Factor XI: structure, function and therapeutic inhibition.

Arterial and venous thromboembolism is a major medical concern that requires therapeutic anticoagulation in various medical fields to prevent its drastic consequences. Despite significant advances in anticoagulant therapy, thrombosis remains a leading cause of morbidity and mortality worldwide. Traditional anticoagulants like heparin and vitamin K antagonists (VKAs) have shown efficacy in preventing and treating thrombosis but come with an inherent risk of bleeding due to their non-specific inhibition of multiple coagulation factors. Subsequent direct oral anticoagulants (DOACs), targeting specific factors such as Xa or thrombin, demonstrated improved safety profiles compared to VKAs, yet bleeding remains a concern. Accordingly, research is focused on developing anticoagulants with improved safety profiles. A safer class of anticoagulants would have broad appeal. The intrinsic pathway of coagulation, involving factor XI (FXI), has attracted attention as a potential target for safer anticoagulants. Preclinical studies and epidemiological data indicate that FXI deficiency or inhibition protects against thrombosis with minimal bleeding. Current research involves evaluating various FXI-directed strategies, and phase 2 studies have shown promising results in orthopedic surgery, atrial fibrillation, end-stage renal disease (ESRD), myocardial infarction, and ischemic stroke. Several agents, such as antisense oligonucleotides, monoclonal antibodies, small synthetic molecules, natural peptides, and aptamers, have been developed to inhibit FXI at different stages, offering potentially safer alternatives to traditional anticoagulants. However, the optimal balance between preventing thrombosis and the risk of bleeding associated with FXI inhibitors requires validation through extensive phase 3 clinical trials using definite clinical endpoints. Several of such trials are currently underway or planned to define the role of FXI inhibitors in clinical practice and determine the most suitable FXI inhibitor for each specific indication. The current review highlights the rationale behind developing FXI inhibitors, presenting the most advanced agents in development, summarizing completed clinical trials, and discussing ongoing research efforts.