Isoxsuprine is used to treat navicular disease and other lower-limb problems in the horse. Isoxsuprine is regulated as a class 4 compound by the Association of Racing Commissioners, International (ARCI) and, thus, requires regulatory monitoring. A gas chromatography-mass spectrometry method utilizing electron impact ionization was developed and validated for the quantitation of isoxsuprine in equine plasma or equine urine. The method utilized robotic solid-phase extraction and tri-methyl silyl ether products of derivatization. Products were bis-trimethylsilyl (TMS) isoxsuprine and tris-TMS ritodrine, which released intense quantifier ions m/z 178 for isoxsuprine and m/z 236 for ritodrine that were products of C-C cleavage. To our knowledge, this procedure is faster and more sensitive than other methods in the literature. Concentrations in urine and plasma of isoxsuprine were determined from a calibrator curve that was generated along with unknowns. Ritodrine was used as an internal standard and was, therefore, present in all samples, standards, and blanks. Validation data was also collected. The limit of detection of isoxsuprine in plasma was determined to be 2 ng/mL, the limit of quantitation of isoxsuprine in plasma was determined to be < 5 ng/mL. The mean coefficient of determination for the calibrator curves for plasma was 0.9925 ± 0.0052 and for calibrator curves for urine 0.9904 ± 0.0075. The recovery efficiencies at concentrations of 50, 200, and 300 ng/mL were 76%, 73%, and 76%, respectively, in plasma and 92%, 89% and 91% in urine.

Document Type


Publication Date


Digital Object Identifier (DOI)


Funding Information

Supported by grants from The Kentucky Equine Drug Council and The Kentucky Racing Commission, Lexington, KY and by research support from the National, Canadian, Kentucky, Charles Town, Ohio, Arkansas, Michigan, Pennsylvania, Ontario, Alabama, Florida, and Nebraska Horsemen's Benevolent and Protective Associations and Mrs. John Hay Whitney.

Related Content

Publication # 263 from the Equine Pharmacology and Experimental Therapeutics Program at the Maxwell H. Gluck Equine Research Center and the Department of Veterinary Science, University of Kentucky.

Published as Kentucky Agricultural Experiment Station Article #02-14-184 with the approval of the Dean and Director, College of Agriculture and Kentucky Agricultural Experiment Station.