Metabolic Fate of Clenbuterol in Calves

Abstract

The metabolic fate of [14C]clenbuterol was studied in two calves, following single or repeated oral administration of the drug at an anabolic dosage (5 μg/kg). Analytical methods were developed to evidence the nature of clenbuterol's metabolites in excreta and tissues. The major biotransformation pathways of clenbuterol were 4-N-oxidation, 4-N-sulfation, and oxidative N-dealkylation of the parent compound. Unchanged clenbuterol accounted for about 20 and 50% of the radioactivity detected in urine and feces, respectively, and for a very large part of the radioactivity detected in organs (≥90%). Retina extracts contained only clenbuterol. Two different extraction techniques and the use of 3H-labeled clenbuterol hydroxylamine demonstrated that this labile compound was not present in liver extracts. Therefore, the toxicity of clenbuterol residues remaining in tissues of cattle treated with this β-agonist toward the consumer is expected to be solely related to unchanged clenbuterol. The implications of the present results on the monitoring of illegal clenbuterol use in cattle, following detection in matrixes such as urine, liver, or retina are discussed. Keywords: Clenbuterol; β-agonist; bovine; metabolism; biotranformation; in vivo