Development of highly potent phosphodiesterase 10A (PDE10A) inhibitors: Synthesis and in vitro evaluation of 1,8-dipyridinyl- and 1-pyridinyl-substituted imidazo[1,5-a]quinoxalines.

Herein we report the synthesis of fluorinated inhibitors of phosphodiesterase 10A (PDE10A) which can be used potentially as lead structure for the development of a (18)F-labeled PDE10A imaging agent for positron emission tomography. The use of ortho-fluoropyridines as residues could potentially enable the introduction of (18)F through nucleophilic substitution for radiolabeling purposes. 2-Fluoropyridines are introduced by a Suzuki coupling at different positions of the molecule. The reference compounds, 1,8-dipyridinylimidazo[1,5-a]quinoxalines and 1-pyridinylimidazo[1,5-a]quinoxalines, show inhibitory potencies at best in the subnanomolar range and selectivity factors greater than 38 against other PDE's. 1,8-Dipyridinylimidazo[1,5-a]quinoxalines are more potent inhibitors than 1-pyridinylimidazo[1,5-a]quinoxalines. Using 2-fluoro-3-pyridinyl as residue provided the most potent inhibitors 16 (IC50 = 0.12 nM), 17 (IC50 = 0.048 nM) and 32 (IC50 = 0.037 nM).