Drug interaction studies and encainide use in renal and hepatic impairment.

The effect of encainide administration on steady-state plasma digoxin levels was evaluated in 17 patients receiving stable doses of digoxin. A paired t test, comparing plasma digoxin levels (mean +/- standard error) before encainide therapy (1.05 +/- 0.14 ng/ml) and after 2 weeks of encainide, 100 mg/day (1.03 +/- 0.11 ng/ml) or 200 mg/day (1.2 +/- 0.2 ng/ml), indicates no significant (p greater than 0.05) change in digoxin levels. These results were confirmed in a second study of 10 patients with severe congestive heart failure. Also, no difference in efficacy of either drug was observed and changes in dosing of digoxin were not required. Plasma concentrations of encainide and its 2 major metabolites, O-demethyl encainide (ODE) and 3-methoxy-O-demethyl encainide, significantly increased by 31.6%, 43.1% and 35.6% after concomitant cimetidine administration in 13 healthy adult men receiving 75 mg/day of encainide. However, a retrospective evaluation of 33 patients receiving both drugs did not reveal any clinically significant interactions. Retrospective evaluation of patients enrolled in clinical studies who received concomitant digoxin (268), antiarrhythmics (118), anticoagulants (78), antidiabetics (40), antipsychotics (23), beta blockers (88), calcium-channel blockers (24) or diuretics (229) did not reveal any clinically significant interactions with encainide. Similarly, in vitro protein binding studies did not reveal any clinically significant interactions with encainide or its major metabolites. Six patients with moderate to severe renal impairment (creatinine clearance 10 to 38 ml/min) received 25 mg of encainide, 3 times/day, for 7 doses. Plasma encainide, ODE and 3-methoxy-O-demethyl concentrations were similar to those observed in normal subjects who had received twice the dose of encainide, and steady-state apparent oral clearance of encainide was reduced by 66% with renal impairment. Based on these data it is recommended that in patients with moderate to severe renal impairment encainide be initiated at one-third the normal dose, or 25 mg once a day. Doses may be elevated in small increments at 1-week intervals if needed for efficacy. The effect of hepatic impairment on the pharmacokinetics of encainide was studied in 7 patients with clinically documented cirrhosis. Compared with normal subjects studied using a similar protocol, the plasma concentrations of encainide were elevated significantly due to a 6-fold decrease in oral clearance. However, since plasma concentrations of the active metabolite ODE were correspondingly lower, specific encainide dosing instructions for patients with hepatic impairment are not indicated.