Design, synthesis, and binding affinities of potential positron emission tomography (PET) ligands for visualization of brain dopamine D3 receptors.

We here report the synthesis of compounds structurally related to the high-affinity dopamine D(3) receptor ligand N-[4-[4-(2,3-dichlorophenyl)piperazin-1-yl]butyl]-7-methoxy-2-benzofurancarboxamide (1). All compounds were specifically designed as potential PET radioligands for brain D(3) receptors visualization, having lipophilicity within a range for high brain uptake and weak nonspecific binding (2 < ClogP < 3.5) and bearing a methoxy substituent for easy access to labeling with the positron emitter isotope (11)C. N-[4-[4-(5-methoxy-2-benzisoxazolyl)piperazin-1-yl]butyl]-4-(4-morpholinyl)benzamide (22), N-[4-[4-(5-methoxy-2-benzisoxazolyl)piperazin-1-yl]butyl]-4-(1H-imidazol-1-yl)benzamide (23), and N-[4-[4-(5-methoxy-2-benzisoxazolyl)piperazin-1-yl]butyl]-5-(2-furanyl)-1H-pyrazole-3-carboxamide (24) displayed good D(3) receptor affinities (K(i) values 38.0, 22.6, and 21.3 nM, respectively) and were selective over D(2) receptor. Moreover, compounds 22-24 were able to permeate the Caco-2 cell monolayer, differently from compound 1. Although the goal to identify potential PET radioligands with subnanomolar affinities for D(3) receptor was not achieved, the proposed strategy could be a starting point for future developments.