Combined Antithrombin and Heparin Supplementation to Blood for Restoring Heparin Responsiveness After Andexanet Alfa Exposure: An In Vitro Model

Background: Andexanet alfa, a Gla-domainless mutant factor Xa (GDXa), reverses oral FXa inhibitors but can cause severe heparin resistance during cardiopulmonary bypass (CPB). Antithrombin (AT) supplementation may mitigate this effect, though dosing evidence is limited. Methods: We evaluated in vitro the effectiveness of combined heparin and AT in restoring heparin responsiveness after andexanet exposure. Whole blood and platelet-poor plasma from surgical patients on CPB were spiked with andexanet at target concentrations of 2.9 and 4 µM, respectively. Heparin responsiveness was assessed using kaolin-activated clotting time (ACT), ellagic acid-activated thromboelastometry (INTEM), and thrombin generation (TG) assays. Heparin alone or with AT (0.8-3.1 µM in whole blood; 1.1-4.4 µM in plasma) simulated clinical AT dosing (25-100 U/kg). Results: -angle and MCF reductions (≤ 16%). In plasma, high-dose AT plus heparin increased TG lag time ∼5-fold, with thrombin peak and velocity reduced by 86%. Conclusion: High-dose AT with heparin mitigates andexanet-induced heparin resistance in vitro, though residual TG and clot formation suggest ongoing thrombotic risk, warranting further study. Trial Registration: The authors have confirmed clinical trial registration is not needed for this submission.