The aim of organ support therapies is to either prevent organ failure or to allow time for the regeneration of native organ function. The aim of the development of the two bioartificial organs that are described next—the bioartificial kidney and bioartificial liver—is to address the largest need in organ transplantation (see Figure 1
), where the kidney and the liver are in highest demand. The goal of the bioartificial kidney is to restore native kidney function and, ultimately, to improve survival. The aim of the bioartificial liver is to bridge patients with liver failure to liver transplantation or to allow time for the recovery of native liver function and thus avoid liver transplantation. While some of the bioartificial kidney and liver systems have already been evaluated in controlled, randomized, multi-center clinical trials, none of them have been approved by the FDA for clinical use. Conclusive clinical trials are required to establish safety and efficacy of these much needed bioartificial systems.
3.3.1. Bioartificial Kidney
The FDA has allowed only one bioartificial kidney to be evaluated in human clinical trials. The renal assist device (RAD), developed by Humes [27
], RenaMed Biologics, Inc. (Lincoln, RI, USA), was an extracorporeal treatment system utilizing a standard hemofiltration cartridge containing approximately 109
renal tubule cells (RTC) grown along the inner surface of the fibers. The RAD was seeded with the RTC derived from human kidneys not suitable for transplantation, mainly due to anatomical defects, and cells were expanded in a culture medium [11
]. The hollow fibers provide support for the cellular system, allow for the transport of essential cell products and nutrients, and prevent the cells from entering the circulatory system. The RAD Circuit consisted of two perfusion loops. The first one was the Continuous Veno-Venous Hemofiltration (CVVH) loop, which is a conventional CVVH system. The second was the RAD loop, which contained the RAD cartridge (Figure 2
). During the RAD treatment, blood from the patient was perfused through a conventional hemofilter, which separates the blood into an ultrafiltrate component and a blood cellular concentrate. A portion of the ultrafiltrate component entered the RAD loop and was perfused through the lumen of the RAD cartridge. Within the RAD cartridge, the ultrafiltrate came into direct contact with the RTC attached to the lumenal wall of the hollow fibers. The blood cellular concentrate circulating around the outside of the hollow fibers was separated from the RTC by a semipermeable hollow fiber membrane, through which only small molecular weight molecules contained in the ultrafiltrate or synthesized by the renal tubule cells can pass. Upon exiting the RAD cartridge, the RAD-treated blood cellular concentrate was recombined with the blood cellular concentrate in the CVVH loop and was then returned to the patient.
The RAD was initially evaluated in a 10-patient Phase I/II study in patients with acute renal failure (ARF) due to acute tubular necrosis (ATN) [30
]. Based on the results of this clinical study, a Phase II, multicenter, randomized, controlled, open-label trial involving 58 patients with ARF was conducted. Forty patients received CVVH and RAD, and 18 received CVVH alone. The trial demonstrated a statistically significant advantage of RAD with respect to survival as compared to the CVVH group [31
]. Unfortunately, these results were not reproduced in the follow-up Phase IIb study.
Other technologies, like an implantable RAD based on microelectromechanical systems, and the transplantable bioengineered kidney based on biological templates seeded with cell lines [20
] hold much promise. At present, while they are in the research and development phase, they are far-off from bringing a clinically available bioartificial kidney to market.
Human clinical trials for the bioartificial kidney have not been initiated or re-initiated. Even in the best case scenario, it will take several years to address technical issues related to cell source and cell viability/functionality, resolve safety issues and manufacturing challenges, meet regulatory expectations, conduct well-designed clinical trials, and secure continuous funding to make these bioengineered systems the standard of care for patients requiring kidney transplantation.
3.3.2. Bioartificial Liver
] summarized over 30 different cell-based liver support devices that have been reported since 1987. More than 14 systems have been evaluated in clinical trials for their capacity to provide liver functions [6
]. Although primary human hepatocytes would seem to be the cells of choice in a bioartificial liver, the availability of these cells is limited. As an alternative, immortalized cell lines of the C3A human hepatoblastoma line have been used in a bioartificial liver device. Primary porcine hepatocytes, which are readily available, have been used in all bioartificial liver systems, with the exception of the Extracorporeal Liver Assist Device (ELAD), which employs the C3A cell line. Despite much progress in understanding the mechanism of action of such systems, none of them have even been reviewed for marketing approval by the FDA. Before progress toward commercialization can be made, multiple variables have to be addressed: the optimization of clinical trial design, the maintenance of cell viability, resolution of regulatory issues, risk mitigation related to xenozoonosis, and outstanding technological challenges.
