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Opportunities for scientific expansion of the deceased donor pool in liver transplantation

2014; Lippincott Williams & Wilkins; Volume: 20; Issue: S2 Linguagem: Inglês

10.1002/lt.24002

1527-6473

A. Matton, Robert J. Porte,

Organ Donation and Transplantation

Over the past decades, liver transplantation has become a successful treatment for patients with end-stage liver disease. A considerable number of patients awaiting liver transplantation, however, die while on the waiting list as a result of the significant global discrepancy between demand and availability of suitable donor livers. In an attempt to expand the number of liver transplantations, physicians are currently pushing the limits by performing split and live liver donations as well as accepting livers from extended criteria donors (ECD).1, 2 In the Western hemisphere, the vast majority of livers used for transplantation, however, remain those from deceased donors. Livers can be donated after either brain death (DBD) or circulatory death (DCD). In most Western countries the number of DBD donations has remained steady or even declined over the last decade, but the number of DCD donations has been increasing.3 The proportion of liver transplantations performed with DCD livers increased from 1.1% in 1995 to 11.2% in 2010 in the United States.4 In the United Kingdom, the percentage of DCD livers was 18% in 2012, whereas in the Netherlands it had increased to 38% in 2013.5,6 Simultaneously, however, the number of unused DCD livers has also been increasing over the past decade as a result of too many concomitant risk factors for graft dysfunction, such as older donor age, high body mass index, and diabetes mellitus in the donor.4 It is not likely that expansion of the deceased donor pool will come from more DBD livers. The largest gain in the number of suitable deceased donor livers could potentially be obtained by maximizing the use of DCD livers. Other types of ECD livers that carry an increased risk of graft failure include steatotic livers and livers from elderly donors.7 A common characteristic of DCD and other types of ECD livers is that they are at greater risk of developing significant ischemia/reperfusion injury, leading to parenchymal, endothelial, and/or biliary injury and subsequent dysfunction (Table 1).8 Biliary injury, in particular, is a significant problem in the transplantation of DCD livers. Bile duct injury can result in leakage and fibrosis of the larger bile ducts, leading to so-called nonanastomotic biliary strictures (NAS; also known as ischemic-type biliary lesions or ischemic cholangiopathy).9 The development of NAS has been reported for up to 30% of DCD livers, and 50% of these patients die or require retransplantation.9, 10 The pathophysiology of NAS is not yet fully understood, but ischemia-related injury, immune-mediated injury, bile salt toxicity, and a lack of regenerative capacity of the bile ducts are thought to be responsible for the development of NAS.11 Ischemia-related injury plays the largest role because biliary epithelial cells are very susceptible to ischemia and are dependent mainly on the oxygen supply through the hepatic artery.11 As a result of the increased rates of graft failure and biliary complications, the costs of DCD transplantations are approximately 30% higher compared with DBD transplantations.12, 13 It has become evident that the current method of organ preservation, which is based on cooling, is not good enough to protect suboptimal donor livers such as those from ECD and DCD donors. The current standard method of organ preservation is static cold storage (SCS), in which the organ is flushed with ice-cold preservation fluid and stored at low temperature (0°C-4°C) in a box with melting ice during transportation from the donor hospital to the transplant center. The advantages of preserving livers using SCS are that it is easily executable, transportable, and inexpensive. However, SCS also causes damage to the organ, frequently resulting in an unacceptably low-quality graft with suboptimal ECD livers (Fig. 1). During SCS, livers are not oxygenated, resulting in adenosine triphosphate depletion, and cold-induced damage occurs. Furthermore, there is no means of assessing the functionality and viability of the organ shortly before implantation. Therefore, optimization of the use of ECD livers should come from novel organ preservation methods. For this end, machine perfusion is the most promising technique. Schematic presentation of the decline in liver graft quality and viability during SCS versus machine perfusion. For ECD liver grafts, SCS results in a rapid decline in organ quality below a level at which the liver can still be transplanted with acceptable outcome. Machine perfusion has the potential to slow the rate at which this decline in quality occurs, resulting in better organ viability after a given period of preservation and potentially allowing for prolongation of the preservation time. In addition, machine perfusion might allow for the resuscitation of liver grafts. Experimental research has indicated that machine perfusion is superior to SCS in the preservation of donor livers. Machine perfusion leads to less ischemia/reperfusion injury,14 allows for prolonged preservation of the organs,15 and has the potential to restore and/or stimulate regeneration of damaged tissue. Moreover, machine perfusion also allows for the ex vivo assessment of graft viability1, 16 and provides the potential for (pharmacological) preconditioning.17, 18 In these ways, machine perfusion can increase the number and quality of donor organs. Disadvantages of machine perfusion, however, are that it is more complex and expensive to perform than SCS (Table 2). The technique of machine preservation and perfusion is still evolving, and several questions remain unanswered (Table 3). It remains to be determined what is the optimal temperature at which organs should be perfused, whether or not an oxygen carrier should be added to the perfusion fluid, how long and at what pressure livers should be perfused, and finally what is the optimal timing of machine perfusion in the period between procurement and transplantation. Furthermore, reliable criteria for the viability assessment of donor livers have yet to be