Pharmacokinetics and disposition of clenbuterol in the horse

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Abstract
The pharmacokinetics of clenbuterol (CLB) following a single intravenous (i.v.) and oral (p.o.) administration twice daily for 7 days were investigated in thoroughbred horses. The plasma concentrations of CLB following i.v. administration declined mono‐exponentially with a median elimination half‐life (t1/2k) of 9.2 h, area under the time–concentration curve (AUC) of 12.4 ng·h/mL, and a zero‐time concentration of 1.04 ng/mL. Volume of distribution (Vd) was 1616.0 mL/kg and plasma clearance (Cl) was 120.0 mL/h/kg. The terminal portion of the plasma curve following multiple p.o. administrations also declined mono‐exponentially with a median elimination half‐life (t1/2k) of 12.9 h, a Cl of 94.0 mL/h/kg and Vd of 1574.7 mL/kg. Following the last p.o. administration the baseline plasma concentration was 537.5 ± 268.4 and increased to 1302.6 ± 925.0 pg/mL at 0.25 h, and declined to 18.9 ± 7.4 pg/mL at 96 h. CLB was still quantifiable in urine at 288 h following the last administration (210.0 ± 110 pg/mL). The difference between plasma and urinary concentrations of CLB was 100‐fold irrespective of the route of administration. This 100‐fold urine/plasma difference should be considered when the presence of CLB in urine is reported by equine forensic laboratories.

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