Abstract

Histamine H₃ receptor (H₃R) antagonists enhance neurotransmitter release and are being developed for the treatment of a variety of neurological and cognitive disorders. Many potent histamine H₃R antagonists contain an imidazole moiety that limits receptor selectivity and the tolerability of this class of compounds. Here we present the in vitro pharmacological data for two novel piperazine amide ligands, A-304121 [4-(3-((2R)-2-aminopropanoyl-1-piperazinyl)propoxy)phenyl)cyclopropylmethanone] and A-317920 [N-((1R)-2-(4-(3-(4-(cyclopropylcarbonyl)phenoxy)propyl)-1-piperazinyl)-1-methyl-2-oxo-ethyl-)-2-furamide], and compare them with the imidazole H₃R antagonists ciproxifan, clobenpropit, and thioperamide. Both A-304121 and A-317920 bind potently to recombinant full-length rat H₃R(pK_i values = 8.6 and 9.2, respectively) but have lower potencies for binding the full-length human H₃R (pK_i values = 6.1 and 7.0, respectively). A-304121 and A-317920 are potent antagonists at rat H₃R in reversing R-α-methylhistamine [(R)-α-MeHA] inhibition of forskolin-stimulated cAMP formation (pK_b values = 8.0 and 9.1) but weak antagonists at human H₃Rs in cyclase (pK_b values = 6.0 and 6.3) and calcium mobilization (pK_b values = 6.0 and 7.3) assays in cells co-expressing Gα_qi5-protein. Both compounds potently antagonize native H₃Rs by blocking histamine inhibition of potassium-evoked [³H]histamine release from rat brain cortical synaptosomes (pK_b values = 8.6 and 9.3) and (R)-α-MeHA reversal of electric field-stimulated guinea pig ileum contractions (pA₂ values = 7.1 and 8.3). A-304121 and A-317920 are also more efficacious inverse agonists in reversing basal guanosine 5′-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding at the human H₃R (pEC₅₀ values = 5.7 and 7.0) than are the imidazole antagonists. These novel and selective piperazine amides represent useful leads for the development of H₃R antagonist therapeutic agents.