Normothermic regional perfusion and ex-situ normothermic perfusion in transplantation: hot times for kidneys ?

The preservation of kidney grafts has historically been based on hypothermia of the organs to decrease the baseline cellular metabolism. According to the Van’t Hoff equation, during preservation at 4°C, cellular metabolism is approximately 10% of the metabolism at 37°C (1). Slowing down the metabolism reduces the deleterious effects of ischaemia, which is defined as the cessation of blood flow to the organ, resulting in a cessation of oxygenation and nutrient supply. Despite the hypothermia, there is still a mismatch between metabolic supply and demand, leading to an alteration in cellular metabolism with the cessation of the mitochondrial respiratory chain (2), the depletion of intra-cellular ATP stocks and the initiation of the anaerobic pathway with the production of lactate. This cascade of events is responsible for acidosis, shutdown of membrane ion pumps, cell oedema, cell apoptosis and ultimately loss of organ function. To reverse this phenomenon, a paradigm shift is currently taking place: restoring aerobic metabolism in normothermic conditions, while providing the elements necessary for proper organ cell function.

Two preservation strategies are based on maintening the organ in normothermic condition: the normothermic regional perfusion (NRP) and the ex-situ normothermic perfusion (EVNP). However, these techniques differ in their position within the renal graft preservation strategy, but also in the means required for their implementation, as well as in their respective objectives. We propose to present the latest data available in the literature on these two techniques.

NRP is only used in donation after circulatory death (DCD), whether in controlled DCD (cDCD) or in uncontrolled DCD (uDCD) procedures. The NRP consists of implementing an extra-coporeal membrane oxygenation (ECMO) after the donor’s circulatory arrest, to re-circulate the donor’s blood in situ while waiting for grafts procurement. The NRP may concern the abdominal organs such as kidney but also the thoracic organs and is mainly performed by femoral cannulas. The aim of NRP is to reduce the ischaemia reperfusion injuries following cardiac arrest in DCD context. Its implementation in clinical practice depends on country-specific procurement protocols.

To date, no randomised controlled trials have been conducted to evaluate PRN.  However, national experience in several countries has been reported with comparative results on matched control groups to demonstrate the safety but also the benefit of implementing NRP in cDCD and uDCD procedures. Oniscu et al (3) reported the results of the UK experience comparing the outcomes of cDCD grafts with or without 2 hours of NRP. 210 transplants from cDCDs with NRP preservation were analysed. They demonstrated that the use of NRP was significantly associated with a decrease in delayed graft function (DGF) (-35%) and that renal function at 12 months was significantly improved (+6.3mL/min/1.73m2).  Padilla et al (4) reported the results of the Spanish experience. The outcomes of grafts from cDCDs with or without the implementation of NRP (duration 60-120 minutes) were compared. 865 transplants were performed from cDCDs with NRP preservation. The implementation of NRP was associated in univariate analysis with a significant decrease of DGF, an increase in renal function, graft, and patient survival at 1 year. In multivariate analysis, only the significant decrease of DGF was highlighted. In France, the implementation of NRP is mandatory in cDCD procedures. Savoye et al (5) reported the results of the French experience by comparing the outcomes of cDCD grafts with NRP to donors after brain death (DBD). In 442 cDCD transplants, the rate of DGF was significantly lower than in DBD transplants. Function and survival of grats at 1 year were identical between the 2 groups. NRP also appears to be of interest in uDCD procedures. Antoine et al. reported French results in this population comparing outcomes with or without NRP over a 3-hour period. 251 uDCD with NRP implementation were compared to 249 uDCD without NRP (6). There observed a significant decrease in primary non-function in the NRP group (6.0% Vs 8.9%), as well as a significant decrease of patients with a renal function at 1 year below 30mL/min or a grat loss.

Overall, in cDCD and uDCD, the implementation of NRP between circulatory arrest and organ procurement appears to be the current standard for reducing DGF. The implementation of NRP also seems to influence the long-term function of renal graft. With these results, NRP is becoming an essential preservation modality in many national procurement protocols.

EVNP consists of an isolated perfusion of the kidney graft after its procurement. Its clinical applications are currently more limited than NRP. There is currently no consensus on the perfusion modality. A perfusion solution composed of an oxygen carrier (Red-blood-cell solutions are most often used, non-heme oxygen carriers are also evaluated) is warmed and oxygenated before being circulated in the graft. EVNP devices are most often adapted from a CEC circuit. Only one EVNP device is currently licensed (Kidney assist, Organ Assist, Netherland). EVNP can have several purposes: rehabilitation and preparation of the graft before transplantation, re-evaluation of an initially rejected graft, or ex-situ treatment of the gaft via targeted therapies. Depending on the purpose, EVNP can be applied throughout preservation or can be applied after a period of preservation in hypothermic condition and before transplantation.

The primary and to date the main use of EVNP are the rehabilitation and the re-evaluation of grafts rejected for transplantation. The first use in the human was performed by the team of Hosgood and Nicholson (7): a kidney graft from a 55-year-old patient rejected for transplantation was perfused thanks to EVNP after a cold storage of 11 hours and for a duration of 35 minutes. EVNP was able to certify the complete revascularization of the graft and the re-start of diuresis of the graft. The transplant was then performed without DGF and with satisfactory renal clearance. Since this first implementation, the safety of EVNP reassessment has been demonstrated with the publication of the first cohorts on recused cDCD (8) and expanded criteria DBD (9) with very encouraging outcomes. The Toronto team (10) has just published the first series of transplants after NEVP in non-recused donors for DBD and DCD transplantation. They compared the outcomes of graft preserved thanks to hypothermic machine perfusion with the addition of 3 hours of NEVP prior to transplantation to a matched cohort of grafts preserved thanks to hypothermic machine perfusion alone. In 12 patients, they demonstrated the feasibility and safety of this preservation modality with encouraging outcomes such as a non-significant decrease of DGF. Finally, a first multicentre randomised controlled phase II trial is underway in the UK (11). This trial compares preservation by static cold storage alone to preservation combined with reperfusion by NEVP 60 minutes before transplantation in cDCD patients. The primary endpoint is the reduction of DGF.

NEVP also seems to be an interesting modality to performed longer preservation duration (>24 hours). Weissenbacher et al (12) demonstrated the technical feasibility of a 48H perfusion by setting up a re-circulation of urine within the NEVP. This first evaluation focused on perfusion and histology parameters and has not yet been followed by a renal transplantation. Finally, the NEVP can be used as a vector to enable the delivery of organ-specific therapy prior to transplantation. Thus, a whole field of research has opened up to allow the regeneration of the graft and prepare it for transplantation, including cell therapy modalities but also gene therapy (13).

In total, NRP and NEVP are 2 preservation modalities implemented under normothermic conditions. NRP is becoming the clinical standard for the procurement of renal graft in cDCD and uDCD conditions. NEVP is currently being evaluated. However, it offers particularly interesting data to assess graft before transplantation. The publication of the first randomised controlled trials will help to better define the implementation of normothermic strategies in the preservation of renal grafts.

Statements and Declarations

Funding: The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Conflict of interests: The other authors have no relevant financial or non-financial interests to disclose.

References

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