Novel bioartificial liver support system: preclinical evaluation

The Roles of Membrane Technology in Artificial Organs: Current Challenges and Perspectives

The recent outbreak of the COVID-19 pandemic in 2020 reasserted the necessity of artificial lung membrane technology to treat patients with acute lung failure. In addition, the aging world population inevitably leads to higher demand for better artificial organ (AO) devices. Membrane technology is the central component in many of the AO devices including lung, kidney, liver and pancreas. Although AO technology has improved significantly in the past few decades, the quality of life of organ failure patients is still poor and the technology must be improved further. Most of the current AO literature focuses on the treatment and the clinical use of AO, while the research on the membrane development aspect of AO is relatively scarce. One of the speculated reasons is the wide interdisciplinary spectrum of AO technology, ranging from biotechnology to polymer chemistry and process engineering. In this review, in order to facilitate the membrane aspects of the AO research, the roles of membrane technology in the AO devices, along with the current challenges, are summarized. This review shows that there is a clear need for better membranes in terms of biocompatibility, permselectivity, module design, and process configuration.

Mass Preparation of Primary Porcine Hepatocytes and the Design of a Hybrid Artificial Liver Module using Spheroid Culture for a Clinical Trial

To isolate a large number of porcine hepatocytes, we originally developed a mass preparation method that combined the usual collagenase perfusion method of a whole liver with a collagenase redigestion method of tissue fragments after liver perfusion. Using a pig of 10kg, collagenase perfusion only resulted in a yield of 63 ± 78 x 108 total cells with a viability of 69.2 ± 25.3 %, but our combined method had a yield of 167 ± 31 x 108 total cells with a viability of 87.9 ± 4.4 % (mean ± SD). Also, the combined method was applied to two pigs of 10kg body weight at the same time, and isolated 387 ± 89 x 108 hepatocytes with a viability of 87.1 ± 6.9 % and a purity of 93.6 ± 2.8 % in 11 experiments.

We designed a large multi-capillary polyurethane foam (MC-PUF) packed-bed module containing 1 x 1010 porcine hepatocytes on a clinical trial scale. The porcine hepatocytes in the module formed spherical multicellular aggregates (spheroids) of 200 – 500 μm diameter. Most hepatocytes forming spheroids were viable judged by fluorescein diacetate and ethidium bromide staining. The activities of ammonia removal, albumin secretion and oxygen consumption of the large MC-PUF module were the same as for a small MC-PUF module containing 2 x 108 porcine hepatocytes, and were maintained for at least 9 days of culture. These results show that a large MC-PUF module is successfully scaled up 50 times.

In conclusion, we succeeded in developing a mass preparation method of porcine hepatocytes and a large hybrid artificial liver module on a clinical trial scale.

A Novel Bioartificial Liver Containing Small Tissue Fragments: Efficiency in the Treatment of Acute Hepatic Failure Induced by Carbon Tetrachloride in Rats

The efficiency of a new bioartificial liver (BAL) containing small tissue fragments in the treatment of acute hepatic failure induced by carbon tetrachloride in rats was evaluated. A day after injection (i.p.) of CCl4 the animals were connected to a BAL containing liver fragments (fragment BAL) and a BAL containing no liver fragments (no-fragment BAL), and extracorporeal hemoperfusion was carried out for 4 h. The activities of alanine transaminase and lactate dehydrogenase as well as the concentrations of ammonia, glucose, urea, and amino acids in plasma were measured. A tendency to the stabilisation of ammonia, glucose, phenylalanine, tyrosine, and other amino acids was revealed at the end of hemoperfusion in poisoned rats connected to the fragment BAL. A statistically significant difference in survival between the animals connected to the fragment BAL and no-fragment BAL was found. The results obtained indicate that the bioreactor containing small liver fragments is effective in the treatment of acute hepatic failure in animals.

Improved survival of porcine acute liver failure by a bioartificial liver device implanted with induced human functional hepatocytes

Acute liver failure (ALF) is a life-threatening illness. The extracorporeal cell-based bioartificial liver (BAL) system could bridge liver transplantation and facilitate liver regeneration for ALF patients by providing metabolic detoxification and synthetic functions. Previous BAL systems, based on hepatoma cells and non-human hepatocytes, achieved limited clinical advances, largely due to poor hepatic functions, cumbersome preparation or safety concerns of these cells. We previously generated human functional hepatocytes by lineage conversion (hiHeps). Here, by improving functional maturity of hiHeps and producing hiHeps at clinical scales (3 billion cells), we developed a hiHep-based BAL system (hiHep-BAL). In a porcine ALF model, hiHep-BAL treatment restored liver functions, corrected blood levels of ammonia and bilirubin, and prolonged survival. Importantly, human albumin and α-1-antitrypsin were detectable in hiHep-BAL-treated ALF pigs. Moreover, hiHep-BAL treatment led to attenuated liver damage, resolved inflammation and enhanced liver regeneration. Our findings indicate a promising clinical application of the hiHep-BAL system.

The use of bioreactors as in vitro models in pharmaceutical research

Bringing a new drug to market is costly in terms of capital and time investments, and any development issues encountered during late-stage clinical trials can often be the result of in vitro-in vivo extrapolations (IVIVE) not accurately reflecting clinical outcome. In the discipline of drug metabolism and pharmacokinetics (DMPK), current in vitro cellular methods do not provide the 3D structure and function of organs found in vivo; therefore, new dynamic methods need to be established to aid improvement of IVIVE. In this review, we highlight the importance of model progression into dynamic systems for use within drug development, focusing on devices developed currently in the areas of the liver and blood-brain barrier (BBB), and the potential to develop models for other organ systems, such as the kidney. We discuss the development of dynamic 3D bioreactor-based systems as in vitro models for use in DMPK studies.

