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Menendez, M.I., Phelps, M.A., Bertone, A.L. Pharmacokinetics of intra-articular betamethasone sodium phosphate and betamethasone acetate and endogenous hydrocortisone suppression in exercising horses. J. vet. Pharmacol. Therap.39, 22–26.To the date, no reports exist of the pharmacokinetics (PK) of betamethasone (BTM) sodium phosphate and betamethasone acetate administered intra-articular (IA) into multiple joints in exercising horses. The purpose of the study was to determine the PK of BTM and HYD concentrations in plasma and urine after IA administration of a total of 30 mg BTM. Eight 4 years old Thoroughbred mares were exercised on a treadmill and BTM was administered IA. Plasma and urine BTM and HYD were determined via high performance liquid chromatography spectrometry for 6 weeks. Concentration-time profiles of BTM and HYD in plasma and urine were used to generate PK estimates for non-compartmental analyses and comparisons among times and HYD concentrations. BTM in plasma had greater Tmax (Tmax 0.8 h) vs. urine (Tmax 7.1 h). Urine BTM concentration (ng/mL) and amount (AUClast; h × ng/mL) were greater than plasma. HYD was suppressed for at least 3 days (<1 ng/mL) for all horses. The time of last quantifiable concentration of BTM (Tlast; hour) was not significantly different in plasma than urine. Use of highly sensitive HPLC-MS/MS assays enabled early detection and prolonged and consistent determination of BTM in plasma and urine.