The active metabolite of prasugrel inhibits adenosine diphosphate- and collagen-stimulated platelet procoagulant activities

BACKGROUND

Prasugrel is a novel antiplatelet prodrug of the same thienopyridine class as clopidogrel and ticlopidine. Metabolism of prasugrel generates the active metabolite R-138727, an antagonist of the platelet P2Y(12) adenosine diphosphate (ADP) receptor, leading to inhibition of ADP-mediated platelet activation and aggregation. ADP also enhances the platelet response to collagen, and these two agonists contribute to the generation of platelet procoagulant activity. We therefore examined whether R-138727 inhibits ADP- and collagen-triggered platelet procoagulant activities.

METHODS AND RESULTS

As shown by whole blood flow cytometry, R-138727 inhibited surface phosphatidylserine expression on ADP plus collagen-stimulated platelets and tissue factor (TF) expression on ADP-, collagen-, and ADP plus collagen-stimulated monocyte-platelet aggregates. R-138727 reduced monocyte-platelet aggregate formation, thereby further inhibiting TF expression. ADP, collagen, and ADP plus collagen accelerated the kinetics of thrombin generation in recalcified whole blood and R-138727 significantly inhibited this acceleration. Clot strength in a modified thromboelastograph system was also inhibited by R-138727 (IC50 0.7 +/- 0.1 microM).

CONCLUSIONS

In addition to its previously known inhibitory effects on platelet activation and aggregation, the active metabolite of prasugrel, R-138727, inhibits platelet procoagulant activity in whole blood (as determined by phosphatidylserine expression on platelets and TF expression on monocyte-platelet aggregates), resulting in the functional consequences of delayed thrombin generation and impaired clot development.

The active metabolite of prasugrel inhibits adenosine diphosphate- and collagen-stimulated platelet procoagulant activities

BACKGROUND

Prasugrel is a novel antiplatelet prodrug of the same thienopyridine class as clopidogrel and ticlopidine. Metabolism of prasugrel generates the active metabolite R-138727, an antagonist of the platelet P2Y(12) adenosine diphosphate (ADP) receptor, leading to inhibition of ADP-mediated platelet activation and aggregation. ADP also enhances the platelet response to collagen, and these two agonists contribute to the generation of platelet procoagulant activity. We therefore examined whether R-138727 inhibits ADP- and collagen-triggered platelet procoagulant activities.

METHODS AND RESULTS

As shown by whole blood flow cytometry, R-138727 inhibited surface phosphatidylserine expression on ADP plus collagen-stimulated platelets and tissue factor (TF) expression on ADP-, collagen-, and ADP plus collagen-stimulated monocyte-platelet aggregates. R-138727 reduced monocyte-platelet aggregate formation, thereby further inhibiting TF expression. ADP, collagen, and ADP plus collagen accelerated the kinetics of thrombin generation in recalcified whole blood and R-138727 significantly inhibited this acceleration. Clot strength in a modified thromboelastograph system was also inhibited by R-138727 (IC50 0.7 +/- 0.1 microM).

CONCLUSIONS

In addition to its previously known inhibitory effects on platelet activation and aggregation, the active metabolite of prasugrel, R-138727, inhibits platelet procoagulant activity in whole blood (as determined by phosphatidylserine expression on platelets and TF expression on monocyte-platelet aggregates), resulting in the functional consequences of delayed thrombin generation and impaired clot development.

Indices of platelet activation and the stability of coronary artery disease

AIM

To determine whether indices of platelet activation are associated with the stability of coronary artery disease (CAD).

METHODS

Platelet function was examined in 677 consecutive aspirin-treated patients presenting for cardiac catheterization. Patients were grouped into recent myocardial infarction (MI), no MI but angiographically documented CAD (non-MI CAD) and no angiographically detectible CAD (no CAD), as well as additional subgroups.

RESULTS

Compared with non-MI CAD or no CAD patients, more patients with recent MI had a shortened platelet function analyzer (PFA)-100 collagen-epinephrine closure time (CT) and increased circulating monocyte-platelet aggregates, neutrophil-platelet aggregates, activated platelet surface GPIIb-IIIa and plasma soluble CD40 ligand (sCD40L). More patients with non-MI CAD had shortened PFA-100 CTs and increased monocyte-platelet aggregates compared with patients with no CAD. Platelet surface P-selectin did not differ among the groups. Subgroup analysis revealed that decreasing PFA-100 CT correlated with the stability of CAD.

CONCLUSIONS

Indices of platelet activation, especially the PFA-100 CT, are associated with the stability of CAD, and may reflect plaque instability, an ongoing thrombotic state and/or reduced responsiveness to aspirin.

Evidence that pre-existent variability in platelet response to ADP accounts for 'clopidogrel resistance'

BACKGROUND

Clopidogrel is a widely used antithrombotic agent that inhibits the platelet P2Y(12) adenosine diphosphate (ADP) receptor. There is increasing interest in 'clopidogrel resistance'.

