Abstract
Life-threatening thrombocytopenia and bleeding, common side effects of clinically available αIIbβ3 antagonists, are associated with the induction of ligand-induced integrin conformational changes and exposure of ligand-induced binding sites (LIBSs). To address this issue, we examined intrinsic mechanisms and structure-activity relationships of purified disintegrins, from Protobothrops flavoviridis venom (i.e., Trimeresurus flavoviridis), TFV-1 and TFV-3 with distinctly different pro-hemorrhagic tendencies. TFV-1 with a different αIIbβ3 binding epitope from that of TFV-3 and chimeric 7 × 103 Fab, i.e., Abciximab, decelerates αIIbβ3 ligation without causing a conformational change in αIIbβ3, as determined with the LIBS antibody, AP5, and the mimetic, drug-dependent antibody (DDAb), AP2, an inhibitory monoclonal antibody raised against αIIbβ3. Consistent with their different binding epitopes, a combination of TFV-1 and AP2 did not induce FcγRIIa-mediated activation of the ITAM-Syk-PLCγ2 pathway and platelet aggregation, in contrast to the clinical antithrombotics, abciximab, eptifibatide, and disintegrin TFV-3. Furthermore, TFV-1 selectively inhibits Gα13-mediated platelet aggregation without affecting talin-driven clot firmness, which is responsible for physiological hemostatic processes. At equally efficacious antithrombotic dosages, TFV-1 caused neither severe thrombocytopenia nor bleeding in FcγRIIa-transgenic mice. Likewise, it did not induce hypocoagulation in human whole blood in the rotational thromboelastometry (ROTEM) assay used in perioperative situations. In contrast, TFV-3 and eptifibatide exhibited all of these hemostatic effects. Thus, the αIIbβ3 antagonist, TFV-1, efficaciously prevents arterial thrombosis without adversely affecting hemostasis.