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Abstract
The in vivo metabolism of both tramadol-N-oxide (TNO) and tramadol was investigated in urine pools obtained from 0–24 h after a single 300 mg kg−1 oral dose administration of each compound to specific pathogen-free and axenic mice. Unchanged TNO (≤42% of the initial drug sample), tramadol, and 23 metabolites from TNO-treated mice and unchanged tramadol (≤15% of the sample) plus 20 metabolites from tramadol-treated mice were profiled, quantified and tentatively identified on the basis of atmospheric pressure ionization mass spectrometry (API-MS) and tandem mass spectrometry (MS/MS) data. Of the tramadol metabolites, five (M1–5) have been previously identified in mice. Of the tramadol and TNO metabolites, six (M18–23) are new metabolites. The tramadol and TNO metabolites were formed via the following seven metabolic pathways: N-oxide reduction (TNO), O/N-demethylation, cyclohexyloxidation, oxidative N-dealkylation, dehydration (TNO), N-oxidation (tramadol), and glucuronidation. Pathways 1–3 appear to be predominant steps forming four major O/N-desmethyl and hydroxycyclohexyl metabolites, and in conjunction with pathway 7, formed six minor glucuronides. Both tramadol-N-oxide and tramadol are extensively metabolized in mice, and no significant qualitative or quantitative differences in metabolism were observed between specific pathogen-free and axenic mice with the exception of a greater amount of unchanged TNO in axenic mice than in specific pathogen-free mice, more M2 in specific pathogen-free mice than in axenic mice in the TNO-dosed mice, and visa versa for M2 of tramadol-dosed mice.
Acknowledgements
The authors thank Dr K. L. Sorgi and Miss L. Scott for synthesizing the tramadol metabolites, and Mr T. L. Kerwin and Miss S. E. Watson for their technical assistance.