Circulating aggregated platelets in coronary artery disease

Effects of P2Y(1) receptor antagonism on the reactivity of platelets from patients with stable coronary artery disease using aspirin and clopidogrel

BACKGROUND AND PURPOSE

P2Y(1) is a purine receptor that triggers platelet aggregation. Its inhibition was studied in patients with stable coronary artery disease (CAD) receiving standard anti-platelet therapy.

EXPERIMENTAL APPROACH

Blood samples from 10 patients on aspirin therapy (ASA, 80 mg·day(-1) ) were withdrawn before and 24 h after the administration of 450 mg clopidogrel (ASA/C) and were anti-coagulated with citrate or hirudin/PPACK in the presence or absence of the P2Y(1 ) inhibitor MRS2179 (M, 100 µM). Platelet responses to ADP (2.5 µM) and TRAP (2.5 µM), and collagen-induced thrombosis under flow conditions were analysed.

KEY RESULTS

Compared with ASA, ASA + M strongly inhibited ADP-induced peak platelet aggregation (88%), late aggregation (84%), P-selectin expression (85%) and α(IIb) β(3) activation (62%) (28%, 65%, 70% and 51% inhibition, respectively, for ASA/C vs. ASA). ASA + M also inhibited platelet/monocyte and platelet/neutrophil conjugate formation by 69% and 71% (57% and 59% for ASA/C vs. ASA). In TRAP-activated blood, ASA + M unexpectedly inhibited α(IIb) b(3) activation by 30%. In blood perfused in collagen-coated glass capillaries (shear rate of 1500 s(-1) ), ASA/C prevented thrombus growth beyond 5 min in relation to thrombus fragments embolization. ASA + M with or without clopidogrel completely prevented thrombus formation. Finally, ex vivo addition of MRS2179 and ASA to the blood of healthy donors markedly blocked thrombus formation on collagen in flow conditions, in contrast to ASA plus the P2Y(12) inhibitor 2-MeSAMP.

CONCLUSIONS AND IMPLICATIONS

Through particularly efficient complementarities with ASA to inhibit platelet activation and thrombus formation, the inhibition of P2Y(1) in the blood of patients with CAD appears to play a more important role than previously anticipated.

Optimizing antiplatelet therapy in acute coronary syndrome and percutaneous coronary intervention

Dual antiplatelet therapy with aspirin and clopidogrel is the standard of care for patients with acute coronary syndrome (ACS) and those undergoing percutaneous coronary intervention (PCI). It is well established that inhibition of platelet aggregation reduces the risk of recurrent thrombotic events and stent thrombosis. However, some patients show a reduced antiplatelet response to standard clopidogrel loading (300 mg) and maintenance (75 mg day(-1)) doses, which has been associated with poorer patient outcomes. Pharmacodynamic and pharmacokinetic studies show that higher-than-standard clopidogrel dosing strategies facilitate more rapid platelet inhibition of a greater intensity as a result of greater plasma concentrations of the clopidogrel active metabolite. Recently completed studies suggest that in patients with ACS undergoing PCI, higher-than-standard clopidogrel dosing regimens provide greater inhibition of platelet function and improved clinical outcomes with a small but significant increase in major bleeding. Newer, more potent antiplatelet agents such as prasugrel and ticagrelor are other alternative strategies that result in more rapid, greater inhibition of platelet function and better outcomes than standard-dose clopidogrel. Whether platelet reactivity-guided therapy or genotyping for cytochrome P450 polymorphisms is useful in managing patients needs to be further defined. Most importantly, early and effective antiplatelet therapy results in the best short- and long-term outcomes for patients with ACS or those undergoing PCI.