ELAD, Vital Therapies, Inc. (San Diego, CA, USA) is an extracorporeal system that utilizes C3A cells [34
]. The cells are placed in the extracapillary space of a modified dialysis cartridge. Safety mechanisms are in place to prevent tumor cells from entering the patient’s blood stream [35
]. The system has been evaluated in several clinical studies in patients diagnosed with Acute Liver Failure (ALF) [36
] and in a Phase III clinical trial in subjects with Alcohol-Induced Liver Decompensation (AILD). Studies in both indications failed to achieve their primary and secondary endpoints. Another Phase III clinical trial on Acute Alcoholic Hepatitis (AAH) is planned based on a post-hoc analysis of the AILD trial. Many years in development, the ELAD system remains in the clinical development stage, still far from entering the marketplace.
HepatAssist™, developed by Demetriou et al. [37
], Circe Biomedical (Lexington, MA, USA) was the first bioartificial liver assist device tested on a large clinical scale in a Phase II/III clinical trial. The device was comprised of porcine hepatocytes cryopreserved until the cells are thawed and placed in an extracorporeal system in the extracapillary space of a hollow fiber membrane. There was no direct contact between the patient’s plasma and porcine cells during the therapy (Figure 3
]. The cells were derived from pigs housed in a specific pathogen-free herd under strict controls and in accordance with regulatory requirements and Circe’s quality program. Cryopreservation allowed for complete microbiological assessment including testing for adventitious agents, and for confirmation of cell viability and relevant functionality prior to clinical application [38
]. The hepatocytes in the device performed many of the metabolic functions of a healthy liver. The membrane had been optimized to maximize transmission of key proteins and to minimize transmission of adventitious agents. The system was first investigated in a Phase I study [39
], which yielded encouraging results on the potential efficacy of the system. It was further evaluated in the first prospective, randomized, controlled Phase II/III multicenter trial conducted in 19 centers across the US and Europe and which included 171 patients with ALF and primary non-function following liver transplantation. Of these, 86 patients were enrolled in the treatment group with HepatAssist system. This was the largest study in the field of liver support. The system demonstrated safety and improved survival in a subgroup of patients with fulminant/sub-fulminant hepatic failure [40
]. To address potential infectivity of PERV, its infectivity was assessed in 103 patients treated with the HepatAssist system in two multicenter clinical trials demonstrating no evidence of PERV transmission [41
]. As the significant survival benefit was identified only in a post hoc subgroup analysis, the HepatAssist device was not approved by the FDA. Further clinical trials required by FDA to proceed to marketing application could not be conducted due to the financial collapse of the system’s developer.
A hybrid liver support system employing porcine hepatocytes with extracorporeal plasma separation and bioreactor perfusion in patients with ALF was developed by Gerlach [43
]. Four separate capillary membrane systems, each forming independent compartments, are woven in order to create a three dimensional network. The bioreactors contained primary hepatocytes obtained from specific pathogen-free pigs. The bioreactor was integrated into a modular extracorporeal liver support (MELS) system, combining biologic liver support with artificial detoxification technology. Development of the MELS system included an eight-patient clinical study [44
]. No PERVs were detected in patients treated with the system [44
]. Despite initial encouraging results, the system never progressed to a prospective, controlled, randomized, clinical trial required for regulatory approval; consequently, the system never reached the marketplace.
A new bioartificial liver assist system called the Spheroid Reservoir Bioartificial Liver (SRBAL) has been developed by Nyberg at Mayo Clinic [46
] as a long-term treatment option to liver transplantation. The SRBAL utilizes primary porcine hepatocytes within three-dimensional spheroid aggregates formed by cell-to cell adhesion mediated by surface molecules. The spheroid structure is thought to protect hepatocytes from apoptosis. The SRBAL is incorporated into an extracorporeal circuit. The system was recently evaluated in a prospective randomized controlled translational study in an ALF animal model. One of the important findings of the study was that the SRBAL maintains functionality of primary pig hepatocytes in the range of normal liver physiology. Further clinical studies that are planned to evaluate the SRBAL system will be the first re-initiation of clinical development of extracorporeal membrane-based bioartificial livers with pig cells after a long period of stasis in this field (see discussion on xenotransplantation that follows).
Lessons learned from using these extracorporeal liver assist systems intended to treat liver failure of various etiologies may guide future research and development of a bioartificial liver that can finally reach patients as an approved therapy.