Factor XI inhibitors for the prevention and treatment of venous and arterial thromboembolism.

Therapeutic anticoagulation is essential to prevent and treat venous and arterial thromboembolism. The available agents target coagulation factors involved in thrombus formation but are associated with an increased risk of bleeding. Factor XI plays a minor role in haemostasis but contributes substantially to thrombus expansion, making it an attractive target to mitigate bleeding while maintaining antithrombotic efficacy. Various novel inhibitors, including antisense oligonucleotides, monoclonal antibodies and small molecules, have been developed. Phase II trials in orthopaedic surgery showed dose-dependent reductions in venous thromboembolism without significantly increasing bleeding compared with enoxaparin. In the first phase III trial of a small-molecule inhibitor of activated factor XI in patients with atrial fibrillation, asundexian was associated with a reduction in bleeding but also a higher risk of stroke, compared with apixaban. Factor XI inhibitors appear safe and hold promise for secondary prevention in myocardial infarction and ischaemic stroke, with ongoing phase III trials assessing their broader efficacy and safety. This Review discusses the rationale, pharmacology, evidence and future directions of factor XI inhibitors across various clinical settings.