Mechanisms of Action

Norpace (disopyramide phosphate) is a Type 1 antiarrhythmic drug (i.e., similar to procainamide and quinidine). In animal studies Norpace decreases the rate of diastolic depolarization (phase 4) in cells with augmented automaticity, decreases the upstroke velocity (phase 0) and increases the action potential duration of normal cardiac cells, decreases the disparity in refractoriness between infarcted and adjacent normally perfused myocardium, and has no effect on alpha- or beta-adrenergic receptors.

Electrophysiology

In man, Norpace at therapeutic plasma levels shortens the sinus node recovery time, lengthens the effective refractory period of the atrium, and has a minimal effect on the effective refractory period of the AV node. Little effect has been shown on AV-nodal and His-Purkinje conduction times or QRS duration. However, prolongation of conduction in accessory pathways occurs.

Hemodynamics

At recommended oral doses, Norpace rarely produces significant alterations of blood pressure in patients without congestive heart failure (see Warnings). With intravenous Norpace, either increases in systolic/diastolic or decreases in systolic blood pressure have been reported, depending on the infusion rate and the patient population. Intravenous Norpace may cause cardiac depression with an approximate mean 10% reduction of cardiac output, which is more pronounced in patients with cardiac dysfunction.

Anticholinergic Activity

The in vitro anticholinergic activity of Norpace is approximately 0.06% that of atropine; however, the usual dose for Norpace is 150 mg every 6 hours and for Norpace CR 300 mg every 12 hours, compared to 0.4 to 0.6 mg for atropine (see Warnings and Adverse Reactions for anticholinergic side effects).

Pharmacokinetics

Following oral administration of immediate-release Norpace, disopyramide phosphate is rapidly and almost completely absorbed, and peak plasma levels are usually attained within 2 hours. The usual therapeutic plasma levels of disopyramide base are 2 to 4 mcg/ml, and at these concentrations protein binding varies from 50% to 65%. Because of concentration-dependent protein binding, it is difficult to predict the concentration of the free drug when total drug is measured.

The mean plasma half-life of disopyramide in healthy humans is 6.7 hours (range of 4 to 10 hours). In six patients with impaired renal function (creatinine clearance less than 40 ml/min), disopyramide half-life values were 8 to 18 hours.

After the oral administration of 200 mg of disopyramide to 10 cardiac patients with borderline to moderate heart failure, the time to peak serum concentration of 2.3 ± 1.5 hours (mean ± SD) was increased, and the mean peak serum concentration of 4.8 ± 1.6 mcg/ml was higher than in healthy volunteers. After intravenous administration in these same patients, the mean elimination half-life was 9.7 ± 4.2 hours (range in healthy volunteers of 4.4 to 7.8 hours). In a second study of the oral administration of disopyramide to 7 patients with heart disease, including left ventricular dysfunction, the mean plasma half-life was slightly prolonged to 7.8 ± 1.9 hours (range of 5 to 9.5 hours).

In healthy men, about 50% of a given dose of disopyramide is excreted in the urine as the unchanged drug, about 20% as the mono-N-dealkylated metabolite, and 10% as the other metabolites. The plasma concentration of the major metabolite is approximately one tenth that of disopyramide. Altering the urinary pH in man does not affect the plasma half-life of disopyramide.

In a crossover study in healthy subjects, the bio-availability of disopyramide from Norpace CR capsules was similar to that from the immediate-release capsules. With a single 300-mg oral dose, peak disopyramide plasma concentrations of 3.23 ± 0.75 mcg/ml (mean ± SD) at 2.5 ± 2.3 hours were obtained with two 150-mg immediate-release capsules and 2.22 ± 0.47 mcg/ml at 4.9 ± 1.4 hours with two 150-mg Norpace CR capsules. The elimination half-life of disopyramide was 8.31 ± 1.83 hours with the immediate-release capsules and 11.65 ± 4.72 hours with Norpace CR capsules. The amount of disopyramide and mono-N-dealkylated metabolite excreted in the urine in 48 hours was 128 and 48 mg, respectively, with the immediate-release capsules, and 112 and 33 mg, respectively, with Norpace CR capsules. The differences in the urinary excretion of either constituent were not statistically significant.

Following multiple doses, steady-state plasma levels of between 2 and 4 mcg/ml were attained following either 150 mg every-6-hour dosing with immediate-release capsules or 300 mg every-12-hour dosing with Norpace CR capsules.

Drug Interactions

Negative Inotropic Properties

General

Drug Interactions

If phenytoin or other hepatic enzyme inducers are taken concurrently with Norpace or Norpace CR, lower plasma levels of disopyramide may occur. Monitoring of disopyramide plasma levels is recommended in such concurrent use to avoid ineffective therapy. Other antiarrhythmic drugs (e.g., quinidine, procainamide, lidocaine, propranolol) have occasionally been used concurrently with Norpace. Excessive widening of the QRS complex and/or prolongation of the Q-T interval may occur in these situations (see Warnings). In healthy subjects, no significant drug-drug interaction was observed when Norpace was coadministered with either propranolol or diazepam. Concomitant administration of Norpace and quinidine resulted in slight increases in plasma disopyramide levels and slight decreases in plasma quinidine levels. Norpace does not increase serum digoxin levels.

Until data on possible interactions between verapamil and disopyramide phosphate are obtained, disopyramide should not be administered within 48 hours before or 24 hours after verapamil administration.

