Hydroxyzine is a first-generation antihistamine and cetirizine, a second-generation antihistamine and active metabolite of hydroxyzine. Hydroxyzine is commonly used in performance horses and as such its use in closely regulated; however, there are no published studies suitable for establishing appropriate regulatory recommendations. In the current study, 12 exercised Thoroughbred research horses received a single oral administration of 500 mg of hydroxyzine. Blood and urine samples were collected prior to and up to 96 hr postdrug administration and concentrations of hydroxyzine and cetirizine determined using liquid chromatography-tandem mass spectrometry. A joint parent/metabolite population 2-compartment pharmacokinetic model with first-order absorption and elimination was utilized to describe the pharmacokinetics of both compounds. Serum hydroxyzine and cetirizine concentrations were above the limit of quantitation (0.1 ng/ml) of the assay at 96 hr (the last time point sampled). The terminal half-life was 7.41 and 7.13 hr for hydroxyzine and cetirizine, respectively. Findings from this study suggest that a prolonged withdrawal time should be observed if this compound is used in performance administered to performance horses and is classified as prohibited substance by the applicable regulatory body.

中文翻译：

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对运动纯种马口服羟嗪后，羟嗪和西替利嗪的药代动力学。

羟嗪是第一代抗组胺药和西替利嗪，第二代抗组胺药和羟嗪的活性代谢产物。羟嗪常用于性能赛马，因此其使用受到严格管制。但是，尚无适合建立适当监管建议的已发表研究。在当前的研究中，有12匹运动的纯种马接受了500 mg羟嗪的单次口服给药。在给药前和给药后96小时收集血液和尿液样品，并使用液相色谱-串联质谱法测定羟嗪和西替利嗪的浓度。具有一级吸收和消除的联合父母/代谢物群体2室药代动力学模型用于描述两种化合物的药代动力学。血清羟嗪和西替利嗪的浓度在96小时（最后采样的时间点）高于测定极限（0.1 ng / ml）。羟嗪和西替利嗪的终末半衰期分别为7.41和7.13小时。这项研究的结果表明，如果此化合物用于对赛马的比赛中使用，并且被适用的监管机构归类为违禁物质，则应延长撤药时间。