Pharmacokinetics of betamethasone in plasma, urine, and synovial fluid following intra-articular administration to exercised thoroughbred horses

The use of corticosteroids, such as betamethasone, in performance horses is tightly regulated. The objective of the current study was to describe the plasma pharmacokinetics of betamethasone as well as time-related urine and synovial fluid concentrations following intra-articular administration to horses. Twelve racing-fit adult Thoroughbred horses received a single intra-articular administration (9 mg) of a betamethasone sodium phosphate and betamethasone acetate injectable suspension into the right antebrachiocarpal joint. Blood, urine, and synovial fluid samples were collected prior to and at various times up to 21 days post drug administration. All samples were analyzed using tandem liquid chromatography-mass spectrometry. Plasma data were analyzed using compartmental pharmacokinetic modeling. Maximum measured plasma betamethasone concentrations were 3.97 ± 0.23 ng/mL at 1.45 ± 0.20 h. The plasma elimination half-life was 7.48 ± 0.39 h. Betamethasone concentrations were below the limit of detection in all horses by 96 h and 7 days in plasma and urine, respectively. Betamethasone fell below the limit of detection in the right antebrachiocarpal joint between 14 and 21 days. Results of this study provide information that can be used to regulate the use of intra-articular betamethasone in the horse.