The Discovery of a Novel Anticoagulant Mechanism: Factor XI Activation Inhibitors

Despite advances in anticoagulant drug discovery to establish a new standard of care, represented by Factor Xa (FXa) inhibitors, additional therapies are needed to address the concomitant bleeding risk posed by these agents. Factor XI (FXI) plays a key role in the intrinsic coagulation cascade contributing to thrombin generation and fibril formation, and there is an increasing weight of evidence supporting a comparable efficacy and superior safety profile of FXI-based oral anticoagulants. A novel allosteric mechanism of action (MoA) for inhibiting FXI activation to Factor XIa (FXIa) was discovered by applying an AS-MS-based screen to the FXI zymogen. Biochemical and biophysical characterization of these structurally diverse small-molecule classes confirmed this mode of inhibition as specific to the FXI zymogen, and a high-resolution FXI-inhibitor cocomplex structure identified the binding pocket that is distal to the FXI cleavage site. These FXI Activation Inhibitors were active in translational functional assays, prolonging activated partial thromboplastin time (aPTT) in human plasma and demonstrating dose-dependent antithrombotic effects in the rabbit arteriovenous-shunt model of thrombosis. The results affirm that FXI Activation Inhibitors are a novel mechanism for the potential treatment of thrombotic disorders.