Although potent inhibitors of cytochrome P450 3A4 (e.g., ketoconazole) have not been studied clinically, in vitro studies have shown that erythromycin and oleandomycin inhibit the metabolism of disopyramide. Cases of life-threatening interactions have been reported for disopyramide when given with clarithromycin and erythromycin indicating that coadministration of disopyramide with inhibitors of cytochrome 3A4 could result in potentially fatal interaction.

Carcinogenesis, Mutagenesis, Impairment of Fertility

Eighteen months of Norpace administration to rats, at oral doses up to 400 mg/kg/day (about 30 times the usual daily human dose of 600 mg/day, assuming a patient weight of at least 50 kg), revealed no evidence of carcinogenic potential. An evaluation of mutagenic potential by Ames test was negative. Norpace, at doses up to 250 mg/kg/day, did not adversely affect fertility of rats.

Pregnancy

Labor and Delivery

It is not known whether the use of Norpace or Norpace CR during labor or delivery has immediate or delayed adverse effects on the fetus, or whether it prolongs the duration of labor or increases the need for forceps delivery or other obstetric intervention.

Nursing Mothers

Studies in rats have shown that the concentration of disopyramide and its metabolites is between one and three times greater in milk than it is in plasma. Following oral administration, disopyramide has been detected in human milk at a concentration not exceeding that in plasma. Because of the potential for serious adverse reactions in nursing infants from Norpace or Norpace CR, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

Pediatric Use

Safety and effectiveness in pediatric patients have not been established (see Dosage and Administration).

Geriatric Use

Clinical studies of Norpace/Norpace CR did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. Other reported clinical experience has not identified differences in responses between the elderly and younger patients. In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

Because of its anticholinergic activity, disopyramide phosphate should not be used in patients with glaucoma, urinary retention, or benign prostatic hypertrophy (medical conditions commonly associated with the elderly) unless adequate overriding measures are taken (see Warnings: Anticholinergic Activity). In the event of increased anticholinergic side effects, plasma levels of disopyramide should be monitored and the dose of the drug adjusted accordingly. A reduction of the dose by one third, from the recommended 600 mg/day to 400 mg/day, would be reasonable, without changing the dosing interval.

This drug is known to be substantially excreted by the kidney, and the risk of toxic reactions to this drug may be greater in patients with impaired renal function. Because elderly patients are more likely to have decreased renal function, care should be taken in dose selection, and it may be useful to monitor renal function (see Precautions: Renal Impairment and Dosage and Administration).

Symptoms

Deliberate or accidental overdosage of oral disopyramide may be followed by apnea, loss of consciousness, cardiac arrhythmias, and loss of spontaneous respiration. Death has occurred following overdosage.

Toxic plasma levels of disopyramide produce excessive widening of the QRS complex and Q-T interval, worsening of congestive heart failure, hypotension, varying kinds and degrees of conduction disturbance, bradycardia, and finally asystole. Obvious anticholinergic effects are also observed.

The approximate oral LD50 of disopyramide phosphate is 580 and 700 mg/kg for rats and mice, respectively.

Treatment

Experience indicates that prompt and vigorous treatment of overdosage is necessary, even in the absence of symptoms. Such treatment may be life-saving. No specific antidote for disopyramide phosphate has been identified. Treatment should be symptomatic and may include induction of emesis or gastric lavage, administration of a cathartic followed by activated charcoal by mouth or stomach tube, intravenous administration of isoproterenol and dopamine, insertion of an intra-aortic balloon for counterpulsation, and mechanically assisted ventilation. Hemodialysis or, preferably, hemoperfusion with charcoal may be employed to lower serum concentration of the drug.

The electrocardiogram should be monitored, and supportive therapy with cardiac glycosides and diuretics should be given as required.

If progressive AV block should develop, endocardial pacing should be implemented. In case of any impaired renal function, measures to increase the glomerular filtration rate may reduce the toxicity (disopyramide is excreted primarily by the kidney).

The anticholinergic effects can be reversed with neostigmine at the discretion of the physician.

Altering the urinary pH in humans does not affect the plasma half-life or the amount of disopyramide excreted in the urine.

Transferring to Norpace or Norpace CR

The following dosage schedule based on theoretical considerations rather than experimental data is suggested for transferring patients with normal renal function from either quinidine sulfate or procainamide therapy (Type 1 antiarrhythmic agents) to Norpace or Norpace CR therapy:

Norpace or Norpace CR should be started using the regular maintenance schedule without a loading dose 6 to 12 hours after the last dose of quinidine sulfate or 3 to 6 hours after the last dose of procainamide.

In patients in whom withdrawal of quinidine sulfate or procainamide is likely to produce life-threatening arrhythmias, the physician should consider hospitalization of the patient. When transferring a patient from immediate-release Norpace to Norpace CR, the maintenance schedule of Norpace CR may be started 6 hours after the last dose of immediate-release Norpace.

Pediatric Dosage

Controlled clinical studies have not been conducted in pediatric patients; however, the following suggested dosage table is based on published clinical experience.

Total daily dosage should be divided and equal doses administered orally every 6 hours or at intervals according to individual patient needs. Disopyramide plasma levels and therapeutic response must be monitored closely. Patients should be hospitalized during the initial treatment period, and dose titration should start at the lower end of the ranges provided below.

Norpace CR capsules should not be used to prepare the above suspension.

Store at 25°C (77°F); excursions permitted to 15–30°C (59–86°F). [See USP Controlled Room Temperature.]