Rivaroxaban and Apixaban Induce Clotting Factor Xa Fibrinolytic Activity.

Background: When bound to an anionic phospholipid-containing membrane, activated clotting factor X (FXa) is sequentially cleaved by plasmin from the intact form, FXa alpha, to FXa and then to Xa33/13. Tissue-type plasminogen activator (t-PA) produces plasmin and is the initiator of fibrinolysis. Both FXa beta and Xa33/13 enhance t-PA-mediated plasminogen activation. Although stable in experiments using purified proteins, Xa33/13 rapidly loses t-PA cofactor function in plasma. Bypassing this inhibition, covalent modification of the FXa alpha active site prevents Xa33/13 formation by plasmin, and the persistent FXa beta enhances plasma fibrinolysis. As the direct oral anticoagulants (DOACs) rivaroxaban and apixaban bind to the FXa active site, we hypothesized that they similarly modulate FXa fibrinolytic function. Methods: DOAC effects on fibrinolysis and the t-PA cofactor function of FXa were studied in patient plasma, normal pooled plasma and purified protein experiments by the use of light scattering, chromogenic assays, and immunoblots. Results: The plasma of patients taking rivaroxaban showed enhanced fibrinolysis correlating with FXa beta. In normal pooled plasma, the addition of rivaroxaban or apixaban also shortened fibrinolysis times. This was related to the cleavage product, FXa beta, which increased plasmin production by t-PA. It was confirmed that these results were not caused by DOACs affecting activated FXIII-mediated fibrin crosslinking, clot ultrastructure and thrombin-activatable fibrinolysis inhibitor activation in plasma. Conclusion: The current study suggests a previously unknown effect of DOACs on FXa in addition to their well-documented anticoagulant role. By enabling the t-PA cofactor function of FXa beta in plasma, DOACs also enhance fibrinolysis. This effect may broaden their therapeutic indications.