The synthetic androgen 17β-19-nortestosterone (β-NT) has been used illegally as a growth promoter in cattle production in the European Union. The elimination of β-NT and its metabolites in plasma and urine was studied in cattle which had received intramuscular injections of its phenylpropionate ester (NTP) at either single or multiple sites at a dose rate of 1 mg/kg bodyweight. In both groups, the plasma concentrations of β-NT, measured by enzyme immunoassay, were consistently greater than the assay's limit of quantification (0.24 ng/ml) during days 1 to 7 of the study. The mean (sd) maximum plasma concentration (Cmax) was significantly greater in the multiply injected animals (4.4 [0.48] v 2.7 [0.15] ng/ml), but other prasma pharmacokinetic parameters, AUC, CL, T1/2β, Tmax and MRT, were not significantly different in the two groups. The equivalent urinary concentrations exceeeded the limit of quantification of the assay (4.5 ng/ml) for up to 24 days after injection. In a second study, the biliary concentrations of β-NT and its 17α-epimer (α-NT) were measured by gas chromatography-high resolution mass spectrometry after cattle were injected intramuscularly at either single or multiple sites with NTPP. Only α-NT was detected in bile for up to 62 days after injection at concentrations above the limit of quantification of the assay (0.7 ng/ml). It is concluded that in some animals, intramuscular injections of NTPP at several sites may decrease the period after injection during which free β-NT and its metabolites are detectable in plasma and urine. After the injection of NTPP, α-NT was detected in bile for longer than it was detected in plasma or urine.
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