Disposition of methylprednisolone acetate in plasma, urine, and synovial fluid following intra‐articular administration to exercised thoroughbred horses

Abstract

Methylprednisolone acetate (MPA) is commonly administered to performance horses, and therefore, establishing appropriate withdrawal times prior to performance is critical. The objectives of this study were to describe the plasma pharmacokinetics of MPA and time-related urine and synovial fluid concentrations following intra-articular administration to sixteen racing fit adult Thoroughbred horses. Horses received a single intra-articular administration of MPA (100mg). Blood, urine, and synovial fluid samples were collected prior to and at various times up to 77days postdrug administration and analyzed using tandem liquid chromatography-mass spectrometry (LC-MS/MS). Maximum measured plasma MPA concentrations were 6.06±1.57 at 0.271days (6.5h; range: 5.0-7.92h) and 6.27±1.29ng/mL at 0.276days (6.6h; range: 4.03-12.0h) for horses that had synovial fluid collected (group 1) and those that did not (group 2), respectively. The plasma terminal half-life was 1.33±0.80 and 0.843±0.414days for groups 1 and 2, respectively. MPA was undetectable by day 6.25±2.12 (group 1) and 4.81±2.56 (group 2) in plasma and day 17 (group 1) and 14 (group 2) in urine. MPA concentrations in synovial fluid remained above the limit of detection (LOD) for up to 77days following intra-articular administration, suggesting that plasma and urine concentrations are not a good indicator of synovial fluid concentrations.