Species Differences in Microsomal Metabolism of Xanthine-Derived A1 Adenosine Receptor Ligands.

Daniela Schneider,Andreas Bauer,Marcus Holschbach,Dirk Bier,Bernd Neumaier

doi:10.3390/ph14030277

Abstract

Tracer development for positron emission tomography (PET) requires thorough evaluation of pharmacokinetics, metabolism, and dosimetry of candidate radioligands in preclinical animal studies. Since variations in pharmacokinetics and metabolism of a compound occur in different species, careful selection of a suitable model species is mandatory to obtain valid data. This study focuses on species differences in the in vitro metabolism of three xanthine-derived ligands for the A1 adenosine receptor (A1AR), which, in their 18F-labeled form, can be used to image A1AR via PET. In vitro intrinsic clearance and metabolite profiles of 8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine (CPFPX), an established A1AR-ligand, and two novel analogs, 8-cyclobutyl-3-(3-fluoropropyl)-1-propylxanthine (CBX) and 3-(3-fluoropropyl)-8-(1-methylcyclobutyl)-1-propylxanthine (MCBX), were determined in liver microsomes from humans and preclinical animal species. Molecular mechanisms leading to significant differences between human and animal metabolite profiles were also examined. The results revealed significant species differences regarding qualitative and quantitative aspects of microsomal metabolism. None of the tested animal species fully matched human microsomal metabolism of the three A1AR ligands. In conclusion, preclinical evaluation of xanthine-derived A1AR ligands should employ at least two animal species, preferably rodent and dog, to predict in vivo behavior in humans. Surprisingly, rhesus macaques appear unsuitable due to large differences in metabolic activity towards the test compounds.

Highlights

The development of novel radioligands for imaging molecular targets via positron emission tomography (PET) is a time-consuming and costly endeavor
The objectives of the present study were two-fold: first, to elucidate the mechanisms underlying the distinct in vitro and in vivo metabolite profiles of CPFPX observed in rodents but not humans; second, to investigate species differences in the metabolism of CPFPX and two novel cyclobutyl analogs, namely 8-cyclobutyl-3-(3-fluoropropyl)-1propylxanthine (CBX, see Table 1) and 3-(3-fluoropropyl)-8-(1-methylcyclobutyl)-1-propylx anthine (MCBX, see Table 1), in hepatic microsomes from humans and commonly used preclinical animal species with the aim to identify suitable animal models for evaluation of xanthine-derived A1 AR radioligands
P450 enzymes and that scaled microsomal clearancehepatic data are in good were preferred over hepatocytes for investigating species differences in

Summary

Introduction

The development of novel radioligands for imaging molecular targets via positron emission tomography (PET) is a time-consuming and costly endeavor. Assessment of imaging characteristics and safety of a candidate compound requires extensive preclinical investigations prior to initial clinical trials. Pharmacokinetics and metabolism are important determinants of the in vivo properties of a novel imaging agent [1]. Radioligand metabolism can lead to radioactive metabolites that impede reliable quantification of the molecular target. Rapid blood clearance of the radioligand may limit target exposure, but on the other hand can enhance the signal-to-background ratio by reducing the amount of radioactivity present in the vascular system [2]. Since most PET radioligands are small lipophilic molecules, metabolism is crucial for the excretion of these compounds from the body and largely determines their biological half-lives

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