OBJECTIVES

To determine whether 'clopidogrel resistance' is accounted for by a pre-existent variability in platelet response to ADP.

METHODS

Platelet response to 20 microm ADP was analyzed by four independent whole blood flow cytometric assays: platelet surface activated GPIIb-IIIa, platelet surface P-selectin, monocyte-platelet aggregates and neutrophil-platelet aggregates. In 25 consecutive, non-aspirin-treated healthy subjects, we studied platelet response before and after clopidogrel administration. In addition, we studied the platelet response in 613 consecutive aspirinated patients with or without coronary artery disease (CAD, as determined by angiography) who had or had not been treated with clopidogrel. In these patients, we tested for homogeneity of variance across all durations of clopidogrel exposure and severity of CAD by estimating the 'goodness of fit' of two independent models.

RESULTS

In the healthy subjects, pre-clopidogrel response to ADP predicted post-clopidogrel response to ADP. In the patients, clopidogrel, as expected, inhibited the platelet response to ADP. However, irrespective of the duration of clopidogrel administration, the severity of CAD, and the dose of aspirin, clopidogrel did not increase the variance in the platelet response to ADP in any of the four assays of platelet response.

CONCLUSIONS

These studies provide evidence that 'clopidogrel resistance' is accounted for by a pre-existent variability in platelet response to ADP and this variability is not increased by clopidogrel administration.

Effects of platelet binding on whole blood flow cytometry assays of monocyte and neutrophil procoagulant activity

BACKGROUND

Monocytes and neutrophils form heterotypic aggregates with platelets initially via engagement of platelet surface P-selectin with leukocyte surface P-selectin glycoprotein ligand-1 (PSGL-1). The resultant intracellular signaling causes the leukocyte surface expression of tissue factor and activation of leukocyte surface Mac-1 (integrin alphaMbeta2, CD11b/CD18). The activation-dependent conformational change in monocyte surface Mac-1 results in the binding of coagulation factor Xa (FXa) and/or fibrinogen to Mac-1. The aim of this study was to develop whole blood flow cytometry assays of these procoagulant activities and to investigate the effects of platelet binding to monocytes and neutrophils.

METHODS

Citrate or D-Phe-Pro-Arg-chloromethylketone (PPACK) anticoagulated whole blood was incubated with monoclonal antibodies against CD14 (PECy5), CD42a (PE), FITC-conjugated test antibody and an agonist, and then fixed with FACS lyse. Appropriate isotype negative controls were prepared in parallel. A BD FACSCalibur was used to analyze monocytes and neutrophils, which were identified based on CD14 fluorescence, forward and 90 degrees light scatter. These populations were further gated into CD42a-positive (platelet-bound) and CD42a-negative (platelet-free). Geometric mean fluorescence and per cent positive data were collected for each subpopulation to measure the binding of test antibodies directed at CD42a, tissue factor, coagulation FXa, bound fibrinogen, activated Mac-1, and CD11b. Compensation controls were prepared on six normal donors prior to the study and these settings were used throughout the 10 donor study. Negative controls verified the lack of cross talk, particularly in the quantified FITC and PE parameters.

RESULTS

The physiologic agonists collagen and ADP increased monocyte-platelet and neutrophil-platelet aggregates and increased leukocyte surface Mac-1/CD11b and surface-bound tissue factor, FXa and fibrinogen. Whereas the increases in Mac-1/CD11b were mainly independent of leukocyte-platelet binding, the increases in surface-bound tissue factor, FXa and fibrinogen were mainly dependent on leukocyte-platelet binding.

CONCLUSIONS

(i) We have developed novel whole blood flow cytometry assays to measure bound tissue factor, coagulation FXa, fibrinogen, activated Mac-1 and CD11b on the surface of monocytes and neutrophils, allowing independent analysis of monocytes and neutrophils with and without surface-adherent platelets. (ii) The monocyte and neutrophil surface binding of tissue factor, FXa and fibrinogen is mainly dependent on platelet adherence to monocytes and neutrophils, whereas the monocyte and neutrophil surface expression of CD11b and activated Mac-1 is mainly independent of platelet adherence to monocytes and neutrophils.

GPIIb-IIIa antagonists reduce thromboinflammatory processes in patients with acute coronary syndromes undergoing percutaneous coronary intervention

OBJECTIVE

To investigate the effects of abciximab, eptifibatide and no GPIIb-IIIa antagonist (control) on soluble CD40 ligand (sCD40L) and the formation of leukocyte-platelet aggregates (LPA) in 98 ACS patients undergoing percutaneous coronary intervention (PCI).

BACKGROUND

sCD40L and LPA are increased in patients with ACS.

METHODS

sCD40L was measured by enzyme-linked immunosorbent assay (ELISA) and LPA by whole blood flow cytometry.

