Abstract

Competition experiments with [³H]mepyramine showed that cetirizine and its enantiomers, levocetirizine and (S)-cetirizine, bound with high affinity and stereoselectivity to human H₁ histamine receptors (K _i values of 6, 3, and 100 nM, respectively). Cetirizine and levocetirizine were 600-fold more selective for H₁ receptors compared with a panel of receptors and channels. Binding results indicated that the interaction between cetirizine, its enantiomers, and histamine is compatible with a competitive behavior, in contrast with the noncompetitive profile of cetirizine and levocetirizine observed in isolated organs. Binding kinetics provided a suitable explanation for this observation, because levocetirizine dissociated from H₁ receptors with a half-time of 142 min; that of (S)-cetirizine was only 6 min, implying that the former could act as a pseudo-irreversible antagonist in functional studies. The carboxylic function of levocetirizine seemed responsible for its long dissociation time. Indeed, hydroxyl or methyl ester analogs dissociated more rapidly from H₁ receptors, with half-times of 31 min and 7 min, respectively. The importance of the carboxylic function of levocetirizine for the interaction with the H₁ receptor was further supported by the results from the mutation of Lys¹⁹¹ to Ala¹⁹¹. This mutation decreased the dissociation half-time of levocetirizine from 142 to 13 min and reduced its affinity from 3 to 12 nM, whereas the affinity and dissociation kinetics of hydroxyl and methyl ester analogs were hardly affected. The mutation of Thr¹⁹⁴ reduced the binding stereoselectivity by selectively enhancing the affinity of the distomer.