CALAN is a calcium ion influx inhibitor (slow-channel blocker or calcium ion antagonist) that exerts its pharmacologic effects by modulating the influx of ionic calcium across the cell membrane of the arterial smooth muscle as well as in conductile and contractile myocardial cells.
Mechanism of action
The precise mechanism of action of CALAN as an antianginal agent remains to be fully determined, but includes the following two mechanisms:
- Relaxation and prevention of coronary artery spasm: CALAN dilates the main coronary arteries and coronary arterioles, both in normal and ischemic regions, and is a potent inhibitor of coronary artery spasm, whether spontaneous or ergonovine-induced. This property increases myocardial oxygen delivery in patients with coronary artery spasm and is responsible for the effectiveness of CALAN in vasospastic (Prinzmetal's or variant) as well as unstable angina at rest. Whether this effect plays any role in classical effort angina is not clear, but studies of exercise tolerance have not shown an increase in the maximum exercise rate–pressure product, a widely accepted measure of oxygen utilization. This suggests that, in general, relief of spasm or dilation of coronary arteries is not an important factor in classical angina.
- Reduction of oxygen utilization: CALAN regularly reduces the total peripheral resistance (afterload) against which the heart works both at rest and at a given level of exercise by dilating peripheral arterioles. This unloading of the heart reduces myocardial energy consumption and oxygen requirements and probably accounts for the effectiveness of CALAN in chronic stable effort angina.
Electrical activity through the AV node depends, to a significant degree, upon calcium influx through the slow channel. By decreasing the influx of calcium, CALAN prolongs the effective refractory period within the AV node and slows AV conduction in a rate-related manner. This property accounts for the ability of CALAN to slow the ventricular rate in patients with chronic atrial flutter or atrial fibrillation.
Normal sinus rhythm is usually not affected, but in patients with sick sinus syndrome, CALAN may interfere with sinus-node impulse generation and may induce sinus arrest or sinoatrial block. Atrioventricular block can occur in patients without preexisting conduction defects (see WARNINGS). CALAN decreases the frequency of episodes of paroxysmal supraventricular tachycardia.
CALAN does not alter the normal atrial action potential or intraventricular conduction time, but in depressed atrial fibers it decreases amplitude, velocity of depolarization, and conduction velocity. CALAN may shorten the antegrade effective refractory period of the accessory bypass tract. Acceleration of ventricular rate and/or ventricular fibrillation has been reported in patients with atrial flutter or atrial fibrillation and a coexisting accessory AV pathway following administration of verapamil (see WARNINGS).
CALAN has a local anesthetic action that is 1.6 times that of procaine on an equimolar basis. It is not known whether this action is important at the doses used in man.
CALAN exerts antihypertensive effects by decreasing systemic vascular resistance, usually without orthostatic decreases in blood pressure or reflex tachycardia; bradycardia (rate less than 50 beats/min) is uncommon (1.4%). During isometric or dynamic exercise, CALAN does not alter systolic cardiac function in patients with normal ventricular function.
CALAN does not alter total serum calcium levels. However, one report suggested that calcium levels above the normal range may alter the therapeutic effect of CALAN.
Pharmacokinetics and metabolism
More than 90% of the orally administered dose of CALAN is absorbed. Because of rapid biotransformation of verapamil during its first pass through the portal circulation, bioavailability ranges from 20% to 35%. Peak plasma concentrations are reached between 1 and 2 hours after oral administration. Chronic oral administration of 120 mg of verapamil HCl every 6 hours resulted in plasma levels of verapamil ranging from 125 to 400 ng/ml, with higher values reported occasionally. A nonlinear correlation between the verapamil dose administered and verapamil plasma levels does exist. No relationship has been established between the plasma concentration of verapamil and a reduction in blood pressure. In early dose titration with verapamil, a relationship exists between verapamil plasma concentration and prolongation of the PR interval. However, during chronic administration this relationship may disappear. The mean elimination half-life in single-dose studies ranged from 2.8 to 7.4 hours. In these same studies, after repetitive dosing, the half-life increased to a range from 4.5 to 12.0 hours (after less than 10 consecutive doses given 6 hours apart). Half-life of verapamil may increase during titration. Aging may affect the pharmacokinetics of verapamil. Elimination half-life may be prolonged in the elderly. In healthy men, orally administered CALAN undergoes extensive metabolism in the liver. Twelve metabolites have been identified in plasma; all except norverapamil are present in trace amounts only.
Norverapamil can reach steady-state plasma concentrations approximately equal to those of verapamil itself. The cardiovascular activity of norverapamil appears to be approximately 20% that of verapamil. Approximately 70% of an administered dose is excreted as metabolites in the urine and 16% or more in the feces within 5 days. About 3% to 4% is excreted in the urine as unchanged drug. Approximately 90% is bound to plasma proteins. In patients with hepatic insufficiency, metabolism is delayed and elimination half-life prolonged up to 14 to 16 hours (see PRECAUTIONS); the volume of distribution is increased and plasma clearance reduced to about 30% of normal. Verapamil clearance values suggest that patients with liver dysfunction may attain therapeutic verapamil plasma concentrations with one third of the oral daily dose required for patients with normal liver function.
After four weeks of oral dosing (120 mg q.i.d.), verapamil and norverapamil levels were noted in the cerebrospinal fluid with estimated partition coefficient of 0.06 for verapamil and 0.04 for norverapamil.
Hemodynamics and myocardial metabolism
CALAN reduces afterload and myocardial contractility. Improved left ventricular diastolic function in patients with Idiopathic Hypertrophic Subaortic Stenosis (IHSS) and those with coronary heart disease has also been observed with CALAN therapy. In most patients, including those with organic cardiac disease, the negative inotropic action of CALAN is countered by reduction of afterload, and cardiac index is usually not reduced. However, in patients with severe left ventricular dysfunction (eg, pulmonary wedge pressure above 20 mm Hg or ejection fraction less than 30%), or in patients taking beta-adrenergic blocking agents or other cardiodepressant drugs, deterioration of ventricular function may occur (see PRECAUTIONS, Drug interactions).