Development of 2-(2-(3-(4-([F]Fluoromethoxy- d)phenyl)-7-methyl-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione for Positron-Emission-Tomography Imaging of Phosphodiesterase 10A in the Brain.

Phosphodiesterase 10A (PDE10A) is a newly identified therapeutic target for central-nervous-system disorders. 2-(2-(3-(4-([F]Fluoroethoxy)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([F]MNI-659, [F]5) is a useful positron-emission-tomography (PET) ligand for imaging of PDE10A in the human brain. However, the radiolabeled metabolite of [F]5 can accumulate in the brain. In this study, using [F]5 as a lead compound, we designed four new F-labeled ligands ([F]6-9) to find one more suitable than [F]5. Of these, 2-(2-(3-(4-([F]fluoromethoxy- d)phenyl)-4-oxo-3,4-dihydroquinazolin-2-yl)ethyl)-4-isopropoxyisoindoline-1,3-dione ([F]9) exhibited high in vitro binding affinity ( K = 2.9 nM) to PDE10A and suitable lipophilicity (log D = 2.2). In PET studies, the binding potential (BP) of [F]9 (5.8) to PDE10A in the striatum of rat brains was significantly higher than that of [F]5 (4.6). Furthermore, metabolite analysis showed much lower levels of contamination with radiolabeled metabolites in the brains of rats given [F]9 than in those given [F]5. In conclusion, [F]9 is a useful PET ligand for PDE10A imaging in brain.