Machine perfusion strategies in liver transplantation

The acceptance of liver transplantation as the standard of care for end-stage liver diseases has led to a critical shortage of donor organs. To expand the organ pool, many countries have liberalized the donor criteria to include a wider donor population. These donor livers tolerate preservation and transport using cold storage (which is the current standard of care) poorly. In recent years, research has focused on optimizing organ preservation techniques by using specialised machines to continuously pump the organ with blood/solution so that they are better preserved.

Research has showed these strategies to confer significant advantages over the current technique. In our review article, we discussed the topic of machine perfusion of liver with an emphasis on current trends. We also reviewed the relevant clinical trials related to this new technique and explored the potential future applications of machine perfusion which may usher in a new era of liver transplant surgery.

This rapid increase in liver transplant programs across 80 countries globally created its own set of problems like donor organ shortage. While the advent of split liver transplantation has potentially doubled the number of available allografts, technical challenges and a shortage of grafts suitable for splitting hinder its universal use.1 The lack of organs available for liver transplantation in many countries has led to liberalization of the selection criteria for acceptable organ donors. More research is now focused on optimizing organ preservation techniques and implementing salvage strategies for organs considered marginal or unsuitable for transplantation.

The concept of ex vivo machine perfusion (MP) of isolated organs dates back to the Lindbergh Apparatus in the 1930s.it deals with organ preservation using static cold storage (SCS). Researchers found that MP techniques using an appropriate perfusate limited allograft damage and led to better preservation of the liver, thereby optimizing the organ before implantation. This has led to a renewed interest in using (and optimizing) MP techniques to protect and preserve these marginal livers. Therefore, global trends in liver transplantation have been towards introducing MP strategies for organ preservation. This review aims to address the topic of MP in liver allograft preservation, with emphasis on current trends in clinical applications and future perspectives.

Basic concepts of machine perfusion
The term “machine perfusion” includes a variety of dynamic, continuous perfusion techniques, which aim to preserve, and possibly improve, the outcomes in liver transplantation. Depending on the temperature of the perfusate, modern MP techniques include the hypothermic machine perfusion (HMP) (2–10 C), subnormothermic machine perfusion (SNMP) (20–25 C) and normothermic machine perfusion (NMP) (35–37.5 C). HMP is the dynamic counterpart of SCS, and it aims to combine the technical simplicity and relative safety of SCS with the positive effects of continuous perfusion. In HMP, the perfusate is not actively oxygenated.

Techniques like hypothermic oxygenated machine Perfusion (HOPE) implement active oxygenation of the hypothermic perfusate. Now, if this oxygenated perfusate is circulated via the portal vein alone, it is termed HOPE. When the oxygenated perfusate is perfused using both the portal vein and hepatic artery, researchers have termed it dual HOPE or DHOPE. Most machines deliver continuous flow through the portal vein and pulsatile flow through the hepatic artery to mimic their physiology. In contrast to HMP, NMP aims to maintain a nearnormal metabolism of the allograft by perfusing it with oxygenated blood-based solutions at body temperatures. It also provides a unique theoretical opportunity for intervention during ex vivo MP by the addition of therapeutic substances into the perfusate.

Therapeutic strategies during ex vivo machine perfusion
Liver preservation using NMP is an evolving field with numerous research being focused on MP therapeutics to recondition the liver prior to transplant. This represents an exciting new frontier in liver transplants. The field of MP therapeutics in liver transplant is focused on defatting cocktails, vasodilators, anti-inflammatory agents, mesenchymal stem cell therapy, and gene therapy to optimize the graft during ex vivo NMP.  Steatotic livers are associated with a higher rate of EAD, therefore, accounting for a large proportion of organ rejection. Therefore, several researchers have examined the possibility using defatting cocktails to recondition steatotic livers prior to transplant.