Molecular fabrications of smart nanobiomaterials and applications in personalized medicine

Recent advances in nanotechnology adequately address many of the current challenges in biomedicine. However, to advance medicine we need personalized treatments which require the combination of nanotechnological progress with genetics, molecular biology, gene sequencing, and computational design. This paper reviews the literature of nanoscale biomaterials described to be totally biocompatible, non-toxic, non-immunogenic, and biodegradable and furthermore, have been used or have the potential to be used in personalized biomedical applications such as drug delivery, tissue regeneration, and diagnostics. The nanobiomaterial architecture is discussed as basis for fabrication of novel integrated systems involving cells, growth factors, proteins, cytokines, drug molecules, and other biomolecules with the purpose of creating a universal, all purpose nanobiomedical device for personalized therapies. Nanofabrication strategies toward the development of a platform for the implementation of nanotechnology in personalized medicine are also presented. In addition, there is a discussion on the challenges faced for designing versatile, smart nanobiomaterials and the requirements for choosing a material with tailor made specifications to address the needs of a specific patient.

Key challenges to the development of extracorporeal bioartificial liver support systems

BACKGROUND

For nearly three decades, extracorporeal bioartificial liver (BAL) support systems have been anticipated as promising tools for the treatment of liver failure. However, these systems are still far from clinical application. This review aimed to analyze the key challenges to the development of BALs.

DATA SOURCE

We carried out a PubMed search of English-language articles relevant to extracorporeal BAL support systems and liver failure.

RESULTS

Extracorporeal BALs face a series of challenges. First, an appropriate cell source for BAL is not readily available. Second, existing bioreactors do not provide in vivo-like oxygenation and bile secretion. Third, emergency needs cannot be met by current BALs. Finally, the effectiveness of BALs, either in animals or in patients, has been difficult to document.

CONCLUSIONS

Extracorporeal BAL support systems are mainly challenged by incompetent cell sources and flawed bioreactors. To advance this technology, future research is needed to provide more insights into interpreting the conditions for hepatocyte differentiation and liver microstructure formation.

Liver regenerative medicine: advances and challenges

Liver transplantation is the standard care for many end-stage liver diseases. However, donor organs are scarce and some people succumb to liver failure before a donor is found. Liver regenerative medicine is a special interdisciplinary field of medicine focused on the development of new therapies incorporating stem cells, gene therapy and engineered tissues in order to repair or replace the damaged organ. In this review we consider the emerging progress achieved in the hepatic regenerative medicine within the last decade. The review starts with the characterization of liver organogenesis, fetal and adult stem/progenitor cells. Then, applications of primary hepatocytes, embryonic and adult (mesenchymal, hematopoietic and induced pluripotent) stem cells in cell therapy of liver diseases are considered. Current advances and challenges in producing mature hepatocytes from stem/progenitor cells are discussed. A section about hepatic tissue engineering includes consideration of synthetic and natural biomaterials in engineering scaffolds, strategies and achievements in the development of 3D bioactive matrices and 3D hepatocyte cultures, liver microengineering, generating bioartificial liver and prospects for fabrication of the bioengineered liver.

Porous and single-skinned polyethersulfone membranes support the growth of HepG2 cells: A potential biomaterial for bioartificial liver systems

In this study, we evaluated a porous and single-layer skin polyethersulfone (PES) membrane as a material for use in hybrid bioartificial liver support systems. The PES membrane has been characterized as a single-layer skin structure, with a rough porous surface. Specifically, we studied the ability of the human hepatoblastoma cell lines (HepG2) to adhere, grow, and spread on the PES membrane. Furthermore, we examined albumin secretion, low-density lipoprotein uptake, and CYP450 activity of HepG2 cells that grew on the membrane. HepG2 cells readily adhered onto the outer surfaces of PES membranes. Over time, HepG2 cells proliferated actively, and confluent monolayer of cells covered the available surface area of the membrane, eventually forming cell clusters and three-dimensional aggregates. Furthermore, HepG2 cells grown on PES membranes maintained highly specific functions, including uptake capability, biosynthesis and biotransformation. These results indicate that PES membranes are potential substrates for the growth of human liver cells and may be useful in the construction of hollow fiber bioreactors. Porous and single-layer skin PES membranes and HepG2 cells may be potential biomaterials for the development of biohybrid liver devices.

Current development of bioreactors for extracorporeal bioartificial liver (Review)

The research and development of extracorporeal bioartificial liver is gaining pace in recent years with the introduction of a myriad of optimally designed bioreactors with the ability to maintain long-term viability and liver-specific functions of hepatocytes. The design considerations for bioartificial liver are not trivial; it needs to consider factors such as the types of cell to be cultured in the bioreactor, the bioreactor configuration, the magnitude of fluid-induced shear stress, nutrients' supply, and wastes' removal, and other relevant issues before the bioreactor is ready for testing. This review discusses the exciting development of bioartificial liver devices, particularly the various types of cell used in current reactor designs, the state-of-the-art culturing and cryopreservation techniques, and the comparison among many today's bioreactor configurations. This review will also discuss in depth the importance of maintaining optimal mass transfer of nutrients and oxygen partial pressure in the bioreactor system. Finally, this review will discuss the commercially available bioreactors that are currently undergoing preclinical and clinical trials.

