Serum APOA4 Pharmacodynamically Represents Administered Recombinant Human Hepatocyte Growth Factor (E3112)

Nonclinical pharmacokinetics of E3112, a recombinant human hepatocyte growth factor, in rats and monkeys by a simple ELISA kit assay

E3112 is a recombinant human hepatocyte growth factor which is under development for the treatment of acute liver failure. Pharmacokinetics (PK) evaluation in experimental animals is important and thus a simple assay for the determination of E3112 in rat and monkey serum has been validated using a commercially available enzyme-linked immunosorbent assay (ELISA) kit. E3112 in rat and monkey serum was quantifiable from 0.313 ng/mL to 15.0 ng/mL without prozone effects. Dilution integrity enabled accurate assay up to 500,000-fold dilution. Accuracy and precision were within the acceptance criteria. PK of E3112 was investigated after intravenous administration to rats and monkeys. PK of E3112 was similar between male and female animals in both species. Nonlinear PK of E3112 was observed in rats after intravenous bolus dose at 1-100 mg/kg while nonlinear PK was not significant in monkeys after intravenous infusion at 0.5-25 mg/kg. These findings suggest that the assay of E3112 in serum using a commercially available ELISA kit was validated and successfully applied to PK studies in rats and monkeys.

Understanding HDL Metabolism and Biology Through In Vivo Tracer Kinetics

HDL (high-density lipoprotein), owing to its high protein content and small size, is the densest circulating lipoprotein. In contrast to lipid-laden VLDL (very-low-density lipoprotein) and LDL (low-density lipoprotein) that promote atherosclerosis, HDL is hypothesized to mitigate atherosclerosis via reverse cholesterol transport, a process that entails the uptake and clearance of excess cholesterol from peripheral tissues. This process is mediated by APOA1 (apolipoprotein A-I), the primary structural protein of HDL, as well as by the activities of additional HDL proteins. Tracer-dependent kinetic studies are an invaluable tool to study HDL-mediated reverse cholesterol transport and overall HDL metabolism in humans when a cardiovascular disease therapy is investigated. Unfortunately, HDL cholesterol-raising therapies have not been successful at reducing cardiovascular events suggesting an incomplete picture of HDL biology. However, as HDL tracer studies have evolved from radioactive isotope- to stable isotope-based strategies that in turn are reliant on mass spectrometry technologies, the complexity of the HDL proteome and its metabolism can be more readily addressed. In this review, we outline the motivations, timelines, advantages, and disadvantages of the various tracer kinetics strategies. We also feature the metabolic properties of select HDL proteins known to regulate reverse cholesterol transport, which in turn underscore that HDL lipoproteins comprise a heterogeneous particle population whose distinct protein constituents and kinetics likely determine its function and potential contribution to cholesterol clearance.

The distinct metabolism between large and small HDL indicates unique origins of human apolipoprotein A4

Apolipoprotein A4's (APOA4's) functions on HDL in humans are not well understood. A unique feature of APOA4 is that it is an intestinal apolipoprotein secreted on HDL and chylomicrons. The goal of this study was to gain a better understanding of the origin and function of APOA4 on HDL by studying its metabolism across 6 HDL sizes. Twelve participants completed a metabolic tracer study. HDL was isolated by APOA1 immunopurification and separated by size. Tracer enrichments for APOA4 and APOA1 were determined by targeted mass spectrometry, and metabolic rates were derived by compartmental modeling. APOA4 metabolism on small HDL (alpha3, prebeta, and very small prebeta) was distinct from that of APOA4 on large HDL (alpha0, 1, 2). APOA4 on small HDL appeared in circulation by 30 minutes and was relatively rapidly catabolized. In contrast, APOA4 on large HDL appeared in circulation later (1-2 hours) and had a much slower catabolism. The unique metabolic profiles of APOA4 on small and large HDL likely indicate that each has a distinct origin and function in humans. This evidence supports the notion that APOA4 on small HDL originates directly from the small intestine while APOA4 on large HDL originates from chylomicron transfer.