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

The study analyzes how the drug Methylprednisolone acetate (MPA) is processed and excreted in the bodies of racing horses after it’s administered into a joint. The results showed that maximum concentrations of the drug in blood occurred within 6.5 to 6.6 hours and became undetectable in blood within 6.25 to 4.81 days and in urine within 17 or 14 days. Despite this, significant levels of the drug could be traced in the synovial fluid of joints for up to 77 days, indicating urine and blood levels may not reliably measure how long the drug remains in a horse’s system after inter-articular administration.

Study Objectives and Methodology

• The study aimed to track the processing and excretion of Methylprednisolone acetate (MPA) in the bodies of racing horses after intra-articular administration.
• To do so, the researchers administered the drug to 16 thoroughbred racing horses, then collected and analyzed blood, urine, and synovial fluid samples over a span of 77 days.
• The liquid chromatography-mass spectrometry (LC-MS/MS) technique was used to analyze these samples.

Results – Plasma Concentrations

• The highest observed MPA concentrations in blood reached 6.06 ± 1.57 ng/mL at about 6.5 hours post-administration in the first group of horses and 6.27 ± 1.29 ng/mL at about 6.6 hours in the second group.
• The drug’s half-life in the bloodstream was found to be about 1.33 ± 0.80 days for the first group and 0.843 ± 0.414 days for the second group.
• By 6.25 ± 2.12 days post-administration in the first group, and 4.81 ± 2.56 days in the second group, MPA became undetectable in blood samples.

Results – Urine Concentrations

• The drug also became undetectable in urine samples collected by day 17 in the first group and by day 14 in the second group.

Results – Synovial Fluid Concentrations

• Significantly, the study found that MPA remained within the limit of detection in the horses’ synovial fluid, up to 77 days following intra-articular administration.
• This suggests that examinations of plasma and urine are not reliable ways to measure the presence and concentrations of MPA in the body after it has been delivered into a joint.