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Synthesis, purification, and chemical characterization of 20-dihydro-6-methylprednisone, an isomeric metabolite of methylprednisolone in the horse, for use as an analytical standard.

Methylprednisolone acetate, pregna-1,4-diene-3,20-dione, 21(acetoxy)-11,17-dihydro-6-methyl-,(6α,11β)-, is a synthetic glucocorticoid pro-drug widely used in equine medicine. It is recognized as a therapeutic medication by many equine organizations, including the American Association of Equine Practitioners (AAEP), the Racing Medication and Testing Consortium (RMTC), the Association of Racing Commissioners International (ARCI), and numerous racing and other regulatory authorities in the United States and elsewhere. As such, methylprednisolone is widely used on horses in training, usually as the long-acting Depo-medrol formulation administered by intra-articular or intra-muscular injection. With respect to racing and other equine performance events, its use as a controlled therapeutic medication on horses in training is regulated by the testing of plasma and/or urine, but usually by application of a quantitative regulatory threshold in plasma. For example, the RMTC lists methylprednisolone as a ‘controlled therapeutic medication’ and sets forth that a 100pg/mL methylprednisolone threshold in plasma or serum corresponds to a seven-day ‘restricted administration time’ (withdrawal time) following intra-articular administration of a ‘100mg total in one intra-articular space’. Approximately 15 metabolites of methylprednisolone have been reported in mammalian systems, though several of them have not been fully characterized due to a lack of authentic reference standards. Of these metabolites, at least three isobaric compounds have been identified in rat urine and four in human urine. It was observed, after reviewing our liquid chromatography-tandemmass spectrometry (LC-MS/MS) drug screening data and the relevant scientific literature, that standard equine analytical procedures commonly used to screen for or analyze methylprednisolone may occasionally have difficulty distinguishing between parent methylprednisolone and isobaric equine metabolites. One isomeric compound which was found in plasma extracts in conjunction with methylprednisolone (Figure 1), after examining mass spectral data, was thought to be 20-dihydro-6-methylprednisone (DHMP), an isomeric metabolite of methylprednisolone previously reported though not unequivocally identified in equine urine. This metabolite has a close structural relationship to methylprednisolone (Figure 2), and has, presuming a similar chromatographic retention time, the potential to interfere with the forensic identification and quantitation of methylprednisolone in equine racing samples. Given these possibilities, an authentic research quality reference standard of DHMP is required to unequivocally determine the