Treatment of acute hepatic failure in mice by transplantation of mixed microencapsulation of rat hepatocytes and transgenic human fetal liver stromal cells

Microencapsulation-mediated cell therapy overcomes the immune incompatibility between donor and recipient in transplantation. The aim of this study was to investigate the effects of transplantation of microcapsules containing a mixture of rat hepatocytes and human fetal liver stromal cells (hFLSCs), engineered to produce basic fibroblast growth factor (bFGF), on acute liver failure (ALF) in mice. In vitro experiments showed that different combinations of microencapsulated rat's hepatocytes and stromal cells survive, grow, and function better in three-dimensional conditions. The metabolic activity of rat hepatocytes co-microencapsulated with hFLSCs, particularly when engineered to produce bFGF (FLSCs/bFGF), is significantly higher than that of microcapsules with rat hepatocytes alone. Intraperitoneal transplantation of the encapsulated hepatocytes with FLSCs/bFGF increased the survival rate and improved liver function of an ALF mouse model induced by a 70% partial hepatectomy in BALB/C mice. Moreover, dramatic liver regeneration was observed 2 days after transplantation in the group that received intraperitoneal transplantations of encapsulated hepatocytes with FLSCs/bFGF. Therefore, transplantation of encapsulated hepatocytes and hFLSCs/bFGF may be a promising strategy to treat ALF or related liver diseases.

Bioartificial liver support systems

A variety of bioartificial liver support systems were developed to replace some of the liver's function in case of liver failure. Those systems, in contrast to purely artificial systems, incorporate metabolically active cells to contribute synthetic and regulatory functions as well as detoxification. The selection of the ideal cell source and the design of more sophisticated bioreactors are the main issues in this field of research. Several systems were already introduced into clinical studies to prove their safety. This review briefly introduces a cross-section of experimental and clinically applied systems and tries to give an overview on the problems and limitations of bioartificial liver support.

Development of a hybrid liver-support device

The development of an extracorporeal hybrid liver-support system using hepatocytes and an artificial device has been long awaited for the treatment of patients with hepatic failure. During the past decade important progress has been made in biotechnology and bioengineering, and a hybrid liver-support device using metabolically active hepatocytes may well become a reality in the near future. This paper outlines recent developments in bioreactor systems used as bioartificial liver-support devices, and focuses on critical issues for bioreactor design, main transport features and culture techniques for hepatocytes. We describe our bioreactor, which uses porcine hepatocytes, and the scaling-up of the device. The biochemical performance of such a device is comparable to that of those developed by other researchers, and we feel encouraged to perform in vivo experiments on animal models in order to evaluate the potential of the device as a bioartificial liver.

Evaluation of a hybrid artificial liver module with liver lobule-like structure in rats with liver failure

We studied the recovery of rats with fulminant hepatic failure (FHF) by treating them with our original hybrid artificial liver support system (HALSS). We developed an original artificial liver module having a liver lobule-like structure (LLS). This module consists of many hollow fibers regularly arranged in close proximity and hepatocyte aggregates (organoids) induced into the extra capillary space of the module by centrifugal force. The LLS module can express some liver specific functions at high levels and maintain them for several months in vitro. In this study, we evaluated the efficacy of our LLS-HALSS by using rats with FHF induced by a method that combined partial hepatectomy with hepatic ischemia. In the animal experiments, blood ammonia levels rapidly increased in the control group (sham-HALSS group). These rats died during or immediately after application of the sham-HALLS. On the other hand, in the LLS module application group (LLS-control group), the increase in blood ammonia was completely suppressed and all rats recovered. Blood constituents at 4 weeks after application were at normal levels, and the weight of the liver was the same as that of a normal rat. These results indicate that HALSS may be useful for treating liver failure patients until liver transplantation can be performed or until regeneration of the native liver occurs.

Hepatocyte hollow-fibre bioreactors: design, set-up, validation and applications

Hepatocytes carry out many vital biological functions, such as synthetic and catabolic reactions, detoxification and excretion. Due to their ability to restore a tissue-like environment, hollow-fibre bioreactors (HFBs) show great potential among the different systems used to culture hepatocytes. Several designs of HFBs have been proposed in which hepatocytes or hepatocyte-derived cell lines can be cultured in suspensions or on a solid support. Currently the major use of hepatocyte HFBs is as bioartificial livers to sustain patients suffering from acute liver failure, but they can also be used to synthesize cell products and as cellular models for drug metabolism and transport studies. Here, we present an overview of the set-up of hepatocyte HFBs and aim to provide potential users with the basic knowledge necessary to develop their own system. First, general information on HFBs is given, including basic principles, transport phenomena, designs and cell culture conditions. The importance of the tests necessary to assess the performance of the HFBs, i.e. the viability and functionality of hepatocytes, is underlined. Special attention is paid to drug metabolism studies and to adequate analytical methods. Finally, the potential uses of hepatocyte HFBs are described.

Porcine acute liver failure model established by two-phase surgery and treated with hollow fiber bioartificial liver support system

AIM: To establish a highly reproducible animal model of acute liver failure (ALF), for assessing the effect of bioartificial liver support system (BALSS).

