Benzophenone derivatives and related compounds as potent histamine H₃-receptor antagonists and potential PET/SPECT ligands

Para-substituted aromatic ethers with benzophenone or related structural elements and a 3-(1H-imidazol-4-yl)propyloxy moiety were prepared by Mitsunobu-type ether synthesis or S_NAr reaction. Most of the title compounds possess high antagonist potency in histamine H₃-receptor assays in vitro as well as in vivo in mouse CNS following oral administration. After defining 4-(3-(1H-imidazol-4-yl)propyloxy)phenyl phenyl methanone as a new lead, structure-activity relationships were investigated for this new class of compounds. Substitution of the meta′-position of the benzophenone moiety with halogen atoms (e.g., iodine, fluorine) led to compounds with high antagonist potency in vitro as well as in vivo (K_i = 9.3 and 4.3 nM, ED₅₀ = 0.7 and 0.47 mg/kg p.o., 18 and 12, respectively). A receptor profile of several functional in vitro assays for several biogenic amine receptors for the meta′-iodinated derivative demonstrated high selectivity toward the histamine H₃ receptor.