Amiodarone is a Potent Inhibitor of Cytochrome P450 Enzymes
Amiodarone functions as a potent inhibitor—not an inducer—of multiple cytochrome P450 enzymes, specifically CYP2C9, CYP2D6, and CYP3A4, as well as P-glycoprotein. 1
Mechanism of Inhibition
Amiodarone blocks the metabolism of other drugs through several CYP pathways 2:
- CYP2C9: Metabolizes warfarin
- CYP2D6: Metabolizes beta blockers and narcotics
- CYP3A4: Metabolizes cyclosporine and calcium channel blockers
- P-glycoprotein: Affects digoxin secretion from renal tubules
The FDA label explicitly states that "Amiodarone is an inhibitor of CYP3A4 and p-glycoprotein" and that "Amiodarone inhibits p-glycoprotein and certain CYP450 enzymes, including CYP1A2, CYP2C9, CYP2D6, and CYP3A4." 1
Clinical Significance: The Metabolite Matters More
Desethylamiodarone (amiodarone's primary metabolite) is actually a more potent inhibitor than the parent drug itself. Research demonstrates that desethylamiodarone has higher 1+Iu/Ki values than amiodarone, meaning drug interactions primarily occur via the metabolite rather than amiodarone alone 3. This metabolite competitively inhibits CYP2D6 with a Ki of 4.5 μM and noncompetitively inhibits CYP2A6, CYP2B6, and CYP3A4 3.
Critical Drug Interactions Requiring Dose Adjustments
Warfarin
- Amiodarone reduces warfarin clearance through CYP2C9 inhibition
- Peak interaction effects occur approximately 7 weeks after starting amiodarone
- Monitor PT/INR at least weekly during the first 6 weeks 2
Digoxin
- Digoxin levels predictably double when co-administered with amiodarone
- Mechanism: Inhibition of P-glycoprotein-mediated renal tubular secretion
- Reduce digoxin dose by 50% when starting amiodarone 2
- Monitor digoxin levels closely
Other Antiarrhythmics
- Quinidine serum concentration increases by 33% after 2 days
- Procainamide and N-acetyl procainamide increase by 55% and 33% respectively
- Reduce quinidine and procainamide doses by one-third 1
Statins
- Simvastatin (CYP3A4 substrate) combined with amiodarone increases myopathy/rhabdomyolysis risk 1
Important Clinical Caveat
The inhibitory effects persist long after discontinuation due to amiodarone's extremely long half-life (up to 100 days). Drug interactions can occur with medications started weeks to months after stopping amiodarone 1.
Common Pitfall to Avoid
Do not confuse amiodarone with enzyme inducers. Grapefruit juice actually increases amiodarone levels by 50% (AUC) and 84% (Cmax), confirming its substrate status for CYP3A4 rather than inducer properties 1. Patients must avoid grapefruit juice during amiodarone therapy.