Differential Regulation of Bilastine Affinity for Human Histamine H1 Receptors by Lys 179 and Lys 191 via Its Binding Enthalpy and Entropy.

Bilastine, a zwitterionic second-generation antihistamine containing a carboxyl group, has higher selectivity for H1 receptors than first-generation antihistamines. Ligand-receptor docking simulations have suggested that the electrostatic interaction between the carboxyl group of second-generation antihistamines and the amino group of Lys179ECL2 and Lys1915.39 of human H1 receptors might contribute to increased affinity of these antihistamines to H1 receptors. In this study, we evaluated the roles of Lys179ECL2 and Lys1915.39 in regulating the electrostatic and hydrophobic binding of bilastine to H1 receptors by thermodynamic analyses. The binding enthalpy and entropy of bilastine were estimated from the van 't Hoff equation using the dissociation constants. These constants were obtained from the displacement curves against the binding of [3H] mepyramine to membrane preparations of Chinese hamster ovary cells expressing wild-type human H1 receptors and their Lys179ECL2 or Lys1915.39 mutants to alanine at various temperatures. We found that the binding of bilastine to wild-type H1 receptors occurred by enthalpy-dependent binding forces and, more dominantly, entropy-dependent binding forces. The mutation of Lys179ECL2 and Lys1915.39 to alanine reduced the affinity of bilastine to H1 receptors by reducing enthalpy- and entropy-dependent binding forces, respectively. These results suggest that Lys179ECL2 and Lys1915.39 differentially contribute to the increased binding affinity to bilastine via electrostatic and hydrophobic binding forces.