RESULTS

There were no baseline differences between the three groups in sCD40L and LPA. At the end of PCI, sCD40L was unchanged in the controls, decreased by 30% (P < 0.001) in the abciximab group and by 11% (P < 0.02) in the eptifibatide group. Eighteen to 24 h after PCI, sCD40L was unchanged in the controls, reduced 30% (P < 0.001) in the abciximab-treated group and 9% (P < 0.01) in the eptifibatide-treated group. At the end of PCI, circulating monocyte-platelet aggregates (MPA) were reduced by 12% (P = NS) in the abciximab-treated group, 13% in the eptifibatide-treated group (P = NS), but slightly increased in the controls (P = NS). Eighteen to 24 h after PCI, MPA were reduced by 41% (P < 0.001) compared to baseline in the abciximab-treated group, by 23% (P = NS) in the eptifibatide-treated group, and 15% (P = NS) in the controls. In contrast to control patients presenting while on clopidogrel, control patients presenting not on clopidogrel demonstrated a reduction in sCD40L and LPA 18-24 h post-PCI (P = NS). At low receptor occupancy, GPIIb-IIIa antagonists did not augment the release of sCD40L or the number of circulating LPA.

CONCLUSIONS

GPIIb-IIIa antagonists reduce circulating sCD40L and LPA formation in patients with ACS undergoing PCI. At low receptor occupancy, GPIIb-IIIa antagonists do not activate platelets.

Leukocyte-platelet aggregation, platelet surface P-selectin, and platelet surface glycoprotein IIIa after percutaneous coronary intervention: Effects of dalteparin or unfractionated heparin in combination with abciximab

BACKGROUND

Plaque disruption with resultant platelet activation and leukocyte-platelet aggregation is a pathophysiologic process common to both acute coronary syndromes and percutaneous coronary interventions. Unfractionated heparin is a standard antithrombotic therapy in patients with both acute coronary syndromes and in those undergoing percutaneous coronary interventions. Low-molecular-weight heparins have been reported to cause less platelet activation than unfractionated heparin.

METHODS

Monocyte-platelet aggregates, neutrophil-platelet aggregates, platelet surface P-selectin, and platelet surface glycoprotein (GP) IIIa were measured serially by whole blood flow cytometry in 40 patients with unstable angina (randomly assigned to either unfractionated heparin 70 U/kg or the low-molecular-weight heparin dalteparin 60 IU/kg) undergoing coronary intervention with planned abciximab administration (in 2, one-half-dose boluses). Assays were performed at baseline, 5 minutes after administration of either type of heparin, 10 minutes after the first bolus of abciximab, 10 minutes after second bolus of abciximab, and 8 to 10 and 16 to 24 hours after administration of either heparin.

RESULTS

No significant differences in clinical outcomes were observed between patients receiving either unfractionated heparin or dalteparin. The number of circulating P-selectin-positive platelets was increased by unfractionated heparin but not dalteparin, and abciximab reversed this increase. The number of circulating P-selectin-positive platelets was reduced below baseline levels in both treatment groups 8 to 10 and 16 to 24 hours after study drug administration. At 8 to 10 and 16 to 24 hours after administration of study drug, platelet degranulation in response to iso-thrombin receptor agonist peptide 1.5 mmol/L was significantly reduced by almost 50% (compared with immediately after study drug administration). Both unfractionated heparin and dalteparin significantly increased the numbers of circulating monocyte-platelet and neutrophil-platelet aggregates, which were subsequently reduced to baseline levels after administration of the second abciximab bolus and to below baseline at both 8 to 10 and 16 to 24 hours in all patients. After both unfractionated heparin and dalteparin administration, platelet surface GP IIIa expression was significantly increased compared with baseline at both 8 to 10 and 16 to 24 hours.

CONCLUSIONS

Dalteparin in combination with abciximab in patients with unstable angina undergoing coronary intervention appears to be safe. Unfractionated heparin, but not dalteparin, degranulates platelets in patients with unstable angina. Both heparins increase the number of circulating monocyte-platelet and neutrophil-platelet aggregates. Abciximab therapy during coronary interventions rapidly reduces the number of degranulated platelets and leukocyte-platelet aggregates.

Circulating monocyte-platelet aggregates are an early marker of acute myocardial infarction

OBJECTIVES

We investigated whether elevated levels of circulating monocyte-platelet aggregates (MPA) can be used to identify patients with acute myocardial infarction (AMI).

BACKGROUND

Commonly used blood markers of AMI reflect myocardial cell death, but do not reflect the earlier pathophysiologic processes of plaque rupture, platelet activation and resultant thrombus formation. Circulating MPA form after platelet activation.

METHODS

In a single center between October 1998 and November 1999, we measured circulating MPA in a blinded fashion by whole blood flow cytometry in 211 consecutive patients who presented to the emergency department (ED) with chest pain and were admitted to rule out AMI. Acute myocardial infarction was diagnosed by a CK-MB fraction greater than three times control.

