The pharmacokinetics and residues of clenbuterol in veal calves

Seven female Brown Swiss calves were used to study the pharmacokinetics of clenbuterol after an effective anabolic dosage of 5 micrograms/kg of BW was given twice daily for 3 wk. Analyses of clenbuterol concentrations in different tissues was done by enzyme immunoassay (EIA). Tissue samples were taken from three calves on the last day of administration and from two more after 3.5 or 14 d of clenbuterol withdrawal. The rate of clenbuterol elimination was dependent on time and tissue. Clenbuterol concentrations in the lung dropped from a mean of 76 ng/g to a level of less than .08 ng/g after 14 d, whereas in the liver the clenbuterol concentrations decreased from 46 ng/g to .6 ng/g within 14 d of withdrawal. Highest levels were always found in the eye: 118 ng/g, 57.5 ng/g, and 15.1 ng/g after 0, 3.5, and 14 d of withdrawal, respectively. These data reveal that different compartments contribute to the elimination of clenbuterol; therefore, concentrations in urine do not follow first order kinetics. An initial rapid decline in the concentration of clenbuterol in urine with a half-life of 10 h is followed by a slower elimination with a half-life of about 2.5 d. Treatments using the anabolic dose of 5 micrograms/kg of BW require longer withdrawal times than the therapeutic dose (.8 micrograms/kg BW).