Synthesis and structural and pharmacological properties of cyclopropane-based conformationally restricted analogs of 4-methylhistamine as histamine H3/H4 receptor ligands

On the basis of the previous results on a histamine H(4) receptor agonist 4-methylhistamine and a cyclopropane-based conformationally restricted analog CEIC (3) with potent H(3)/H(4) receptor antagonistic effect, 4-methylhistamine analogs 4 and 5 of CEIC were designed and synthesized. Compound 4 showed strong affinity (K(i)=38.7 nM) for the H(3) receptor, which was more potent than a well-known H(3) antagonist thioperamide. Stable tautomer and conformation of 3 and 4, which can affect the pharmacological activity, were analyzed by ab initio calculations.