Intratracheal clenbuterol in the horse: its pharmacological efficacy and analytical detection

Clenbuterol, a beta2 agonist/antagonist, is the only bronchodilator approve d 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 sa nctions. Anecdotal reports suggest that clenbuterol may have been administe red by intratracheal (IT) injection to obtain beneficial effects and avoid post-race detection. The objectives of this study were (1) to measure the p harmacological efficacy of IT dose of clenbuterol and (2) to determine the analytical findings in urine in the presence and absence of furosemide. Whe n administered intratracheally (90 mug/horse) to horses suffering from chro nic 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 concentr ations in the urine were also measured after IT dose in the presence and ab sence of furosemide. Four horses were administered i.v. furosemide (5 mg/kg ), and four horses were administered saline (5 mL). Two hours later, all ho rses were administrated clenbuterol (IT, 90 mug), and the furosemide-treate d horses received a second dose of furosemide (2.5 mg/kg, i.v.). Three hour s after clenbuterol dose (1 h after hypothetical 'post-time'), the mean spe cific gravity of urine samples from furosemide-treated horses was 1.024, we ll above the 1.010 concentration at which furosemide is considered to inter fere with drug detection. There was no interference by furosemide with 'enh anced' ELISA screening of clenbuterol equivalents in extracted and concentr ated samples. Similarly, furosemide had no effect on mass spectral identifi cation or quantification of clenbuterol in these samples. These results sug gest that the IT dose of clenbuterol (90 mug) is, in pharmacological terms, indistinguishable from the dose of saline, and that, using extracted sampl es, clenbuterol dose is readily detectable at 3 h after dosing. Furthermore , concomitant dose of furosemide does not interfere with detection or confi rmation of clenbuterol.