Unique binding pocket for KW-4679 in the histamine H1 receptor

Abstract The histamine H 1 receptor has an aspartate (Asp) residue in transmembrane helix 3 (TM3), which is well-conserved among biogenic amine receptors. The Asp residue is one of the most crucial amino acids for ligand binding. The tested histamine H 1 receptor antagonists with tri- and tetracyclic structures were not selective for histamine H 1 receptors and showed affinity for several other biogenic amine receptors. In contrast, KW-4679 (( Z )-11-[3-(dimethylamino)propylidene]-6,11-dihydrodibenz[ b , e ]oxepin-2-acetic acid hydrochloride), a tricyclic compound, was a selective histamine H 1 receptor antagonist. [ 3 H]KW-4679 had high affinity ( K d value of 2.5±0.12 nM) for wild-type human histamine H 1 receptors. In the [ 3 H]KW-4679 binding assay, replacement of Asp 107 by alanine by site-directed mutagenesis greatly reduced the affinities (280–2100-fold) of tri- and tetracyclic compounds, whereas this mutation led to a comparatively small reduction (14-fold) in KW-4679 affinity. These results demonstrate that the tested tri- and tetracyclic histamine H 1 receptor antagonists which have a tight interaction with the Asp residue are not selective for the histamine H 1 receptor. Furthermore, the high selectivity of KW-4679 might be explained by a unique binding pocket, which consists of the Asp residue and other acceptor sites, in the histamine H 1 receptor.