Methadone metabolism in the rat in vivo: identification of a novel formamide metabolite.

Deuterium-labelled methadone and metabolites were used for the g.l.c.-mass spectrometry detection and identification of biliary conjugated methadone metabolites in rats. After beta-glucuronidase hydrolysis the bile extract contained an unknown metabolite that was not ring hydroxylated and retained an intact keto group. Chemical oxidation of the methadone metabolite 2-ethylidene-N,5-dimethyl-3,3-diphenylpyrrolidine, perchlorate salt (EDDP) with m-chloroperbenzoic acid in chloroform, gave a compound identical by g.l.c.-mass spectrometry to the new metabolite. The chemical oxidation product was identified as 2-(4',4'-diphenylheptan-5'-one-2'-yl)oxaziridine by spectroscopic methods. The oxaziridine was shown to quantitatively isomerize to a secondary formamide (2-formamido-4,4-diphenyl-5-heptanone) during g.l.c.-mass spectrometry analysis. The formamide was also isolated by flash column chromatography after reflux of the oxaziridine in m-xylene, and then characterized by spectroscopy. The formamide and oxaziridine g.l.c.-mass spectrometry characteristics were identical. It was concluded on the basis of g.l.c.-mass spectrometry that the metabolite is the secondary formamide.