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 endo‐genous 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 1/2 α ), beta ( t 1/2 β ), and gamma ( t 1/2 γ ) 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.