Gillard Michel, Van Der Perren Christy, Moguilevsky Nicole, Massingham Roy, Chatelain Pierre
UCB S.A. Pharma Sector, In Vitro Pharmacology, Braine l'Alleud, Belgium.
Mol Pharmacol. 2002 Feb;61(2):391-9. doi: 10.1124/mol.61.2.391.
Competition experiments with [(3)H]mepyramine showed that cetirizine and its enantiomers, levocetirizine and (S)-cetirizine, bound with high affinity and stereoselectivity to human H(1) histamine receptors (K(i) values of 6, 3, and 100 nM, respectively). Cetirizine and levocetirizine were 600-fold more selective for H(1) 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 H(1) 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 H(1) receptors, with half-times of 31 min and 7 min, respectively. The importance of the carboxylic function of levocetirizine for the interaction with the H(1) receptor was further supported by the results from the mutation of Lys(191) to Ala(191). 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 Thr(194) reduced the binding stereoselectivity by selectively enhancing the affinity of the distomer.
用[³H]美吡拉敏进行的竞争实验表明，西替利嗪及其对映体左西替利嗪和（S）-西替利嗪与人类H₁组胺受体具有高亲和力和立体选择性结合（Kᵢ值分别为6、3和100 nM）。与一组受体和通道相比，西替利嗪和左西替利嗪对H₁受体的选择性高600倍。结合结果表明，西替利嗪及其对映体与组胺之间的相互作用符合竞争行为，这与在离体器官中观察到的西替利嗪和左西替利嗪的非竞争特征相反。结合动力学为这一观察结果提供了合适的解释，因为左西替利嗪从H₁受体解离的半衰期为142分钟；（S）-西替利嗪的半衰期仅为6分钟，这意味着前者在功能研究中可作为一种拟不可逆拮抗剂。左西替利嗪的羧基功能似乎是其长解离时间的原因。事实上，羟基或甲酯类似物从H₁受体解离得更快，半衰期分别为31分钟和7分钟。将赖氨酸（Lys）191突变为丙氨酸（Ala）191的结果进一步支持了左西替利嗪羧基功能对与H₁受体相互作用的重要性。这种突变将左西替利嗪的解离半衰期从142分钟降至13分钟，并将其亲和力从3 nM降至12 nM，而羟基和甲酯类似物的亲和力和解离动力学几乎未受影响。苏氨酸（Thr）194的突变通过选择性增强异构体的亲和力降低了结合立体选择性。