METHODS: A two-phase complete liver devascularization procedure was performed in eight loco-hybrid pigs. Blood biochemical index and liver biopsy were studied every 2 h after surgery, and survival time was recorded. The BALSS constructed with high volume recirculating technique was a hollow fiber circulating system consisting of a hepatocyte reactor-hollow fiber module inoculated with microcarrier-adhering hepatocytes, and a double pump, heparinized, thermostabilized, micro-capsulized activated carbon-adsorbing plasmapheresis system. Twelve pigs undergoing two-phase surgery were randomized into: control group (perfused without hepatocytes, n = 6) and treatment group (perfused with hepatocytes, n = 6). Intergroup liver biochemical indexes, survival time, and liver pathological changes were analyzed at regular intervals.

RESULTS: Two-phase surgery was performed in all the experimental pigs, and there was no obvious difference between their biochemical indexes. After 3 h of phase II surgery, ammonia (Amm) increased to (269±37) μmol/L. After 5 h of the surgery, fibrinogen (Fib) decreased to (1.5±0.2) g/L. After 7 h of the surgery, ALT, AST, Tbil and PT were (7.6±1.8) nka/L, (40±5) nka/L, (55±8) μmol/L and (17.5±1.7) nka/L respectively. After 9 h of surgery, ALB and Cr were (27±4) g/L and (87±9) μmol/L. After 13 h of surgery, BUN was (3.5±0.9) μmol/L. All the above values were different from those determined before surgery. Survival time of pigs averaged 13.5±1.4 h. ALF pigs in the other group were treated with BALSS. The comparison analysis between the treated and control animals showed the changes of Tbil, PT, Alb, BUN, Cr, Fib, and Amm (P<0.01), but there was no change of ALT and AST. The survival time was statistically different (P<0.01), and there was no significant difference in histological changes.

CONCLUSION: The porcine ALF model established by two-phase devascularized surgery is valid and reproducible. The hollow fiber BALSS can meet the needs of life support and is effective in treating ALF.

Liver support for fulminant hepatic failure: is it time to use the molecular adsorbents recycling system in children?

OBJECTIVE

To describe the main liver support devices used for fulminant hepatic failure (FHF) and to review data on the Molecular Adsorbents Recycling System (MARS) and assess its efficiency in children.

DATA SOURCE

Studies were identified through selected readings and a MEDLINE search from 1975 and 2004 using fulminant hepatic failure, acute liver failure, primary graft dysfunction, liver support, MARS, and extracorporeal liver assist device as key words.

STUDY SELECTION

All original studies, including case reports, relating to the use of the MARS or albumin dialysis system were included. Additional attention was put on prognosis criteria of FHF severity in children.

DATA EXTRACTION

Study design, numbers and diagnoses of patients, definite or bridging treatment, outcome measures, and complications were extracted and compiled. Results of individual trials were combined on the risk ratio scale.

DATA SYNTHESIS

Nine randomized trials including 354 patients were identified. However, liver support failed to significantly affect mortality when compared with standard medical therapy. Albumin dialysis, and particularly MARS, emerges as an easily applicable technique for temporary liver support. Some well-designed studies have characterized its efficiency in a few indications, such as in intractable pruritus in chronic liver disease, in acute or chronic liver diseases, and in decompensated cirrhosis with hepatorenal syndrome. In adults and children with FHF, anecdotal reports suggest that MARS may stabilize the patient. However, no randomized controlled study has validated its use in this indication. A randomized controlled study is ongoing in adults with FHF. Such a trial seems to be unfeasible in children for several methodologic reasons.

CONCLUSIONS

Although promising preliminary results suggest that MARS may have a significant position in the therapeutic arsenal for FHF, no sufficient data exist to justify its use in children. For as long as the results of the ongoing adult trial are not available, the indications of this expensive technique in children with FHF are limited.

Rapid, large-scale formation of porcine hepatocyte spheroids in a novel spheroid reservoir bioartificial liver

We have developed a novel bioreactor based on the observation that isolated porcine hepatocytes rapidly and spontaneously aggregate into spheroids under oscillation conditions. The purpose of this study was to characterize the influence of oscillation frequency (0.125 Hz, 0.25 Hz), cell density (1-10 x 10(6) cells/mL), and storage condition (fresh, cryopreserved) of porcine hepatocytes on the kinetics of spheroid formation. The viability and metabolic performance of spheroid hepatocytes was also compared to monolayer culture. We observed that both fresh and cryopreserved porcine hepatocytes began formation of spheroids spontaneously at the onset of oscillation culture. Spheroid size was directly related to cell density and time in culture, though inversely related to oscillatory frequency. Spheroid formation by fresh porcine hepatocytes was associated with decreased cell death (lactate dehydrogenase release, 1.3 +/- 1.0 vs. 3.1 +/- 0.7 U/mL, P < 0.05) and increased metabolic performance (albumin production, 14.7 +/- 3.3 vs. 4.6 +/- 1.4 fg/c/h, P < 0.0001; ureagenesis from ammonia, 267 +/- 63 vs. 92 +/- 13 micromol/L/h, P < 0.001) compared with monolayer culture. In conclusion, based on the favorable properties of rapid spheroid formation, increased hepatocellular function, and ease of scale-up, the spheroid reservoir bioreactor warrants further investigation as a bioartificial liver for support of liver failure.

History of xenotransplantation

The present historical review reports the clinical experiences of transplantations from animal to human. The first transplantation attempts were made without any knowledge of the species barrier. The pioneers of xenotransplantation realized xenotransfusions as early as the 16th century, then cell and tissue xenotransplantations in the 19th century. At the beginning of the 20th century, xenotransplantation of testicles became the latest craze. At the same time, and later in the 1960s, organ xenotransplantations were attempted, with disappointing results. Mathieu Jaboulay, Serge Voronoff, Keith Reemtsma, James Hardy, Denton Cooley, Thomas Starzl, Christiaan Barnard and Leonard Bailey were among the pionneers of xenotransplantation. Recent trials concerned above all tissue and cell xenotransplantations. Nowadays, with encapsulation, transgenesis, and cloning, great advances have been made for controlling xenograft rejection, but ethical questions linked to the risk of infections have become a major pre-occupation within the scientific community and the general population.

