Kidney Preservation Strategies to Improve Transplant Outcomes.

The Machine Perfusion Hype The inability to preserve organs beyond several hours contributes to the scarcity of organs for transplantation. To exacerbate the difficulty in organ preservation and to convene with the ever-growing organ demand, grafts donated after circulatory death or retrieved from extended criteria donors are increasingly used. These organs are more susceptible to ischemia-reperfusion injury, and storage/preservation on ice was associated with higher risk of delayed graft function (DGF).1 DGF is a manifestation of AKI unique to the transplant process but hampered by marked variation in definition and diagnosis. Unfortunately, despite the nonconsensual definition, DGF is the most used end point in machine perfusion studies. A key issue in transplantation science is the timing and duration of interventions used to limit ischemia-reperfusion injury. Such strategies can be divided into preconditioning and postconditioning. Preconditioning strategies include donor management before organ recovery, such as intermittent hypoxia, dietary restriction, and therapeutic hypothermia. Postconditioning aims to improve graft quality ex vivo after procurement (e.g., machine perfusion) or to enhance the recovery of the recipient (e.g. enhanced recovery after surgery pathways). In animals, preconditioning was shown to reduce oxidative stress, preserve cellular energy stores, decrease the inflammatory response, and improve survival.2 In this context, previous work by Niemann et al.3 showed that DGF was reduced in recipients of kidneys from donors preconditioned using mild hypothermia (28%), compared with the standard normothermia (39%), followed by an uncontrolled period of static cold storage. Following up on these results, the same investigators tested whether mild donor hypothermia (34–35°C), established through noninvasive external cooling devices, could improve donated kidney graft function.4 Donor hypothermia was compared with hypothermic machine perfusion of the donated organ ex vivo, or a combination of the two, in a multisite study conducted in the United States. DGF occurred in 30% of the patients in the hypothermia group, in 19% of the patients in the machine perfusion group, and in 22% of patients in the combination therapy group. Although likely underpowered, 1-year graft survival was similar in the three groups. Surprisingly, this study only included brain-dead kidney donors, who were mostly (80%) standard criteria donors with a mean Kidney Donor Profile Index of 46%. Thus, this cohort is less likely to benefit from preconditioning and postconditioning strategies, including hypothermic machine perfusion. The authors concluded that in these kidney grafts, therapeutic hypothermia was inferior to ex vivo machine perfusion in reducing DGF. The combination of hypothermia and machine perfusion did not provide additional protection. In this perspective, in light of this recent trial,4 we review the current evidence for hypothermic kidney machine perfusion and future preservation/perfusion strategies to improve the outcome after kidney transplantation. Hypothermic Nonoxygenated Machine Perfusion Hypothermia slows the metabolism of cells by about half for every 10°C drop in temperature. Static cold storage works by removing blood and microthrombi from the kidney and replacing this with an acellular preservation solution in a hypothermic environment. Although the mechanisms underlying the benefits of hypothermic machine perfusion remain to be fully elucidated, pulsatile preservation upregulates nitric oxide production by the vascular endothelium as well as clearing the debris and toxic metabolites. Maintenance of hemodynamic stimulus by exerting shear stress forces on the vessel is likely to be also required for endothelial cells to maintain physiologic functions. Altogether, machine perfusion is associated with lower intrarenal resistance at the time of in vivo reperfusion, improving earlier transplant function, manifested by reduction in the DGF rate.5 There are now several commercially available perfusion devices that are broadly similar to minor variations in perfusion temperature (4–10°C), flow (pulsatile versus nonpulsatile), and provision of oxygenation (oxygenated versus nonoxygenated). The machines currently under study include the LifePort (Organ Recovery Systems; Itasca, IL), the KidneyAssist (OrganAssist; Gronigen, The Netherlands), and the Waters RM3 system (Rochester, MN). Once the kidney has been removed from the donor, the kidney is flushed manually with a preservation solution (e.g., Belzer MPS UW). After flushing, a cannula is inserted into the renal artery and secured and then connected to a disposable circuit designed specifically for the device. The preservation solution is then continuously recirculated through the donor kidney while being transported to a suitable recipient. In a landmark study including 672 kidney recipients from 60 European centers, hypothermic machine perfusion reduced the rate of DGF (odds ratio, 0.52) compared with static cold storage.5 These results were confirmed by later meta-analyses, demonstrating that hypothermic perfusion reduces the incidence of DGF in all types of donors.1 Because kidneys retrieved after circulatory death have a higher overall DGF rate, fewer perfusions are needed to prevent one episode of DGF. In fact, a recent Cochrane meta-analysis1 concluded that the number of perfusions to prevent one episode of DGF was 7.26 and 13.6 in recipient of