title: Pharmacokinetics of methocarbamol and phenylbutazone in exercised Thoroughbred horses.
abstract: Methocarbamol (MCBL) is commonly used in performance horses for the treatment of skeletal muscle disorders. Current regulatory recommendations for show horses and racehorses are based on a single oral dose of 5 g, although doses in excess of this are often administered. The goal of the current study was to characterize the disposition of MCBL following higher dose administration and administration in combination with another commonly used drug in performance horses, phenylbutazone (PBZ). Exercised Thoroughbred horses were administered various doses of MCBL as a sole agent and MCBL in combination with PBZ. Blood samples were collected at various times, concentrations of MCBL and PBZ measured using LC-MS/MS and pharmacokinetic parameters calculated using compartmental analysis. Following administration of 15 g of MCBL, either as part of a single- or multiple-dose regimen, a number of horses exceeded the Association of Racing Commissioners International and the United States Equestrian Federation's recommended regulatory threshold at the recommended withdrawal time. There was not a significant difference between horses that received only MCBL and those that received MCBL and PBZ. Results of the current study support an extended withdrawal guideline when doses in excess of 5 g are administered.
authors.affiliation: K.L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, CA, USA.|Department of Veterinary Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA, USA.
authors.forename: H K
authors.initials: HK
authors.lastname: Knych
authors.affiliation: K.L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, CA, USA.|Department of Veterinary Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, CA, USA.
authors.forename: S D
authors.initials: SD
authors.lastname: Stanley
authors.affiliation: K.L. Maddy Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, CA, USA.
authors.forename: K N
authors.initials: KN
authors.lastname: Seminoff
authors.affiliation: Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA, USA.
authors.forename: D S
authors.initials: DS
authors.lastname: McKemie
authors.affiliation: Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA, USA.
authors.forename: P H
authors.initials: PH
authors.lastname: Kass
mesh_terms: D000284:Administration, Oral; D000818:Animals; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004359:Drug Therapy, Combination; D005260:Female; D006736:Horses / Q000097:blood / Q000378:metabolism; D007275:Injections, Intravenous; D008297:Male; D008721:Methocarbamol / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D009125:Muscle Relaxants, Central / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D009824:Ointments / Q000008:administration & dosage; D010653:Phenylbutazone / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D010805:Physical Conditioning, Animal
chemical_list: D000894:Anti-Inflammatory Agents, Non-Steroidal; D009125:Muscle Relaxants, Central; D009824:Ointments; D008721:Methocarbamol; D010653:Phenylbutazone