Clenbuterol plasma pharmacokinetics in cattle.

The pharmacokinetics of clenbuterol (Cb) were investigated to determine the extent to which analysis of plasma concentration can be used to discriminate between therapeutic and illicit growth promoting treatment of cattle. Analysis of plasma concentration enabled assessment of the extent of differences in pharmacokinetics between such dosing regimens. Cattle were treated with Cb using either a therapeutic (20 calves, 0.8 microgram Cb kg-1, twice daily in feed for 10 days), or growth promoting (30 calves, 10 micrograms Cb kg-1, twice daily by drench for 20 days) dosing regimens. Blood samples were collected by jugular venepuncture, and plasma Cb concentrations determined by direct enzyme immunoassay. To determine plasma pharmacokinetics, use of a two compartment model was applied to the data and revealed that steady state kinetics were reached after 3 and 5 days following initiation of therapeutic and growth promoting dosing regimens, respectively. Tolerance limit analysis of concentrations during the therapeutic regimen indicated that a plasma Cb concentration greater than 1.63 ng ml-1 would be indicative (p < 0.01) of a growth promoting dose.