Biotransformation of moclobemide in humans

The structure of the urinary metabolites formed after moclobemide administration in human was elucidated, and the pattern compared with that in the plasma. The metabolic pathways of moclobemide were also compared with those of structurally related substances. After oral moclobemide administration, on average 95% of the dose was recovered in the urine within 4 days, with a mean of 92% being excreted during the first 12 h. The drug is extensively metabolized: less than 1% of the dose was excreted unchanged. A total of 19 metabolites, accounting together for about 64% of the dose, was isolated and all metabolites accounting for more than 1% of the dose were identified. Consistent with other morpholine-containing compounds, metabolic pathways of moclobemide include mainly oxidative attack on the morpholine moiety, leading to a multitude of oxidation products. Four primary metabolic reactions were identified: morpholine N-oxidation, aromatic hydroxylation, morpholine C-oxidation and deamination. The major metabolites in urine are 4 carboxylic acids (M7A and M7B, M8, M9) that account for 49% of the dose. Only 2 metabolites (M3, M10) were found to be hydroxylated on the aromatic nucleus. They were excreted completely as conjugates of glucuronic and/or sulfuric acid. Conjugation in general, however, seems to be of minor importance in the overall biotransformation of the drug. The metabolite pattern in plasma was found to be qualitatively but not quantitatively similar to that observed in urine. Almost all of the main urinary metabolites were found in plasma as well. The unchanged parent compound and 2 primary oxidation products of the morpholine ring (M1, M15), which were present in urine only in trace amounts, could easily be detected in plasma.