To the Editor:
We read with interest the recent article by Alix et al. describing the effect of M101 (a marine worm hemoglobin) added to the static cold storage (SCS) solution for liver preservation.1 In this study, the authors compared SCS+M101 to SCS alone and to hypothermic oxygenated machine perfusion (HOPE). The authors claim that adding the oxygen carrier M101 to SCS could be an alternative to HOPE for liver graft preservation. Although the authors should be commended for investigating an innovative approach to prevent ATP depletion during graft preservation, there are several important methodological issues to address.
First, we disagree with the author's definition of HOPE and believe that this control group is flawed. The authors state that, during HOPE, a continuous oxygen flow was delivered to the surface of the preservation solution. However, this approach is substantially different from the previously described (and currently clinically used) method of HOPE, where a membrane oxygenator in the machine perfusion circuit allows for active oxygenation of the preservation solution to achieve partial oxygen pressures (pO2) of (at least) 70 kPa (500 mmHg).2,3 Delivering oxygen only to the surface of the preservation solution is unlikely to achieve an adequate pO2. Unfortunately, there is no information on the oxygen content in the perfusate.
Secondly, the different oxygenation strategy may account for the decreasing ATP levels during machine perfusion, which is a very unlikely phenomenon during HOPE if the preservation solution is adequately oxygenated. In fact, several groups have previously shown increased ATP levels at the end of both single and dual HOPE, in experimental and clinical studies (Table 1).2,[4], [5], [6], [7], [8], [9], [10] The observed decrease in ATP levels during machine perfusion supports our hypothesis that the HOPE methodology applied by the authors does not achieve adequate perfusate pO2 levels. Adequate oxygenation under hypothermic conditions remains the key strategy to decrease reperfusion injury, as mitochondria are reprogrammed by cold oxygenation, leading to metabolism of accumulated succinate and NADH with significant increases in ATP levels within just 1–2 hours of HOPE.11
Table 1.
Studies using HOPE for donor liver preservation with measurement of ATP.
| Reference | Protocol | Fold ATP increase |
|---|---|---|
| Brüggenwirth et al. (2020)5 | 20-hour dual HOPE of discarded human livers | 5 |
| Van Rijn et al. (2017)2 | 2-hour dual HOPE of human livers | 11 |
| Westerkamp et al. (2016)6 | 2-hour dual HOPE of discarded human livers | 15 |
| Monbaliu et al. (2012)7 | 24-hour dual HOPE of discarded human livers | 2 |
| Stegeman et al. (2009)8 | 90-minute HOPE of rat livers | 6 |
| Vekemans et al. (2007)9 | 24-hour HOPE of porcine livers | 1 |
| Dutkowski et al. (2006)10 | 1-hour HOPE of rat livers | 1 |
ATP, adenosine triphosphate; HOPE, hypothermic oxygenated machine perfusion.
Third, in the results presented by the authors, mitochondrial activity and post-transplantation injury markers are similar between livers preserved with SCS supplemented with M101 vs. HOPE. However, we postulate that by using HOPE with an adequate pO2 of the preservation solution, mitochondrial function would have been substantially improved. Notably, high (100%) oxygen delivery best preserves functional and structural liver integrity compared to low (20%) oxygen delivery.12 In addition, machine perfusion in the absence of oxygen is not able to prevent reperfusion injury and triggers hepatocyte death.12,13
Fourth, the effect of M101 supplementation in SCS solutions should be further tested in experimental models, such as animal models or preclinical research models using severely injured grafts, e.g. in livers exposed to extended donor warm ischemia, or in severely steatotic livers.
Fifth, we believe there are several other benefits of HOPE, such as the ability to assess liver viability or extend preservation time, which are currently unlikely to be achieved by SCS.
Taken together, we believe that it is currently overstated that M101 supplemented to the SCS solution could be an alternative to HOPE for liver graft preservation. Based on the results presented, supplementation of M101 to the SCS solution seems better than SCS alone, but this requires more research. Additionally, this method still does not achieve the results that can be obtained with HOPE.
Financial support
The authors received no financial support to produce this manuscript.
Authors' contributions
IB, RP, VM wrote the manuscript in consultation with OL, MM, PD, DM and PM. All authors critically revised the final manuscript and approved the submitted version.
Conflict of interest
The authors declare no conflicts of interest that pertain to this work.
Please refer to the accompanying ICMJE disclosure forms for further details.
Footnotes
Author names in bold designate shared co-first authorship
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jhepr.2020.100194.
Supplementary data
References
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