Platelet glycoprotein IIb/IIIa receptor inhibition in primary angioplasty for acute myocardial infarction: The new paradigm of direct revascularization

Acute myocardial infarction results from thrombotic occlusion superimposed on a ruptured athersoclerotic plaque. Immediate restoration of normal flow in the infarct-related artery can be achieved either with fibrinolytic or with direct mechanical revascularization. Primary PTCA has been shown to be superior to fibrinolytic therapy with respect to mortality, reinfarction, non-fatal stroke and length of hospitalization. Its results can be further improved by the addition of potent platelet inhibitors directed against the final common component of all stimuli for platelet aggregation, the glycoprotein (GP) IIb/IIIa receptor. In randomized clinical trials, primary angioplasty with adjunctive abciximab - a monoclonal antibody against the GP IIb/IIIa - was better than conventional primary angioplasty with heparin only. Abciximab use was associated with a significant reduction in reinfarction, need for urgent target vessel revascularization, microcirculatory dysfunction and regional left ventricular dysfunction as well as with a strong trend towards a reduction in mortality, even in patients receiving coronary stents.

Isosorbide dinitrate inhibits platelet adhesion and aggregation in nonthrombolyzed patients with acute myocardial infarction

BACKGROUND

Apart from their vasodilatatory properties, nitrates have been shown to inhibit platelet aggregation. The effects of nitrates on platelet adhesion have not been studied. Nonselected patients with acute myocardial infarction (AMI) have been suggested to gain no benefit from administration of nitrates. However, the importance of nitrates may be greater in a subgroup of nonthrombolyzed patients with AMI.

HYPOTHESIS

Isosorbide dinitrate (ISDN) decreases platelet adhesion and aggregation in nonthrombolyzed patients with AMI.

METHODS

Consecutive 48 men with AMI, not eligible for thrombolytic therapy because of late presentation (> 12 h), were prospectively randomized 2:1 to double-blind ISDN (mean dose 2.4 +/- 0.9 mg/h) (n = 33) or placebo (0.9% sodium chloride) (n = 15) infusion. All patients received aspirin. Blood samples were taken at baseline (no study medication) and 3 h into ISDN or placebo infusion. Platelet adhesion to collagen was measured in the ethylene diamine tetraacetic acid (EDTA)-platelet rich plasma by recording changes in light transmission with an optical aggregometer. Platelet aggregation was measured using the Born's method.

RESULTS

Isosorbide dinitrate significantly decreased both platelet adhesion and aggregation. No effect was seen in the placebo group.

CONCLUSIONS

In patients with AMI who do not receive thrombolytic therapy, ISDN effectively inhibits platelet adhesion and aggregation. These effects of nitrates may be of therapeutic and prognostic significance in this group of patients.

Platelet aggregation in acute coronary syndromes: use of a new aggregometer with laser light scattering to assess platelet aggregability

OBJECTIVE

Platelet aggregation has been implicated in the pathogenesis of acute coronary syndromes. Small aggregates consisting of < or = 100 platelets cannot be quantified with a conventional aggregometer employing optical density. Using a recently developed aggregometer based on laser light scattering, we studied platelet aggregability in patients with acute coronary syndromes.

METHODS

Peripheral blood samples were obtained from 39 patients with acute myocardial infarction or unstable angina who had received no prior antiplatelet or anticoagulant therapy, to be assayed immediately using a PA-100 platelet aggregometer. Blood samples from 14 healthy volunteers were used as controls.

RESULTS

Spontaneous formation of platelet aggregates was observed only in patients with acute coronary syndromes. The size of these aggregates was small, consisting of < or = 100 platelets (primary aggregation). Agonist-induced aggregation consisted of two phases. In the first few minutes, the number of small aggregates increased markedly (primary aggregation), followed by an increase in larger aggregates (secondary aggregation). The EC50 of epinephrine for primary aggregation was nearly 50 times lower in acute coronary patients than in controls (P < 0.001), while the EC50 for secondary aggregation was only 2 times lower (P < 0.001).

CONCLUSIONS

Aggregometry using light scattering suggests that platelet hyperaggregability and hypersensitivity in acute coronary syndromes may occur in primary but not secondary aggregation.

