstrate any survival benefit. Encouraged by the progress on techniques that maintain long-term cultures of hepatocytes, more recent efforts have been directed at the use of hepatocytes as the basis of liver support. This review takes a critical look at past and present concepts in the development of artificial liver supports and both qualitatively and quantitatively evaluates the advantages and disadvantages of the available methodology.

Evolution of the bioartificial liver: the need for randomized clinical trials

The pursuit of a bioartificial liver is well documented in the literature. Early techniques of artificial liver support that have undergone clinical testing included simple exchange transfusions, extracorporeal xenogeneic or allogeneic liver perfusion, cross-circulation, hemodialysis, charcoal hemoperfusion, and plasmapheresis with plasma exchange. These techniques failed because they were unable to adequately support those hepatic functions essential for survival and because they lacked a back-up therapy, such as liver transplantation, for irreversible forms of liver disease. The concept evolved that hepatic functions essential for survival would be best performed by hepatocytes in an apparatus that allowed sustained or repetitive application. The best results have been achieved with bioartificial liver technologies that employ hepatocytes as implantable systems or extracorporeal devices. Implantable bioartificial liver systems include hepatocytes that have been on coated microcarrier beads, within microencapsulated gel droplets, within biodegradable polymeric substrates, or as spheroid hepatocyte aggregates. Extracorporeal systems include hepatocytes in suspension, on flat plates, and in hollow fiber bioreactors. Several extracorporeal systems have undergone extensive animal testing and are entering the early stages of human clinical trials. Randomized trials are needed to establish the value of bioartificial liver support in the treatment of patients with acute hepatic failure or as a bridge to liver transplantation.

Dialysis against a recycled albumin solution enables the removal of albumin-bound toxins

The removal of protein-bound substances of pathogenetic relevance from blood is of therapeutic interest for drug intoxications, renal and liver failure, and metabolic disorders. Current methods using adsorbents are effective but often not specific enough. This work presents an alternative method that enables the dialyzability of albumin-bound toxins from plasma by the use of a high-flux dialyzer (F 60 Fresenius) and an albumin solution circulating on the dialysate side to increase selectively the affinity for albumin-bound toxins. This method resulted in effective removal of unconjugated bilirubin, drugs with a high protein-binding ratio (sulfobromophthalein, theophylline), and a protein-bound toxin (phenol). The additional removal of PBS could extend the applicability of dialysis, for example, to drug intoxications and liver failure or could improve the elimination of protein-bound uremic toxins in chronic renal failure.

Long-term cultures of adult mammalian hepatocytes in hollow fibers as the cellular component of extracorporeal (hybrid) liver assist devices

A discussion of the treatment of liver insufficiency with extracorporeal (hybrid) liver assist devices (LADs) should address a definition of the types of liver failure susceptible to being treated by these devices as well as the modalities of in vivo and in vitro testing. Relevant to the first subject is the subject of pathogenesis of hepatic coma, which should be the target for the design of these LADs. Although this modality of therapy is new, it can be predicted that these devices will demand minimal safety conditions, i.e., the seeding with cells that are not tumorigenic or carrying viral particles. Among other topics to be considered in the development of LADs is the proper choice of hollow fiber to be used and the testing on proper animal models of hepatic failure. It is our philosophy that the long-term culture of adult mammalian hepatocytes in hollow fibers is the basis for appropriate designs of this type of temporary liver support.

Enzymatic detoxification using lipophilic hollow-fiber membranes: III. Oxidation reactions of sulfides

A lipophilic hollow-fiber membrane preparation that was previously described for glucuronidation and sulfation reactions was used for the enzymatic oxidation of sulfides. Endogenous and exogenous toxins in buffer solution or in serum or blood of intoxicated animals were circulated through the internal lumen of lipophilic hollow fibers. Native liver microsomes of the rabbit were circulated on the outside of the hollow fibers. Lipophilic toxins accumulate in and penetrate through the lipophilic membrane and the toxins are oxidized by the mixed function oxygenase system of liver microsomes. The oxidized products cannot rediffuse to the donor side. The endogenous toxin dimethylsulfide (DMS) was converted on the enzyme side to dimethylsulfoxide (DMSO) and small amounts of dimethylsulfone, which are hydrophilic and nontoxic substances. Other sulfide compounds, ethylmethylsulfide (EMS) and s-butylmethylsulfide (BMS), have also been converted to their oxidized forms. The enzymatic clearance of the hollow-fiber module for DMS in in vivo experiments in the rabbit was found to be 1.30 nmol/h/mg protein/cm2 hollow-fiber surface. The transmembranous enzymatic clearance of the in vitro oxidation reactions of DMS, EMS, and BMS in buffer solutions (open circuit) were measured, respectively, as 1.63, 3.45, and 5.16 nmol/h/mg protein/cm2 hollow-fiber surface. This technique allows the enzymatic oxidation of sulfur compounds with liver microsomes in vitro and in vivo without immunological hazards, and it is suited for artificial liver support.

Hepatic assist: present and future

Fulminant hepatic failure due to acute massive liver cell necrosis is a complex pathophysiological entity, and treatment is still unsatisfactory. Artificial liver supports such as hemodialysis, hemoperfusion, and plasmapheresis have recently been used clinically to treat fulminant hepatic failure. However, survival rate has not improved as expected, although the consciousness of the patient has improved frequently. In this article the present status of clinical artificial liver support and basic research of hybrid artificial liver will be discussed. Moreover, the future aspects of total artificial liver support and hepatocyte transplantation for chronic liver failure will be introduced.

Enzymatic detoxification using lipophilic hollow-fiber membranes: II. Sulfation reactions

A lipophilic hollow-fiber technique was used in the enzymatic sulfation of lipophilic toxins. Endogenous and exogenous toxins, for which glucuronidation reactions already were demonstrated with this technique, were submitted to sulfate transferase reaction as an alternative phase-II detoxification route. Native enzyme was circulated on the external side of a lipophilic hollow-fiber membrane while the toxin-containing media (serum or aqueous solution) were circulated inside the hollow fiber. Sulfation reactions were catalyzed by rabbit liver cytosol, with a specific sulfate transferase activity of 626 nmol/min-mg protein. Clearance of the hollow-fiber module for phenol, p-cresol, paracetamol, 2-aminophenol, and 5-hydroxyindol were determined to be 38.1, 47.2, 2.7, 5.3, and 3.2 pmol/mg protein/h/cm2 hollow-fiber surface, respectively. This technique allows sulfation reactions with crude enzyme preparations over long periods without loss of activity from covalent immobilization and without immunological hazards.