What are the elimination half‑life, P‑glycoprotein interactions, CYP2D6 metabolism, effects on QT interval, and effects on cardiac conduction of flecainide?

Flecainide Pharmacological Properties

Flecainide has an elimination half-life of approximately 20 hours (range 12-27 hours), is metabolized by CYP2D6, interacts with P-glycoprotein as a substrate, causes QT prolongation primarily through QRS widening rather than true repolarization delay, and significantly slows cardiac conduction throughout the heart with the greatest effects on the His-Purkinje system. 1

Half-Life

The apparent plasma half-life of flecainide averages 20 hours (range 12-27 hours) after multiple oral doses in patients with premature ventricular contractions 1. With multiple dosing, steady-state levels are approached in 3-5 days due to this long half-life 1.

Important age-related variations exist:

• Infants at birth: up to 29 hours
• 3 months of age: 11-12 hours
• 1 year of age: 6 hours
• Children 1-12 years: approximately 8 hours
• Adolescents 12-15 years: 11-12 hours
• Adults 20-80 years: slight increase with advancing age 1

The half-life is prolonged in patients with renal impairment and those with NYHA class III congestive heart failure (mean 19 hours vs 14 hours in healthy subjects) 1.

CYP2D6 Metabolism

Flecainide is extensively metabolized by CYP2D6, with approximately 30% (range 10-50%) excreted unchanged in urine 1. The two major urinary metabolites are meta-O-dealkylated flecainide (MODF, which is active but about one-fifth as potent) and the meta-O-dealkylated lactam of flecainide (inactive) 1.

CYP2D6 genetic polymorphism significantly affects flecainide metabolism:

• Patients with poor CYP2D6-mediated metabolism (heterozygous extensive metabolizers and intermediate/poor metabolizers) show age-related reduction in flecainide metabolism because metabolism is taken over by CYP1A2, whose activity decreases with age 2
• However, recent evidence suggests that when using standardized CYP2D6 classifications, intermediate metabolizers and normal metabolizers show similar rates of toxicity and recurrence at standard dosing (100 mg every 12 hours), challenging current dose adjustment recommendations 3
• Poor metabolizers may actually have better outcomes, raising questions about whether standard dosing is subtherapeutic for other groups 3

Critical drug interactions via CYP2D6 inhibition:

• Verapamil increases flecainide levels and requires monitoring 4
• Amiodarone, ritonavir, saquinavir, and tipranavir all interact with flecainide 4
• Paroxetine (a CYP2D6 inhibitor) significantly increases QTc prolongation when combined with flecainide, particularly in extensive and intermediate metabolizers 5

P-Glycoprotein Interactions

Flecainide is a P-glycoprotein substrate 4. Verapamil, which inhibits P-glycoprotein, requires monitoring of flecainide concentrations during coadministration 4. This P-glycoprotein interaction is clinically relevant because:

• P-gp-mediated drug interactions can lead to increased gastrointestinal absorption and accumulation in cardiomyocytes 6
• This mechanism may enhance the inhibitory effect on hERG channels and increase QT prolongation risk 6
• Digoxin levels should be monitored when used with flecainide due to P-glycoprotein interactions 4, 1

Effects on QT Interval

Flecainide causes QT prolongation, but this is primarily due to QRS widening rather than true repolarization delay. 1

The QT interval widens about 8%, but 60-90% of this widening is due to QRS prolongation 1. The JT interval (QT minus QRS) only widens about 4% on average, and significant JT prolongation occurs in less than 2% of patients 1. This distinguishes flecainide from class III agents that cause true repolarization delay.

Key monitoring parameters:

• QRS increases on average about 25% (0.02 seconds) and as much as 150% in some patients 1
• Many patients develop QRS complexes ≥0.12 seconds 1
• Caution is warranted when QRS increases to ≥0.18 seconds 1
• Torsades de pointes is rare with flecainide, unlike class III agents 1

The relationship between plasma levels and ECG changes:

• Plasma levels of 0.2-1 mcg/mL may be needed for maximal therapeutic effect 1
• Levels above 0.7-1 mcg/mL are associated with higher rates of cardiac adverse events including conduction defects 1
• Mean QRS duration during a dosing interval at steady state correlates with mean plasma concentration 7

Effects on Cardiac Conduction

Flecainide produces dose-related decreases in intracardiac conduction throughout all parts of the heart, with the greatest effect on the His-Purkinje system (H-V conduction). 1

Specific conduction effects:

• PR interval increases on average about 25% (0.04 seconds), with increases up to 118% in some patients 1
• Approximately one-third of patients develop new first-degree AV block (PR ≥0.2 seconds) 1
• 4% of patients develop new bundle branch block 1
• Effects on AV nodal and intra-atrial conduction are present but less pronounced than ventricular effects 1

Clinically significant conduction disturbances occur at these rates:

• Sinus node dysfunction (sinus pause, arrest, symptomatic bradycardia): 1.2% 1
• Second-degree AV block: 0.5% 1
• Third-degree AV block: 0.4% 1

Critical precautions:

• If second- or third-degree AV block, or right bundle branch block with left hemiblock occur, flecainide should be discontinued unless a pacemaker is in place 1
• Use with extreme caution in sick sinus syndrome due to risk of sinus bradycardia, pause, or arrest 1
• Flecainide increases endocardial pacing thresholds and may suppress ventricular escape rhythms 1
• Pacing thresholds should be determined before therapy, after one week, and at regular intervals; doubling voltage or pulse width is usually sufficient to regain capture 1

Important contraindications based on conduction effects:

• Avoid in patients with preexisting advanced AV block (second- or third-degree) or significant conduction system disorders (left bundle branch block, or right bundle branch block with left hemiblock) 8
• Contraindicated in sinus or AV conduction disease without a pacemaker 4, 8

The proarrhythmic risk with flecainide is monomorphic ventricular tachycardia (not torsades de pointes), particularly in patients with structural heart disease, ventricular conduction delays (QRS >120 ms), ventricular scar tissue, or left ventricular dysfunction 9. This was demonstrated in the CAST trial where flecainide increased mortality in post-MI patients 9.