In vitro and in vivo metabolism of the anabolic-androgenic steroid oxandrolone in the horse

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.