ISOPTIN (verapamil HCl) 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

Essential Hypertension

ISOPTIN 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 ISOPTIN does not alter systolic cardiac function in patients with normal ventricular function. ISOPTIN does not alter total serum calcium levels. However, one report suggested that calcium levels above the normal range may alter the therapeutic effect of ISOPTIN.

Other Pharmacological Actions of ISOPTIN Include the Following

ISOPTIN (verapamil HCI) 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 ISOPTIN 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.

ISOPTIN regularly reduces the total systemic resistance (afterload) against which the heart works both at rest and at a given level of exercise by dilating peripheral arterioles.

Electrical activity through the AV node depends, to a significant degree, upon calcium influx through the slow channel. By decreasing the influx of calcium, ISOPTIN prolongs the effective refractory period within the AV node and slows AV conduction in a rate related manner.

Normal sinus rhythm is usually not affected, but in patients with sick sinus syndrome, ISOPTIN 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).

ISOPTIN does not alter the normal atrial action potential or intraventricular conduction time, but depresses amplitude, velocity of depolarization and conduction in depressed atrial fibers. ISOPTIN may shorten the antegrade effective refractory period of accessory bypass tracts. 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).

ISOPTIN 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.

Pharmacokinetics and Metabolism: With the immediate release formulation, more than 90% of the orally administered dose of ISOPTIN 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 ISOPTIN 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.

In early dose titration with verapamil a relationship exists between verapamil plasma concentrations and the 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. No relationship has been established between the plasma concentration of verapamil and a reduction in blood pressure.

Aging may affect the pharmacokinetics of verapamil. Elimination half-life may be prolonged in the elderly.

In multiple dose studies under fasting conditions the bioavailability measured by AUC of ISOPTIN SR was similar to ISOPTIN immediate release; rates of absorption were, of course, different. In a randomized, single-dose, crossover study using healthy volunteers, administration of 240 mg ISOPTIN SR with food produced peak plasma verapamil concentrations of 79 ng/mL, time to peak plasma verapamil concentration of 7.71 hours, and AUC (0-24 hr) of 841 ng-hr/mL. When ISOPTIN SR was administered to fasting subjects, peak plasma verapamil concentration was 164 ng/mL; time to peak plasma verapamil concentration was 5.21 hours; and AUC (0-24 hr) was 1,478 ng-hr/mL. Similar results were demonstrated for plasma norverapamil. Food thus produces decreased bioavailability (AUC) but a narrower peak to trough ratio. Good correlation of dose and response is not available, but controlled studies of ISOPTIN SR have shown effectiveness of doses similar to the effective doses of ISOPTIN (immediate release).

In healthy man, orally administered ISOPTIN 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 steadystate 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 of immediate release verapamil 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. 

In ten healthy males, administration of oral verapamil (80 mg every 8 hours for 6 days) and a single oral dose of ethanol (0.8 g/kg) resulted in a 17% increase in mean peak ethanol concentrations (106.45 ± 21.40 to 124.23 ± 24.74 mg•hr/dL) compared to placebo. The area under the blood ethanol concentration versus time curve (AUC over 12 hours) increased by 30% (365.67 ± 93.52 to 475.07 ± 97.24 mg•hr/dL). Verapamil AUCs were positively correlated (r=0.71) to increased ethanol blood AUC values. (See PRECAUTIONS: Drug Interactions.)

Hemodynamics and Myocardial Metabolism: ISOPTIN reduces afterload and myocardial contractility. Improved left ventricular diastolic function in patients with IHSS and those with coronary heart disease has also been observed with ISOPTIN therapy. In most patients, including those with organic cardiac disease, the negative inotropic action of ISOPTIN is countered by reduction of afterload and cardiac index is usually not reduced. However, in patients with severe left ventricular dysfunction (e.g., pulmonary wedge pressure above 20 mmHg 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 DRUG INTERACTIONS).

Pulmonary Function: ISOPTIN does not induce bronchoconstriction and hence, does not impair ventilatory function.