Secretin Therapy During Ex-Situ Normothermic Liver Machine Perfusion: A Critical Factor for Restoration of Bile Duct Physiology and the Protective "Bicarbonate Umbrella"

Liver transplantation is the only curative treatment option for patients with end-stage liver disease, but it is limited by a large gap between the number of patients in need and donor organs available. Strikingly, in 2021, 38% of available donor livers were disposed being assessed as non-transplantable in The Netherlands, while approximately 20% of patients on the liver transplant waitlist died or became too sick to be transplanted. Similar or even higher discard rates are observed in other countries, such as the US. Acceptance of extended criteria donor (ECD) livers, referring to suboptimal grafts from older, obese, or otherwise comorbid donors and organs from donation after circulatory death (DCD), is proposed to meet this growing demand. However, transplantation of these livers is associated with a higher rate of post-operative complications, increased hospital costs and reduced graft survival, compared to standard grafts.

Machine perfusion (MP) is a dynamic, isolated platform to preserve liver grafts out-of-the-body by circulation of an oxygenated perfusate. While dual hypothermic oxygenated perfusion (DHOPE) reconditions the graft in a hypometabolic state, normothermic MP (NMP) allows full assessment of metabolic function at physiological temperatures. The sequential protocol of both perfusion techniques, with controlled oxygenated rewarming for 60 minutes (COR), named DHOPE-COR-NMP is currently used in clinical practice for viability assessment of high-risk ECD-livers prior to transplantation. The decision moment whether a liver is suitable for transplantation is after 150 minutes of NMP. Unfortunately, one third of these tested livers are currently discarded, mainly because of not meeting the predefined criteria for cholangiocyte viability (own, unpublished data). The platform of MP allows graft reconditioning in an isolated circuit, with no systemic effects of administered therapeutics in the recipient as the liver is thoroughly flushed out before implantation in the recipient, thereby limiting side-effects.

Secretin is a hormone with a short half-life that is produced in the duodenum, with systemic effects. In the liver, secretin stimulates cholangiocytes through the secretin receptor. This leads to an increase in intracellular cyclic AMP (cAMP), subsequently activating Protein kinase A (PKA), eventually leading to a downstream excretion of chloride through the CFTR channels. Chloride is a leading force in bicarbonate excretion, as this excreted chloride is resorbed through Anion-Exchanger-2 (AE2) and exchanged for bicarbonate. In a physiological setting, cholangiocytes are protected from bile-acid injury by this "bicarbonate umbrella". With the excretion of bicarbonate, water is also excreted through Aquaporin-1 (AQP-1), and thus increasing total bile production. The increase in bile production is one of the main reasons for secretin to be administered during Magnetic Resonance Cholangio-Pancreatography (MRCP) imaging of the biliary tree in its current application as diagnostic tool.

The perfusate that is used for NMP, is based on an oxygen carrier (currently red blood cells), colloids and certain supplements to reach a near-physiological environment in oncotic pressure and nutrients. In physiological conditions, bile contains high amounts of bicarbonate, resulting in an alkalotic fluid. This earlier mentioned "bicarbonate umbrella" protects the cholangiocytes from bile acid-induced injury. During NMP, bile pH and bile bicarbonate levels are lower compared to a physiological setting (own, unpublished data). The investigators hypothesize that cholangiocytes during NMP are not able to demonstrate their full potential, as they are not stimulated by secretin like in-vivo physiology. As explained earlier, the increase in bile production is a result of increasing bicarbonate excretion, and thereby biliary pH, creating a more physiological environment for the liver and bile ducts. Possible further injury of the bile duct from bile acid toxicity could be prevented by adding secretin to the perfusate during COR-NMP. This however, can only be researched after this initial validation of the safety and efficacy of secretin as a missing component to reach physiological conditions during ex-situ liver NMP.