Diltiazem Treatment Impairs Hepatic Drug Oxidation: Studies of Antipyrine

To evaluate the effect of diltiazem on antipyrine disposition and metabolism, 10 healthy subjects received 1.2 gm antipyrine on two occasions, once while taking no other medications and once during long-term oral diltiazem, 120 mg three times daily. Antipyrine oral clearance was markedly reduced from (mean +/- SEM) 41.7 +/- 4.1 to 29.9 +/- 2.8 ml/min (P less than 0.01) during diltiazem treatment, resulting in prolongation of antipyrine elimination t1/2 from 12.2 +/- 1.0 to 16.7 +/- 1.3 hours (P less than 0.01), with no change in apparent volume of distribution (42.1 +/- 4.0 vs. 41.3 +/- 3.1 L; not significant). Measurement of urinary antipyrine and metabolites excreted in the urine during 24 hours after the antipyrine dose (percent of total 24-hour excretion) showed increased antipyrine (4.4% +/- 1.0% vs. 7.8% +/- 1.6%; P less than 0.01) during diltiazem treatment with no significant change in proportion of 4-hydroxyantipyrine, 3-hydroxymethylantipyrine, and norantipyrine excretion between trials. Chronic oral diltiazem in therapeutic doses markedly impairs antipyrine oxidation. Diltiazem may therefore impair the clearance of other coadministered drugs that undergo hepatic oxidation.