RESULTS

Patients with AMI (n = 61), as compared with those without AMI (n = 150), had significantly higher numbers of circulating MPA (11.6 +/- 11.4 vs. 6.4 +/- 3.6, mean +/- SD, p < 0.0001). After controlling for age, the adjusted odds of developing AMI for patients in the 2nd, 3rd and 4th quartiles of MPA, in comparison with patients in the lowest quartile (odds ratio = 1.0), were 2.1 (95% confidence interval [CI]: 0.7, 6.8), 4.4 (95% CI: 1.5, 13.1) and 10.8 (95% CI: 3.6, 32.0), respectively. The number of circulating MPA in patients with AMI presenting within 4 h of symptom onset (14.4) was significantly greater than those presenting after 4 h (9.4) and after 8 h (7.0), (p < 0.001). Of the 61 patients with AMI, 35 (57%) had a normal creatine kinase isoenzyme ratio at the time of presentation to the ED, but had high levels of circulating MPA (13.3).

CONCLUSIONS

Circulating MPA are an early marker of AMI.

Circulating monocyte-platelet aggregates are a more sensitive marker of in vivo platelet activation than platelet surface P-selectin: studies in baboons, human coronary intervention, and human acute myocardial infarction

BACKGROUND

Platelet surface P-selectin is considered the "gold standard" marker of platelet activation. Degranulated, P-selectin-positive platelets, however, aggregate with leukocytes in vitro and rapidly lose surface P-selectin in vivo.

METHODS AND RESULTS

Flow cytometric tracking of autologous, biotinylated platelets in baboons enabled us to directly demonstrate for the first time in vivo that (1) infused degranulated platelets very rapidly form circulating aggregates with monocytes and neutrophils, and (2) 30 minutes after infusion of the degranulated platelets, the percentage of circulating monocytes aggregated with infused platelets persist at high levels, whereas the percentage of circulating neutrophils aggregated with infused platelets and the platelet surface P-selectin of nonaggregated infused platelets return to baseline. We therefore performed 2 clinical studies in patients with acute coronary syndromes. First, after percutaneous coronary intervention (n=10), there was an increased number of circulating monocyte-platelet (and to a lesser extent, neutrophil-platelet) aggregates but not P-selectin-positive platelets. Second, of 93 patients presenting to an Emergency Department with chest pain, patients with acute myocardial infarction (AMI) (n=9) had more circulating monocyte-platelet aggregates (34.2+/-10.3% [mean+/-SEM]) than patients with no AMI (n=84, 19.3+/-1.4%, P<0.05) and normal control subjects (n=10, 11.5+/-0.8%, P<0.001). Circulating P-selectin-positive platelets, however, were not increased in chest pain patients with or without AMI.

CONCLUSIONS

As demonstrated by 3 independent means (in vivo tracking of activated platelets in baboons, human coronary intervention, and human AMI), circulating monocyte-platelet aggregates are a more sensitive marker of in vivo platelet activation than platelet surface P-selectin.

Dissociation of glycoprotein IIb/IIIa antagonists from platelets does not result in fibrinogen binding or platelet aggregation

BACKGROUND

The primary mechanism of action of glycoprotein (GP) IIb/IIIa antagonists is inhibition of the final common pathway of platelet aggregation: fibrinogen binding to the GP IIb/IIIa complex. However, it has been reported that induction of fibrinogen binding and platelet aggregation is an intrinsic prothrombotic property of low-dose GP IIb/IIIa antagonists. These apparently paradoxical results have been extensively referenced in the cardiology literature.

METHODS AND RESULTS

By platelet aggregation and flow cytometry, we demonstrate that (1) dissociation of GP IIb/IIIa antagonists (abciximab, tirofiban, eptifibatide, or xemilofiban) from platelets does not result in platelet aggregation; (2) tirofiban and eptifibatide can induce a fibrinogen-binding-competent conformation of the GP IIb/IIIa receptor, but stable fibrinogen binding does not occur without fixation; (3) the slow off-rate of abciximab exposes only a small proportion of unblocked GP IIb/IIIa receptors at any time, and these also fail to stably bind fibrinogen; and (4) the GP IIb/IIIa antagonist-induced fibrinogen binding in some previously reported experiments was probably the result of artifactual thrombin generation.

CONCLUSIONS

Under physiological conditions, GP IIb/IIIa antagonists currently in clinical use do not have an intrinsic activating property that results in platelet aggregation or stable fibrinogen binding to GP IIb/IIIa.

Twenty-two year (1975 to 1997) trends in the incidence, in-hospital and long-term case fatality rates from initial Q-wave and non-Q-wave myocardial infarction: a multi-hospital, community-wide perspective

OBJECTIVES

The goal of this study was to examine long-term trends in the incidence, in-hospital and long-term mortality patterns in patients with an initial non-Q-wave myocardial infarction (NQWMI) as compared with those with an initial Q-wave myocardial infarction (QWMI).

