Inhibitory effects of verapamil and diltiazem on simvastatin metabolism in human liver microsomes.

AIMS: To determine the effects of verapamil and diltiazem on simvastatin metabolism in human liver microsomes and to compare their inhibitory potencies and CYP3A4 inactivation parameters with those reported previously for mibefradil. METHODS: Simvastatin metabolism was investigated in human liver microsomes in the presence and absence of verapamil or diltiazem (0.1-250 microM). Kinetics of CYP3A4 inactivation by verapamil and diltiazem were determined using testosterone as the substrate. RESULTS: When verapamil was coincubated with simvastatin, IC50 values ranged from 23 to 26 microM for all major metabolites. The IC50 values ranged from 4.8 to 5.6 microM on preincubation of verapamil for 30 min in the presence of an NADPH-generating system. Corresponding IC50 values for diltiazem ranged from 110-127 microM and from 21-27 microM, respectively. Verapamil and diltiazem inhibited testosterone 6beta-hydroxylation in a time- and concentration-dependent manner, key features of mechanism-based inactivation. Values for the inactivation parameters kinact and KI were 0.15 +/- 0.04 min-1 (mean +/- s.d.) and 2.9 +/- 0.6 microM, respectively, for verapamil and 0.07 +/- 0.01 min-1 and 3.3 +/- 1.5 microM, respectively, for diltiazem. CONCLUSIONS: The IC50 values for coincubation of verapamil and diltiazem were 46- and 220-fold higher, respectively, than those reported previously for mibefradil, and 16- and 71-fold higher, respectively, for preincubation. Thus, the results of this study suggest that verapamil and diltiazem are less likely than mibefradil to cause acute drug interactions with simvastatin in vivo. However, verapamil and diltiazem are moderate mechanism-based inhibitors of CYP3A4 and therefore may still cause significant inhibition of simvastatin metabolism in vivo during chronic therapy.

Enhanced cholesterol reduction by simvastatin in diltiazem-treated patients.

AIMS: To investigate whether an interaction between diltiazem and the 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor simvastatin may enhance the cholesterol-lowering response to simvastatin in diltiazem-treated patients. METHODS: One hundred and thirty-five patients attending the Sheffield hypertension clinic who started consecutively on simvastatin for primary or secondary prevention of coronary heart disease (CHD) during the 2 years June, 1996-May 1998 were surveyed. From the clinic records we extracted and recorded absolute and percentage cholesterol responses to the starting dose of simvastatin and coprescription of diltiazem. RESULTS: The cholesterol reduction for the 19 patients on diltiazem was 33.3% compared with 24.7% in the remaining 116 patients (median difference 8.6%, 95% CI 1.1-12.2%, P<0.02). The interindividual variability of cholesterol response to simvastatin was greater for patients not taking diltiazem than for those patients taking diltiazem. The ratio of the variances in response for the nondiltiazem group relative to the diltiazem group was 1.34 at 10 mg simvastatin daily (not significant, 95% CI 0.16-4.11), and 3.42 at 20 mg daily (P<0.01, 95% CI 1.26-7.18). Concurrent diltiazem therapy (P<0.04), age (P=0.001) and starting dose of simvastatin (P=0.002) were found to be significant independent predictors of percentage cholesterol response. CONCLUSIONS: Patients who take both simvastatin and diltiazem may need lower doses of simvastatin to achieve the recommended reduction in cholesterol. The pharmacokinetic and pharmacodynamic aspects of this interaction need further study to confirm an enhanced effect on cholesterol reduction, and exclude an increased risk of adverse events.