Abstract
In recent years, radiofluorinated alkyl azides have been reported for click radiolabeling and pretargeted PET imaging, but only little is known about the biodistribution and metabolism of these compounds. In this work, we present a significantly improved procedure for the synthesis of [18F]fluoroethyl azide and reinvestigated this radiolabeled probe in detail showing poor stability and very restricted suitability for in vivo application. Therefore, modified low-molecular-weight [18F]fluoroalkyl azides were developed. Propargyl-tagged endomorphin-1 (as model compound) was successfully radiolabeled in high yield and short reaction time making these probes useful and efficient bioorthogonal tools for rapid radiolabeling. Biodistribution, pharmacokinetics and in vivo stability were studied by preclinical PET/MR scanning and metabolite analysis. The results of this study revealed only limited applicability of [18F]fluoroalkyl azides for in vivo application.
Highlights
Positron emission tomography (PET) allows non-invasive molecular imaging and quantification of metabolic processes
Radiosynthesis of [18F]1 is commonly carried out using K2CO3/ Kryptofix-222 (K222)-mediated nucleophilic radiofluorination and synthesis of precursor 3, and subsequent radiolabeling affording [18F]1. [(i) NsCl, N(Et)[3]; (ii) 18F-HF, K2CO3, Kryptofix-222]
Probes prepared by rapid radiolabeling using [18F]1 are known to exhibit high metabolic stability in vivo.[27]
Summary
Positron emission tomography (PET) allows non-invasive molecular imaging and quantification of metabolic processes. Spatial and temporal resolution made PET an indispensable diagnostic tool in the clinic and in biomedical research. The versatility of this imaging modality is highly dependent on the availability of radiolabeled probes. Short-lived nuclides are generally favored due to low patient radiation doses and high specific activities.[1] sufficient time is still necessary for production, quality control, delivery and application of radiolabeled probes. Fluorine-18 (half-life of 109.8 min) is considered to be the ideal PET radioisotope enabling the radiochemist to conduct multistep synthesis and purifications using chromatographic methods.[2,3,4]
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