Ex Vivo Reversal of FXI/FXIa Inhibition in Human Whole Blood by Coagulation Factor Concentrates: Insights from Flow Chamber and Global Hemostasis Assays

Introduction Factor XI (FXI) has emerged as a promising target for anticoagulation, as it plays a key role in thrombosis while being less critical for normal hemostasis [ 1 ] [ 2 ]. Several FXI/FXIa inhibitors are currently in advanced clinical development. However, unlike direct factor Xa or thrombin inhibitors, no specific reversal agents are available, raising concerns regarding the management of bleeding or urgent interventions [ 3 ] [ 4 ]. This study evaluates whether non-specific reversal agents, such as prothrombin complex concentrates (PCCs) and activated PCCs (aPCCs), can restore coagulation in the presence of FXI inhibition. Method In this ongoing study (n=10), ethical approval was granted by the Ethics Committee of RWTH Aachen University (Ethikkommission RWTH Aachen, Germany). Whole blood samples were spiked ex vivo with different FXI inhibitors, including small molecule inhibitors and monoclonal antibody inhibitors. For non-specific reversal experiments, anticoagulated samples were treated with different concentrations of PCC and aPCC at clinically relevant levels, with or without the presence of an FXI inhibitor. A panel of coagulation parameters, including activated partial thromboplastin time (aPTT), prothrombin time (PT), thrombin generation (lag time, peak thrombin, and endogenous thrombin potential (ETP)); as well as whole-blood thromboelastometry (ROTEM Delta, IN-Test) and flow chamber assays under defined shear conditions, were performed. Results Baseline FXI activity and aPTT were 87.5%±17.5% and 27.7 s±1.8 s, respectively. After spiking with a small molecule inhibitor, values changed to 16.8%±2.2% and 63.9 s±7.1 s; with a monoclonal antibody to 1.8%±0.5% and 66.6 s±0.3 s, confirming effective inhibition. Similarly, whole-blood thromboelastometry showed a marked prolongation of clotting time, confirming effective FXI inhibition. Following anticoagulation with FXI/FXIa inhibitors, a prolongation of lag time and a reduction in ETP and thrombin peak height were observed, effects that were reversed by PCC administration ([ Fig. 1 ]). These preliminary findings align with whole-blood flow experiments showing restored thrombus formation under shear conditions after PCC treatment ([ Fig. 2 ]). Fig. 1 Effect of PCC on Thrombin Generation after FXI/FXIa Inhibition. Thrombin generation in platelet-poor plasma samples following spiking with either a small-molecule FXI inhibitor (800 ng/mL) or a monoclonal antibody (Ab) FXI/FXIa inhibitor (50 µg/mL). Subsequently, reversal of the anticoagulant effect was assessed after addition of 50 U of PCC. Both the small-molecule and the monoclonal Ab inhibitors markedly reduced thrombin generation compared with the control (shown in grey) and demonstrated effective reversal following PCC administration. (FXI/FXIa: Factor XI/activated Factor XI; PCC: prothrombin complex concentrate). Fig. 2 Flow Chamber Analysis of Thrombus Formation and Reversal of FXI/FXIa Inhibition . Representative flow chamber analysis of FXI/FXIa inhibition and its reversal by PCC. Whole blood from a healthy donor was treated with a small-molecule or monoclonal FXI/FXIa inhibitor, with or without PCC. After incubation, AlexaFluor 647 was added to label fibrinogen. Samples were perfused over tissue factor (0.01 pM)/collagen (50–100 μg/mL)-coated channels at 10 dyn/cm ² . Real-time imaging showed impaired thrombus formation restored by PCC. Conclusion Effective reversal of FXI/FXIa inhibitors is essential for their safe clinical use, particularly in acute bleeding or urgent surgery settings. Our preliminary data indicate that (a)PCCs can partially restore coagulation in the presence of FXI inhibition, supporting their potential as non-specific reversal strategies (3). Publication History Article published online: 17 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany