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| Home > Publications database > Relevance of in vitro metabolism models for PET radiotracer development |
| Home > Publications database > Relevance of in vitro metabolism models for PET radiotracer development |
| Dissertation / PhD Thesis | FZJ-2017-05313 |
2017
Abstract: The application of positron emission tomography (PET) as a tool for molecular imaging of the brain islimited by the availability of suitable radiotracers. In radiotracer development, success or failure of anovel compound is determined by numerous physicochemical and pharmacological factors of whichmetabolism represents a particularly important one. Hence, an initial assessment of the metabolicproperties of a candidate radiotracer at an early stage of the development process prior to expensiveand time-consuming animal studies is desirable. Various in vitro metabolism models are available forthe evaluation of candidate drugs in the pharmaceutical sector, however, since drug and tracerapplications differ fundamentally (especially with regard to the amount of administered substance) thequestion rises whether these models are also capable of providing relevant and valid data for theevaluation of radiotracers. In the present work, this question was examined using three xanthinederivedadenosine A1 receptor (A1AR) ligands, 8-cyclopentyl-3-(3-fluoropropyl)-1-propylxanthine(CPFPX), 8-cyclobutyl-3-(3-fluoropropyl)-1-propylxanthine (CBX) and 3-(3-fluoropropyl)-8-(1-methylcyclobutyl)-1-propylxanthine (MCBX) as model compounds. Radioligands for imaging adenosinereceptors in vivo are of particular interest for biomedical research since this receptor system has beenassociated with numerous physiological functions (e.g. regulation of sleep) and pathophysiologicalconditions (e.g. Alzheimer’s and Parkinson’s disease). In vivo metabolic stability and metabolitepatterns of the three A1AR ligands were investigated in the rat model and compared to in vitro datagenerated in rat liver microsomes. Prior to the characterisation of the microsomal metabolism of thetest compounds, the influence of incubation conditions on the in vitro test results was assessed and themicrosomal assay was optimised in order to provide physiologically relevant data. Metabolic activity inliver microsomes was strongly influenced by the pH value of the buffer medium and the type andconcentration of the organic solvent used to solubilise the test compounds. Variations of incubationmatrix composition resulted in variations of individual substrate depletion values of up to 500%,whereas substrate depletion ratios proved to be highly constant. In vitro microsomal half-lives (t1/2) ofthe compounds were determined and expressed as ratios for the purpose of later in vitro-in vivocomparison. The t1/2 ratios (± SE) of CBX:CPFPX, MCBX:CPFPX and CBX:MCBX were 3.1 ± 0.11,1.4 ± 0.029 and 2.2 ± 0.033. In vivo metabolic stability of the 18F-labelled compounds were assessed inanaesthetised rats via blood analysis. Plasma clearance (CL) values were calculated and expressed asinversed ratios to facilitate direct comparison with in vitro t1/2 ratios. The inversed CL ratios (± SE) of[18F]CBX:[18F]CPFPX, [18F]MCBX:[18F]CPFPX and [18F]CBX:[18F]MCBX were 2.6 ± 0.12, 0.82 ± 0.019 and3.1 ± 0.15. In vitro t1/2 ratios deviated between 19 and 71% from inverse CL ratios. These deviationscan be considered small in view of the reduced complexity of the microsomal model and the multitudeof physiological parameters affecting in vivo pharmacokinetics of a substance. Visual comparison ofmetabolite profiles generated in vitro and in vivo revealed a high degree of similarity. In conclusion,both quantitative and qualitative aspects of radiotracer metabolism could be reasonably well predictedby microsomal data. This result encourages the implementation of in vitro metabolism studies as anintegral part of PET radiotracer development.
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