Exploring the Effect of Cyclization of Histamine H1 Receptor Antagonists on Ligand Binding Kinetics

There is an increasing interest in guiding hit optimization by considering the target binding kinetics of ligands. However, compared to conventional structure–activity relationships, structure–kinetics relationships have not been as thoroughly explored, even for well-studied archetypical drug targets such as the histamine H1 receptor (H1R), a member of the family A G-protein coupled receptor. In this study, we show that the binding kinetics of H1R antagonists at the H1R is dependent on the cyclicity of both the aromatic head group and the amine moiety of H1R ligands, the chemotypes that are characteristic for the first-generation H1R antagonists. Fusing the two aromatic rings of H1R ligands into one tricyclic aromatic head group prolongs the H1R residence time for benchmark H1R ligands as well as for tailored synthetic analogues. The effect of constraining the aromatic rings and the basic amines is systematically explored, leading to a coherent series and detailed discussions of structure–kinetics relationships. This study shows that cyclicity has a pronounced effect on the binding kinetics.