Graft assessment of the ex vivo perfused porcine kidney using hyperpolarized [1-13 C]pyruvate

Ex-Vivo Human-Sized Organ Machine Perfusion: A Systematic Review on the Added Value of Medical Imaging for Organ Condition Assessment

Machine perfused ex-vivo organs offer an excellent experimental platform, e.g., for studying organ physiology and for conducting pre-clinical trials for drug delivery. One main challenge in machine perfusion is the accurate assessment of organ condition. Assessment is often performed using viability markers, i.e., lactate concentrations and blood gas analysis. Nonetheless, existing markers for condition assessment can be inconclusive, and novel assessment methods remain of interest. Over the last decades, several imaging modalities have given unique insights into the assessment of organ condition. A systematic review was conducted according to accepted guidelines to evaluate these medical imaging methods, focussed on literature that use machine perfused human-sized organs, that determine organ condition with medical imaging. A total of 18 out of 1,465 studies were included that reported organ condition results in perfused hearts, kidneys, and livers, using both conventional viability markers and medical imaging. Laser speckle imaging, ultrasound, computed tomography, and magnetic resonance imaging were used to identify local ischemic regions and quantify intra-organ perfusion. A detailed investigation of metabolic activity was achieved using 31P magnetic resonance imaging and near-infrared spectroscopy. The current review shows that medical imaging is a powerful tool to assess organ condition.

Ex Vivo Simultaneous H215O Positron Emission Tomography and Magnetic Resonance Imaging of Porcine Kidneys-A Feasibility Study

The aim was to establish combined H215O PET/MRI during ex vivo normothermic machine perfusion (NMP) of isolated porcine kidneys. We examined whether changes in renal arterial blood flow (RABF) are accompanied by changes of a similar magnitude in renal blood perfusion (RBP) as well as the relation between RBP and renal parenchymal oxygenation (RPO).

METHODS

Pig kidneys (n = 7) were connected to a NMP circuit. PET/MRI was performed at two different pump flow levels: a blood-oxygenation-level-dependent (BOLD) MRI sequence performed simultaneously with a H215O PET sequence for determination of RBP.

RESULTS

RBP was measured using H215O PET in all kidneys (flow 1: 0.42-0.76 mL/min/g, flow 2: 0.7-1.6 mL/min/g). We found a linear correlation between changes in delivered blood flow from the perfusion pump and changes in the measured RBP using PET imaging (r2 = 0.87).

CONCLUSION

Our study demonstrated the feasibility of combined H215O PET/MRI during NMP of isolated porcine kidneys with tissue oxygenation being stable over time. The introduction of H215O PET/MRI in nephrological research could be highly relevant for future pre-transplant kidney evaluation and as a tool for studying renal physiology in healthy and diseased kidneys.

Lactate and Lactylation: Clinical Applications of Routine Carbon Source and Novel Modification in Human Diseases

Cell metabolism generates numerous intermediate metabolites that could serve as feedback and feed-forward regulation substances for posttranslational modification. Lactate, a metabolic product of glycolysis, has recently been conceptualized to play a pleiotropic role in shaping cell identities through metabolic rewiring and epigenetic modifications. Lactate-derived carbons, sourced from glucose, mediate the crosstalk among glycolysis, lactate, and lactylation. Furthermore, the multiple metabolic fates of lactate make it an ideal substrate for metabolic imaging in clinical application. Several studies have identified the crucial role of protein lactylation in human diseases associated with cell fate determination, embryonic development, inflammation, neoplasm, and neuropsychiatric disorders. Herein, this review will focus on the metabolic fate of lactate-derived carbon to provide useful information for further research and therapeutic approaches in human diseases. We comprehensively discuss its role in reprogramming and modification during the regulation of glycolysis, the clinical translation prospects of the hyperpolarized lactate signal, lactyl modification in human diseases, and its application with other techniques and omics.

