Direct Thrombin Inhibitors

Direct Thrombin Inhibitors

1. Definition & overview

• Direct thrombin inhibitors (DTIs) are anticoagulants that bind directly to thrombin (factor IIa) and inactivate it, without requiring antithrombin (AT-III) as a cofactor — this is the defining contrast with heparin/LMWH/fondaparinux, which are indirect (AT-III–dependent) thrombin/factor-Xa inhibitors (KDT 8e Ch.45, pp.666–7; G&G 14e Ch.36, p.715).
• Thrombin (factor IIa) is the central effector protease of coagulation: it is the final protease generated in the zymogen-activation cascade and it (i) converts soluble fibrinogen → fibrin, (ii) activates platelets via PAR-1/PAR-4 receptors, and (iii) provides positive feedback amplification by activating factors V, VIII and XI — so inhibiting thrombin attenuates fibrin formation, platelet activation, and further thrombin generation simultaneously (G&G 14e Ch.36, pp.709–10).
• Because thrombin is so pivotal, a direct thrombin block is a mechanistically "downstream and broad" anticoagulant strategy; DTIs inhibit both circulating (free) and clot-bound thrombin, whereas the heparin–AT-III complex is too large to access fibrin-bound thrombin efficiently — a key pharmacodynamic advantage of DTIs (KDT 8e Ch.45, p.666; G&G 14e Ch.36, p.715).
• DTIs are split by route and chemistry into:
  • Parenteral DTIs — bivalirudin, argatroban, and the recombinant hirudins desirudin and lepirudin (G&G 14e Ch.36, pp.715–6).
  • Oral direct thrombin inhibitor (oDTI) — dabigatran etexilate, the only marketed oral agent, a member of the direct oral anticoagulant (DOAC) group alongside the oral factor-Xa inhibitors (G&G 14e Ch.36, p.718; KDT 8e Ch.45, pp.671–2).
• The parent natural lead compound is hirudin, a 65-amino-acid polypeptide anticoagulant secreted by the salivary glands of the medicinal leech (Hirudo medicinalis); desirudin, lepirudin and (structurally) bivalirudin are derived from or modelled on it (KDT 8e Ch.45, p.666; G&G 14e Ch.36, p.715).
• All anticoagulants — DTIs included — increase bleeding risk; this is the universal class adverse effect and dominates the risk–benefit calculus (G&G 14e Ch.36, p.709).