Analysis of low-speed rotational atherectomy for the reduction of platelet aggregation

Platelet activation is an important determinant of acute outcomes of percutaneous intervention. The objective of this study was to assess the effect of rotational atherectomy on platelet activation in an in vitro model. Freshly collected heparinized porcine blood was exposed to a 2.0-mm Rotablator burr rotating at one of three speeds: 180,000, 140,000, or 0 rpm. The specimens were analyzed immediately for concentration and size of platelet aggregates and plasma-free hemoglobin. There were significantly more platelet aggregates of >20-microm diameter at higher speeds (7,434+/-2,193 at 180,000, vs. 2,269+/-627 at 140,000, vs. 633+/-258 aggregates/ml at 0 rpm; P < 0.001). Plasma-free hemoglobin, a simple measure of cell damage, decreased with decreasing rotational speed (429+/-168 mg/dl at 180,000, vs. 88+/-44 mg/dl at 140,000, vs. 9+/-9 mg/dl at 0 rpm; P < 0.0001). In vitro, platelet activation decreases with decreasing burr speed, suggesting that the use of the Rotablator system at its minimum approved speed (140,000 rpm) could prove clinically beneficial.

Circulating aggregated platelets, number of platelets per aggregate, and platelet size during acute myocardial infarction

Circulating aggregated platelets (total, reversibly and irreversibly aggregated), the number of platelets per aggregate and "big" platelets were measured by a modification of the Wu and Hoak method in 42 patients on the first, second and fifth day of acute myocardial infarction (AMI). Among them, 30 had an uncomplicated course and 12 patients had complications that occurred between the 5th and 10th day of hospitalization (7 patients had reinfarction and 5 died). In all patients the measured parameters were elevated compared with those of control subjects. There was a significant increase, especially after the first observation day, in the values of total aggregated platelets (37 +/- 11% vs 26 +/- 12%, p < 0.001), reversibly aggregated platelets (27 +/- 9% vs 17 +/- 8%, p < 0.001) and the average platelets per aggregate (8.6 +/- 0.3 vs. 2.3 +/- 0.4) in patients with versus without complications. In considering the role of platelets in the development of AMI, these findings may add information to the role of platelets in determining the course of AMI.

Lovastatin therapy in heterozygous familial hypercholesterolaemic patients: effect on blood rheology and fibrinogen levels

Thirteen heterozygous familial hypercholesterolaemic patients were treated with lovastatin for 1 year, and were investigated for the effect on lipid profile, blood rheology and fibrinogen levels. A significant dose-dependent reduction in serum levels of total and LDL-cholesterol, Apo B and the ratio of total cholesterol to HDL-cholesterol was noted. Improvement in red blood cell filterability and an increase in fibrinogen levels were also observed. We conclude that the hypocholesterolaemic effect of lovastatin in familial hypercholesterolaemia is accompanied by changes in blood rheology. While some of these haemorheological changes may be considered beneficial, others may be regarded as unfavourable. The net effect of lovastatin therapy on the coronary risk of familial hypercholesterolaemic patients warrants further investigation.

Lovastatin therapy in hypercholesterolemia: effect on fibrinogen, hemorrheologic parameters, platelet activity, and red blood cell morphology

The effect of lovastatin therapy on blood rheology was investigated in 26 hypercholesterolemic patients. Treatment with lovastatin was associated with a significant improvement in whole blood filtration time and a tendency toward normalization in red blood cell morphology. A significant increase was observed in fibrinogen level, in ADP-induced platelet aggregation, in the percentage of "big" platelets, and in platelet count. The viscosity of whole blood and plasma and the percentage of aggregated platelets did not change significantly. The cause for these hemorrheologic changes and their role in influencing the coronary risk of lovastatin-treated hypercholesterolemic patients should be further investigated.

Temporal aggregate size distributions from simulation of platelet aggregation and disaggregation

A modification of the Smoluchowski collision theory for platelet aggregation is proposed with additional kinetic terms to accommodate observed disaggregation behavior. This model, consisting of a set of coupled, nonlinear, first-order differential equations, approximates size distributions with time for normal human platelets in plasma after the addition of a stimulus. Parameters controlling the kinetics of the formation and breakup of aggregates are numerically investigated. The aggregation coefficient, predominant during the aggregation phase, is strongly dependent on both time and aggregating agent doses. For the disaggregation phase, the disaggregation rate constants are a function of aggregate size, with a time-dependent disaggregation coefficient. Numerical results generated by the model are compared with experimental volume-size distribution curves from the literature.