kidneys donated after circulatory and brain death, respectively. Kidney graft survival was significantly higher after hypothermic machine perfusion versus static cold storage, both at 1 and 3 years after transplantation in standard and extended criteria grafts.1,5 In both the European setting and in the United States, the total expenditure was reduced after hypothermic machine perfusion, in association with the lower risk of DGF and rate of dialysis.1 Both economic evaluations were based on the results from Moers et al.,5 which represent an obvious limitation.1 There is no significant difference between hypothermic machine perfusion and static storage in the duration of DGF and hospitalization, acute rejection, patient survival, and long-term kidney transplant function, and data are contradictory for primary graft dysfunction.1 Further studies looking solely at the effect of hypothermic machine perfusion on DGF incidence are not necessary.1 The Future of Machine Perfusion Hypothermia precludes ex vivo optimization or assessment of graft function, leaving the cumulative effects of donor quality, preservation, and reperfusion injury to be revealed only after implantation. In addition, the absence of reliable viability assessment often led centers to conservative organ selection and underuse of available kidneys. During liver transplantation, two large randomized controlled trials recently demonstrated that blood-based normothermic machine perfusion (NMP) reduced ischemia-reperfusion injury, early allograft dysfunction, and reduced organ discard.6 To date, no randomized controlled trial has been published that includes a normothermic kidney perfusion arm. Although cold anoxic storage aims to arrest cell metabolism, ex vivo perfusion at physiologic normothermic temperature (37°C) provides a continuous flow of warmed, oxygenated perfusate containing nutritional substrates, thereby maintaining the metabolic activity of the tissue. Beyond maintaining energy equilibrium, normothermic perfusion minimizes cold ischemia time/injuries and promotes cell repair mechanisms. In the first clinical studies, NMP might be associated with an improvement in organ utilization.7,8 Beyond its diagnostic applicability, perfusion at 37°C might serve as a superior preservation strategy and a platform for active organ reconditioning. Early human data suggest that normothermic perfusion might reduce DGF compared with static cold storage.7,8 The first randomized trials comparing normothermic with hypothermic machine perfusion or static cold storage are underway (ISRCTN15821205 and NCT05782543). In a porcine preclinical model of donation after circulatory death, normothermic perfusion improved early postoperative creatinine and urea clearance.9 From a cost standpoint, NMP systems are more challenging, including the purchase and maintenance of the perfusion device, perfusate components, disposables, ancillary personnel and support staff, and facility fees. In this context, some concerns were raised that a more complex perfusion machine might further widen disparities and access to transplant. However, cost savings could be realized if perfusion technology turned out to reduce the incidence of postoperative complications and length of hospital stays. Importantly, the cost of machine perfusion was fully reimbursed through the US Centers for Medicare & Medicaid Services under the organ acquisition cost report during liver transplantation.6 Beyond the cost, normothermic perfusion requires a complex heating system, tight pH and glucose control, and might lead to red blood cell hemolysis, infection, and immunization. Furthermore, failure of the perfusion machine would lead to a rapid intragraft thrombosis and loss of the organ. Subnormothermic (21–22°C) machine perfusion was proposed as an alternative to perfusion at 37°C.10 Subnormothermia aims to avoid cold-induced graft injury without increasing metabolism to a level requiring oxygen carriers for adequate oxygenation.10 Perfusion of a cell-free, oxygenated perfusate at 22°C promoted mitochondrial respiration and ATP stores before transplantation. Overall, preclinical studies suggest that 22°C might be the optimal temperature to protect against ischemia-reperfusion injury in the kidney while avoiding complex machines used for normothermic perfusion.11 In porcine kidneys retrieved after circulatory death, subnormothermic perfusion with blood and PlasmaLyte reduced acute tubular necrosis and improved kidney function compared with normothermic perfusion. Importantly, subnormothermia might also be used for the delivery of pharmacological agents. Finally, a controlled oxygenated rewarming strategy has been proposed as a safer gradual transition from cold to warm before reperfusion, avoiding a sudden heat shock. Concluding Remarks Hypothermic machine perfusion has greatly impeded progress in mitigating DGF and improving kidney organ utilization.1 The use of (sub)normothermic perfusion is a rapidly evolving field that is poised to transform organ preservation, reconditioning/repair, and expand the utilization of organs that were previously considered untransplantatable (e.g., old). While we await the first clinical trial investigating normothermic versus hypothermic perfusion and static cold storage, long-term data of available studies comparing hypothermic machine perfusion to static cold storage should be reported. Finally, future (sub)normothermic perfusion trials should investigate the use of perfusion parameters to assess organ viability and improve outcomes.