In this issue of Transplantation, attention is focused on a perennial challenge, organ preservation and its associated ischemia-reperfusion injury (IRI). The interview of Dr James Southard (see Special Feature in this issue1) reminds us that research objectives have changed remarkably little in the last 40 years, and that fundamental scientific principles—understanding the biology and using the right models—remain as important as ever. However, although the broad objectives may be the same, it is notable how the specific targets of research in this field have moved. Very few recent publications describe new chemical compositions for static cold storage (SCS) solutions (indeed there is no consistent evidence that any more recent product is superior to University of Wisconsin solution introduced in the 1980s). There is, however, renewed interest in machine perfusion (a technology first used clinically in the 1960s) and also to the application of modern biological technologies to the twin challenges of clinical demand and donor organ quality.
What are the problems that researchers in the field are trying to solve? The science that underpins organ preservation is essentially that of ischemia-reperfusion—a physiological process that thwarts the best intentions of transplant surgeons and scientists, and one that is particularly problematic in suboptimal donor organs. Much is now known about the pathology: this should enable us to devise the means to manage it, using organ preservation technology as the platform to deliver this (eg, see Jain et al,2 Cucchiari et al3 in this issue). Approaches to organ preservation can be categorized by the following: (1) method of delivery (static versus machine perfusion, continuous versus combination); (2) temperature (subzero, hypothermic, subnormothermic, normothermic); (3) oxygen (±nutrient) delivery; (4) location (ex situ versus in situ).
Despite many years of use in kidney transplantation, evidence for the benefit of hypothermic machine perfusion (HMP) was slow to emerge, but is now substantiated, at least in extended criteria donor organs.4 Early enthusiasm for HMP in liver transplantation led to oxygenated systems and encouraging evidence of benefit in high risk and donation after circulatory death (DCD) organs.5 Normothermic perfusion is inherently more complex and expensive: systems for lung and liver perfusion reached clinical implementation ahead of those for kidney and heart. Pancreas and intestinal perfusion remains within the domain of laboratory research, although the potential application of such technology here is widely agreed.
Temperature is a “hot” topic. HMP is intrinsically safe: if the perfusion fails, the organ defaults to SCS. The addition of oxygen has been shown to be effective even at cold temperatures, believed to be by the avoidance of mitochondrial succinate accumulation and consequent reactive oxygen species generation at reperfusion.6 Very encouraging data are coming from clinical studies in kidney and liver DCD transplants.7,8
Normothermic machine perfusion (NMP) is a much more recent entrant to the transplant environment but is now prominent in clinical trials involving heart, lung, liver, and kidney patients. As a preservation method, there are strong theoretical advantages to storing organs in a physiological, functioning state: the organ can be resuscitated from hypoxia, or other acute injury; cellular adenosine triphosphate (ATP) levels can be restored; organ function can be used to predict viability; preservation times can be prolonged. These advantages have been (partially) exploited in the NMP systems deployed to date.
NMP is not the only “new kid on the block.” Although not yet supported by published clinical data, “supercooling” (the technique of preserving organs at below ice temperature) has many attractions. This was described by the group of Berendsen et al9 in the liver, using machine perfusion and static storage in sequence: in this issue, Que et al10 describe the preservation of mouse hearts at –8°C, using a hyperosmolar perfusion solution to avoid freezing. Compared with UW-preserved controls, supercooling reduced the rate of ATP consumption, measured in tissue at 24 hours preservation. Heterotopic heart transplantation was successfully achieved after 96 hours of preservation (much longer than could be achieved using conventional SCS).
Our increased understanding of the cellular and molecular processes that take place during organ retrieval, preservation, and transplantation is generating a plethora of novel approaches to therapy: indeed, the challenge is to know which strategies to test and in what context. With the opportunities opened by machine perfusion (particularly under normothermic conditions), we must now determine what it is that we require this technology to deliver.
Machine perfusion can be used to test organ viability: there are advantages to the normothermic systems because of the physiological temperature and organ function inherent to this method. Viability assessment may involve simple injury markers (eg, transaminase levels during liver perfusion), functional markers (eg, left ventricular function during cardiac NMP, see Ribeiro et al,11 this issue), or genetic markers (eg, miRNA, see Parker,12 this issue). The combination of a stable, physiologically perfused organ and a validated algorithm based on objective viability biomarkers is likely to transform our clinical approach to the marginal donor organ.
NMP can be used to treat the donor organ after retrieval and before transplantation: this enables therapies to be delivered that might not be effective if given systemically. For example, delivery of mesenchymal stem cells may be much more effective if delivered directly to the organ during NMP, rather than by intravenous delivery to the patient (with sequestration in the lungs). It is these, more ambitious, objectives which researchers are beginning to explore, particularly the prospect of delivering targeted therapies to abrogate IRI. These strategies include using siRNA and miRNA methods to block IRI (see Elgharably et al13 and Parker12 in this issue). Others are researching the use of pharmacological agents to protect the organ from IRI: we may even see the repurposing of some old drugs, including cyclosporine (see Haam et al14 in this issue) and metformin (see Westerkamp et al15 in this issue). For a review of the wide range of cellular, genetic, and pharmacological interventions relating to perfusion therapeutics, see Xu et al16 in this issue.