In 2019, Boteon et al. infused PPARa (Peroxisome proliferator-activated receptor) ligands combined with other pharmacological modulators of lipid metabolism (defatting cocktail) to achieve decreased lipid content of human livers within 6 h. This was accompanied by a successful functional recovery and decreased expression of markers of reperfusion injury. Since liver sinusoidal endothelial cells, compared with hepatocytes, are more prone to injury during cold storage, researchers have also examined the use of prostaglandin E1 (PGE1) to improve the microcirculation.

Hara et al. first used PGE1 in a rodent model and observed that treated rodent livers had significantly improved bile production, in addition to decreased levels of liver injury markers.Echeverri and colleagues studied the effects of adding BQ123 (an endothelin receptor agonist) and verapamil (a calcium channel blocker) to the perfusate in a porcine transplantation model. Consistent to other studies, they demonstrated an improved hepatic arterial flow and lower hepatocyte injury markers during NMP.The usage of gene modulation agents, such as antisense oligonucleotides (ASOs) and small interfering RNA (siRNA) is especially exciting. In 2017, Goldaracena et al. demonstrated the use of ASO to silence miRNA-122, which is a necessary factor for HCV replication. They were able to induce HCV resistance in a porcine model. In 2020, researchers from Queen Elizabeth Hospital, Birmingham experimented with infusion of multi-potent adult progenitor cells into the right hepatic artery of six discarded allografts during NMP during perfusion.

They observed a trans-endothelial migration of these stem cells with subsequent secretion of anti-inflammatory and immunomodulatory agents.86 Protection of mitochondria is crucial to prevent IRI. As discussed above, accumulation of succinate is a metabolic marker of ischemia and is responsible for mitochondrial ROS production during reperfusion. Pharmacological strategies targeted towards decreasing succinate accumulation is sufficient to ameliorate ischemia-reperfusion injury in murine models of heart attack and stroke. NMP protocols holds great potential to effectively restore mitochondrial function and avoid redox stress-induced IRI.

The possibility of repairing these poor-quality livers sufficiently to enable transplantation requires their preserving for several days. Most of the current approaches for NMP have been used only for a relatively short period of time (median perfusion time of 9 h). This is not sufficient time to allow for ex-vivo therapeutics. So, Eshmuminov and colleagues sought to develop an ultra-long perfusion system with the aim to preserve 10 discarded human livers for seven days. Their perfusion system consisted of a dual vascular supply with high pressure, oxygen-rich arterial blood entering through the hepatic artery and low pressure, oxygen-reduced blood entering via the portal vein.

To mimic the physiological uptake of nutrients and the enterohepatic circulation of bile, they injected parenteral nutrition and ursodeoxycholic acid into the portal vein line of their perfusion machine. They also incorporated an integrated dialysis unit for removal of metabolic waste products from the blood. Their proprietary algorithm automatically adjusted the dialysate flow, and controlled hematocrit in the perfusate. Automated insulin and glucagon administration was used to maintain physiological blood glucose levels. Finally, balloons to mimic diaphragm oscillations were also integrated into the system to produce continuous movement of the liver.

With this new perfusion system, the researchers were able to maintain viability of six out of ten human livers up to seven days of ex-vivo perfusion without the need for additional blood products or perfusate exchange.87,88 The novel platform for long-term NMP and ex vivo therapeutics of human livers has the potential to be one of the most significant breakthroughs in transplant surgery of recent decades, enabling organ repair on the pump and it provides an exciting new direction for the world of liver transplants.

Over the last decade, there has been significant progress in the field of allograft preservation techniques. Despite this increased interest in the field of MP, and many clinical trials demonstrating favorable outcomes, only a minority of liver transplant centers worldwide currently own a liver MP device for use them outside of clinical trials (Table 6). A re-evaluation of some logistical aspects of organ allocation may be required to achieve wider adoption of MP techniques. HOPE and NMP methods arepromising approaches to optimize the marginal liver allografts.

Both allow for viability testing of the allografts, which may prove important in predicting the quality and safety of the organ for transplantation. Ultimately these strategies may improve the outcomes of ECD liver transplants. Ex-vivo therapeutics provides a tremendous potential to further salvage the fragile allografts and increase the donor pool.