Tambocor

Drug Interactions.   TAMBOCOR has been administered to patients receiving digitalis preparations or beta-adrenergic blocking agents without adverse effects. During administration of multiple oral doses of TAMBOCOR to healthy subjects stabilized on a maintenance dose of digoxin, a 13%-19% increase in plasma digoxin levels occurred at six hours postdose.

In a study involving healthy subjects receiving TAMBOCOR and propranolol concurrently, plasma flecainide levels were increased about 20% and propranolol levels were increased about 30% compared to control values. In this formal interaction study, TAMBOCOR and propranolol were each found to have negative inotropic effects; when the drugs were administered together, the effects were additive. The effects of concomitant administration of TAMBOCOR and propranolol on the PR interval were less than additive. In TAMBOCOR clinical trials, patients who were receiving beta blockers concurrently did not experience an increased incidence of side effects. Nevertheless, the possibility of additive negative inotropic effects of beta blockers and flecainide should be recognized.

Flecainide is not extensively bound to plasma proteins. In vitro studies with several drugs which may be administered concomitantly showed that the extent of flecainide binding to human plasma proteins is either unchanged or only slightly less. Consequently, interactions with other drugs which are highly protein bound (e.g., anticoagulants ) would not be expected. TAMBOCOR has been used in a large number of patients receiving diuretics without apparent interaction. Limited data in patients receiving known enzyme inducers ( phenytoin, phenobarbital, carbamazepine ) indicate only a 30% increase in the rate of flecainide elimination. In healthy subjects receiving cimetidine (1 gm daily) for one week, plasma flecainide levels increased by about 30% and half-life increased by about 10%.

When amiodarone is added to flecainide therapy, plasma flecainide levels may increase two-fold or more in some patients, if flecainide dosage is not reduced.

Drugs that inhibit cytochrome P450IID6, such as quinidine , might increase the plasma concentrations of flecainide in patients that are on chronic flecainide therapy; especially if these patients are extensive metabolizers.

There has been little experience with the coadministration of TAMBOCOR and either disopyramide or verapamil. Because both of these drugs have negative inotropic properties and the effects of coadministration with TAMBOCOR are unknown, neither disopyramide nor verapamil should be administered concurrently with TAMBOCOR unless, in the judgment of the physician, the benefits of this combination outweigh the risks. There has been too little experience with the coadministration of TAMBOCOR with nifedipine or diltiazem to recommend concomitant use.