The use of ELISA tests and immunoaffinity chromatography combined with reversed-phase high-performance liquid chromatography for dexamethasone detection in equine urine

Pharmacokinetics of dexamethasone with pharmacokinetic/pharmacodynamic model of the effect of dexamethasone on endogenous hydrocortisone and cortisone in the horse

A compartmental model was used to describe the pharmacokinetics of dexamethasone (DXM) and changes in the plasma concentration of endogenous cortisone (COR) and hydrocortisone (HYD) following intravenous (i.v.) administration of DXM (0.05 mg/kg) in horses. Quantification of DXM, COR and HYD in equine plasma was achieved using liquid chromatography interfaced with triple spray quadrupole quantum tandem mass spectrometry (LC/TSQ-MS/MS). The median alpha (t(1/2alpha)), beta (t(1/2beta)), and gamma (t(1/2gamma)) half-lives were 0.33, 2.2, and 10.7 h respectively. The area under the DXM plasma concentration curve (AUC) was 113.5 ng.h/mL. At 72 h post-DXM administration, the plasma concentration of DXM in all horses was below the level of quantification (100 pg/mL). The baseline plasma concentration of COR was 3.5 +/- 0.69 ng/mL and declined significantly (P < 0.02) to 2.9 +/- 0.86 ng/mL at 1 h. The nadir in COR plasma concentration was 0.65 +/- 0.12 ng/mL at 28.8 +/- 9.0 h, and the DXM plasma concentration was 0.19 +/- 0.13 ng/mL. COR concentration returned to baseline at 96 h. Baseline plasma concentration of HYD was 58.8 +/- 11.7 ng/mL and declined significantly (P < 0.001) to 41.1 +/- 14.9 ng/mL at 1 h following DXM administration but recovered to baseline at 96 h. The sensitivity of LC/TSQ-MS/MS allowed complete description of the pharmacokinetics of DXM and its effect on plasma concentrations of both COR and HYD.

Hydrocortisone concentrations in post-race urine from horses

As hydrocortisone is an endogenous substance, it is first necessary to establish its normal concentrations so as to be able to control its use in racing animals. This study was designed to establish the hydrocortisone concentrations in post-race urine samples of horses racing in Brazil and also to evaluate the results in relation to the international threshold set for this drug. Urine samples were analysed by HPLC-UV. The results were evaluated according to the concentration range as well as sex and time of sample collection (afternoon or evening races). The results showed a high degree of variation in the concentrations of hydrocortisone in the urine (93 +/- 69 ng/ml). The maximum concentration observed was 646 ng/ml, although only a few horses (around 1%) showed levels within the range 500-650 ng/ml, 91% being in the range 0-150 ng/ml. The data suggested a normal distribution curve. Statistical analysis showed no significant influence of sex or time of sample collection.