Comparison of the effects of injections of nortestosterone phenylpropionate at single and multiple sites in cattle on the detection of its residues in plasma, urine and bile

The synthetic androgen 17 beta-19-nortestosterone (beta-NT) has been used i llegally as a growth promoter in cattle production in the European Union. T he elimination of beta-NT and its metabolites in plasma and urine was studi ed in cattle which had received intramuscular injections of its phenylpropi onate ester (NTPP) at either single or multiple sites at a dose rate of 1 m g/kg bodyweight. In both groups, the plasma concentrations of beta-NT, meas ured by enzyme immunoassay, were consistently greater than the assay's limi t of quantification (0.24 ng/ml) during days 1 to 7 of the study. The mean (sd) maximum plasma concentration (C-max) was significantly greater in the multiply injected animals (4.4 [0.48] v 2.7 [0.15] ng/ml), but other plasma pharmacokinetic parameters, AUC, CL, T1/2 beta, T-max and MRT, were not si gnificantly different in the two groups. The equivalent urinary concentrati ons exceeded 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 beta-NT and its 17 alpha-epimer (alpha-NT) were measured by gas chromatogra phy-high resolution mass spectrometry after cattle were injected intramuscu larly at either single or multiple sites with NTPP. Only cr-NT was detected in bile for up to 62 days after injection at concentrations above the limi t 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 beta-NT and its metabolites are d etectable in plasma and urine. After the injection of NTPP, alpha-NT was de tected in bile for longer than it was detected in plasma or urine.