A 70-year-old woman with chronic atrial fibrillation is anticoagulated with rivaroxaban. She is started on amiodarone for rate control. Two weeks later, her laboratory results show unexpectedly elevated rivaroxaban plasma levels with increased bleeding risk. What is the mechanism of this interaction?

• A Amiodarone inhibits CYP1A2 which is the primary metabolic pathway for rivaroxaban
• B Amiodarone and its metabolite desethylamiodarone inhibit both CYP3A4 and P-glycoprotein (P-gp), which are the major elimination pathways for rivaroxaban, leading to increased rivaroxaban exposure ✓
• C Amiodarone displaces rivaroxaban from albumin binding, increasing free drug concentration
• D Amiodarone reduces renal tubular secretion of rivaroxaban by competing for the same renal transporter

Explanation

Rivaroxaban is a direct oral anticoagulant (DOAC) eliminated by two parallel pathways: approximately one-third is metabolized by CYP3A4/CYP2J2, and approximately two-thirds is excreted unchanged via P-glycoprotein (P-gp)-mediated renal and fecal excretion. Amiodarone and its major active metabolite desethylamiodarone are potent inhibitors of both CYP3A4 and P-gp (as well as CYP2D6). This dual inhibition significantly reduces rivaroxaban clearance, increasing its AUC and peak plasma levels, thereby elevating bleeding risk. This is a clinically important interaction; dose adjustment or switching anticoagulants should be considered. Similar interactions occur with other P-gp inhibitors like verapamil and dronedarone with all DOACs.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.