Oxidation of hyperpolarized [1-13 C]pyruvate in isolated rat kidneys

Kidneys play a central role in numerous disorders but current imaging methods have limited utility to probe renal metabolism. Hyperpolarized (HP) 13 C magnetic resonance imaging is uniquely suited to provide metabolite-specific information about key biochemical pathways and it offers the further advantage that renal imaging is practical in humans. This study evaluated the feasibility of hyperpolarization examinations in a widely used model for analysis of renal physiology, the isolated kidney, which enables isolation of renal metabolism from the effects of other organs and validation of HP results by independent measurements. Isolated rat kidneys were supplied with either HP [1-13 C]pyruvate only or HP [1-13 C]pyruvate plus octanoate. Metabolic activity in both groups was confirmed by stable renal oxygen consumption. HP [1-13 C]pyruvate was readily metabolized to [13 C]bicarbonate, [1-13 C]lactate, and [1-13 C]alanine, detectable seconds after HP [1-13 C]pyruvate was injected. Octanoate suppressed but did not eliminate the production of HP [13 C]bicarbonate from [1-13 C]pyruvate. Steady-state flux analyses using non-HP 13 C substrates validated the utilization of HP [1-13 C]pyruvate, as observed by HP 13 C NMR. In the presence of octanoate, lactate is generated from a tricarboxylic acid cycle intermediate, oxaloacetate. The isolated rat kidney may serve as an excellent model for investigating and establishing new HP 13 C metabolic probes for future kidney imaging applications.

Magnetic resonance imaging during warm ex vivo kidney perfusion

BACKGROUND

The shortage of donor organs for transplantation remains a worldwide problem. The utilization of suboptimal deceased donors enlarges the pool of potential organs, yet consequently, clinicians face the difficult decision of whether these sub-optimal organs are of sufficient quality for transplantation. Novel technologies could play a pivotal role in making pre-transplant organ assessment more objective and reliable.

METHODS

Ex vivo normothermic machine perfusion (NMP) at temperatures around 35-37°C allows organ quality assessment in a near-physiological environment. Advanced magnetic resonance imaging (MRI) techniques convey unique information about an organ's structural and functional integrity. The concept of applying magnetic resonance imaging during renal normothermic machine perfusion is novel in both renal and radiological research and we have developed the first MRI-compatible NMP setup for human-sized kidneys.

RESULTS

We were able to obtain a detailed and real-time view of ongoing processes inside renal grafts during ex vivo perfusion. This new technique can visualize structural abnormalities, quantify regional flow distribution, renal metabolism, and local oxygen availability, and track the distribution of ex vivo administered cellular therapy.

CONCLUSION

This platform allows for advanced pre-transplant organ assessment, provides a new realistic tool for studies into renal physiology and metabolism, and may facilitate therapeutic tracing of pharmacological and cellular interventions to an isolated kidney.

Unraveling metabolism during kidney perfusion using tracer studies, a systematic review

BACKGROUND

Understanding kidney metabolism during perfusion is vital to further develop the technology as a preservation, viability assessment, and resuscitation platform. We reviewed the evidence on the use of labeled metabolites (tracers) to understand "on-pump" kidney behavior.

METHODS

PubMed, Embase, Web of Science, and Cochrane databases were systematically searched for studies evaluating metabolism of (non)radioactively labeled endogenous compounds during kidney perfusion.

RESULTS

Of 5899 articles, 30 were included. All were animal studies [rat (70%), dog (13%), pig (10%), rabbit (7%)] perfusing but not transplanting kidneys. Perfusion took place at hypothermic (4-12°C) (20%), normothermic (35-40°C) (77%), or undefined temperatures (3%). Hypothermic perfusion used albumin or a clinical kidney preservation solution, mostly in the presence of oxygen. Normothermic perfusion was mostly performed with oxygenated crystalloids often containing glucose and amino acids with unclear partial oxygen tensions. Active metabolism of carbohydrate, amino acid, lipids, and large molecules was shown in hypothermic and normothermic perfusion. Production of macromolecules, such as prostaglandin, thromboxane, and vitamin D, takes place during normothermic perfusion. No experiments compared differences in metabolic activity between hypothermic and normothermic perfusion. One conference abstract showed increased anaerobic metabolism in kidneys donated after circulatory death by adding labeled glucose to hypothermically perfused human kidneys.

CONCLUSIONS

Tracer studies during kidney perfusion contribute to unraveling kidney metabolic behavior in pre-clinical models. Whether findings are truly translational needs further investigation in large animal models of human kidneys. Furthermore, it is essential to better understand how ischemia changes this metabolic behavior.

Renal Normothermic Machine Perfusion: The Road Toward Clinical Implementation of a Promising Pretransplant Organ Assessment Tool

The increased utilization of high-risk renal grafts for transplantation requires optimization of pretransplant organ assessment strategies. Current decision-making methods to accept an organ for transplantation lack overall predictive power and always contain an element of subjectivity. Normothermic machine perfusion (NMP) creates near-physiological conditions, which might facilitate a more objective assessment of organ quality before transplantation. NMP is rapidly gaining popularity, with various transplant centers developing their own NMP protocols and renal viability criteria. However, to date, no validated sets of on-pump viability markers exist nor are there unified NMP protocols. This review provides a critical overview of the fundamentals of current renal NMP protocols and proposes a framework to approach further development of ex vivo organ evaluation. We also comment on the potential logistical implications of routine clinical use of NMP, which is a more complex procedure compared with static cold storage or even hypothermic machine perfusion.

