Normothermic machine perfused organs as models of drug pharmacokinetics and therapeutic delivery
Keywords: drug delivery
normothermic machine perfusion
pharmacokinetics
clearance

Less than 10% of drug candidates successfully pass through clinical trials and gain regulatory approval. One of the causes of translational failure is poor drug pharmacokinetics which are inadequately predicted by preclinical studies. Despite improvements in pharmacokinetic prediction in recent years, the circulating kinetics of many novel biologic agents, nanomedicines and drug delivery systems which exhibit transporter-mediated clearance, and size-dependent uptake by the reticuloendothelial system (RES), do not scale between species in a predictable manner and suffer with inadequate delivery to the therapeutic target. This hinders clinical translation, thus highlighting the need for more accurate preclinical testing to predict human drug disposition and facilitate improved drug delivery.
Normothermic machine perfusion (NMP) is a method of organ preservation whereby oxygenation and nutrient provision to an organ is enabled via circulation of a cellular or acellular perfusate at body temperature. This maintains organs in a quasi-physiological state which we hypothesise will preserve the pharmacokinetic processes of drug distribution, metabolism and excretion, reflecting drug disposition in a close-to-man environment. The primary aim of this thesis is to determine whether human and porcine NMP clearance organs (kidney, spleen and liver) serve as a tool for predicting human drug clearance and for assessing drug delivery.
Perfusion protocols were developed to support prolonged perfusion of i) human NMP liver and kidney, ii) laboratory-procured, porcine NMP liver and kidney and iii) abattoir-derived, porcine NMP liver and spleen. Systematic comparison of the different approaches demonstrated a robust physiological platform for profiling therapeutic pharmacokinetics and delivery.
A small molecule therapeutic drug was delivered to the human and laboratory-procured, porcine liver and kidney models with assessment of pharmacokinetics. Both human and porcine organs were able to successfully support distribution, metabolism, hepatobiliary and renal excretion of the drug. Organ specific and total body clearance were calculated using physiological and pharmacokinetic data from the NMP organs and compared to human clinical data. Human NMP organs, which are discarded for transplantation with varying degrees of injury, did not accurately predict human clearance. However, pristine porcine NMP liver and kidney kidneys enabled accurate prediction of human intrinsic organ and total body clearance; the most important measure used to establish the safe dosing regimen when translating a novel drug into patients. This represents the first successful validation of a large mammalian normothermically preserved organ model against clinical data for an approved drug.
Finally, the NMP spleen, liver and kidney were used to investigate the kinetics and delivery of a model polymeric nanoparticle and a viral vector; two classes of agent commonly captured by the RES and poorly translated from bench to bedside. PEGylated and non-PEGylated nanoparticle kinetics measured in NMP organs correspond to existing small animal data and provide evidence for nanoparticle capture in large animal organs for which data is limited. The NMP porcine liver was also able to support the circulating kinetics and integration of a viral vector into host hepatocyte DNA; a first for drug delivery studies in the ex vivo NMP liver and an exciting prospect for the delivery of hepatocyte-targeted gene therapies.
In conclusion, despite limitations such as the effect of ischaemia reperfusion injury on organ physiology and the low throughput nature of the technology, this work demonstrates the utility of NMP clearance organs as a tool for predicting human pharmacokinetics and assessing drug delivery. The richness of data generated from the models has the potential to serve as an adjunct to existing preclinical technologies, for accelerating the development of novel therapeutics for patient benefit.