Synthesis, Radiofluorination, and In Vitro Evaluation of Pyrazolo[1,5-a]pyridine-Based Dopamine D4 Receptor Ligands: Discovery of an Inverse Agonist Radioligand for PET

A series of fluoro-substituted analogs structurally derived from the aminomethyl-substituted pyrazolo[1,5-a]pyridine lead compounds 9 (FAUC 113) and 10 (FAUC 213) were synthesized and evaluated as high-affinity D4 receptor (D4R) ligands (3a−3h, K i = 1.3–28 nM). The para-fluoroethoxy-substituted derivatives 3f and 3h revealed an outstanding D4 subtype selectivity of more than 3 orders of magnitude over both congeners D2 and D3 combined with inverse agonism at D4R. The corresponding 18F-labeled radioligands revealed high serum stability in vitro and log P values of 2–3. In vitro rat brain autoradiography showed specific binding of [18F]3h in distinct brain regions, including the gyrus dentate of the hippocampus, that were inhibited by both eticlopride (65–80%) and the selective D4R antagonist 10 (78–93%). The observed binding pattern was mainly consistent with the known D4R distribution in the rat brain. Thus, [18F]3h (FAUC F41) represents a potential radioligand for studying the D4R in vivo by positron emission tomography (PET).