Clinical application of bioartificial liver support systems

OBJECTIVE

To review the present status of bioartificial liver (BAL) devices and their obtained clinical results.

BACKGROUND

Acute liver failure (ALF) is a disease with a high mortality. Standard therapy at present is liver transplantation. Liver transplantation is hampered by the increasing shortage of organ donors, resulting in high incidence of patients with ALF dying on the transplantation waiting list. Among a variety of liver assist therapies, BAL therapy is marked as the most promising solution to bridge ALF patients to liver transplantation or to liver regeneration, because several BAL systems showed significant survival improvement in animal ALF studies. Until today, clinical application of 11 different BAL systems has been reported.

METHODS

A literature review was performed using MEDLINE and additional library searches. Only BAL systems that have been used in a clinical trial were included in this review.

RESULTS

Eleven BAL systems found clinical application. Three systems were studied in a controlled trial, showing no significant survival benefits, in part due to the insufficient number of patients included. The other systems were studied in a phase I trial or during treatment of a single patient and all showed to be safe. Most BAL therapies resulted in improvement of clinical and biochemical parameters.

CONCLUSIONS

Bioartificial liver therapy for bridging patients with ALF to liver transplantation or liver regeneration is promising. Its clinical value awaits further improvement of BAL devices, replacement of hepatocytes of animal origin by human hepatocytes, and assessment in controlled clinical trials.

Oxygen Consumption in a Hollow Fiber Bioartificial Liver-Revisited

Oxygen consumption dynamics in a hollow fiber, hepatocyte-loaded bioartificial liver are investigated both theoretically and experimentally. The theoretical model is based upon the Krogh cylinder, which approximates the bioreactor as a collection of cylindrical elements comprised of an inner fiber lumen for media perfusion, the fiber wall through which oxygen can diffuse, and an annular region of hepatocytes surrounding the fiber. The primary non-dimensional parameters that describe the system are: (i) the Peclet number, Pe, which is the ratio of convective oxygen transport through the lumen to diffusive oxygen transport to the fiber walls; (ii) the hepatocyte saturation parameter, theta, which is the ratio of the inlet oxygen partial pressure to the Michaelis-Menten half-rate oxygen partial pressure; (iii) the Thiele modulus, phi2, which is the ratio of oxygen consumption rate to oxygen diffusion rate in the hepatocyte annulus; (iv) the hepatocyte permeability ratio, beta31, which is the ratio of oxygen permeability in the hepatocyte cell mass to oxygen permeability in the perfusing lumen medium; and (v) the hepatocyte annular thickness, rho3, which is the ratio of the exterior hepatocyte annular radius to the fiber lumen radius. Only Pe and theta are easily manipulated operating variables. phi2, beta31, and rho3 are engineering design parameters that are set when a bioreactor is fabricated. The model results are expressed as the effective hepatocyte utilization ratio, Vratio, which is the ratio of the observed oxygen consumption rate to the intrinsic hepatocyte oxygen consumption rate. Large regions of Vratio > 0.9, which is deemed an acceptable effective hepatocyte utilization are found for parameter values consistent with standard hollow fiber cartridges used in bioartificial liver fabrication. The extent of the Vratio > 0.9 region increases to a plateau with increasing Pe, increases with increasing theta, decreases with increasing phi2, increases with increasing beta31, and decreases with increasing rho3. The theoretical results indicate that Vratio > 0.9 is found whenever the experimentally observed fractional oxygen consumption from the perfusing medium, is less than 0.25. Combination of the theoretical and experimental results indicate that intrinsic, per cell oxygen consumption in the hollow fiber system may decrease as hepatocyte cell density increases and that this decrease may be due to lower intrinsic oxygen requirements in denser suspensions and not due to diffusion limitations in oxygen transport in the hollow fiber system as might be expected from two-dimensional, monolayer culture oxygen consumption measurements.

Bioartificial liver support devices: historical perspectives

Fulminant hepatic failure (FHF) is an important cause of death worldwide. Despite significant improvements in critical care therapy there has been little impact on survival with mortality rates approaching 80%. In many patients the cause of the liver failure is reversible and if short-term hepatic support is provided, the liver may regenerate. Survivors recover full liver function and a normal life expectancy. For many years the only curative treatment for this condition has been liver transplantation, subjecting many patients to replacement of a potentially self-regenerating organ, with the lifetime danger of immunosuppression and its attendant complications, such as malignancy. Because of the shortage of livers available for transplantation, many patients die before a transplant can be performed, or are too ill for operation by the time a liver becomes available. Many patients with hepatic failure do not qualify for liver transplantation because of concomitant infection, metastatic cancer, active alcoholism or concurrent medical problems. The survival of patients excluded from liver transplantation or those with potentially reversible acute hepatitis might be improved with temporary artificial liver support. With a view to this, bioartificial liver support devices have been developed which replace the synthetic, metabolic and detoxification functions of the liver. Some such devices have been evaluated in clinical trials. During the last decade, improvements in bioengineering techniques have been used to refine the membranes and hepatocyte attachment systems used in these devices, in the hope of improving function. The present article reviews the history of liver support systems, the attendant problems encountered, and summarizes the main systems that are currently under evaluation.