BACKGROUND

Limited data are available describing trends in the incidence and mortality from an initial QWMI and NQWMI from a multi-hospital community-wide perspective.

METHODS

Our study was an observational study of 5,832 metropolitan Worcester, Massachusetts residents (1990 census = 437,000) hospitalized with validated initial acute MI in all greater Worcester hospitals during 11 annual periods between 1975 and 1997.

RESULTS

The incidence of QWMI progressively decreased between 1975/78 (incidence rate = 171/100,000 population) and 1997 (101/100,000 population). In contrast, the incidence of NQWMI progressively increased between 1975/78 (62/100,000 population) and 1997 (131/100,000 population). Hospital death rates were 19.5% for patients with QWMI and 12.5% for those with NQWMI. After controlling for various covariates, patients with QWMI remained at significantly increased risk for hospital mortality (adjusted odds ratio = 1.63; 95% confidence interval: 1.35, 1.97). While the hospital mortality of QWMI has progressively declined over time (1975/78 = 24%; 1997 = 14%), the in-hospital mortality for NQWMI has remained the same (1975/78 = 12%; 1997 = 12%). These trends remained after adjusting for potentially confounding prognostic factors. The multivariable adjusted two-year mortality after hospital discharge declined over time for patients with QWMI and NQWMI.

CONCLUSIONS

Despite impressive declines in the incidence, in-hospital and long-term mortality associated with QWMI, NQWMI is increasing in frequency and has the same in-hospital mortality now as it did 22 years ago.

Laser scanning cytometry: a novel method for the detection of platelet--endothelial cell adhesion

BACKGROUND

Adherence of platelets to endothelial cells may be a significant event in the development of vascular thrombosis. Existing models, which examine platelet-endothelial cell interactions, compromise endothelial cell integrity or use radioactivity to identify platelets that adhere to endothelial cells. We report a novel method for in vitro detection of platelet-endothelial cell adhesion that allows endothelial cells to remain as an intact monolayer and for visualization of individual platelets.

METHODS

Fluorescently labeled platelets were incubated with a confluent monolayer of endothelial cells. Laser scanning cytometry (LSC) identified platelets bound to endothelial cells based on their fluorescent signals.

RESULTS

LSC detection of platelets reliably reproduced well-described findings of thrombin-induced platelet-endothelial cell adhesion. Results demonstrating reduced adhesion with a glycoprotein IIb-IIIa-specific blocking monoclonal antibody confirmed the specificity of the LSC detection of platelet-endothelial cell adhesion.

CONCLUSIONS

LSC is a novel method for detecting platelet--endothelial cell adhesion. Its advantages over other methods are: (a) endothelial cells remain undisturbed and adherent throughout; (b) the ability to detect individual bound platelets and subpopulations; (c) the ability to store images and slides and then relocate, revisualize, and reanalyze individual cells or cell populations of interest; and (d) no radioactivity.

The cleaved peptide of PAR1 results in a redistribution of the platelet surface GPIb-IX-V complex to the surface-connected canalicular system

The only known function of the 41 amino acid cleaved peptide (TR1-41) of the seven transmembrane domain thrombin receptor (PARI) is to activate platelets (as determined by aggregation, surface P-selectin, and fibrinogen binding to activated GPIIb-IIIa). We now demonstrate that TR1-41 results in a concentration-dependent decrease in the platelet surface expression of each component of the GPIb-IX-V complex, as determined by flow cytometry with a panel of monoclonal antibodies (including 6D1, directed against the von Willebrand factor binding site on GPIbalpha, and TM60, directed against the thrombin binding site on GPIbalpha). TR1-41 also decreased ristocetin-induced platelet agglutination. Immunoblotting after incubation of platelets with TR1-41 revealed neither a loss of platelet GPIb nor increase in supernatant GPIb fragments. As demonstrated by immunoelectron microscopy, TR1-41 resulted in a redistribution of GPIb, GPIX, and GPV from the platelet surface to the surface-connected canalicular system (SCCS). In summary, the cleaved peptide (TR1-41) of PAR1 results in a redistribution of the platelet surface GPIb-IX-V complex to the SCCS, thereby negatively regulating the GPIbalpha binding sites for von Willebrand factor and thrombin.

