Summary. Background: Utilization of glycoprotein IIb-IIIa (GPIIb-IIIa) inhibitors improves outcomes of patients with acute coronary syndromes (ACS), including those undergoing percutaneous coronary intervention (PCI). These results may be related to the ability of the inhibitors to destabilize coronary thrombi, reduce microembolization, and restore vessel patency. Objective: To evaluate in vitro the ability of GPIIb-IIIa antagonists, abciximab and eptifibatide, to promote the disaggregation of platelet-rich thrombus. Methods: Antagonist-induced disaggregation was assayed in plasma by aggregometry, as well as in whole blood by point of care and capillary perfusion systems. Fibrinogen dissociation from the platelet surface was quantified by flow cytometry. Results: Significant disaggregation of 5 lM ADP-induced aggregates was observed after addition of either agent. The maximum extent and rate of disaggregation were significantly higher with eptifibatide than with abciximab. Both antagonists also dispersed 2 lg mL )1 collagen-induced aggregates, again with eptifibatide having a greater effect. Eptifibatide, but not abciximab (up to 10 lg mL )1 ), was efficient at dissociating aggregates to single platelets in whole blood and dispersing aggregates that had been aged for 30 min before treatment. Eptifibatide also reduced existing thrombus burden in the perfusion model under arterial flow conditions. A key mechanism of aggregate dispersal was antagonist-induced displacement of platelet-bound fibrinogen, which was greater with eptifibatide, a competitive inhibitor of fibrinogen binding, than with the noncompetitive inhibitor, abciximab. Conclusions: These results suggest that drug concentration and residence time, along with thrombus extent and age, may be critical determinants in promoting timely recanalization.

Atherosclerotic plaques can lead to partial vascular occlusions that produce abnormally high levels of arterial wall shear stress. Such pathophysiological shear stress can promote shear-induced platelet aggregation (SIPA), which has been linked to acute myocardial infarction, unstable angina, and stroke. This study investigated the role of the tyrosine kinase Syk in shear-induced human platelet signaling. The extent of Syk tyrosine phosphorylation induced by pathophysiological levels of shear stress (100 dyn/cm(2)) was significantly greater than that resulting from physiological shear stress (10 dyn/cm(2)). With the use of phospho-Syk specific antibodies, these data are the first to show that key regulatory sites of Syk at tyrosines 525/526 (Y525/526) and tyrosine 352 (Y352) were phosphorylated in response to pathophysiological shear stress. Increased phosphorylation at both sites was attenuated by pharmacological inhibition of Syk using two different Syk inhibitors, piceatannol and 3-(1-methyl-1H-indol-3-yl-methylene)-2-oxo-2,3-dihydro-1H-indole-5-sulfonamide (OXSI-2), and by inhibition of upstream Src-family kinases (SFKs). Shear-induced response at the Syk 525/526 site was ADP dependent but not contingent on glycoprotein (GP) IIb-IIIa ligation or the generation of thromboxane (Tx) A(2). Pretreatment with Syk inhibitors not only reduced SIPA and Syk phosphorylation in isolated platelets, but also diminished, up to 50%, the platelet-mediated thrombus formation when whole blood was perfused over type-III collagen. In summary, this study demonstrated that Syk is a key molecule in both SIPA and thrombus formation under flow. Pharmacological regulation of Syk may prove efficacious in treating occlusive vascular disease.

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Based on these data, the ICHOR/ Plateletworks utilized under modified guidelines may serve as a surrogate for LTA when rapid measurements are necessary.A rapid platelet function measurement method as a surrogate for light transmission aggregometry (LTA), the current gold standard, is ideal in order to have the option for GPIIb-IIIa antagonist dose monitoring or adjustment prior to or during a coronary intervention. A newly modified rapid platelet function analysis system (ICHOR/Plateletworks was evaluated and compared to the results obtained with traditional light transmission aggregometry (LTA), and the Ultegra/RPFA system. Blood samples collected in D-Phe-Pro-Arg-chloromethyl ketone dihydrochloride (PPACK) anticoagulant were treated with increasing concentrations of eptifibatide, tirofiban or abciximab. LTA was carried out in conjunction with the ICHOR/Plateletworks, using a modified method, and Accumetrics Ultegra with RPFA cartridges. Based on these data, the ICHOR/Plateletworks utilized under modified guidelines may serve as a surrogate for LTA when rapid measurements are necessary.