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INTRODUCTION: Recently, [¹⁸F]FE@SUPPY and [¹⁸F]FE@SUPPY:2 were introduced as the first positron emission tomography (PET) tracers for the adenosine A₃ receptor. Thus, aim of the present study was the metabolic characterization of the two adenosine A₃ receptor PET tracers. METHODS: In vitro carboxylesterase (CES) experiments were conducted using incubation mixtures containing different concentrations of the two substrates, porcine CES and phosphate-buffered saline. Enzymatic reactions were stopped by adding acetonitrile/methanol (10:1) after various time points and analyzed by a high-performance liquid chromatography (HPLC) standard protocol. In vivo experiments were conducted in male wild-type rats; tracers were injected through a tail vein. Rats were sacrificed after various time points (n=3), and blood and brain samples were collected. Sample cleanup was performed by an HPLC …