Direct Thrombin Inhibitors

Introduction & definition

• Direct thrombin inhibitors (DTIs) are anticoagulants that bind directly to thrombin (factor IIa) and inactivate it without requiring antithrombin (AT-III) as a cofactor — the defining contrast with heparin/LMWH/fondaparinux, which are indirect (AT-III–dependent) inhibitors.
• Why thrombin — thrombin is the central effector protease of coagulation — the final protease generated in the cascade. It converts fibrinogen → fibrin, activates platelets via PAR-1/PAR-4, and provides positive feedback by activating factors V, VIII and XI; so a direct thrombin block attenuates fibrin formation, platelet activation and further thrombin generation simultaneously.
• Key advantage — DTIs inhibit both circulating (free) and clot-bound thrombin, whereas the bulky heparin–AT-III complex cannot access fibrin-bound thrombin efficiently — a major pharmacodynamic advantage.
• Two groups — Parenteral DTIs — bivalirudin, argatroban, and the recombinant hirudins desirudin & lepirudin; oral DTI (oDTI) — dabigatran etexilate, the only marketed oral agent and a member of the direct oral anticoagulant (DOAC) group.
• Parent lead — hirudin — a 65-amino-acid polypeptide from the salivary glands of the medicinal leech (Hirudo medicinalis); desirudin, lepirudin and (structurally) bivalirudin derive from or are modelled on it. As with all anticoagulants, bleeding is the universal class adverse effect.