Successful ex vivo liver perfusion system for hepatic failure pending liver regeneration or liver transplantation

Long-term normothermic machine preservation of human livers: what is needed to succeed?

Although short-term machine perfusion (≤24 h) allows for resuscitation and viability assessment of high-risk donor livers, the donor organ shortage might be further remedied by long-term perfusion machines. Extended preservation of injured donor livers may allow reconditioning, repairing, and regeneration. This review summarizes the necessary requirements and challenges for long-term liver machine preservation, which requires integrating multiple core physiological functions to mimic the physiological environment inside the body. A pump simulates the heart in the perfusion system, including automatically controlled adjustment of flow and pressure settings. Oxygenation and ventilation are required to account for the absence of the lungs combined with continuous blood gas analysis. To avoid pressure necrosis and achieve heterogenic tissue perfusion during preservation, diaphragm movement should be simulated. An artificial kidney is required to remove waste products and control the perfusion solution's composition. The perfusate requires an oxygen carrier, but will also be challenged by coagulation and activation of the immune system. The role of the pancreas can be mimicked through closed-loop control of glucose concentrations by automatic injection of insulin or glucagon. Nutrients and bile salts, generally transported from the intestine to the liver, have to be supplemented when preserving livers long term. Especially for long-term perfusion, the container should allow maintenance of sterility. In summary, the main challenge to develop a long-term perfusion machine is to maintain the liver's homeostasis in a sterile, carefully controlled environment. Long-term machine preservation of human livers may allow organ regeneration and repair, thereby ultimately solving the shortage of donor livers.

Future of bioartificial liver support

Many different artificial liver support systems (biological and non-biological) have been developed, tested pre-clinically and some have been applied in clinical trials. Based on theoretical considerations a biological artificial liver (BAL) should be preferred above the non-biological ones. However, clinical application of the BAL is still experimental. Here we try to analyze which hurdles have to be taken before the BAL will become standard equipment in the intensive care unit for patients with acute liver failure or acute deterioration of chronic liver disease.

Liver support devices

PURPOSE OF REVIEW

Liver support devices are used either as a bridge to liver transplantation or liver recovery in patients with acute or acute-on-chronic liver failure. The review analyzes the recent literature and asks if the current enthusiasm for these devices is justified.

RECENT FINDINGS

Many liver support devices exist and are discussed. Clinical data on artificial devices are rapidly emerging, especially on the molecular adsorbents recirculating system, and fractionated plasma separation and adsorption (Prometheus). While hepatic encephalopathy is improved by the molecular adsorbents recirculating system and probably Prometheus too, neither system has been shown to improve survival. Less clinical data exist for bioartificial support devices. These may use human hepatocytes, such as the extracorporeal liver assist device, although most devices use porcine hepatocytes, such as HepatAssist.

SUMMARY

Enthusiasm in liver support devices is justified as many nonrandomized studies have suggested some biochemical and clinical benefits. The results of several ongoing multicenter randomized controlled trials are anxiously awaited. Meanwhile, because mortality without liver transplantation remains high despite the use of liver support devices, these devices should only be used in the research setting or by experts proficient in their use and as a bridge to liver transplantation rather than liver recovery.

Functional analysis of encapsulated hepatic progenitor cells

A major challenge in developing therapies based on progenitor or stem cell populations (from sources other than bone marrow) involves developing a mode to deliver these cells in a manner that optimizes their viability, engraftment, proliferation, and differentiation. We have previously isolated a hepatic progenitor cell (HPC) population from adult liver tissue that differentiates into hepatic and biliary cell subtypes. We postulated that, using electrostatic encapsulation, we could reproducibly generate an ex vivo environment for the HPCs. We also theorized that this approach would foster cellular viability and function of the progenitor cell population. Using this encapsulation process, we consistently produced beads with uniform diameters between 200 and 700 microm. In vitro analysis of the encapsulated beads demonstrated extended periods of viability and function based on albumin production, urea metabolism, and glycogen storage. In conclusion, HPC encapsulation fosters the subsequent differentiation of HPCs into functional cells while maintaining their viability in long-term culture. These results demonstrate the efficacy of this method using somatic-derived progenitor cell populations and pave the way for clinical therapies.

Technology insight: liver support systems

Emergency orthotopic liver transplantation (OLT) is currently the only standard treatment for fulminant hepatic failure (FHF). The waiting time for transplantation can exceed a week-using a liver assist device to bridge patients with FHF to OLT might therefore decrease the mortality rate. Several liver support systems have been described, but no system has gained FDA approval or widespread clinical acceptance. Although the results of many experimental and clinical trials are encouraging, the field is still in its initial stages. Using nonbiologic liver support is based on the assumption that several toxins that cause hepatic coma can be removed from the circulation by blood or plasma sorption methods. As these toxins could be involved in many FHF complications recovery without the need for transplantation is the ultimate aim. Biologic liver support uses xenogeneic livers or hepatocytes to support the failed human liver, exploiting biological cell functions, namely detoxification, metabolism, and biosynthesis. The classical nonbiologic dialysis methods could decrease mortality in patients with acute-on-chronic liver failure, but definitive conclusions are impossible to draw because of the small number of patients studied and inadequate follow-up. Larger studies performed in specialty centers should provide conclusive data about the role of the bioartificial liver support system as a possible universal bridge to OLT. This article presents an overview of published experience with liver support systems since the 1960s.

Liver Function During Extracorporeal Whole Liver Perfusion in a Pig Model of Acute Ischemic Liver Failure

The shortage of livers for transplant has renewed interest in the potential of temporary liver support such as extra corporeal whole liver perfusion. In an ischemic induced liver failure model we perfused an extra corporeal liver through only a portal vein and assessed the function of this ex vivo liver by using hepatic tests to estimate elimination as well as synthesis capacities. Acute liver failure was performed in five control pigs by a hepatic devascularization associated to an end to side portocaval shunt. In a treated group, 5 to 6 h after this hepatic devascularization, animals were connected to an extra corporeal liver perfused via the portal vein with blood withdrawn from the ischemic liver animal from its portal vein. Devascularization of the liver induced an increase in liver enzymes and ammonia, a drop in the ratio of branched chain amino acids to aromatic amino acids, and a decrease in blood urea and indocyanine green and galactose clearances. In treated animals, urea, amino acid ratio, and clearances increased after the ex vivo liver perfusion. In this group, mean bile production and mean liver oxygen consumption were 13.7 +/- 3.6 ml/h and 16.1 +/- 7.7 ml/min, respectively. In an acute ischemic liver failure pig model, an extra corporeal whole liver perfusion demonstrated detoxification properties as well as synthesis capacities.