Synthesis and in Vitro Pharmacology of a Series of New Chiral Histamine H3-Receptor Ligands:  2-(R and S)-Amino-3-(1H-imidazol-4(5)-yl)propyl Ether Derivatives

To investigate stereospecificity and the mechanism of activation of the histamine H3-receptor, a series of 2-(R and S)-amino-3-(1H-imidazol-4(5)-yl)propyl ether derivatives were synthesized. In these compounds, the structures of the well-known antagonist iodoproxyfan and the full agonists R- or S-(α)-methylhistamine were combined in one molecule. The obtained “hybrid” molecules were tested for H3-receptor affinity on rat cerebral cortex. Some selected compounds were further screened for H3-receptor functional activity with GTPγ[35S] autoradiography studies using rat brain tissue sections. The affinity of all the synthesized compounds (−log K i = 5.9−7.9) was lower than that found for iodoproxyfan or two of its analogues; however, the compounds showed stereospecificity. The S-configuration of the series of 2-amino-3-(1H-imidazol-4(5)-yl)propyl ether derivatives, which resembles the stereochemistry of R-(α)-methylhistamine, was more favorable. Incorporation of an amino group in the propyl chain of iodoproxyfan and analogues did not alter the antagonistic behavior for compounds with an aromatic side chain. However, when also the aromatic moiety was replaced by a cyclohexyl group, the compounds behaved as agonists. This indicates that an interaction between the side chain amino group and the H3-receptor protein is involved in H3-receptor activation. The 2-(S)-amino-3-(1H-imidazol-4(5)-yl)propyl cyclohexylmethyl ether (23) has H3-receptor agonistic properties with high affinity for the histamine H3-receptor (−log K i = 7.9 ± 0.2) and might serve as a useful tool for further studies concerning drug design and receptor−ligand interactions.