Pharmacokinetics and Anti-Inflammatory Effects of Intramuscular Betamethasone in Exercised Thoroughbred Horses.
Abstract: The pharmacokinetics and pharmacodynamics of betamethasone following intra-articular administration to horses have been described; however, studies characterizing intramuscular administration are lacking. Twenty-four horses received an intramuscular dose of 12 mg betamethasone sodium phosphate/betamethasone acetate. Blood and urine were collected at post administration for up to 408 h. Concentrations of betamethasone were determined using LC-MS/MS and pharmacokinetic parameters determined using a Population PK three-compartment model. The duration of pharmacodynamic effects was assessed by measuring changes in cortisol and inflammatory biomarkers utilizing an ex vivo model. The C, T, and terminal half-life of betamethasone were 6.43 ± 1.70 ng/mL, 0.75 (0.5-2.0 h; median and range), and 30.5 ± 20.4 h, respectively. Covariates were not found to have significant effects on the variability of pharmacokinetic parameters. Based on Monte Carlo simulations, for 1000 horses, a detection time of 23 days is recommended for concentrations to fall below the screening limit of 10 pg/mL in 99% of the population. Urine concentrations were above the limit of quantitation in 2/24 horses at 408 h. Suppression of cortisol lasted for 360 h. Effects on inflammatory biomarker production lasted for a prolonged period. An extended withdrawal time for intramuscular administration prior to competition is warranted.
© 2026 The Author(s). Journal of Veterinary Pharmacology and Therapeutics published by John Wiley & Sons Ltd.
Publication Date: 2026-02-07 PubMed ID: 41653445DOI: 10.1111/jvp.70052Google Scholar: Lookup
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Summary
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This research article investigates the kinetics and anti-inflammatory effects of betamethasone, a steroid medication, when administered intramuscularly in racehorses. It suggests that the drug’s effects last longer than previously thought, and thus, intramuscular administration should be done with a longer withdrawal period before a horse enters a competition.
Methodology
- The investigation involved twenty-four racehorses, each administered an intramuscular dosage of 12 mg of a combination of betamethasone sodium phosphate and betamethasone acetate.
- Sampling involved gathering the horses’ blood and urine at various intervals after drug administration, for up to 408 hours (approximately 17 days).
- The concentrations of betamethasone in the samples were determined using a method known as liquid chromatography–mass spectrometry (LC-MS/MS).
- The researchers employed a Population PK three-compartment model to determine the pharmacokinetic parameters.
- The duration and effectiveness of the anti-inflammatory impact of the drug were evaluated through measuring levels of cortisol and other inflammatory biomarkers, using an ex vivo model. This method allows for the observation of biological processes outside the organism, typically inside a test tube or petri dish.
Results
- The research concluded that the maximum betamethasone blood concentration (C), time until this maximum concentration (T), and the terminal half-life of the drug are 6.43 ± 1.70 ng/mL, 0.75 hours (range of 0.5-2 hours), and 30.5 ± 20.4 hours, respectively.
- No significant effects of covariates on the variability of pharmacokinetic parameters were found in this study.
- Monte Carlo simulations were carried out to predict the detection time — the time for which the drug is detectable in the body. The simulation suggested a detection time of 23 days for the drug concentration to fall below the screening limit of 10 pg/mL in 99% of horses.
- Two out of twenty-four horses had urine concentrations above the limit of quantification at 408 hours after administration.
- Suppression of cortisol, an inflammatory biomarker, was sustained for up to 360 hours, or 15 days.
- Effects on the production of inflammatory biomarkers were found to last for a significantly extended period.
Conclusions
- Applying these findings, the study advocates an extended withdrawal time for intramuscular betamethasone administration in horses before they participate in competitions.
- This conclusion is crucial in maintaining fair and ethical practices in equestrian sports, ensuring horses are not racing under the influence of medications that could give them undue strength or resilience.
Cite This Article
APA
Sullivan J, Blea J, McKemie DS, Kass PH, Knych HK.
(2026).
Pharmacokinetics and Anti-Inflammatory Effects of Intramuscular Betamethasone in Exercised Thoroughbred Horses.
J Vet Pharmacol Ther.
https://doi.org/10.1111/jvp.70052 Publication
Researcher Affiliations
- K.L Maddy Equine Analytical Chemistry Laboratory (Pharmacology Section), School of Veterinary Medicine, University of California, Davis, California, USA.
- School of Veterinary Medicine, University of California, Davis, California, USA.
- K.L Maddy Equine Analytical Chemistry Laboratory (Pharmacology Section), School of Veterinary Medicine, University of California, Davis, California, USA.
- Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, California, USA.
- K.L Maddy Equine Analytical Chemistry Laboratory (Pharmacology Section), School of Veterinary Medicine, University of California, Davis, California, USA.
- Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, California, USA.
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