Abstract
Therapeutic dose (20, 40 and 80 μg/man) of clenbuterol hydrochloride, a β2-adrenergic stimulant, was orally administered to healthy volunteers, and the unmetabolized drug in plasma and urine was determined by enzyme immunoassay. The plasma levels of clenbuterol reached the maximum value of 0.1, 0.2 and 0.35 ng/ml, respectively, in a dose-dependent manner within 2.5h, which lasted for over 6 h after the administration. The half-life of clenbuterol in plasma was estimated to be about 35 h. When the drug was orally administered repeatedly to men twice a day, the plasma level reached the plateau within 4 d after the initial administration. At that time, the plasma levels of the unchanged form were 0.2 to 0.3 ng/ml and 0.5 to 0.6 ng/ml at doses of 20 and 40 μg/man, respectively. The bound ratio of the drug to plasma protein was estimated to be 89-98% at a single administration of 80 μg of the drug. The cumulative urinary excretion of unchanged compound corresponded to about 20% of the administered dose as measured at 72 h following a single oral administration. When clenbuterol hydrochloride was orally administered to rats at a dose of 2μg/kg, the plasma level reached the maximum at about 1 h after the administration. In rabbits, the plasma concentrations reached the maximum value of about 0.2 and 0.8 ng/ml within 2 h following administration of clenbuterol hydrochloride at doses of 0.5 and 2 μg/kg, respectively. The half-life of clenbuterol in plasma was about 30 h in rats and about 9 h in rabbits.
