J. vet. Pharmacol. Therap. 23, 251–260, 2000. THERAPEUTICS Intratracheal clenbuterol in the horse: its pharmacological efficacy and analytical detection Harkins, J. D., Robinson, N. E., Woods, W. E., Lehner, A. F., Smith, M. D., Gates, R. S., Fisher, M., Tobin, T. Intratracheal clenbuterol in the horse: its pharmacological efficacy and analytical detection. J. vet. Pharmacol. Therap. 23, 251–260. Clenbuterol, a  2 agonist/antagonist, is the only bronchodilator approved by the US Food and Drug Administration for use in horses. The Association of Racing Commissioners International classifies clenbuterol as a class 3 agent, and, as such, its identification in post-race samples may lead to sanctions. Anecdotal reports suggest that clenbuterol may have been administered by intratracheal (IT) injection to obtain beneficial effects and avoid post-race detection. The objectives of this study were (1) to measure the pharmacologi- cal efficacy of IT dose of clenbuterol and (2) to determine the analytical findings in urine in the presence and absence of furosemide. When adminis- tered intratracheally (90 g/horse) to horses suffering from chronic obstructive pulmonary disease (COPD), clenbuterol had effects that were not significantly different from those of saline. In parallel experiments using a behavior chamber, no significant effects of IT clenbuterol on heart rate or spontaneous locomotor activity were observed. Clenbuterol concentrations in the urine were also measured after IT dose in the presence and absence of furosemide. Four horses were administered i.v. furosemide (5 mg/kg), and four horses were administered saline (5 mL). Two hours later, all horses were administrated clenbuterol (IT, 90 g), and the furosemide-treated horses received a second dose of furosemide (2.5 mg/kg, i.v.). Three hours after clenbuterol dose (1 h after hypothetical ‘post-time’), the mean specific gravity of urine samples from furosemide-treated horses was 1.024, well above the 1.010 concentration at which furosemide is considered to interfere with drug detection. There was no interference by furosemide with ‘enhanced’ ELISA screening of clenbuterol equivalents in extracted and concentrated samples. Similarly, furosemide had no effect on mass spectral identification or quantification of clenbuterol in these samples. These results suggest that the IT dose of clenbuterol (90 g) is, in pharmacological terms, indistinguishable from the dose of saline, and that, using extracted samples, clenbuterol dose is readily detectable at 3 h after dosing. Furthermore, concomitant dose of furosemide does not interfere with detection or confirmation of clenbuterol. J. Daniel Harkins, 108 Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY 40506 -0099, USA. E -mail: dharkins@ca.uky.edu J. D. HARKINS* N. E. ROBINSON ¶ W. E. WOODS* A.F. LEHNER* M. D. SMITH † R. S. GATES ‡ M. FISHER § & T. TOBIN* *Maxwell H. Gluck Equine Research Center and the Department of Veterinary Science, ¶ Department of Large Animal Clinical Sciences, Veterinary Medical Center, Michigan State University, East Lansing, MI, USA; † Department of Statistics, ‡ Biosystems and Agricultural Engineering, University of Kentucky, Lexington, KY 40506, USA; § The Kentucky Racing Commission, Ironworks Pike, Lexington, KY, USA 22Published as c268 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 c 99 -14 -167 with the approval of the Dean and Director, College of Agriculture and Kentucky Agricultural Experiment Station INTRODUCTION Clenbuterol is a  2 agonist/antagonist bronchodilator approved by the American Association of Equine Practitioners and is the only  2 agonist approved by the United States Food and Drug Administration for use in horses. As a bronchodilator, it may have the potential to alter the athletic performance of animals, particularly if the horse has bronchospasm. Furthermore, clen- buterol may stimulate the cardiac and central nervous systems, which could translate to positive effects on the performance of racing horses. Clenbuterol is also classified by the Association of Racing Commissioners International as a class 3 agent, and its detection in post-race samples may lead to significant sanctions against trainers. In 1998, highly sensitive screening procedures © 2000 Blackwell Science Ltd 251