SummaryDual antiplatelet therapy with aspirin and clopidogrel decreases the rate of stent thrombosis in patients undergoing percutaneous coronary intervention (PCI). However, despite intensified antiplatelet treatment, up to 4.7% of the patients undergoing coronary stenting develop thrombotic stent occlusion, suggesting incomplete platelet inhibition due to clopidogrel resistance. We evaluated the percentage of clopidogrel nonresponders among 105 patients with coronary artery disease (CAD) undergoing elective PCI. All patients were treated regularly with aspirin 100 mg/d and received a loading dose of 600 mg clopidogrel followed by a maintenance dose of 75 mg/d before PCI. Clopidogrel non-responders were defined by an inhibition of ADP (5 and 20 μMol/L) induced platelet aggregation that was less than 10% when compared to baseline values 4 h after clopidogrel intake. Semi-responders were identified by an inhibition of 10 to 29%. Patients with an inhibition over 30% were regarded as responders. We found that 5 (ADP 5 μMol/L) to 11% (ADP 20 μMol/L) of the patients were non-responders and 9 to 26% were semi-responders. Among the group of nonresponders there were two incidents of subacute stent thrombosis after PCI. We conclude that a subgroup of patients undergoing PCI does not adequately respond to clopidogrel, which may correspond to the occurrence of thromboischemic complications. Point-of-care testing may help to identify these patients who may then benefit from an alternative antiplatelet therapy.

Platelet-collagen interactions play a fundamental role in the process of arterial thrombosis. The major platelet collagen receptor is the glycoprotein VI (GPVI). Here, we determined the effects of a soluble dimeric form of GPVI on platelet adhesion in vitro and in vivo. We fused the extracellular domain of GPVI with the human immunoglobulin Fc domain. The soluble dimeric form of GPVI (GPVI-Fc) specifically bound to immobilized collagen. Binding of GPVI-Fc to collagen was inhibited competitively by soluble GPVI-Fc, but not control Fc lacking the external GPVI domain. GPVI-Fc inhibited the adhesion of CHO cells that stably express human GPVI and of platelets on collagen and attenuated thrombus formation under shear conditions in vitro. To test the effects of GPVI-Fc in vivo, arterial thrombosis was induced in the mouse carotid artery, and platelet-vessel wall interactions were visualized by intravital fluorescence microscopy. Infusion of GPVI-Fc but not of control Fc virtually abolished stable arrest and aggregation of platelets following vascular injury. Importantly, GPVI-Fc but not control Fc, was detected at areas of vascular injury. These findings further substantiate the critical role of the collagen receptor GPVI in the initiation of thrombus formation at sites of vascular injury and identify soluble GPVI as a promising antithrombotic strategy.

Platelet adhesion and activation at the vascular wall are the initial steps leading to arterial thrombosis and vascular occlusion. Prostacyclin and nitric oxide inhibit platelet adhesion, acting via cyclic adenosine monophosphate (cAMP)-and cyclic guanosine monophosphate (cGMP)-dependent protein kinases. A major downstream target for both cAMP-and cGMPdependent protein kinases is the vasodilator-stimulated phosphoprotein (VASP). To test the significance of VASP for the regulation of platelet adhesion in vivo, we studied platelet-vessel wall interactions using VASP-deficient (VASP ؊/؊ ) mice. Under physiologic conditions, platelet adhesion to endothelial cells was significantly enhanced in VASP null mutants when compared with wild-type mice (P < .05). Platelet recruitment in VASP null mice involved P-selectin and the fibrinogen receptor glycoprotein IIb-IIIa (GPIIb-IIIa). Under pathophysiologic conditions, the loss of VASP increased platelet adhesion to the postischemic intestinal microvasculature, to the atherosclerotic endothelium of ApoE-deficient mice, and to the subendothelial matrix following endothelial denudation (P < .05 vs wild type). Importantly, platelet adhesion in VASP null mutants was unresponsive to nitric oxide. These data show for the first time in vivo that VASP is involved in down-regulation of platelet adhesion to the vascular wall under both physiologic and pathophysiologic conditions. ( IntroductionThe adhesion of platelets to the vascular wall is central to the pathogenesis of atherogenesis and arterial thrombosis. 1,2 Nitric oxide (NO) and prostacyclin are of major importance for the regulation of platelet-vessel wall interactions. They activate soluble guanylyl cyclase and adenylate cyclase, respectively, initiating a subsequent rise in platelet cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP). [3][4][5] Cyclic GMPdependent protein kinase I (cGKI) and cAMP-dependent protein kinase (cAK) are thought to be the major downstream targets of the NO/cGMP and prostacyclin/cAMP signaling cascades in platelets. 3,6,7 A common substrate of both cAK and cGK is the vasodilatorstimulated phosphoprotein (VASP). [8][9][10] VASP was isolated initially from human platelets but it is also expressed in a wide variety of other cells and tissues. 11 VASP is the founding member of a family of proline-rich proteins designated the Ena/VASP protein family, which comprises VASP, Drosophila Enabled (Ena), a substrate of the Abelson tyrosine kinase (Abl), the mammalian Ena homolog Mena, and the Ena-VASP-like protein Evl. 12-14 These proteins share highly homologous N-terminal and C-terminal domains (Ena-VASP homology domains 1 and 2, designated EVH1 and EVH2) and proline-rich central domains. 12-14 VASP has been found to be associated with focal adhesions, stress fibers, cell-cell contacts, and highly dynamic membrane regions in platelets, smooth muscle cells, endothelial cells, and fibroblasts. 11,15 VASP directly binds to profilin, zyxin, and to the focal adhesion and cell-cell cont...