Niyanta Dattani interviews Citi Research analyst Andrew Baum on the subject of factor XI inhibitors. Heart attacks and strokes are leading causes of death in the western world, a result of abnormal clotting (thrombosis) preventing oxygenated blood-flow to the heart and brain. While the $24bn DOAC class was a major upgrade to warfarin in the 3.2m patients at risk of a stroke from atrial fibrillation, there is still a material increase in major and non-major bleeding. More importantly for many high-risk indications, DOACs are under-utilized or not indicated. Factor XI inhibitors (FXIi) promise to prevent thrombosis yet still allow haemostasis (repair injury to prevent bleeding). The phase III ready FXIi market has the potential to exceed $55bn by 2035 (consensus <$5bn), even with generic DOACs post 2026 (Eliquis, Xarelto), and around 75% of the commercial opportunity for FXIi comes from patients not currently treated with blood thinners such as DOACs.