Gabapentin, 1-(Aminomethyl)cyclohexaneacetic acid, MW 171.240, is a frequently prescribed high dose human medication that is also used recreationally. Gabapentin is orally absorbed; the dose can be 3,000 mg/day and it is excreted essentially unchanged in urine. Gabapentin is stable in the environment and routinely detected in urban wastewater. Gabapentin randomly transfers from humans to racing horses and is at times detected at pharmacologically ineffective / trace level concentrations in equine plasma and urine. In Ohio racing between January 2019 and July 2020,18 Gabapentin identifications, all less than 2 ng/ml in plasma, were reported. These identifications were ongoing because the horsemen involved were unable to pin down and therefore avoid the source of these identifications. Given that 44 ng/ml or less is an Irrelevant Plasma Concentration (IPC) of Gabapentin in horses, we proposed a 5 ng/ml plasma interim Screening Limit of Detection for Gabapentin identifications in Ohio racing, and an essentially similar 8 ng/ml plasma Screening Limit of Detection was suggested by a scientific advisor to the Ohio Horse Racing Commission. As such, an analytical Screening Limit of 8 ng /ml in plasma is an appropriate and pharmacologically conservative analytical "cut-off" or Screening Limit of Detection (SLOD) for Gabapentin in equine competitive events to avoid the calling of "positive" identifications on random unavoidable trace level identifications of this widely prescribed human therapeutic medication in equine forensic samples.
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Supported by grant funding from the Equine Health and Welfare Alliance, Inc, Versailles, Kentucky and the United States Trotting Association, Columbus, Ohio. Further supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch Program under project KY014066 Accession Number 7001029, the National Horsemen's Benevolent and Protective Association, and the Alabama, Arizona, Arkansas, Ontario, Canada; Charles Town, WV; Florida, Indiana, Iowa, Kentucky, Louisiana, Michigan, Minnesota, Nebraska, Ohio, Oklahoma, Oregon, Pennsylvania, Tampa Bay Downs, Florida, Texas, Washington State, and West Virginia Horsemen’s Benevolent and Protective Associations.
Published as paper #505 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.
Brewer, Kimberly; Machin, Jacob; Maylin, George; Fenger, Clara; Morales-Briceño, Abelardo; and Tobin, Thomas, "Gabapentin, a human therapeutic medication and an environmental substance transferring at trace levels to horses: a case report." (2022). Maxwell H. Gluck Equine Research Center Faculty Publications. 68.