Variability in plasma concentrations of methylprednisolone 6 days after intrasynovial injection of methylprednisolone acetate in racing horses: A field study.
Authors: Machin J, Duer W, Maylin G, Fenger C, Wilson D, Ivey M, Berthold B, Allison S, Tobin T
Journal: Equine veterinary journal
Summary
# Editorial Summary Methylprednisolone acetate is routinely injected into synovial structures to manage joint inflammation, yet its clearance from plasma varies considerably—a factor with significant implications for competition withdrawal times in regulated equine sports. Researchers measured plasma methylprednisolone concentrations at six days post-injection in 76 racing horses (Thoroughbreds, Standardbreds and Quarter Horses) across different joints, with some animals also receiving concurrent hyaluronic acid administration. Plasma concentrations ranged widely (mean 96 ± 104 pg/mL), but crucially, distal tarsal joints, cunean bursa and medial femorotibial joints showed substantially higher circulating drug levels than carpal joints, whilst co-administration with hyaluronic acid significantly prolonged clearance compared to methylprednisolone acetate alone. Practitioners should recognise that anatomical location of injection and adjunctive therapies materially influence systemic drug exposure at the six-day checkpoint; this variability necessitates careful consideration when advising clients on realistic competition return timelines, particularly for horses in regulated disciplines where plasma concentration thresholds determine eligibility.
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Practical Takeaways
- •Injection site affects drug clearance—distal tarsal and medial femorotibial joints retain methylprednisolone longer in the bloodstream than carpal joints, potentially affecting competition withdrawal times
- •Adding hyaluronic acid to methylprednisolone acetate injections prolongs plasma drug concentration; practitioners should account for this when advising on competition eligibility in regulated sports
- •Withdrawal times before competition should be individualized based on which synovial structure was injected and whether adjuvant therapies were used
Key Findings
- •Mean plasma methylprednisolone concentration at 6 days post-injection was 96±104 pg/mL across all horses
- •Carpal joint injections produced lower plasma MP concentrations than distal tarsal or medial femorotibial joints (P<0.05)
- •Co-administration of hyaluronic acid with MP acetate resulted in higher plasma MP concentrations compared to MP acetate alone (P<0.05)
- •Metacarpophalangeal joint injections showed 86% censored data, indicating rapid clearance from plasma