Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/11/2006
Publication Date: 6/14/2006
Citation: Shelver, W.L., Smith, D.J. 2006. Tissue residues and urinary excretion of zilpaterol in sheep treated for 10-days with dietary zilpaterol. Journal of Agricultural and Food Chemistry 54:4155-4161.
Interpretive Summary: Zilpaterol is a growth promoter approved for use in cattle in Mexico and South Africa but not in other countries. Zilpaterol is forbidden for use in animal products in the European Union and numerous other countries and consequently it is necessary to have valid analytical methods capable of demonstrating the animal products are free from Zilpaterol. Since the beneficial effects of using growth promoters are also found useful in species other than cattle, this study examined the zilpaterol levels obtained when sheep were fed a supplement containing zilpaterol and measured the decline in zilpaterol levels after the growth promoter was removed. The study demonstrated that zilpaterol levels climbed rapidly and leveled off in about 5 days. After stopping the Zilpaterol the levels of the drug dropped rapidly with 95% eliminated from muscle, liver, and kidney after 3 days. Comparison of three analytical methods indicated all three produced the same results demonstrating that the ELISA method developed in this lab which is capable of high throughput was a method of choice.
Technical Abstract: Zilpaterol is a beta-adrenergic growth promoter approved in Mexico and South Africa for use in cattle. Understanding the rates of zilpaterol depletion from tissues and urine are of interest for the development of strategies to detect the off label use of zilpaterol. Eight sheep were fed 0.15 mg/kg/day dietary zilpaterol HCl (Zilmax®) for ten consecutive days; two sheep each were slaughtered 0, 2, 5, and 9 days after discontinuing exposure to the zilpaterol containing diet. Two control sheep received an identical diet without added zilpaterol. Tissue zilpaterol levels rapidly decreased during the withdrawal period. Based on LC-MS/MS-ES measurements, liver zilpaterol residues in sheep were 29.3, 1.5, 0.13, and 0.10 ng/g after 0, 2, 5, and 9 d withdrawal periods, respectively; kidney residues were 29.6, 1.10, 0.09, and 0 ng/g and muscle residues were 13.3, 0.86, 0.12, and 0.08 at the same respective withdrawal periods. Urinary zilpaterol concentrations were measured at days 1, 3, 5, and 8 during the feeding period and at days 0, 1, 2, 5, 7, and 9 of the withdrawal period. Between-animal variation in urinary zilpaterol concentrations during the feeding period was considerable, although zilpaterol concentrations converged somewhat as steady state was reached. During the first 3 days of the withdrawal period, zilpaterol elimination followed a first order excretion pattern having an average elimination half life of 12.7 ± 4.0 hours. Urinary zilpaterol concentrations during the withdrawal period were determined using ELISA, HPLC-Fluorescence, LC-MS/MS-ES (external standard) and LC-MS/MS-IS (internal standard). Comparison of these methods showed a high correlation with each other. With the exception of LC-MS/MS-IS, the regression coefficients of the linear equations with a zero intercept were between 0.90 and 1.25 indicating the near equivalence of the methods. Because of its simplicity, ELISA is a convenient assay for determining zilpaterol levels in urine giving similar results to HPLC-Fluorescence and LC-MS/MS-ES without requiring the extensive cleanup of the latter methods.