Background: Methylprednisolone (MP) acetate is a commonly used corticosteroid for suppression of inflammation in synovial structures in horses. Its use is often regulated in equine sports by plasma MP concentrations. Objectives: To describe variability in MP plasma concentrations after MP acetate injection in different synovial structures and with co-administration with hyaluronic acid (HA). Study design: Field study in actively racing horses in three disciplines (Thoroughbred, Standardbred and Quarter Horse). Methods: Seventy-six horses (15 Thoroughbreds, 20 Standardbreds and 41 Quarter Horses) were included in the study. Injection of any synovial structure with a total body dose of 100 mg MP acetate was permitted, data were grouped according to the synovial structure injected and coadministration with HA. Plasma was collected before injection and at 6 days post-injection. Per cent censored data (below the limit of quantification) for each synovial structure were determined, and summary statistics generated by Robust Regression on Order. Differences between synovial structures and co-administration with HA were identified by ANOVA with Tukey’s post hoc testing. Results: Metacarpophalangeal (MCP) plasma concentrations contained 86% censored data and could not be included in the statistical analysis. The carpal joints (CJO) group had a lower plasma MP concentration (P < 0.05) than the distal tarsal joints (DTJ) or medial femorotibial (MFT), the no HA (NHA) group had a lower plasma MP concentration (P < 0.05) than HA. Main limitations: The synovial structures injected varied by racing discipline, so this study was unable to identify any differences between disciplines. Conclusions: Practitioners should be aware that injection of DTJ, CS and MFT joints, and combining MP acetate with HA may prolong its clearance, and withdrawal times for competition in regulated equine sports.

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Funding for sample collection was provided by the North American Association of Racetrack Veterinarians, and funding for determination of drug concentrations was provided by the Equine Health and Welfare Alliance, Oklahoma Quarter Horse Racing Association, Ohio Horsemen’s Benevolence and Protection Association, the Indiana Horsemen’s Benevolence and Protection Association, the Ohio Harness Horsemen’s Association, the Illinois Harness Horsemen’s Association, the Thoroughbred Racing Association of Oklahoma and the Morrisville Auxiliary Fund.

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Published as paper #486 from T Tobin and the Equine Pharmacology, Therapeutics and Toxicology Program at the Maxwell H. Gluck Equine Research Center and Department of Veterinary Science, University of Kentucky. The information reported in this paper is part of a project of the Kentucky Agricultural Experiment Station (KAES publication #17-16-088) and is published with the approval of the Director. The authors would like to thank Ms. Sarah Sanford for technical assistance.