ARC19499, an aptamer, inhibits the tissue factor pathway inhibitor (TFPI), facilitating clot initiation and propagation through the extrinsic pathway. It binds specifically to TFPI, blocking its inhibition of both factor Xa and the TF/factor VIIa complex, thereby correcting thrombin generation in hemophilia A and B plasma and restoring clotting in FVIII-neutralized whole blood.