Normothermic Oxygenated Perfusion (NMP) Viability Testing Before Transplantation of Discarded Livers

Liver transplantation (LT) is a highly effective treatment for end-stage liver disease and early-stage primary liver cancer. As such, the demand for donor livers greatly exceeds supply; in 2021 in France, 12.9% of patients on the waitlist either died or were delisted for worsening of their condition.

Demand for liver grafts has driven the wider use of extended criteria donors (ECD). Grafts from ECD donors are associated with an increased risk of primary non-function, which is difficult to predict on an individual basis. The suitability of donor livers is determined based on donor history, age, weight, biological parameters of liver function, radiologic features, as well as the graft's macroscopic aspect at the time of procurement. The predictive value of these parameters to detect grafts unsuitable for transplantation is low, especially in the group of grafts obtained from ECD. Hence, the viability of donor livers can only be assessed after the fact, which could be life threatening for recipients of ECD livers.

As a result, a substantial number of perfectly viable organs are wrongly discarded based on a highly subjective assessment as the level of acceptance varies widely depending on the physician's judgement. In France, 253 (20%) of proposed livers were rejected in 2021.

Per current practices, livers are made available for rescue allocation after 5 refusals from 5 different transplant centers before being definitely discarded. Although the reasons for refusal are sometimes evident, (i.e. suspicious tumor or cirrhosis on the donor's CT-scan), most of the time refusal is based on the donor's clinical and biological characteristics.

This multicentric prospective non-randomized trial aims to evaluate the potential of NMP to provide an objective viability assessment of discarded livers. The co-primary endpoints will be (i) the rate of initially discarded livers eventually rescued after evaluation on a normothermic machine and (ii) the rate of functional grafts at 3 months among transplanted rescued livers.

A graft is considered functional at 3 months if the recipient remains alive and no retransplantation has been necessary within that period.

The device that will be used to perfuse the grafts is the OrganOx metra. It perfuses the donor liver with blood, oxygen and nutrients, as well as a number of medications (bile salt, insulin, heparin and prostacyclin), at normal body temperature to mimic ideal physiological conditions and preserve the organ for up to 24 hours. The device provides information as to the haemodynamic, synthetic and metabolic function of the liver to assist the clinician in assessing the organ's suitability for transplantation.

For a liver to be considered viable it has to meet specific viability criteria that are easily assessed using the perfusion device. Several teams demonstrated that NMP provides a tool to assess organ viability pre-transplantation as the liver is able to maintain an almost physiological metabolism. This is especially important as NMP allows an objective assessment of grafts driving the decision to transplant based upon actual graft function rather than a rather superficial risk assessment, which is what is being done currently. Mergental et al. proposed in 2016 lactate clearance (i.e. < 2.5 mmol/l) as a marker of viability in high-risk or discarded livers while Watson et al. suggested that viability is a combination of several characteristics including transaminases, glucose metabolism, lactate clearance, and acid-base balance. In addition, the ability of the liver to maintain acid-base homeostasis was shown to be predictive of post-operative outcomes. Sutton et al. also suggested that bile output may differentiate viable from non-viable livers. Other proposed criteria include hemodynamic parameters (hepatic artery and portal vein flow) and bile composition during NMP.

In 2020, Mergental et al. reported validation of lactate clearance as a viability criteria (i.e. < 2.5 mmol/l within 4 hours) allowing transplantation to be performed using 22 of 31 discarded livers with 100% 90-day survival. Another trial reported successful transplantation after NMP using a custom-built device of 15 livers out of 21 discarded grafts. These preliminary results came from small samples, 45% of which originated from donation after circulatory death (DCD). They need confirmation in a larger sample of organs from donors with brainstem death (DBD), adapted to the French liver allocation system. Grafts from DCD donors will not be included because, in France, DCD donors are subjected to in situ normothermic regional perfusion before procurement. Only liver grafts procured from DBD donors will be included in this study.