Recent Research Developments in the Direct Inhibition of Coagulation Proteinases - Inhibitors of the Initiation Phase

Physiologic clotting is a defensive action. The new cell-based model of hemostasis proposes three steps - initiation, amplification and propagation - occurring on specific cell surfaces to generate a thrombus in a tightly regulated manner. The initiation phase relies on key players including tissue factor (TF), factor VIIa (fVIIa), platelets, Ca2+, phospholipids, and factor X/Xa (fX/fXa). Exposure of TF on sub-endothelial and other blood cells triggers a coagulation response, which may have to be inhibited to prevent a deleterious thrombotic effect. Inhibiting TF-initiated coagulation, akin to ‘nipping coagulation in the bud’, is predicted to have major advantages, including a more efficient separation of the antithrombotic and hemorrhagic responses. The availability of crystal structures of TF, fVIIa and TF- fVIIa complex makes structure-based drug design feasible. Although no initiation phase small molecule inhibitor has reached the clinic as yet, several molecules have displayed promise. We discuss recent results on the discovery of inhibitors of the initiation phase with special emphasis on peptides, peptidomimetics and organic small molecules.