The identification of urinary metabolites of doxepin in patients

The metabolism of doxepin was investigated in three patients who provided cumulative urine samples on each of 3 successive days. These samples were examined by means of stereoselective HPLC or HPLC combined with mass spectrometry through a plasmaspray interface (LCMS). In addition, sufficient quantities of the major metabolites were isolated from the urine by column chromatography. The isolated metabolites were examined by HPLC, proton nuclear magnetic resonance spectroscopy (1H-NMR), and chemical ionization and/or electron impact mass spectrometry (EIMS). The resultant spectra and chromatographic properties were compared with authentic reference standards. The metabolites were identified as (E)-2-hydroxydoxepin, (E)-2-hydroxy-N-desmethyldoxepin, (Z)- and (E)-N-desmethyldoxepin, and (Z)- and (E)-doxepin N-oxide. There was no evidence of hydroxylation at the oxymethylene bridge. A further metabolite previously unreported was tentatively identified by LCMS and EIMS as an aromatic hydroxy-N-desmethyldoxepin hydrated at the exocyclic double bond. This metabolite was present in very low amounts, precluding its analysis by 1H-NMR. Moreover, this type of compound dehydrated readily in vitro, and numerous attempts to synthesize reference materials were unsuccessful. The tentative identification of this hydrated metabolite lends significant support to a possible mechanism responsible for the enrichment of cis-N-desmethyldoxepin over time in plasma.