Development of (18)F-Labeled Radiotracers for PET Imaging of the Adenosine A(2A) Receptor: Synthesis, Radiolabeling and Preliminary Biological Evaluation

The adenosine A(2A) receptor (A(2A)R) represents a potential therapeutic target for neurodegenerative diseases. Aiming at the development of a positron emission tomography (PET) radiotracer to monitor changes of receptor density and/or occupancy during the A(2A)R-tailored therapy, we designed a library of fluorinated analogs based on a recently published lead compound (PPY). Among those, the highly affine 4-fluorobenzyl derivate (PPY1; K(i)(hA(2A)R) = 5.3 nM) and the 2-fluorobenzyl derivate (PPY2; K(i)(hA(2A)R) = 2.1 nM) were chosen for (18)F-labeling via an alcohol-enhanced copper-mediated procedure starting from the corresponding boronic acid pinacol ester precursors. Investigations of the metabolic stability of [(18)F]PPY1 and [18F]PPY2 in CD-1 mice by radio-HPLC analysis revealed parent fractions of more than 76% of total activity in the brain. Specific binding of [(18)F]PPY2 on mice brain slices was demonstrated by in vitro autoradiography. In vivo PET/magnetic resonance imaging (MRI) studies in CD-1 mice revealed a reasonable high initial brain uptake for both radiotracers, followed by a fast clearance.