Meperidine metabolites: identification of N-hydroxynormeperidine and a hydroxy-methoxy derivative of meperidine in biological fluids.

Gas chromatographic and gas chromatographic-mass spectrometric techniques were used to identify non-acidic metabolites of meperidine (N-methyl-4-phenyl-4-carbethoxypiperidine) excreted in human, rat, and guinea pig urine. Following enzymic hydrolysis N-hydroxynormeperidine was identified in the urine of all three species in addition to the expected metabolites normeperidine and meperidine N-oxide. In rat urine the p-hydroxyphenyl metabolite of meperidine was present in appreciable amounts. Also present in small quantity was a new phenolic metabolite of meperidine containing both hydroxyl and O-methoxyl substituents in the phenyl ring of the parent drug. The latter two metabolites were excreted as conjugates in the rat.