Machine Perfusion as a Strategy to Decrease Ischemia-Reperfusion Injury and Lower Cancer Recurrence Following Liver Transplantation

Abstract

Liver transplantation (LT) is a key treatment for primary and secondary liver cancers, reducing tumor burden with concurrent improvement of liver function. While significant improvement in survival is noted with LT, cancer recurrence rates remain high. Mitochondrial dysfunction caused by ischemia-reperfusion injury (IRI) is known to drive tumor recurrence by creating a favorable microenvironment rich in pro-inflammatory and angiogenic factors. Therefore, strategies that decrease reperfusion injury and mitochondrial dysfunction may also decrease cancer recurrence following LT. Machine perfusion techniques are increasingly used in routine clinical practice of LT with improved post-transplant outcomes and increased use of marginal grafts. Normothermic (NMP) and hypothermic oxygenated machine perfusion (HOPE) provide oxygen to ischemic tissues, and impact IRI and potential cancer recurrence through different mechanisms. This article discussed the link between IRI-associated inflammation and tumor recurrence after LT. The current literature was screened for the role of machine perfusion as a strategy to mitigate the risk of cancer recurrence. Upfront NMP ("ischemia free organ transplantation") and end-ischemic HOPE were shown to reduce hepatocellular carcinoma recurrence in retrospective studies. Three prospective randomized controlled trials are ongoing in Europe to provide robust evidence on the impact of HOPE on cancer recurrence in LT.

Keywords: disease-free survival; hepatocellular carcinoma; hypothermic oxygenated machine perfusion (HOPE); ischemia-reperfusion injury; liver transplantation; microenvironment; mitochondrial injury; normothermic machine perfusion; tumor recurrence.

Figure 1

Risk factors and mechanism of ischemia-reperfusion injury (IRI) and cancer recurrence. Tumor biology, recipient factors, surgical factors, and donor features all contribute to recurrence of cancer. Tumor biology such as size, number, vascular invasion, and low-grade differentiation all highlight features of an aggressive tumor with increased risk of recurrence. Recipient factors such as older age, higher body mass index (BMI), underlying co-morbidities, and post-transplant immunosuppression play a role in recurrence risk. Surgical factors such as duration of surgery, intraoperative hypotension, and medical treatment may impact the level of IRI. Finally, donor features, such as macro-steatosis, donation after circulatory death (DCD), prolonged warm and cold ischemia, and small graft size may increase risk of IRI and tumor recurrence. IRI is characterized by two phases: ischemia and reperfusion. During ischemia, cell injury and anaerobic respiration occur resulting in acidosis and buildup of sodium, calcium, and respiration substrates, such as succinate. Upon reperfusion, these factors stimulate release of reactive oxygen species (ROS) at mitochondrial complex I and damage-associated molecular patterns (DAMPs) from hepatocytes, hepatic sinusoidal epithelial cells (HSEC), and Kupffer cells (KC), attracting recipient neutrophils to the site. Inflammatory cytokines such as TNF-α, IL-6, and IL-12 are released stimulating a pro-inflammatory milieu consisting of lymphoid cells and macrophages. Increased vascular endothelial growth factor (VEGF), hypoxia inducible factor (HIF-1a), and matrix metalloproteinases (MMP) allow for remodeling and stimulate angiogenesis. This allows for circulating tumor cells (CTC) to migrate and settle, resulting in recurrence. Significant IRI after liver implantation is also seen in lungs with initial inflammation and edema potentially contributing to the elevated HCC recurrence in lungs after liver transplantation. In addition, this pro-inflammatory environment stimulates early liver fibrosis and ischemic biliary strictures. Created with BioRender.com.

Figure 2

Decreased tumor recurrence with machine perfusion techniques. (1) Upfront normothermic machine perfusion (NMP) is utilized in “ischemia free liver transplantation” (IFLT). Immediate and continuous NMP without initial cold flush and cold storage maintain mitochondrial and other cell function to prevent further mitochondrial injury as seen with reperfusion under normothermic conditions after standard cold storage (SCS). As some injury already occurs in the donor, some reactive oxygen species (ROS) and flavin mononucleotide (FMN) are released from complex 1. The reduction in additional injury can reduce the downstream inflammation and cancer recurrence. (2) Hypothermic oxygenated perfusion (HOPE) reprograms the mitochondria and improves complex protein activity by restoring the electron transport chain (ETC) function. This allows for clearance of NADH and respiratory substrates such as succinate with concomitant ATP recharging. The direct consequence is a significant decrease in ROS and FMN release from complex 1 at implantation or subsequent NMP. (3) Downstream inflammation is also reduced with less DAMPs and cytokine release and less recruitment of pro-inflammatory cells. The releases of mediators like vascular endothelial growth factor (VEGF), hypoxia inducible factor (HIF-1a), and membrane metalloprotease (MMP) are reduced and there is decreased mobilization and engraftment of circulating tumor cells (CTC). This results in decreased tumor recurrence. Created with BioRender.com.