Use of Ultrathin Shell Microcapsules of Hepatocytes in Bioartificial Liver-Assist Device

We previously encapsulated hepatocytes in ultrathin shell microcapsules and showed them to have enhanced differentiated functions over cells cultured in monolayer. Here we have used these microencapsulated hepatocytes in a bioartificial liver-assisted device (BLAD) with a rat hepatectomy model. Primary rat hepatocytes were encapsulated in 150- to 200-microm microcapsules, using an electrostatic droplet generator. The microencapsulated hepatocytes exhibited good in vitro urea synthesis activity in plasma from rats with fulminant hepatic failure (FHF). The ex vivo hemoperfusion was conducted in FHF rats by perfusing plasma at a rate of 1-2 mL/min through 1.5-2 x 10(8) encapsulated hepatocytes packed into a packed-bed bioreactor. Hemoperfusion with the bioreactor was initiated 5 h after operative induction of liver failure and continued for 7 h. The BLAD-treated rats showed improvements over the control groups in survival time and metabolic indicators, including ammonia and total bilirubin levels. Furthermore, expanded bed adsorption (EBA) detoxification technology using Streamline-SP resin was explored to complement the bioreactor with microencapsulated hepatocytes. In vitro experiments indicated that serum ammonia could be specifically removed in dose-dependent manner, whereas the total serum proteins were unaffected by the resin. In ex vivo experiments, hemoperfusion with the resin was initiated 3 h after operative induction of liver failure and continued for 7 h. The resin-treated rats showed obvious serum ammonia removal with no observable total blood protein and blood cell adsorption. Therefore, Streamline-SP resin can potentially be integrated into a BLAD for improved efficacy.

Liver stem cells: prospects for treatment of inherited and acquired liver diseases

It is now understood that there are three cell compartments which physiologically contribute to vertebrate liver parenchymal maintenance and regeneration after injury: mature liver cells (hepatocytes, cholangiocytes), intraorgan stem/progenitor cells (cells of the proximal biliary tree, periductal cells) and extraorgan stem cells (from the circulation and the bone marrow). All of these cell populations, as well as other, non-physiologic stem cells (e.g., mesenchymal stromal cells from the bone marrow, fetal hepatoblasts, embryonic stem [ES] cells), may be used therapeutically for treatment of inherited and acquired liver diseases. This article will summarise our current understanding of these various cell populations, and review possible approaches to their therapeutic use, including cell transplantation, bioartificial liver devices (BLDs), gene therapy and administration of exogenous factors to stimulate normal physiological responses to repair.

Recovery of rats with fulminant hepatic failure by using a hybrid artificial liver support system with polyurethane foam/rat hepatocyte spheroids

We studied the recovery of rats with fulminant hepatic failure (FHF) by treating them with our original hybrid artificial liver support system (HALSS). FHF was induced by a two-thirds partial hepatectomy and 10 minutes of hepatic ischemia. Rats with FHF were treated with a polyurethane foam/spheroid HALSS including 2.0 x 10(8) hepatocytes for 1 hour (HALSS group, n = 5), and with the same system without hepatocytes in the artificial liver module as a control experiment (sham-HALSS group, n = 3). The level of blood constituents, ammonia, glucose and creatinine, showed no major difference between the two groups at the end of treatment. All rats in the sham-HALSS group died within 5 hours after treatment. However, the level of blood constituents of rats with FHF in the HALSS group improved with time, and all rats in the HALSS group recovered. Liver tissue of rats treated with HALSS showed cell mitosis and improvement from injury. These results indicated that our HALSS has a strong possibility to induce recovery from hepatic failure.

Improving the next generation of bioartificial liver devices

Several extracorporeal bioartificial liver (BAL) devices are currently being evaluated as an alternative or adjunct therapy for liver disease. While these hybrid systems show promise, in order to become a clinical reality, BAL devices must clearly demonstrate efficacy in improving patient outcomes. Here, we present aspects of BAL devices that could benefit from fundamental advances in cell and developmental biology. In particular, we examine the development of human hepatocyte cell lines, strategies to stabilize the hepatocyte phenotype in vitro, and emphasize the importance of the cellular microenvironment in bioreactor design. Consideration of these key components of BAL systems will greatly improve next generation devices.

Preclinical evaluation of the Excorp Medical, Inc, Bioartificial Liver Support System

BACKGROUND

Acute liver failure has no medically recognized effective therapy other than orthotopic liver transplantation. Development of bioartificial livers for support of patients with acute liver failure requires meaningful preclinical evaluation before clinical trials.

STUDY DESIGN

Complete results from preclinical safety and efficacy evaluation of the Excorp Medical Bioartificial Liver Support System (BLSS) using a D-galactosamine (D-gal) canine liver failure model are presented. From a total cohort of 23 purpose-bred male hounds, 18 animals were administered a lethal dose (1.5 g/kg) of D-gal. The 18 animals were divided into four treatment groups: no BLSS treatment (n = 6), BLSS treatment starting at 24 to 26 hours post D-gal (n = 5), BLSS treatment starting at 16 to 18 hours post D-gal (n = 4), and "mock support" treatment with a BLSS system containing no hepatocytes (n = 3). The animals were treated until death or death equivalent, or euthanized at 60 hours. Physiologic parameters were continuously monitored. Blood chemistries were obtained every 8 hours.

