Oxandrolone is an anabolic-androgenic steroid with favourable anabolic to androgenic ratio, making it an effective anabolic agent with less androgenic side effects. Although its metabolism has been studied in humans, its phase I and II metabolism has not been previously reported in the horse. The purpose of this study was to investigate the in vitro metabolism of oxandrolone (using both equine liver microsomes and S9) and in vivo metabolism following oral administration (three daily doses of 50 mg of oxandrolone to a single Thoroughbred horse), using both gas and liquid chromatography-mass spectrometry techniques. The in vitro phase I transformations observed included 16-hydroxylated (two epimers), 17-methyl-hydroxylated and 16-keto metabolites. In addition to parent oxandrolone and these hydroxylated metabolites, the 17-epimer and a 17,17-dimethyl-18-norandrost-13-ene analogue were detected in biological samples following the administration. 16-keto-oxandrolone was only observed in urine. The 16- and 17-methyl-hydroxylated oxandrolone metabolites were predominantly excreted as sulfate conjugates in urine, whereas parent oxandrolone, its epimer and 17,17-dimethyl-18-norandrost-13-ene derivative were found predominantly in the unconjugated urine fraction. The most abundant analyte detected in both plasma and urine was parent oxandrolone. However, the longest detection period using the developed analytical method was provided by 17-hydroxymethyl-oxandrolone in both matrices. The results of this study provided knowledge of how best to detect the use of oxandrolone in regulatory samples.

中文翻译：

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马体内合成代谢-雄激素类固醇氧雄龙的体外和体内代谢

Oxandrolone 是一种合成代谢雄激素类固醇，具有良好的合成代谢与雄激素比例，使其成为一种有效的合成代谢剂，具有较少的雄激素副作用。尽管已经在人类中研究了它的代谢，但它的 I 期和 II 期代谢以前没有在马身上报道过。本研究的目的是研究氧雄龙的体外代谢（同时使用马肝微粒体和 S9）和口服给药后的体内代谢（对一匹纯种马每天 3 次服用 50 毫克氧雄龙），同时使用气体和液相色谱-质谱技术。体外的观察到的 I 期转化包括 16-羟基化（两种差向异构体）、17-甲基-羟基化和 16-酮代谢物。除了母体 oxandrolone 和这些羟基化代谢物外，给药后在生物样品中检测到 17-差向异构体和 17,17-二甲基-18-norandrost-13-ene 类似物。仅在尿液中观察到 16-酮氧雄龙。16-和 17-甲基羟基化氧甲氢龙代谢物主要作为硫酸盐结合物在尿液中排泄，而母体氧甲氢龙、其差向异构体和 17,17-二甲基-18-诺雄龙-13-烯衍生物主要在未结合的尿液部分中发现。在血浆和尿液中检测到的最丰富的分析物是母体 oxandrolone。然而，17-羟甲基-氧雄龙在两种基质中提供了使用开发的分析方法的最长检测期。这项研究的结果提供了有关如何最好地检测监管样本中氧甲氢龙使用的知识。