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.

YK‐11 is a steroidal selective androgen receptor modulator, a compound class prohibited in both equine racing and human sports because of their potentially performance enhancing properties. YK‐11 is easily accessible via internet‐based supplement vendors making this compound a possible candidate for doping; however, its phases I and II metabolism has not yet been reported in the horse. The purpose of this study was to investigate the in vivo metabolites of YK‐11 in urine and plasma following oral administration with three daily doses of 50 mg to two Thoroughbred horses. In vitro incubations with equine liver microsomes/S9 were also performed for use as metabolite reference materials; however, this resulted in the formation of 79 metabolites with little overlap with the in vivo metabolism. In plasma, parent YK‐11 and seven phase I metabolites were detected, with five of them also observed in vitro. They were present nonconjugated in plasma, with one metabolite also indicating some glucuronide conjugation. In urine, 11 phase I metabolites were observed, with four of them also observed in vitro and six of them also detected in plasma. Nine metabolites were excreted non‐conjugated in urine, with two of them also indicating some sulfate conjugation. Two minor metabolites were detected solely as sulfate conjugates. The most abundant analytes in urine were a mono‐O‐demethylated breakdown product and di‐O‐demethylated YK‐11. The most abundant analytes in plasma were two isomers of the breakdown product with an additional hydroxylation reaction, which also provided the longest detection time in both matrices.

Calcium dobesilate (CD) is a synthetic venoactive drug used in veterinary medicine to treat equine navicular disease. Etamsylate is a haemostatic agent used in horses for the treatment of exercise‐induced pulmonary haemorrhage. Both etamsylate and CD dissociate in the circulatory system with 2,5‐HBSA as the active drug. The aim of the research was to be able to provide detection time (DT) advice from pharmacokinetic (PK) studies in Thoroughbred horses to better inform trainers, and their veterinary surgeons, prescribing these substances for treatment of Thoroughbred racehorses. Two (pilot study) and six (final study) horses were given 28 and 9 repeated dose of CD (3 mg/kg BID) respectively. Two horses were each given a single intravenous (IV) dose of etamsylate (10 mg/kg). Plasma and urine 2,5‐HBSA concentrations were measured by liquid chromatography–tandem mass spectrometry (LC–MS/MS). The CD pilot study revealed that steady state could be reached with a few days and that 2,5‐HBSA in plasma and urine shows instability during storage at −20°C but appears stable at −80°C. A novel holistic non‐linear mixed‐effects three‐compartmental PK model was developed that described both plasma and urine concentrations of 2,5‐HBSA, from either CD or etamsylate administration. Typical values for 2,5‐HBSA clearance and bioavailability were 2.0 mL/min/kg and 28% respectively. Using the parameters obtained from this PK model, in conjunction with methodology developed by Toutain, afforded a possible screening limit (SL) that can regulate for a DT of 3 days in urine; however, a corresponding SL in plasma would be below current levels of detection. However, it is the responsibility of the individual racing authorities to apply their own risk management with regard to SLs and DTs.