Temperature has profound and not always predictable, implications in the context of transplant organs. Instead of regarding temperature as a binary variable (cold or warm), we should think of it as a continuous variable. Already there is evidence that the rate of rewarming is important, with clear benefits demonstrated by a gradual transition from HMP to NMP.17 A similar effect is seen with respect to oxygen transition: progressive rather than sudden change is important in oxygen delivery too.
Hyperthermia may have therapeutic applications in transplantation: in this issue, Chen et al18 describe the potential benefits of “triple progressive thermopreconditioning.” Rat hearts previously subjected to 3 sequential immersions in a 42°C water bath were relatively protected from the effects of prolonged occlusion of the left anterior descending artery, an effect associated with induction of protective proteins. In the context of NMP, von Horn et al19 previously showed that rat livers are similarly protected from reperfusion injury by a 10-minute exposure to 42°C during machine perfusion.
As well as perfusing the isolated organ (ex situ), transplant organs can be perfused within the donor (in situ). Increasing dependency on DCD organs in many countries has focused attention upon in situ perfusion, “normothermic regional perfusion” (NRP) following circulatory arrest. This development has been led by centers in Spain and France, and more recently the United Kingdom. In this issue, Savier et al20 describe a matched control study comparing the outcomes of liver transplantation from controlled DCD donors managed with NRP (n = 50) with those from donation after brain death (DBD) controls (n = 100). The markers of IRI (peak transaminase in the first 4 d, early allograft dysfunction, acute kidney injury rate) were better in the DCD + NRP cohort than in the DBD cohort. Notably, there was no difference in biliary complications (a major concern in DCD donor liver transplants), and no difference in medium-term outcomes. This implies that NRP substantially abrogates the deleterious effects of warm ischemia. It also corroborates the positive data from controlled DCD + NRP liver transplantation from the United Kingdom.21 The combination of NRP and NMP has not yet been reported clinically, but clear evidence of the potential of this was shown in a porcine liver transplant model.22
Other combinations of perfusion methods are in use. The complexity of initiating machine perfusion from the start of preservation is logistically challenging: this requires the perfusion device to be transported to the donor hospital, together with the expertise to operate it. It is much simpler to transport the organ in an ice-box and initiate perfusion at the transplant hospital. There is undoubtedly a theoretical advantage to continuous perfusion from the outset of preservation compared with “poststatic cold storage” perfusion, but this remains to be quantified.
The progressive adoption of perfusion systems into clinical service (as opposed to clinical trials) is revealing an emerging pattern of use: there are 3 broad indications for this technology. First, indications that relate to the quality of the donor organ—to improve utilization of marginal donor organs. Improved utilization has not been systematically shown in HMP (possibly because of the lack of proven viability metrics) but has been shown in NMP of the liver, both in a large European randomized trial23 and in a recent study of previously discarded livers.24 Increasing organ utilization is a major objective of any new preservation method: it is likely that this will be key to any future health-economic justification for this technology.
Second, indications that relate to the recipient—to facilitate exceptionally difficult cases. An example is described in this issue25 by Carvalheiro et al: a highly complex redo liver transplant requiring cardiac as well as abdominal surgery that was enabled by an overall preservation time of >30 hours (including 23 h of NMP).
Third, indications that relate to hospital logistics—to enable more flexibility of transplant timing. This may allow planning of multiple transplants, longer travel times for donor organs (or recipients), or simply to make transplantation less onerous and more sustainable. In this issue, Cardini et al26 describe the benefits of planning liver transplantation as an essentially daytime activity even for extended criteria livers, as a result of the adoption of NMP. The recent publication from the Zurich group27 of 7-day NMP of both discarded human and pig livers opens up an exciting prospect of semielective liver transplantation, together with all the therapeutic interventions that a 7-day window would enable.
Perfusion technology is complex and expensive and the interventions that it enables will require skilled operatives. Rather than provide these in every transplant center, where the technology and expertise would be, at best, intermittently used, it may be better to concentrate these within strategically located “Assessment and Repair Centres.” Organs would be retrieved and transported rapidly from donor hospitals to such centers, where the organs would be managed by specialized personnel before being sent out to transplant hospitals. Organ recovery centers have been established in North America for donor lungs that have failed acceptance criteria: for other organs this is currently a point of discussion, but one which may soon require a decision as new perfusion systems become more widely accepted.
The interests of basic scientists, biomedical engineers, and transplant clinicians are converging in this area of vital importance to our patients. The crucial challenge is now to translate the most promising techniques into changes in clinical practice, through well-designed clinical trials. This is indeed a time of exciting opportunities.
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