RESULTS

Although survival times for BLSS-supported animals were significantly greater than for the unsupported group, the greatest impact on delaying progression of liver disease was time of intervention. Intervention at 16 to 18 hours post D-gal administration showed significant delay in increasing blood ammonia, lactate, and prothrombin time as compared with untreated animals. Elevated intracranial pressure was found in two of six untreated animals, but in none of the treated animals (zero of nine). Healthy animals supported by the BLSS system evidenced no significant safety problems.

CONCLUSIONS

Results suggest the BLSS impacts the course of liver failure in the animal model. Phase I clinical safety evaluation is underway.

Plasma versus whole blood perfusion in a bioartificial liver assist device

The ramifications of using whole blood or plasma for perfusion off an hepatocyte containing bioartificial liver bioreactor in which the hepatocytes are separated by a membrane or other physical barrier from the perfusate stream on the rate of change of patient blood concentrations are explored through dynamic modeling of whole blood perfusion as a two compartment system (patient tissue and blood compartments), and plasma perfusion as a three compartment system (patient tissue and blood compartments, and a plasma reservoir). The whole blood perfusion model is described by three dimensionless parameters: the Damkohler number, Da, which represents the ratio of the rate of conversion by the bioreactor to the rate of perfusion; kappa, which represents the ratio of the rate of internal reequilibration between the tissue and blood compartments and the rate of perfusion; and Vtb, the tissue/blood volume ratio. The plasma perfusion model has three additional dimensionless parameters: f, the fraction of plasma withdrawn from the blood in a plasma separator; alpha, the ratio of the plasma perfusion rate in the bioreactor to the blood draw rate; and Vbr, the blood/plasma reservoir volume ratio. Within the physiologic range of parameters, the rate of reduction in blood concentration in both the whole blood-perfused and plasma-perfused systems are sensitive to Damkohler number up to Da approximately 2. Neither system is sensitive to variations in kappa, and the plasma perfusion system has little sensitivity to alpha. Given bioreactors of equivalent activity, a greater rate of blood concentration reduction and lower endpoint blood concentration at equivalent perfusion times will be achieved with whole blood perfusion. There are two physical reasons for this. The first is that the plasma perfused system is only processing a fraction, f, of the blood compared with the whole blood perfusion system. The second reason is that, although the blood-perfused system is limited by overall bioreactor performance, the plasma-perfused system is mass transfer limited to the rate of blood concentration dilution into the plasma reservoir rather than limited by the overall bioreactor performance.

Extracorporeal support of the failing liver

Successes in machine-based extracorporeal support for different organ functions stimulated research in the field of liver support approximately 50 years ago. Initial failure to improve outcome using detoxification methods like dialysis, blood and plasma exchange, or plasmapheresis over sorbents fueled interest in biologic liver support concepts using bioreactors or combined methods. New device configurations, technical improvement of existing detoxification methods, and the refinement in cell culture techniques led to a boost in research on biologic and nonbiologic approaches. Currently, many systems are in the preclinical phase or have entered clinical studies. A number of completed clinical trials have reported a favorable therapeutic impact of the most advanced solutions on the course and outcome of liver failure. Often, findings must be reconfirmed. However, current knowledge suggests that extracorporeal liver support can successfully stabilize liver function, improve the clinical condition of patients, and considerably improve survival in certain subgroups of patients with fulminant hepatic failure and acute decompensation of chronic hepatic failure. Although the initial focus of liver support methods was bridging to liver transplantation, bridging to recovery of organ function and treatment of intractable pruritus are now valuable indications.

Recent developments on human cell lines for the bioartificial liver

Most bioartificial liver (BAL) devices contain porcine primary hepatocytes as their biological component. However, alternatives are needed due to xenotransplantation associated risks. Human liver cell lines have excellent growth characteristics and are therefore candidates for application in BAL devices. Tumour-derived cell lines HepG2 and C3A express a variety of liver functions, but some specific liver functions, like ammonia detoxification and ureagenesis are insufficient. Immortalised human hepatocytes might offer better prospects. The balance between immortalisation and transformation with dedifferentiation of cells seems controllable by conditional immortalisation and/or the use of telomerase as immortalising agent. Another promising approach will be the use of embryonic or adult human stem cells. Rodent stem cells have been directed to hepatic differentiation in vitro, which might be applicable to human stem cells. However, both functionality and safety of immortalised human liver cell lines and differentiated stem cells should be improved before successful use in BAL devices becomes reality.

First clinical use of a novel bioartificial liver support system (BLSS)

The first clinical use of the Excorp Medical Bioartificial Liver Support System (BLSS) in support of a 41-year-old African-American female with fulminant hepatic failure is described. The BLSS is currently in a Phase I/II safety evaluation at the University of Pittsburgh/UPMC System. Inclusion criteria for the study are patients with acute liver failure, any etiology, presenting with encephalopathy deteriorating beyond Parson's Grade 2. The BLSS consists of a blood pump; a heat exchanger to control blood temperature; an oxygenator to control oxygenation and pH; a bioreactor; and associated pressure and flow alarm systems. Patient liver support is provided by 70-100 g of porcine liver cells housed in the hollow fiber bioreactor. The patient exhibited transient hypotension and thrombocytopenia at initiation of perfusion. The only unanticipated safety event was a lowering of patient glucose level at the onset of perfusion with the BLSS that was treatable with intravenous glucose administration. Moderate changes in blood biochemistries pre- and post perfusion are indicative of liver support being provided by the BLSS. While the initial experience with the BLSS is encouraging, completion of the Phase I/II study is required in order to more fully understand the safety aspects of the BLSS.

