Platelet Glycoprotein IIb/IIIa Inhibitors

Glycoprotein IIb/IIIa (GPIIb-IIIa) complexes (integrin aIIbb3) mediate platelet aggregation by binding fibrinogen or von Willebrand factor (vWF), protein cofactors that form bridges between adjacent platelets. The cross-linked adhesive proteins assemble platelets into the aggregate. Agents that block the function of the GPIIb-IIIa complex of platelets constitute a powerful new generation of antithrombotic drugs.1 Among the shortand long-term aims of such drugs are (1) to provide immediate relief in the case of ongoing arterial thrombosis and (2) to eliminate excessive platelet reactivity in diseased vessels so that occlusive thrombi and restenosis do not occur, while allowing sufficient hemostasis to prevent spontaneous bleeding. It should be emphasized that stenosis and partial occlusion are both prothrombotic, with increased shear stress promoting platelet activation. Under these conditions, vWF plays a major role in the mediation of thrombus formation, interacting with GPIIbIIIa and the adhesion receptor GPIb.2 Otherwise, fibrinogen is the major cofactor of platelet aggregation, essentially binding through a dodecapeptide sequence (aa400 to aa411) present at the carboxy terminus of each g chain. Binding of vWF and other adhesive proteins, such as fibronectin, to GPIIb-IIIa is mediated by the Arg-Gly-Asp (RGD) sequence, a universal mediator of cellular interactions with the extracellular matrix.1–3 Anti–GPIIb-IIIa drugs block this final step of the platelet aggregation process. They also block the “outside-in” signaling that follows the binding of adhesive proteins to activated GPIIb-IIIa and the onset of platelet aggregation.3 This signaling may promote events such as secretion, clot retraction, and the expression of procoagulant activity; therefore, its inhibition extends the influence of anti–GPIIb-IIIa drugs beyond the blocking of platelet-to-platelet cohesion.