Perspectives: what we are learning from factor XI/factor XIa inhibitor trials

Over the past decade, a diverse pipeline of factor XI (FXI)/XIa-targeting agents has entered clinical development, including oral small-molecule inhibitors, monoclonal antibodies, antisense oligonucleotides, and small interfering RNAs. Across early- and late-phase clinical trials, these agents have generally shown a favorable safety profile, with consistently low rates of major bleeding, even when used with antiplatelet therapy or compared with standard anticoagulants. However, efficacy signals have varied by clinical setting. In venous thromboembolism prophylaxis after orthopedic surgery and in secondary stroke prevention, FXI/XIa inhibition has shown encouraging antithrombotic activity. In contrast, trials in atrial fibrillation, acute coronary syndrome, and cancer-associated thrombosis have yielded mixed efficacy results, highlighting potential limitations of intrinsic pathway inhibition in conditions with high thrombin generation or platelet-driven thrombosis. Collectively, these findings position FXI/XIa inhibitors as a potential paradigm shift in antithrombotic therapy for targeted indications, with ongoing phase 3 studies expected to further define their clinical role and optimal use across diverse thrombotic disorders.