Residues and metabolism of 19-nortestosterone laurate in steers†

Abstract

The illegal use of 19-nortestosterone (19NT; 4-estren-17β-ol-3-one; nandrolone) and its esters in livestock, for growth promotion purposes, has been widely reported in the European Union. The target residues for surveillance of abuse in bovine urine and bile samples are 17α- and 17β-19NT, although this choice of target residues is not based on in vivo radiotracer biotransformation data. In this study, four steers were administered [³H₂]- and [²H₃] 17β-19NT laurate (2 mg kg^–1 body mass) by intramuscular injection and blood, urine, faeces and bile samples were taken for 30 d until slaughter, after which tissues were sampled for total residue analysis. Total plasma radiolabelled residues reached a maximum of 56.3 ± 15.9 pmol ml^–1 at 36 h and were still appreciable (13.3 ± 1.6 pmol ml^–1) 30 d after treatment. Throughout the study period, total residue concentrations in bile (about 2–16 nmol ml^–1), urine and faeces (0.5–3 nmol ml^–1 or g^–1) were higher than in other tissues sampled at slaughter. At slaughter there was evidence of residue accumulation in pigmented eye tissue (33.1 ± 6.1 pmol g^–1) and in white (13.4 ± 3.4 pmol g^–1) and black hair (28.9 ± 8.9 pmol g^–1). Evaluation of radio-HPLC profiles of urine and bile extracts generally indicated that 19NT and 19NT laurate residues were present in relatively small amounts among a complex mixture of metabolites. GC-MS analysis of glucuronidase-hydrolysed bile extracts indicated that the major metabolites were 5β-estrane-3α,17α-diol, 5α-estrane-3β,17α-diol, 5α-estran-3α-ol-17-one (norandrosterone) and estra-1,3,5(10)-triene-3,17α-diol (17α-estradiol).