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Vol. 61, Issue 2, 391-399, February 2002
UCB S.A. Pharma Sector, In Vitro Pharmacology, Braine l'Alleud,
Belgium (M.G., C.V.D.P., R.M., P.C.); and Laboratory of Applied
Genetics, Institute of Molecular Biology and Medicine, Free University
of Brussels, Gosselies, Belgium (N.M.)
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
(Ki 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.
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