Bioartificial Liver Systems: Why, What, Whither?

Abstract

Acute liver disease is a life-threatening condition for which liver transplantation is the only recognized effective therapy. While etiology varies considerably, the clinical course of acute liver failure is common among the etiologies: encephalopathy progressing toward coma and multiple organ failure. Detoxification processes, such as molecular adsorbent recirculating system (MARS) and Prometheus, have had limited success in altering blood chemistries positively in clinical evaluations, but have not been shown to be clinically effective with regard to patient survival or other clinical outcomes in any Phase III prospective, randomized trial. Bioartificial liver systems, which use liver cells (hepatocytes) to provide metabolic support as well as detoxification, have shown promising results in early clinical evaluations, but again have not demonstrated clinical significance in any Phase III prospective, randomized trial. Cell transplantation therapy has had limited success but is not practicable for wide use owing to a lack of cells (whole-organ transplantation has priority). New approaches in regenerative medicine for treatment of liver disease need to be directed toward providing a functional cell source, expandable in large quantities, for use in various applications. To this end, a novel bioreactor design is described that closely mimics the native liver cell environment and is easily scaled from microscopic (<1 ml cells) to clinical ( approximately 600 ml cells) size, while maintaining the same local cell environment throughout the bioreactor. The bioreactor is used for study of primary liver cell isolates, liver-derived cell lines and stem/progenitor cells.

The use of bioreactors as in vitro models in pharmaceutical research

“…Using primary human cells in these assays is the ideal gold standard [6]; however freshly isolated cells, cell cultures or tissue fragments cannot be used to study effects that occur over a longer time period due to the loss of functional integrity from drug transporter loss and enzyme depletion after a few hours [7] [8]. In addition, primary human cells are a scarce and precious resource.…”

“…The liver is the main excretory organ within the body, but as well as eliminating waste products through the bile, it carries out a variety of highly specialised functions; oxidative detoxification, intermediate metabolism and supply of nutrients, modulation of immune and hormonal systems and protein and macromolecule synthesis [6]. Disruption in these functions due to liver disorders such as acute or chronic liver failure can lead to a possible 90% mortality rate if essential hepatic functions are not restored during the critical phase of liver failure [23].…”

“…Popular liver derived cell lines such as the HepG2/C3A cell line derived from hepatomas have been used in pharmacological studies, recently being utilised in microfluidic bioreactors to examine the sensitivity of the models in response to various xenotoxins [32] [33]. However, compared to primary hepatocytes, functionality is often lost or altered in these cell lines [6]. For instance, phase I enzymes were expressed at significantly lower levels in HepG2 cells relative to primary human hepatocytes [34].…”

“…Primary human adult hepatocytes are considered to be the gold standard for biocompatibility and functionality within BAL systems [6] [43], but exhibit a rapid, timedependent, general loss of function in vitro [7]; including reduced cytochrome P450 and other biotransformation enzyme activities [44] and the down regulation of liver enriched transcription factors such as CCAAT/enhancer binding protein alpha [43]. However, the use of complex chemically defined media has allowed hepatocytes to go through a number of cycles in culture as well as stabilize hepatocyte morphology, survival, and liver-specific functions [45] [38] [46] [43].…”

“…During culture, hepatocytes flatten and spread upon attachment, whether grown on a flat surface or a scaffold. Although cell-cell junctions can be formed, the cytoskeleton is largely disrupted, and leads to loss of cell polarity, and ultimately liver specific functions [6].…”