N (α)-Methylated phenylhistamines exhibit affinity to the hH(4)R-a pharmacological and molecular modelling study

Histamine H(1)-receptor agonists and antagonists exhibit affinity to the human histamine H(4)-receptor (hH(4)R). However, the pharmacological profiles between hH(1)R and hH(4)R exhibit similarities and differences. Since suprahistaprodifen and trifluoromethylphenylhistamine show significant affinity to hH(4)R, the aim of this study was to analyse a large number of new phenylhistamines, histaprodifens and phenoprodifens at hH(4)R to extend the pharmacological profile of these compound classes at hH(4)R. The hH(4)R-RGS19 fusion protein was co-expressed with G(αi2) and G(β1γ2) in Sf9 insect cells, and [(3)H]histamine competition binding as well as GTPase assays were performed. Based on adequate crystal structures, homology models of hH(4)R were generated. Molecular modelling studies, including molecular dynamics and prediction of Gibbs energy of ligand binding, were performed in order to explain the pharmacological data at hH(4)R on molecular level. The exchange of the phenyl moiety of phenylhistamines into the diphenylpropyl moiety of histaprodifens acts, in contrast to hH(1)R, as partial agonism-inverse agonism switch at hH(4)R. Based on our studies, some phenylhistamine derivatives with significantly higher affinity at hH(4)R than at hH(1)R were identified. The molecular dynamic simulations revealed two different conformations for the highly conserved Trp(6.48), suggested to be involved in receptor activation. Furthermore, the predicted Gibbs energy of ligand binding for six selected phenylhistamines was in very good agreement with the experimentally determined affinities. We identified phenylhistamine derivatives with higher affinity at hH(4)R than at hH(1)R. Besides, we have identified partial agonism-inverse agonism switch between phenylhistamines and histaprodifens at hH(4)R. These results are very important to understand selectivity between hH(1)R and hH(4)R and to design new potent H(1)R and/or H(4)R receptor ligands.