An enzyme-linked immunosorbent assay to identify inhibitors of activation of platelet integrin αIIbβ3

Abstract

The affinity of integrin α IIb β 3 for adhesive ligands is tightly regulated by the platelet such that fibrinogen binding is observed only after platelet activation. Ligand binding is necessary for platelet aggregation, which contributes to vascular occlusion in pathological states. Therefore, we have developed an ELISA assay to screen for compounds that inhibit α IIb β 3 activation. Washed platelets were incubated in microtitre wells with potential inhibitory compounds and stimulated with an agonist to activate α IIb β 3. After the addition of biotin-PAC1, a fibrinogen-mimetic monoclonal antibody, the activation state of α IIb β 3 was measured by sedimenting the platelets and quantitating the residual biotin-PAC1 in the cell-free supernatant in a streptavidin-based ELISA. This assay detected (1) specific PAC1 binding to activated platelets in response to a variety of agonists, and (2) dose-dependent inhibition of PAC1 binding by function-blocking anti- α IIb β 3 monoclonal antibodies, by the tetrapeptide, RGDS, and by an α IIb β 3-selective RGD peptidomimetic. Furthermore, the assay detected inhibition of PAC1 binding by intracellular inhibitors of platelet activation, including bisindolylmaleimide, a selective protein kinase C antagonist, and wortmannin, an inhibitor of phosphatidylinositol 3-kinase. These studies demonstrate that this integrin activation ELISA can detect pharmacological blockade of platelet α IIb β 3 by extracellular and intracellular inhibitors. Its use may facilitate the search for clinically useful anti-platelet drugs.