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Dexamethasone serum concentrations after intravenous administration in horses during race training

In: Comparative Exercise Physiology
Authors:
S. McClure Midwest Equine Surgery and Sports Medicine, 2615 Eastgate Drive, Boone, IA 50036, USA.
Department of Veterinary Clinical Sciences College of Veterinary Medicine, Iowa State University, 1800 Christensen Drive, Ames, IA 50011-1134, USA.

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C. Fenger Equine Integrated Medicine, PLC, 4904 Ironworks Rd., Georgetown, KY 40324, USA.

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K. Kersh Department of Veterinary Clinical Sciences College of Veterinary Medicine, Iowa State University, 1800 Christensen Drive, Ames, IA 50011-1134, USA.

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B. Brown Bradley Brown, private practice, 425 Sacree Rd., Shelbyville, KY 40065, USA.

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G. Maylin New York Drug Testing and Research Program, Morrisville State College, 777 Warren Rd, Ithaca, NY 14853, USA.

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W. Duer Duer Forensic Toxicology LLC., 1621 Gulf Blvd #102, Clearwater, FL 33767-2928, USA.

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L Dirikolu Department of Comparative Biomedical Sciences, School of Veterinary Medicine, Louisiana State University, Skip Bertman Drive, Baton Rouge, LA 70803, USA.

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K. Brewer Kimberly Brewer, private practice, 15775 Cypress Creek Lane, Wellington, FL 33414, USA.

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J. Machin The Maxwell H. Gluck Equine Research Center, University of Kentucky, 1400 Nicholasville Rd, Lexington, KY 40503, USA.

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T. Tobin The Maxwell H. Gluck Equine Research Center, University of Kentucky, 1400 Nicholasville Rd, Lexington, KY 40503, USA.

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Dexamethasone (DXM) sodium phosphate is a widely used corticosteroid for inflammatory conditions in horses, regulated in racing jurisdictions in the USA by a 0.005 ng/ml serum/plasma threshold. This study seeks to describe serum concentrations of DXM at 48 and 72 h after intravenous administration of 20 mg DXM sodium phosphate over 1 to 5 days, and to identify a possible source of DXM overages. 74 horses (39 Thoroughbreds, 13 Standardbreds, 22 Quarter Horses) in active race training received 20 mg DXM sodium phosphate. Serum was collected before injection, at 48 and 72 h post last injection, and analysed by LC/MS-MS (limit of quantification (LOQ) = 2.5 pg/ml). No differences were identified by ANOVA (P≤0.05) for racing breeds, age, gender or the number of days of DXM sodium phosphate administration, so data were pooled for each time point. The DXM serum concentration at 48 h (mean ± standard deviation, range) was 2.18±1.56 pg/ml (<2.5 to 40 pg/ml). Summary statistics could not be derived for 72 h DXM serum concentration data owing to censored data, but ranged from <2.5 to 95.8 pg/ml. There was one extreme outlier (Tukey) at 48 h, and two extreme outliers at 72 h. A separate study was conducted using sedentary experimental horses to determine the likelihood that positive DXM samples could result from environmental transfer. Urine was collected from a mare 2 to 3 h post administration of 20 mg DXM. Hay with 100 ml of the DXM (17 ng/ml) containing urine was offered to each of six experimental horses and blood was collected at 0, 4, 8, 12, 16, 20 and 24 h. All six horses had plasma DXM concentration above the limit of detection and five of six had plasma DXM concentrations above the LOQ for at least one sample time.

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