Studies on the Metabolism of Diltiazem in Man

The human urinary metabolites of diltiazem were analyzed by thin-layer chromatography (TLC) and gas chromatography-mass spectrometry. Diltiazem was metabolized by deacetylation, N-demethylation, O-demethylation and conjugation. Metabolite MA, N- monodemethyl -diltiazem, was identified as a new major metabolite in human urine, and four metabolites were identified as deacetyl-diltiazem (M1), deacetyl-N- monodemethyl -diltiazem (M2), deacetyl-O-demethyl-diltiazem (M4), deacetyl-N,O-demethyl-diltiazem (M6) which were known as rat urinary metabolites. Metabolite M2, M4 and M6 were converted in part to glucuronides and/or sulfates. Unchanged diltiazem and metabolite MA were determined in human plasma and urine by TLC-densitometry. Diltiazem and metabolite MA excreted in 24-h urine were 44.4 and 48.5% of the total unconjugated form, respectively. The mean plasma level of metabolite MA was approximately one-third of diltiazem level. On the basis of these findings, a probable metabolic pathway of diltiazem in man is presented.