Histaprodifens:: Synthesis, pharmacological in vitro evaluation, and molecular modeling of a new class of highly active and selective histamine H1-receptor agonists

A new class of histamine analogues characterized by a 3,3-diphenylpropyl substituent at the 2-position of the imidazole nucleus has been prepared outgoing from 4,4-diphenylbutyronitrile (4b) via cyclization of the corresponding methyl imidate 5b with 2-oxo-4-phthalimido-1-butyl acetate or 2-oxo-1,4-butandiol in liquid ammonia, followed by standard reactions. The title compounds displayed partial agonism on contractile H-1 receptors of the guinea-pig ileum and endothelium-denuded aorta, respectively, except 10 (histaprodifen; 2-[2-(3,3-diphenglpropyl)-1H-imidazol-4-yl]ethanamine) which was a full agonist in the ileum ass ay. While 10 was equipotent with histamine (1), methylhistaprodifen (13) and dimethylhistaprodifen (14) exceeded the functional potency of 1 by a factor of 3-5 (13) and 2-3 (14). Compounds 10 and 13-17 relaxed precontracted rat aortic rings (intact endothelium) with relative potencies of 3.3- up to 28-fold (compared with 1), displaying partial agonism as well. Agonist effects were sensitive to blockade by the selective Hi-receptor antagonist mepyramine (pA(2) approximate to 9 (guineapig) and pA(2) approximate to 8 (rat aorta)). The affinity of 10 and 13-17 for guinea-pig H-1 receptors increased 20- to 100-fold compared with 1. Two lower homologues of 10 were weak partial H-1-receptor agonists while two higher homologues of 10 were silent antagonists endowed with micromolar affinity for rat and guinea-pig H-1 receptors. In functional selectivity experiments, 10, 13, and 14 did not stimulate H-2, H-3, and several other neurotransmitter receptors. They displayed only low to moderate affinity for these sites (pA(2) < 6). For a better understanding of structure-activity relationships, the interaction of 1 and 10, 13 and 14 within the transmembrane (TM) domains of the human histamine H-1 receptor were studied using molecular dynamics simulations. Remarkable differences were found between the binding modes of 10, 13, and 14 and that of 1. The imidazole ring of 10, 13, and 14 was placed 'upside down' compared with 1, making the interaction of the N-pi-atom with Tyr431 possible. This new orientation was mainly caused by the space filling substitution at the 2-position of the imidazole ring and influenced the location of the protonated N-alpha-atom which was positioned more between TM III and TM VI. This orientation can explain both the increased relative potency and the maximum effect of 10, 13, and 14 compared with 1. Compound 13 (methylhistaprodifen; N-alpha-methyl-2-[2-(3,3-diphenylpropyl)-1H-imidazol-4-yl]ethanamide) is the most potent histamine H-1-receptor agonist reported so far in the literature and may become a valuable tool for the study of physiological and pathophysiological H-1-receptor-mediated effects.