Evaluating reversal strategies for long-acting factor XI antibodies: RFVIIa®, FEIBA®, Kcentra®, novel FXI mutants, and an anti-idiotypic antibody

Abstract Background: Factor XI (FXI) and activated FXI (FXIa) inhibitor antibodies are emerging antithrombotic therapies due to their lower potential for bleeding complications compared with conventional anticoagulants. However, since FXI deficiency (hemophilia C) can result in a bleeding diathesis in some individuals, optimal strategies for reversing their antihemostatic effects during clinically relevant bleeding events must be investigated. As these new agents enter clinical trials involving medically complex patient populations with higher bleeding risks, the ideal reversal strategy will become ever more important. Understanding the mechanisms by which various reversal agents restore coagulation in the presence of these targeted FXI inhibitors with differing MOAs is critical for clinical translation and future peri-procedural management. Methods: We evaluated the ability of multiple potential reversal or bypassing agents to reduce anticoagulation in human plasma (SSC/ISTH Secondary Coagulation Standard Lot #5) spiked with FXI or FXIa antibodies at a clinically relevant concentration (100 nM). The antibodies tested were the FXI A2 domain inhibitor gruticibart (AB023), the FXI A3 domain inhibitor AB011 (a.k.a. BAY1831865), and the active site binding FXIa inhibitor osocimab. Reversal agents included recombinant factor VIIa (NovoSeven®), activated prothrombin complex concentrate (FEIBA®), non-activated PCC and (Kcentra®), a FXI/PKA chimera that is not recognized by gruticibart, a FXI mutant that is not recognized by AB011, and an antibody that inhibits gruticibart (AB032). Here we performed global coagulation assays including activated partial thromboplastin time (aPTT), prothrombin time (PT), thrombin generation assay (TGA) using 0.2 pM tissue factor and contact activators ellagic acid and kaolin, and viscoelastic testing of whole blood using ROTEM (NATEM protocol). Results: FXI inhibitors prolonged aPTT without affecting PT. All inhibitors reduced thrombin generation across activation methods in the TGA. When activated with low dose TF, AB011 suppressed thrombin generation to levels comparable to FXI immune depleted plasma (FXI-/-), followed by osocimab > gruticibart. With contact activation (ellagic acid or kaolin), the relative potency of thrombin inhibition shifted: AB011 again showed the strongest inhibition on par with FXI-/- plasma, followed by gruticibart, and osocimab. Among the bypassing agents, NovoSeven® and FEIBA® consistently demonstrated concentration-dependent restoration of aPTT and thrombin generation in the presence of all FXI inhibitors, but also shortened PT. Kcentra® demonstrated no reversal of aPTT prolongation or thrombin generation parameters and slightly prolonged PT at the highest concentration tested (2 U/mL). Targeted reversal strategies showed mechanistic specificity. AB032 and FXI/PKA (for gruticibart reversal), and the FXI mutant (for AB011 reversal) provided partial to complete normalization of aPTT and improved thrombin generation profiles, without shortening PT. ROTEM analysis under NATEM conditions revealed comparable anticoagulant effects across all FXI inhibitors. Complete reversal was achieved using NovoSeven® (45 nM) and FEIBA® (2 U/mL). Addition of the gruticibart anti-idiotypic antibody AB032 at a 20x excess to gruticibart partially normalized ROTEM parameters, aligning with similar results from aPTT measurements. Conclusion: FXI inhibitors produce anticoagulant effects across global coagulation assays that can be partially or completely reversed by bypassing agents, particularly NovoSeven® and FEIBA®; however PT, TGA and viscoelastic testing also reveal the potential to generate a transient procoagulant/prothrombotic state if plasma concentrations of these commercially available agents are too high. The magnitude of the procoagulant activity is dependent on the bypassing agent as well as the specific FXI inhibitor, suggesting that reversal strategies should be tailored to the mechanism of the different FXI inhibitors (i.e. A2, A3, or catalytic domain) to achieve optimal outcomes. Antibody-specific and mutation-based reversal agents also demonstrate selective reversal effects but without measurable procoagulant activities. These findings provide a framework for developing specific FXI inhibitor reversal strategies, and support management of FXI-directed anticoagulation with commercially available bypassing agents based on the specific MOA of the FXI(a) inhibitor.