A Review of Current and Emerging Trends in Donor Graft-Quality Assessment Techniques

The number of patients placed on kidney transplant waiting lists is rapidly increasing, resulting in a growing gap between organ demand and the availability of kidneys for transplantation. This organ shortage has forced medical professionals to utilize marginal kidneys from expanded criteria donors (ECD) to broaden the donor pool and shorten wait times for patients with end-stage renal disease. However, recipients of ECD kidney grafts tend to have worse outcomes compared to those receiving organs from standard criteria donors (SCD), specifically increased risks of delayed graft function (DGF) and primary nonfunction incidence. Thus, representative methods for graft-quality assessment are strongly needed, especially for ECDs. Currently, graft-quality evaluation is limited to interpreting the donor’s recent laboratory tests, clinical risk scores, the visual evaluation of the organ, and, in some cases, a biopsy and perfusion parameters. The last few years have seen the emergence of many new technologies designed to examine organ function, including new imaging techniques, transcriptomics, genomics, proteomics, metabolomics, lipidomics, and new solutions in organ perfusion, which has enabled a deeper understanding of the complex mechanisms associated with ischemia-reperfusion injury (IRI), inflammatory process, and graft rejection. This review summarizes and assesses the strengths and weaknesses of current conventional diagnostic methods and a wide range of new potential strategies (from the last five years) with respect to donor graft-quality assessment, the identification of IRI, perfusion control, and the prediction of DGF.

Ischemia-Reperfusion Injuries Assessment during Pancreas Preservation

Maintaining organ viability between donation and transplantation is of critical importance for optimal graft function and survival. To date in pancreas transplantation, static cold storage (SCS) is the most widely practiced method of organ preservation. The first experiments in ex vivo perfusion of the pancreas were performed at the beginning of the 20th century. These perfusions led to organ oedema, hemorrhage, and venous congestion after revascularization. Despite these early hurdles, a number of factors now favor the use of perfusion during preservation: the encouraging results of HMP in kidney transplantation, the development of new perfusion solutions, and the development of organ perfusion machines for the lung, heart, kidneys and liver. This has led to a resurgence of research in machine perfusion for whole organ pancreas preservation. This review highlights the ischemia-reperfusion injuries assessment during ex vivo pancreas perfusion, both for assessment in pre-clinical experimental models as well for future use in the clinic. We evaluated perfusion dynamics, oedema assessment, especially by impedance analysis and MRI, whole organ oxygen consumption, tissue oxygen tension, metabolite concentrations in tissue and perfusate, mitochondrial respiration, cell death, especially by histology, total cell free DNA, caspase activation, and exocrine and endocrine assessment.

Development of a human heart-sized perfusion system for metabolic imaging studies using hyperpolarized [1-13 C]pyruvate MRI

PURPOSE

Increasing worldwide demand for cardiac transplantation has spurred new developments to increase the donor pool. Normothermic preservation of heart grafts for transplantation is an emerging strategy to improve the utilization of marginal grafts. Hyperpolarized MR using metabolic tracers such as [1-¹³ C]pyruvate, provide a novel means of investigating metabolic status without the use of ionizing radiation. We demonstrate the use of this methodology to examine ex vivo perfused porcine heart grafts.

METHODS

Hearts from three 40-kg Danish domestic pigs were harvested and subsequently perfused in Langendorff mode under normothermic conditions, using an MR-compatible perfusion system adapted to the heart. Proton MRI and hyperpolarized [1-¹³ C]pyruvate were used to investigate and quantify the functional and metabolic status of the grafts.

RESULTS

Hearts were perfused with whole blood for 120 min, using a dynamic contrast-enhanced perfusion experiment to verify successful myocardial perfusion. Hyperpolarized [1-¹³ C]pyruvate MRI was used to assess the metabolic state of the myocardium. Functional assessment was performed using CINE imaging and ventricular pressure data. High lactate and modest alanine levels were observed in the hyperpolarized experiment. The functional assessment produced reduced functional parameters. This suggests an altered functional and metabolic profile compared with corresponding in vivo values.

CONCLUSION

We investigated the metabolic and functional status of machine-perfused porcine hearts. Utilizing hyperpolarized methodology to acquire detailed myocardial metabolic information-in combination with already established MR methods for cardiac investigation-provides a powerful tool to aid the progress of donor heart preservation.