Clinical and laboratory evaluation of the safety of a bioartificial liver assist device for potential transmission of porcine endogenous retrovirus

BACKGROUND

The potential risk of transmission of porcine endogenous retroviruses (PERV) from xenogeneic donors into humans has been widely debated. Because we were involved in a phase I/II clinical trial using a bioartificial liver support system (BLSS), we proceeded to evaluate the biosafety of this device.

MATERIALS AND METHODS

The system being evaluated contains primary porcine hepatocytes freshly isolated from pathogen-free, purpose-raised herd. Isolated hepatocytes were installed in the shell, which is separated by a semipermeable membrane (100-kD nominal cutoff) from the lumen through which the patients' whole blood is circulated. Both before and at defined intervals posthemoperfusion, patients' blood was obtained for screening. Additionally, effluent collected from a clinical bioreactor was analyzed. The presence of viral particles was estimated by reverse transcriptase-polymerase chain reaction (RT-PCR) and RT assays. For the detection of pig genomic and mitochondrial DNA, sequence-specific PCR (SS-PCR) was used. Finally, the presence of infectious viral particles in the samples was ascertained by exposure to the PERV-susceptible human cell line HEK-293.

RESULTS

PERV transcripts, RT activity, and infectious PERV particles were not detected in the luminal effluent of a bioreactor. Culture supernatant from untreated control or mitogen-treated porcine hepatocytes (cleared of cellular debris) also failed to infect HEK-293 cell lines. Finally, RT-PCR, SS-PCR, and PERV-specific RT assay detected no PERV infection in the blood samples obtained from five study patients both before and at various times post-hemoperfusion.

CONCLUSION

Although longer patient follow-up is required and mandated to unequivocally establish the biosafety of this device and related bioartificial organ systems, these analyses support the conclusion that when used under standard operational conditions, the BLSS is safe.

Advances in bioartificial liver assist devices

Rapid advances in development of bioartificial liver assist devices (BLADs) are exciting clinical interest in the application of BLAD technology for support of patients with acute liver failure. Four devices (Circe Biomedical HepatAssist, Vitagen ELAD, Gerlach BELS, and Excorp Medical BLSS) that rely on hepatocytes cultured in hollow-fiber membrane technology are currently in various stages of clinical evaluation. Several alternative approaches for culture and perfusion of hepatocytes have been evaluated in preclinical, large animal models of liver failure, or at a laboratory scale. Engineering design issues with respect to xenotransplantation, BLAD perfusion, hepatocyte functionality and culture maintenance, and ultimate distribution of a BLAD to a clinical site are delineated.

The role of a bioengineered artificial kidney in renal failure

Renal failure continues to carry substantial burden of morbidity and mortality in both acute and chronic forms, despite advances in transplantation and dialysis. There is evidence to suggest that the kidney has metabolic, endocrine, and immune effects transcending its filtration functions, even beyond secretion of renin and erythropoietin. Our laboratory has developed experience in the tissue culture of renal parenchymal cells, and has now been able to demonstrate the metabolic activity of these cells in an extracorporeal circuit recapitulating glomerulotubular anatomy. We have observed active transport of sodium, glucose, and glutathione. We describe the design and initial preclinical testing of the bioartificial kidney, as well as future directions of our research.

Safety observations in phase I clinical evaluation of the Excorp Medical Bioartificial Liver Support System after the first four patients

A Phase I clinical safety evaluation of the Excorp Medical, Inc, Bioartificial Liver Support System (BLSS) is in progress. Inclusion criteria are patients with acute liver failure of any etiology, presenting with encephalopathy deteriorating beyond Parson's Grade 2. The BLSS consists of a blood pump, heat exchanger to control blood temperature, oxygenator to control oxygenation and pH, bioreactor, and associated pressure and flow alarm systems. Patient liver support is provided by 70-100 g of porcine liver cells housed in the hollow fiber bioreactor. A single support period evaluation consists of 12 hour extracorporeal perfusion with the BLSS sandwiched between 12 hours of pre (baseline) and 12 hours of post support monitoring. Blood chemistries and hematologies are obtained every 6 hours during monitoring periods and every 4 hours during perfusion. Physiologic parameters are monitored continuously. The patient may receive a second treatment at the discretion of the clinical physician. Preliminary evaluation of safety considerations after enrollment of the first four patients (F, 41, acetaminophen induced, two support periods; M, 50, Wilson's disease, one support period; F, 53, acute alcoholic hepatitis, two support periods; F, 24, chemotherapy induced, one support period) is presented. All patients tolerated the extracorporeal perfusion well. All patients presented with hypoglycemia at the start of perfusion, treatable by IV dextrose. Transient hypotension at the start of perfusion responded to an IV fluid bolus. Only the second patient required heparin anticoagulation. No serious or unexpected adverse events were noted. Moderate biochemical response to support was noted in all patients. Completion of the Phase I safety evaluation is required to fully characterize the safety of the BLSS.

Current status of pediatric liver transplantation

Increased survival for young liver transplant recipients has greatly improved. Increasing success has led to broader indications, thereby increasing the number of potential recipients. Pediatric liver centers are developing new strategies to cope with the ever-increasing demands for suitable size appropriate grafts. UNOS is in the process of updating guidelines to regulate the sharing of organs which become available from new surgical techniques. In the future, alternative therapies, such as artificial liver assist devices and techniques of cellular transplantation and genetic modification of hepatocytes, may decrease the number of children who die while waiting for a suitable organ or even obviate the need for the liver transplantation.