Metabolism of norethisterone in the greyhound

Norethisterone has been used as a successful oral contraceptive in humans for many years. It was recently permitted for use as an oestrus suppressant in racing greyhounds. To monitor the use of norethisterone as part of a routine drug surveillance programme, knowledge of its metabolism was required to enable detection.

Gas chromatography/mass spectrometry and selective derivatisation techniques have been used to identify urinary metabolites of norethisterone following oral administration to the greyhound. Metabolites were extracted using solid-phase and liquid-liquid extraction techniques.

Several metabolites were identified, including reduced, mono-, di- and trihydroxylated steroids. The major metabolites observed were 17α-ethynyl-5β-estrane-3α,17β-diol, 17α-ethynyl-5α-estrane-3β,17β-diol, three 17α-ethynylestranetriol stereoisomers and two 17α-ethynylestranetetrol stereoisomers. The major metabolites were predominantly excreted as glucuronic acid conjugates and detection of the administration of norethisterone was possible for up to 8 days post-dose using the methods described. The nandrolone metabolites, 19-norepiandrosterone, estranediol and 19-noretiocholanolone, were also identified in the post-administration samples collected up to 8 h after dosing the treated animals.

The urinary metabolites identified in this study have further increased the knowledge of steroid metabolism in the greyhound, providing information to support routine drug testing programmes for greyhound racing.