Ten-year trends in the incidence, treatment, and outcome of Q-wave myocardial infarction

The benefits of coronary reperfusion and antiplatelet therapy for patients with Q-wave acute myocardial infarction (Q-AMI) are well established in the context of randomized, controlled trials. The use and recent impact of these and other therapies on the broader, community-wide population of patients with Q-AMI is less well established. Residents of the Worcester, Massachusetts, metropolitan area (1990 census population 437,000) hospitalized with confirmed Q-AMI in all metropolitan Worcester, Massachusetts, hospitals in 4 1-year periods between 1986 and 1997 comprised the sample of interest. We examined the rates of occurrence, use of reperfusion strategies, and hospital mortality in a cohort of 711 patients with Q-AMI treated early in the reperfusion era (1986 and 1988) in comparison to 669 patients with Q-AMI treated a decade later (1995 and 1997). The percentage of Q-AMI among all hospitalized patients with AMI decreased over the decade of reperfusion therapy: 52% in 1986 and 1988 versus 35% in 1995 and 1997 (p < 0.001). Use of reperfusion therapy for patients with Q-AMI increased from 22% to 57%, with a marked increase in the use of primary angioplasty over time (1% vs 16%). The profile of patients receiving reperfusion therapy also changed significantly over the study period. Marked increases in use of antiplatelet therapy, beta blockers, angiotensin-converting enzyme inhibitors, and decreased use of calcium channel blockers, were observed over time. The crude in-hospital case fatality rate declined from 19% (1986 and 1988) to 14% (1995 and 1997) in patients with Q-AMI. Results of a multivariable regression analysis showed lack of reperfusion therapy, older age, anterior wall AMI, and cardiogenic shock to be independent predictors of in-hospital mortality in patients with Q-AMI. Thus, the percentage of all AMI's presenting as Q-AMI, and hospital mortality after Q-AMI, has decreased significantly in the past 10 years. The decrease in mortality occurs in the setting of broader use of reperfusion and adjunctive therapy (including primary angioplasty).

GPIIb-IIIa antagonist-induced reduction in platelet surface factor V/Va binding and phosphatidylserine expression in whole blood

In addition to inhibition of platelet aggregation, GPIIb-IIIa antagonists may reduce thrombotic events via other mechanisms. In a novel whole blood flow cytometric system, we investigated the effects of GPIIb-IIIa antagonists, in the presence or absence of thrombin inhibitors, on platelet surface-bound factor V/Va and platelet surface phospholipids. Diluted venous blood was incubated with either buffer or a GPIIb-IIIa antagonist (abciximab, tirofiban, or eptifibatide). Some samples were pre-incubated with clinically relevant concentrations of unfractionated heparin (UFH), a low molecular weight heparin, a direct thrombin inhibitor, or buffer only. Platelets were then activated and labeled with mAb V237 (factor V/Va-specific) or annexin V (binds phosphatidylserine), fixed, and analyzed by flow cytometry. In the absence of thrombin inhibitors, GPIIb-IIIa antagonists (especially abciximab) significantly reduced agonist-induced platelet procoagulant activity, as determined by reduced binding of V237 and annexin V. At high pharmacologic concentrations, unfractionated heparin and enoxaparin, but not hirudin, further reduced factor V/Va binding to the surface of activated platelets in the presence of GPIIb-IIa antagonists. Agonist-induced platelet procoagulant activity was reduced in a patient with Glanzmann's thrombasthenia. We conclude that GPIIb-IIIa antagonists reduce platelet procoagulant activity in whole blood and heparin and enoxaparin augment this reduction. Fibrinogen binding to GPIIb-IIIa is important in the generation of platelet procoagulant activity.

Antithrombotic therapy in the cardiac catheterization laboratory: focus on antiplatelet agents

Pharmacologic advances in the use of antithrombotic agents have paralleled the technologic innovations used in patients undergoing coronary interventions. The recognition of the central role of platelets in the development of complications related to coronary interventions led to the investigation and subsequent routine use of several antiplatelet agents as adjuvants to coronary intervention. Thus, the oral agents aspirin and either ticlopidine or clopidogrel are routinely administered after coronary stenting. Intravenous glycoprotein (GP) IIb/IIIa antagonists have been extensively studied and reduce adverse cardiac events in patients undergoing coronary interventions, especially those receiving intracoronary stents. Despite the growing use of GP IIb/IIIa antagonists, much information remains unknown as to the proper dosing and the effects these agents have on other elements of the hemostatic and vascular systems.

Platelet glycoprotein ibalpha is a counterreceptor for the leukocyte integrin Mac-1 (CD11b/CD18)

The firm adhesion and transplatelet migration of leukocytes on vascular thrombus are both dependent on the interaction of the leukocyte integrin, Mac-1, and a heretofore unknown platelet counterreceptor. Here, we identify the platelet counterreceptor as glycoprotein (GP) Ibalpha, a component of the GP Ib-IX-V complex, the platelet von Willebrand factor (vWf) receptor. THP-1 monocytic cells and transfected cells that express Mac-1 adhered to GP Ibalpha-coated wells. Inhibition studies with monoclonal antibodies or receptor ligands showed that the interaction involves the Mac-1 I domain (homologous to the vWf A1 domain), and the GP Ibalpha leucine-rich repeat and COOH-terminal flanking regions. The specificity of the interaction was confirmed by the finding that neutrophils from wild-type mice, but not from Mac-1-deficient mice, bound to purified GP Ibalpha and to adherent platelets, the latter adhesion being inhibited by pretreatment of the platelets with mocarhagin, a protease that specifically cleaves GP Ibalpha. Finally, immobilized GP Ibalpha supported the rolling and firm adhesion of THP-1 cells under conditions of flow. These observations provide a molecular target for disrupting leukocyte-platelet complexes that promote vascular inflammation in thrombosis, atherosclerosis, and angioplasty-related restenosis.