Unstable angina

Unstable angina can manifest as an array of symptom complexes. In some patients, medical therapy will stabilize the episodes of angina, and only predismissal exercise testing or angiography (or both) will be necessary. At the other end of the spectrum are patients with rest angina or multiple episodes of silent ischemia who are refractory to medical therapy and experience undetected microinfarction. Most of these patients require immediate catheterization and subsequent intervention with intra-aortic balloon pulsation, percutaneous transluminal coronary angioplasty, or coronary artery bypass grafting. An entire spectrum of manifestations exists between these two extremes. One challenge during the 1990s will be better stratification of patients with unstable angina so that safe, efficient, cost-effective treatment strategies can be appropriately applied to all patients.

Deaggregation of human platelets in vitro by an RGD analog antagonist of platelet glycoprotein IIb/IIIa receptors

Binding of fibrinogen to platelet glycoprotein (GP) IIb/IIIa receptors is essential for normal platelet aggregation. We investigated whether inhibition of GP IIb/IIIa receptors with arginine-glycine-aspartate-O-methyltyrosine amide (RGDY), an analog of the receptor recognition sequence found in fibrinogen, is associated with platelet deaggregation. Platelets from five healthy human subjects that were maximally aggregated by addition of adenosine diphosphate to platelet-rich plasma were deaggregated in a dose-dependent manner by subsequent addition of RGDY (86.5 +/- 6.2%, 68.2 +/- 4.3%, 44.9 +/- 6.4%, and 31.6 +/- 4.1% for RGDY concentrations of 400, 200, 133, and 67 microM, respectively vs. 7.8 +/- 2.9% for saline control samples, p less than 0.0001). The extent of deaggregation was decreased as the time of addition of RGDY (400 microM) after maximal aggregation increased (85.6 +/- 6.7%, 58.5 +/- 12.6%, 47.1 +/- 2.7%, and 37.1 +/- 4.2% for 0, 1, 3, and 5 minute intervals, respectively, vs. 8.3 +/- 3.1% in control samples, n = 4, p less than 0.0001) consistent with the occurrence of irreversible aggregation. Thus, RGDY can rapidly and extensively deaggregate human platelets under certain conditions which may enhance its antithrombotic efficacy.

Angiographic morphology of coronary artery stenoses in prolonged rest angina: evidence of intracoronary thrombosis

Previous clinical and angiographic/histopathologic correlative studies have demonstrated that angiographic findings of occlusive thrombus, intraluminal filling defects and complex lesion morphology indicate the presence of intracoronary thrombosis. The purpose of this study was to determine whether the presence of these descriptors of intracoronary thrombosis is associated with the syndrome of prolonged rest angina. The coronary angiograms of 50 patients with prolonged rest angina without myocardial infarction (group I) and 42 concurrent patients with stable angina (group II) were reviewed without knowledge of the clinical syndrome. Patients with prior myocardial infarction, coronary angioplasty or coronary artery bypass graft surgery were excluded, as were patients with important aortic stenosis. Each coronary artery stenosis in a major epicardial vessel was evaluated for the presence or absence of intracoronary thrombus (defined using standard criteria), complex lesion morphology (defined as the presence of haziness, a smudged appearance or irregular lesion margins) and eccentricity, and the frequency of each of these findings in groups I and II was compared. Intracoronary thrombus was present significantly more often in group I patients (42%) than in group II patients (17%) (chi 2 5.77; p less than 0.02). Complex lesion morphology was also present significantly more often in group I (44%) than in group II (14%) patients (chi 2 8.17; p less than 0.01). Either standard criterion for intracoronary thrombus or complex morphology was present in 70% of group I but only 21% of group II patients (chi 2 19.7; p less than 0.001). These results support a strong association of the angiographic descriptors of intraluminal thrombosis with the clinical syndrome of prolonged rest angina.(ABSTRACT TRUNCATED AT 250 WORDS)