confirmed in the clinical setting. With respect to timing, machine perfusion can be performed in the donor (normothermic regional perfusion),19 immediately after procurement, and/or during or after the storage and transportation of the organ (Fig. 2). Schematic overview of the various combinations and types of liver machine perfusion that have been described. The optimal combination of different machine perfusion techniques remains to be determined and may vary with type of donor liver. Large numbers of animal experiments have been performed to explore the feasibility and potential benefits of machine perfusion. In one study, hypothermic oxygenated machine perfusion of porcine DCD livers has been shown to prevent arteriolonecrosis of the peribiliary vascular plexus, potentially reducing posttransplant biliary ischemia and leading to faster and more efficient regeneration of the biliary epithelium.20 Another study recently suggested that normothermic machine perfusion also improves biliary epithelial regeneration in a porcine model of DCD livers.21 Moreover, there is evidence from an experimental study that gradual warming of DCD liver grafts is superior to SCS and hypothermic machine perfusion.22 The first clinical application of liver machine perfusion was reported by Guarerra et al. in 2010.23 This study of 20 patients involved dual (portal vein and hepatic artery) nonoxygenated hypothermic machine perfusion of the donor liver before transplantation. This method resulted in lower cellular damage markers and less ischemia/reperfusion injury after transplantation.24, 25 A second clinical trial was reported by Dutkowski et al. in 2014.26 These investigators reported on the feasibility and safety of hypothermic oxygenated machine perfusion through the portal vein in DCD livers and achieved excellent early outcome after transplantation in 8 patients. Our group has recently initiated a pilot study on hypothermic oxygenated machine perfusion using dual perfusion of both the portal vein and the hepatic artery in DCD livers (Netherlands Trial Registry ID: NTR4493; www.trialregister.nl). This trial is ongoing, but the initial results are encouraging. More clinical trials will be needed to elucidate whether the different methods of machine perfusion are beneficial in the prevention of graft failure and biliary complications after transplantation, especially in DCD liver grafts. A multicenter, randomized, controlled clinical trial will soon be initiated by our group to compare hypothermic dual oxygenated machine perfusion with SCS in DCD liver grafts. A primary endpoint in this trial will be the development of NAS. Another randomized, controlled clinical trial has been initiated to evaluate the effects of hypothermic oxygenated perfusion through the portal vein alone in DBD livers (ClinicalTrials.gov ID: NCT01317342). In addition, a randomized controlled clinical trial on normothermic machine perfusion (Controlled-Trials.com ID: ISRCTN39731134) will soon be launched, and a pilot study of normothermic regional perfusion in DCD organ donors was recently completed.19 The largest potential gain to be obtained in expanding the deceased donor pool lies in the utilization of ECD livers; there are increasing numbers of unused DCD livers compared with a stable or even declining number of DBD livers. It is a crucial that measures are taken to improve the quality of ECD donor livers, especially of livers obtained from DCD donors. DCD livers already account for a substantial proportion of all liver transplantations performed in countries such as the United Kingdom and the Netherlands. Increased utilization of DCD livers may contribute significantly to the number of available deceased donor livers in other countries as well. Moreover, improving the quality of DCD livers could lead to a substantial reduction in the rate of early graft failure after transplantation. Assessing the viability of livers, in particular suboptimal ECD livers, before transplantation would also lead to a more careful selection of transplantable livers. This would theoretically result not only in better outcomes after transplantation but also in expansion of the number of available donor livers. A common characteristic of DCD and other types of ECD livers is that they have suffered a higher degree of injury prior to transplantation, explaining the higher risk of early graft failure after transplantation. It has become evident that the current method of organ preservation, which is based on cooling and SCS, is not sufficient to preserve these preinjured ECD and DCD livers adequately. If we want to improve the numbers and success rate of transplantation of livers from DCD and ECD donors, we have to introduce more sophisticated methods of organ preservation. Machine perfusion is receiving increasing attention as an alternative preservation method (Fig. 1). Experimental studies have indicated that machine perfusion provides better protection of DCD livers, and the first clinical trials have been initiated and reported. The potential role of machine perfusion in expanding the deceased donor pool is twofold. First, machine perfusion can be used for resuscitation of liver grafts prior to transplantation, thereby not only improving the quality of DCD transplants but also increasing the number of transplantable ECD livers. Second, machine perfusion can be used to assess the function and viability of liver grafts prior to transplantation, allowing for careful selection of transplantable livers from a pool of currently discarded ECD livers. Various protocols for machine perfusion have been described, but it remains to be established which method provides the best protection of DCD livers (Fig. 2). The optimal and most cost-effective strategy for liver preservation based on machine perfusion may be a combination of different techniques for the different phases of organ preservation and transportation (Fig. 3). An important outcome parameter for determining the efficacy of machine perfusion will be the degree of biliary injury and the rate of biliary complications (ie, NAS) after DCD liver transplantation. The optimal and most cost-effective strategy of liver preservation based on machine perfusion technology may be a combination of different techniques for the different phases of organ preservation and transportation.