Abstract

Previous studies revealed that phenylhistamines and histaprodifens possess higher potency and affinity at guinea pig histamine H₁-receptor (gpH₁R) than at human histamine H₁-receptor (hH₁R). However, we recently identified an imidazolylpropylguanidine [N¹-(3-cyclohexylbutanoyl)-N²-[3-(1H-imidazol-4-yl)-propyl]guanidine (UR-AK57)] with higher potency and efficacy at hH₁R compared with gpH₁R. The aim of this study was to reveal the molecular basis for the species differences of synthetic ligands. We studied 11 novel phenylhistamines and phenoprodifens. H₁R species isoforms were expressed in Sf9 insect cells, and [³H]mepyramine competition binding and GTPase assays were performed. We identified bulky phenylhistamines with higher potency and affinity at hH₁R compared with gpH₁R. Molecular dynamics simulations of ligand-H₁R interactions revealed four potential binding modes for phenylhistamines possessing an additional histamine moiety; the terminal histamine moiety showed a high flexibility in the binding pocket. There are striking similarities in ligand properties in bulky phenylhistamines and UR-AK57. Comparison of bulky phenylhistamine binding mode with binding mode of UR-AK57 suggests that only one of these four binding modes should be established. The higher potency is explained by more effective van der Waals interaction of the compounds with Asn^2.61 (hH₁R) relative to Ser^2.61 (gpH₁R). In addition, two stable binding modes for phenoprodifens with different orientations in the binding-pocket were identified. Depending on phenoprodifen orientation, the highly conserved Trp^6.48, part of the toggle switch involved in receptor activation, was found in an inactive or active conformation, respectively. We identified the first phenylhistamines with higher potency at hH₁R than at gpH₁R and obtained insight into the binding mode of bulky phenylhistamines and imidazolylpropylguanidines.