Andexanet alfa (andexanet) is the only Food and Drug Administration–approved antidote for direct FXa (factor Xa) inhibitors but has been reported to cause resistance to unfractionated heparin (UFH). This has delayed anticoagulation for procedures requiring cardiopulmonary bypass. The mechanism, andexanet and UFH dose dependence, and thrombotic risk of andexanet-associated heparin resistance are unknown.

The effect of andexanet in vitro was determined using activated clotting times and thromboelastography. Ex vivo cardiopulmonary bypass circuits were used to determine whether andexanet impaired anticoagulation for extracorporeal circulation. Kinetics of AT (antithrombin) inhibition of FXa and thrombin were measured in the presence of andexanet. Equilibrium modeling and thrombin generation assay validation were used to predict the role of andexanet, AT, and UFH concentrations in andexanet-associated heparin resistance.

Andexanet prevented UFH-mediated prolongation of activated clotting times and thromboelastography times. At lower concentrations of andexanet, heparin resistance could be overcome with suprapharmacologic doses of UFH, but not at higher andexanet concentrations. Andexanet rendered standard doses of UFH inadequate to prevent circuit thrombosis, and suprapharmacologic UFH doses were only partially able to overcome this. Scanning electron microscopy demonstrated coagulation activation in circuits. Andexanet prevented UFH enhancement of AT-mediated inhibition of FXa and thrombin. Equilibrium modeling and thrombin generation assay validation demonstrated that andexanet creates a triphasic equilibrium with UFH and AT: initial UFH unresponsiveness, normal UFH responsiveness when andexanet is depleted, and finally AT depletion. Sufficient cardiopulmonary bypass heparinization can only occur at low therapeutic andexanet doses and normal AT levels. Higher andexanet doses or AT deficiency may require high UFH doses and potentially AT supplementation.

Andexanet causes heparin resistance due to redistribution of UFH-bound AT. If andexanet cannot be avoided before heparinization and direct thrombin inhibitors are undesirable, our in vitro study suggests excess UFH should be considered as a potential strategy before AT supplementation.