Evaluation of platelet function by flow cytometry

Platelet function in whole blood can be comprehensively evaluated by flow cytometry. Flow cytometry can be used to measure platelet reactivity, circulating activated platelets, platelet-platelet aggregates, leukocyte-platelet aggregates, procoagulant platelet-derived microparticles, and calcium flux. Clinical applications of whole blood flow cytometric assays of platelet function in disease states (e.g., acute coronary syndromes, angioplasty, and stroke) may include identification of patients who would benefit from additional antiplatelet therapy and prediction of ischemic events. Circulating monocyte-platelet aggregates appear to be a more sensitive marker of in vivo platelet activation than circulating P-selectin-positive platelets. Flow cytometry can also be used in the following clinical settings: monitoring of GPIIb-IIIa antagonist therapy, diagnosis of inherited deficiencies of platelet surface glycoproteins, diagnosis of storage pool disease, diagnosis of heparin-induced thrombocytopenia, and measurement of the rate of thrombopoiesis.

Platelet GP IIIa Pl(A) polymorphisms display different sensitivities to agonists

BACKGROUND

Both inherited predisposition and platelet hyperreactivity have been associated with ischemic coronary events, but mechanisms that support genetic differences among platelets from different subjects are generally lacking. Associations between the platelet Pl(A2) polymorphism of GP IIIa and coronary syndromes raise the question as to whether this inherited variation may contribute to platelet hyperreactivity.

METHODS AND RESULTS

In this study, we characterized functional parameters in platelets from healthy donors with the Pl(A) (HPA-1) polymorphism, a Leu (Pl(A1)) to Pro (Pl(A2)) substitution at position 33 of the GP IIIa subunit of the platelet GP IIb/IIIa receptor (integrin alpha(IIb)beta(3)). We studied 56 normal donors (20 Pl(A1,A1), 20 Pl(A1,A2), and 16 Pl(A2,A2)). Compared with Pl(A1,A1) platelets, Pl(A2)-positive platelets showed a gene dosage effect for significantly greater surface-expressed P-selectin, GP IIb/IIIa-bound fibrinogen, and activated GP IIb/IIIa in response to low-dose ADP. Surface expression of GP IIb/IIIa was similar in resting platelets of all 3 genotypes but was significantly greater on Pl(A2,A2) platelets after ADP stimulation (P=0.003 versus Pl(A1,A1); P=0.03 versus Pl(A1,A2)). Pl(A1,A2) platelets were more sensitive to inhibition of aggregation by pharmacologically relevant concentrations of aspirin and abciximab.

CONCLUSIONS

Pl(A2)-positive platelets displayed a lower threshold for activation, and platelets heterozygous for Pl(A) alleles showed increased sensitivity to 2 antiplatelet drugs. These in vitro platelet studies may have relevance for in vivo thrombotic conditions.

Chronic venous insufficiency is associated with increased platelet and monocyte activation and aggregation

PURPOSE

This study assessed whether the increased numbers of platelet-monocyte aggregates observed in patients with venous stasis ulceration (VSU) represent a response to dermal ulceration or if it is a condition associated with underlying chronic venous insufficiency (CVI). We also analyzed the expression of CD11b in patients with CVI to determine whether leukocyte activation, known to occur in VSU, is a precursor of or a response to ulceration.

METHODS

Patients with varying classes of CVI (n = 24) and healthy control subjects (n = 15), whose status was documented by means of duplex scanning, stood upright and stationary for 10 minutes. Two aliquots of blood, drawn from a distal leg vein and an antecubital fossa vein, were incubated with either buffer or one of three platelet agonists. After fixation, these samples were further incubated with fluorescent-labeled monoclonal antibodies (f-MoAb) specific for CD14 (monocytes) and CD61 (platelets). The activated leukocyte assay was performed by incubating another aliquot of the blood samples with f-MoAb specific for CD11b and CD14. All samples were evaluated by means of flow cytometry.

RESULTS

We observed significantly more platelet-monocyte aggregates throughout the circulation in patients with CVI than in control subjects (29% vs. 8%; P <.0002). Furthermore, patients with CVI formed significantly more of these aggregates in response to all platelet agonists than did control subjects. There were no significant differences between baseline numbers of aggregates or response to agonists in patients who had CVI with (n = 10) or without (n = 14) ulceration. Patients with CVI had more circulating platelet-neutrophil aggregates than control subjects (7.2% vs. 3.6%; P =.05). The addition of platelet agonists to the blood of patients with CVI resulted in more platelet-neutrophil aggregates than in control subjects. Monocyte CD11b expression was higher in patients with CVI than in control subjects (7.5 vs. 3.7; P <.01), with no differences noted in CD11b expression between patients with or without ulceration. Neutrophil CD11b expression was low and similar in control subjects and patients with CVI.

