Clinical pharmacokinetics of verapamil

Verapamil is widely used in the treatment of supraventricular tachyarrhythmias as well as for hypertension and control of symptoms in angina pectoris. Unlike other calcium antagonists, detailed pharmacokinetic data are available for verapamil. Plasma concentrations of verapamil appear to correlate with both electrophysiological and haemodynamic activity after either intravenous or oral drug administration, although considerable intra- and intersubject variation has been found in the intensity of pharmacological effects resulting at specific plasma drug levels. Verapamil is widely distributed throughout body tissues; animal studies suggest that drug distribution to target organs and tissues is different with parenteral administration from that found after oral administration. The drug is eliminated by hepatic metabolism, with excretion of inactive products in the urine and/or faeces. An N-demethylated metabolite, norverapamil, has been shown to have a fraction of the vasodilator effect of the parent compound in in vitro studies. After intravenous administration, the systemic clearance of verapamil appears to approach liver blood flow. The high hepatic extraction results in low systemic bioavailability (20%) after oral drug administration. Multicompartmental kinetics are observed after single doses; accumulation occurs during multiple-dose oral administration with an associated decrease in apparent oral clearance. Norverapamil plasma concentrations approximate those of verapamil following single or multiple oral doses of the parent drug. Because of the complex pharmacokinetics associated with multiple-dose administration and the variation in individual patient responsiveness to the drug, 'standard' dosing recommendations are difficult to determine; use of verapamil must be titrated to a clinical end-point. Further, the potential for alteration in verapamil's disposition by the presence of hepatic dysfunction or cardiovascular disorders which result in altered hepatic blood flow is only now becoming apparent. A potentially toxic interaction has been reported between verapamil and digoxin, in which renal excretion of the glycoside is impaired, but the true clinical significance of this remains debatable. Combination therapy with verapamil and beta-adrenoceptor blocking compounds has been advocated by some investigators, but may be hazardous because of the additive negative inotropic and chronotropic effects inherent in both agents.