Biotransformation of 17-alkylsteroids in the equine: gas chromatographic-mass spectral identification of ten intermediate metabolites of methyltestosterone.

The metabolism of the orally active anabolic steroid methyltestosterone in the equine was investigated by administration of the drug along with a tritiated radiolabel tracer. In this study some of the metabolites were identified and a radio immunoassay screen and immunoaffinity chromatography gel for methyltestosterone were also evaluated. Pathway intermediates, in particular the 17-methylandrostanediols, were studied to gain an insight into the most likely stereochemistry of the major metabolites. The predominant phase I biotransformations involve reduction of the A ring 3-oxo and 4-ene groups to yield predominantly 3 beta-hydroxy-5 alpha-androstane products and hydroxylation of the steroid nucleus at several positions. Epimerisation of the 17 alpha-methyl group also occurred. Ten steroids could be positively identified by comparison with authentic reference materials and many other triol, tetrol and pentols were also observed. Phase II metabolites and sulphate conjugates in particular, were common.