Hydroxyzine 2HCl is an antagonist of histamine H1-receptor that inhibits the binding of [3H]pyrilamine and [3H]desloratadine to histamine H1 receptor with IC50s of 10 nM and 19 nM.
Hydroxyzine Dihydrochloride is the first generation H1 receptor antagonist drug. Hydroxyzine is an antihistaminic with sedative properties used in the control of anxiety and emesis[3]. Hydroxyzine provided sustained pain relief to six hours, whereas meperidine produced analgesia up to two hours. The combination produced additive analgesia only during the first 2 hr[4]. The mean systemic availability of oral hydroxyzine was 72%. Hydroxyzine was rapidly converted to cetirizine regardless of the route of administration. The mean area-under-the-curve was eight and ten times higher for cetirizine than hydroxyzine after intravenous and oral dosing, respectively. After oral administration of hydroxyzine, the mean peak concentration of cetirizine was approximately 2.2 mg/mL and that of hydroxyzine 0.16 mg/mL. The terminal half-life for cetirizine varied between 10 and 11 h after intravenous and oral administration of hydroxyzine. Pharmacodynamic modelling predicted that maximal antihistamine effect would occur with twice daily oral administration of hydroxyzine at 2 mg/kg[5]. Hydroxyzine reduced carbachol-induced serotonin release from rat bladder in vitro through a mechanism which was unrelated to its H1 receptor antagonistic properties. The ability of hydroxyzine to inhibit bladder mast cell activation by neurogenic stimuli along with its anticholinergic, anxiolytic and analgesic properties[6].
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