Antiplatelet effects of clopidogrel compared with aspirin after myocardial infarction: enhanced inhibitory effects of combination therapy.

OBJECTIVES: We sought to compare the inhibitory effects of the combination of two doses of aspirin plus clopidogrel with either drug alone on platelet aggregation and activation. BACKGROUND: Enhanced platelet inhibitory effects of clopidogrel by aspirin on platelet aggregation and activation are suggested by experimental studies but have not been shown in humans. METHODS: The effects of clopidogrel 75 mg or aspirin 100 (300) mg on platelet aggregation and activation by flow cytometry after stimulation with various agonists were determined in 30 patients with a past history of myocardial infarction. RESULTS: Clopidogrel alone or in combination with aspirin markedly inhibited adenosine diphosphate (ADP)-mediated platelet aggregation compared with monotherapy with aspirin (24.6 +/- 3.3% or 26.6 +/- 2.7% vs. 44.7 +/- 2.9%; p < 0.001). Combined treatment significantly inhibited collagen-induced aggregation compared with aspirin and clopidogrel (16.4 +/- 2.4%, 36.5 +/- 4.2% and 59.3 +/- 5.1%, respectively;, p < 0.001) and resulted in considerable inhibition of aggregation induced by thrombin receptor agonist peptide (TRAP, p < 0.03). Clopidogrel with or without aspirin significantly suppressed expression of platelet activation markers CD 62p, CD 63 and PAC-1 after stimulation with ADP or thrombin (p < 0.001). In addition, the combined treatment was more effective than either agent alone after activation with low dose thrombin (p < 0.05). Both doses of aspirin equally potentiated the platelet inhibitory effects of clopidogrel. CONCLUSIONS In this prospective clinical ex vivo platelet study, clopidogrel was more effective than aspirin in inhibiting ADP-mediated platelet aggregation and activation. Clopidogrel in combination with aspirin showed synergistic inhibitory effects after stimulation with collagen and thrombin compared with monotherapies. Thus, this dual antiplatelet treatment strategy deserves further evaluation in clinical trials for secondary prevention of acute myocardial infarction or unstable angina.

Association of two silent polymorphisms of platelet glycoprotein Ia/IIa receptor with risk of myocardial infarction: a case-control study.

BACKGROUND: The platelet membrane glycoprotein Ia/IIa plays a major part in platelet function as a primary receptor for collagen. A previous report showed a variation of glycoprotein Ia/IIa receptor density and function associated with two silent and linked polymorphisms (807C/T and 873G/A) within the glycoprotein Ia gene. Because platelet thrombus formation is implicated in the pathogenesis of acute myocardial infarction, we investigated these polymorphisms among patients who had had a myocardial infarction. METHODS: We did a 2/1 case-control study including 177 patients (median age 57.0 [range 32-72] years) and 89 controls with same age and sex. Distributions of the 807C/T and 873G/A polymorphisms were investigated by genotyping DNA by PCR, single-strand conformation polymorphism analysis, and sequencing. FINDINGS: The prevalence of the homozygous 807T/873A genotype was 2.9 times higher among patients with myocardial infarction than among controls (16.4% vs 5.6%, p=0.022). There was an association between patients homozygous for the 807T/873A allele and myocardial infarction (odds ratio 3.3 [95% CI 1.2-8.8]), which was strongest in a subgroup of smokers. The homozygous 807T/873A genotype remained an independent risk factor for myocardial infarction (p=0.005) when age, sex, smoking, hypertension, diabetes, body-mass index, LDL-cholesterol and HDL-cholesterol, and fibrinogen were adjusted for by logistic regression. INTERPRETATION: The 807T/873A homozygosity of the platelet glycoprotein Ia/IIa gene polymorphism, associated with differences in surface collagen receptor density and activity, appears to be an independent risk factor for acute myocardial infarction. Our findings need to be confirmed in a larger, prospective study that includes patients from different populations and cardiovascular risk groups.

Fine structural and functional consequences of deglycosylation of the platelet adhesion receptor GPIb-IX (CD 42b).

To investigate the role of the glycosylation of the platelet receptor glycoprotein Ib (GPIb, CD 42b), platelets and purified GPIb were deglycosylated by neuraminidase, O- and N-glycosidases. N-deglycosylation and neuraminic-acid cleavage had little effect on ristocetin and botrocetin-induced platelet agglutination. However, O-deglycosylation reduced the response by approximately 50%, and total deglycosylation (the combination of all three glycosidases) fully abolished the response to ristocetin. Interestingly, binding of von Willebrand Factor (vWF) to purified GPIb in the presence of ristocetin and botrocetin in a standardized microtiter plate assay was not altered by partial or even by total deglycosylation. Electron microscopy indicated that the normally stretched approximately 50 nm long molecule was approximately 32 nm after N-deglycosylation, approximately 20 nm after O-deglycosylation, and reduced in a approximately 15 nm long collapse by total deglycosylation. These results suggest that deglycosylation has major structural impacts on GPIb, strongly impairing platelet-vWF interactions; however, vWF binding to isolated GPIb remains unaffected.