The pharmacokinetics and pharmacodynamics of diltiazem and its metabolites in healthy adults after a single oral dose.

A potential complicating factor in the characterization of the pharmacokinetics and pharmacodynamics of diltiazem after an oral dose in the presence of two metabolites, N-demethyldiltiazem and desacetyldiltiazem, in plasma. Both N-demethyldiltiazem and desacetylditiazem have been shown to have pharmacologic activity in animal tissues. It is therefore possible that these metabolites contribute to the pharmacologic effect of diltiazem, but this possibility has not been explored. The purpose of this study was to investigate the pharmacokinetics and pharmacodynamics of diltiazem, N-demethyldiltiazem, and desacetyldiltiazem. Particular attention was paid to the effect of diltiazem on atrioventricular conduction. Six healthy men received a 120 mg oral dose of diltiazem. Concentrations of diltiazem, N-demethyldiltiazem, and desacetyldiltiazem in plasma and urine were measured by a sensitive HPLC method. Measures of pharmacologic response (heart rate, blood pressure, and PR interval) were obtained at each blood sampling time. Mean (+/- SD) peak plasma concentrations of diltiazem, N-demethyldiltiazem, and desacetyldiltiazem were 174.3 +/- 72.7, 42.6 +/- 10.0, and 14.9 +/- 3.3 ng/ml, respectively. The apparent half-lives of diltiazem, N-demethyldiltiazem, and desacetyldiltiazem were 6.5 +/- 1.4 hours, 9.4 +/- 2.2 hours, and 18 +/- 6.2 hours, respectively. Both N-demethyldiltiazem and diltiazem were eliminated by net secretion, whereas the renal clearance of desacetyldiltiazem did not exceed clearance by filtration. Both N-demethyldiltiazem and desacetyldiltiazem are bound to plasma proteins, with unbound fractions of 0.323 +/- 0.035 and 0.230 +/- 0.021. These values are similar to the unbound fraction of diltiazem (0.254 +/- 0.027). No significant effect of diltiazem on blood pressure or heart rate was noted. However, a prolongation of the PR interval was observed in all six subjects. Furthermore, an apparent clockwise hysteresis in the concentration-effect relationship was found in four of the six subjects. These findings suggest that some form of acute tolerance to the electrophysiologic effect of diltiazem develops, but the results of pharmacodynamic modeling suggest that this is not caused by the antagonistic effects the metabolites.

The pharmacokinetics of the enantiomers of atenolol.

A number of studies have demonstrated that lipophilic beta-adrenoceptor blocking agents, eliminated almost exclusively by hepatic metabolism, are stereoselectively metabolized in human beings. Previous studies in our laboratory have demonstrated that pindolol, a beta-adrenoceptor blocking agent of intermediate lipophilicity that is eliminated by both hepatic metabolism and renal excretion, is eliminated stereoselectively in the kidney. In the present study we examined the pharmacokinetics of the enantiomers of atenolol, a hydrophilic cardioselective beta-adrenoceptor blocking agent that is eliminated almost exclusively by the kidney. A single 100 mg oral dose of racemic atenolol was administered to six healthy adult men. Concentrations of d- and l-atenolol in plasma and urine were measured by a stereospecific HPLC analytic procedure. In each subject the peak concentration of d-atenolol was greater than the peak concentration of l-atenolol (mean +/- SD of 420 +/- 81 ng/ml vs 366 +/- 61 ng/ml; p less than 0.05). The peak concentration of both enantiomers was reached at the same time in each subject (between 2 and 3 hours). The renal clearances of d- and l-atenolol were not significantly different (109.7 +/- 33.5 ml/min vs 112.5 +/- 36.7 ml/min), probably because the major route of renal elimination is glomerular filtration. The half-lives of d- and l-atenolol were not significantly different (mean +/- SD of 4.6 +/- 1.1 hours vs 5.2 +/- 0.9 hours). However, both the AUC and the amount excreted unchanged in the urine in 24 hours Ae [0-24]) were significantly greater for d-atenolol than for l-atenolol (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)