Achievement of behavioral efficacy and improved potency in new heterocyclic analogs of benzofuran H3 antagonists

Cherry-picked ligands at histamine receptor subtypes

Histamine, a biogenic amine, is considered as a principle mediator of multiple physiological effects through binding to its H1, H2, H3, and H4 receptors (H1-H4Rs). Currently, the HRs have gained attention as important targets for the treatment of several diseases and disorders ranging from allergy to Alzheimer's disease and immune deficiency. Accordingly, medicinal chemistry studies exploring histamine-like molecules and their physicochemical properties by binding and interacting with the four HRs has led to the development of a diversity of agonists and antagonists that display selectivity for each HR subtype. An overview on H1-R4Rs and developed ligands representing some key steps in development is provided here combined with a short description of structure-activity relationships for each class. Main chemical diversities, pharmacophores, and pharmacological profiles of most innovative H1-H4R agonists and antagonists are highlighted. Therefore, this overview should support the rational choice for the optimal ligand selection based on affinity, selectivity and efficacy data in biochemical and pharmacological studies. This article is part of the Special Issue entitled 'Histamine Receptors'.

Highly Potent Fluorescence-Tagged Nonimidazole Histamine H3 Receptor Ligands

Different (3-phenoxypropyl)piperidine derivatives have been coupled to fluorescent moieties (5-dimethylaminonaphthalene-1-sulfonyl, carbazol-9-ylcarbonyl, 2-cyanoisoindol-1-yl, 2-cyanobenzo[f]isoindol-1-yl, 2,4-dinitrobenzen-1-yl, 2,4-diaminophenyl, 7-nitrobenzofurazan-4-yl, 7-aminosulfonylbenzofurazan-4-yl, 4-methylcoumarin-6-yl) as novel histamine H(3) receptor ligands. They have been synthesised starting from piperidine in a few steps. The compounds display good to excellent histamine hH(3) receptor affinities with K(i) values ranging from 13.4 to 0.048 nM. Some of the new compounds belong to the most potent ligands known so far and may act as tools for identification and understanding of the binding site on the histamine H(3) receptor. In vivo screening on selected derivatives of Sanger's reagent showed antagonist potencies with ED(50) values from 7.9 to 0.39 mg kg(-1), p.o.

Synthesis and biological evaluation of new non-imidazole H3-receptor antagonists of the 2-aminobenzimidazole series

A novel series of non-imidazole H(3)-receptor antagonists was developed, by chemical modification of a potent lead H(3)-antagonist composed by an imidazole ring connected through an alkyl spacer to a 2-aminobenzimidazole moiety (e.g., 2-[[3-[4(5)-imidazolyl]propyl]amino]benzimidazole), previously reported by our research group. We investigated whether the removal of the imidazole ring could allow retaining high affinity for the H(3)-receptor, thanks to the interactions undertaken by the 2-aminobenzimidazole moiety at the binding site. The imidazole ring of the lead was replaced by a basic piperidine or by a lipophilic p-chlorophenoxy substituent, modulating the spacer length from three to eight methylene groups; moreover, the substituents were moved to the 5(6) position of the benzimidazole nucleus. Within both the 2-alkylaminobenzimidazole series and the 5(6)-alkoxy-2-aminobenzimidazole one, the greatest H(3)-receptor affinity was obtained for the piperidine-substituted compounds, while the presence of the p-chlorophenoxy group resulted in a drop in affinity. The optimal chain length was different in the two series. Even if the new compounds did not reach the high receptor affinity shown by the imidazole-containing lead compound, it was possible to get good H(3)-antagonist potencies with 2-aminobenzimidazoles having a tertiary amino group at appropriate distance.