Andexanet alfa reverses apixaban and rivaroxaban anticoagulation. How does it differ mechanistically from idarucizumab (used for dabigatran reversal)?

• A Andexanet alfa is a modified recombinant factor Xa decoy protein that sequesters free anti-Xa drugs; idarucizumab is a monoclonal antibody Fab fragment that binds dabigatran with higher affinity than thrombin ✓
• B Andexanet alfa regenerates free factor Xa by displacing apixaban from its active site via competitive kinetics; idarucizumab catalyzes dabigatran hydrolysis
• C Andexanet alfa activates vitamin K epoxide reductase to regenerate active clotting factors depleted by anti-Xa drugs; idarucizumab activates prothrombin
• D Andexanet alfa directly binds tissue factor, bypassing the need for Xa; idarucizumab blocks PAR-1 receptors on platelets

Explanation

Andexanet alfa is a recombinant modified human factor Xa decoy protein (with active site serine → alanine substitution to eliminate its own pro-coagulant activity, and Gla domain deletion to prevent prothrombinase complex assembly). It acts as a 'molecular sponge' — it binds free anti-Xa direct oral anticoagulants (apixaban, rivaroxaban) and low-molecular-weight heparin in plasma with high affinity, sequestering them away from endogenous factor Xa, thereby rapidly restoring factor Xa activity and haemostasis. In contrast, idarucizumab is a humanized monoclonal antibody Fab fragment that binds dabigatran (a direct thrombin inhibitor) with approximately 350-fold higher affinity than thrombin does, forming a tight non-covalent complex and neutralizing its anticoagulant effect within minutes. Both reversal agents work within 5–10 minutes.

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

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