CONCLUSION

All classes of CVI are associated with significantly increased percentages of platelet-monocyte aggregates and increased percentages of platelet-neutrophil aggregates throughout the circulation. The presence of more of these aggregates and the increased propensity to form aggregates in the presence of platelet agonists in all classes of CVI suggests an underlying state of platelet activation and increased reactivity that is independent of the presence of ulceration. The increased expression of monocyte CD11b throughout the circulation in all classes of CVI suggests that although systemic monocyte activation occurs in CVI, its presence is independent of VSU as well.

Laboratory markers of platelet activation and their clinical significance

Whole blood flow cytometry is a powerful new laboratory technique for assessment of platelet activation and function. Flow cytometry can be used to measure platelet hyperreactivity, circulating activated platelets, leukocyte-platelet aggregates, and procoagulant platelet-derived microparticles in a number of clinical settings, including acute coronary syndromes, angioplasty, cardiopulmonary bypass, acute cerebrovascular ischemia, peripheral vascular disease, diabetes mellitus, preeclampsia, and Alzheimer's disease. Clinical applications of whole blood flow cytometric assays of platelet function in these diseases may include identification of patients who would benefit from additional antiplatelet therapy and prediction of ischemic events. Circulating monocyte-platelet aggregates appear to be a more sensitive marker of in vivo platelet activation than circulating P-selectin-positive platelets. Flow cytometry can also be used in the following clinical settings: monitoring of glycoprotein IIb-IIIa antagonist therapy, diagnosis of inherited deficiencies of platelet surface glycoproteins, diagnosis of storage pool disease, diagnosis of heparin-induced thrombocytopenia, and measurement of the rate of thrombopoiesis.

Patients with venous stasis ulceration have increased monocyte-platelet aggregation

PURPOSE

Leukocyte activation has been implicated in the pathogenesis of venous stasis ulceration, but the involvement of activated platelets and leukocyte-platelet aggregates has not been previously investigated. The purpose of this study was to determine whether patients with venous stasis ulceration have increased platelet activation and a propensity toward formation of leukocyte-platelet aggregates.

METHODS

Blood was drawn from the superficial veins of the leg just proximal to a venous stasis ulcer and from an antecubital vein in 14 patients with venous stasis ulceration. Blood was also drawn from the antecubital vein of 14 volunteers without evidence of venous disease. Whole-blood flow cytometry was used to analyze the samples before and after activation with a panel of agonists for evidence of platelet activation and the formation of leukocyte-platelet aggregates.

RESULTS

Patients with venous stasis ulceration had a greater number of monocyte-platelet aggregates in both the arm and leg samples than did the control subjects (p < 0.01). Furthermore, antecubital blood samples from patients with venous stasis ulceration stimulated with either thrombin-receptor agonist peptide, adenosine diphosphate, or phorbol myristate acetate formed more monocyte-platelet aggregates than did control samples (p < 0.05). No differences in platelet activation or neutrophil-platelet aggregate formation were noted among the three sample groups.

CONCLUSIONS

Patients with venous stasis ulceration have an increase in the number of monocyte-platelet aggregates in systemic venous blood as well as in venous blood adjacent to a venous stasis ulcer, implicating the monocyte as the leukocyte involved in the pathogenesis of venous stasis ulceration. No association was identified between the presence of a venous stasis ulcer and either neutrophil-platelet aggregation or the activation of individual platelets. Because platelet activation is necessary for the formation of monocyte-platelet aggregates, these data also suggest that monocyte-platelet aggregation is a more sensitive marker for in vivo platelet activation than is the identification of individual activated platelets.

The cleaved peptide of the thrombin receptor is a strong platelet agonist

Thrombin cleaves its G-protein-linked seven-transmembrane domain receptor, thereby releasing a 41-aa peptide and generating a new amino terminus that acts as a tethered ligand for the receptor. Peptides corresponding to the new amino terminal end of the proteolyzed seven-transmembrane domain thrombin receptor [TR42-55, SFLLRNPNDKYEPF, also known as TRAP (thrombin receptor-activating peptide)], previously have been demonstrated to activate the receptor. In this study, we demonstrate that the 41-aa cleaved peptide, TR1-41 (MGPRRLLLVAACFSLCGPLLSARTRARRPESKATNATLDPR) is a strong platelet agonist. TR1-41 induces platelet aggregation. In whole-blood flow cytometric studies, TR1-41 was shown to be more potent than TR42-55 and almost as potent as thrombin, as determined by the degree of increase in: (i) platelet surface expression of P-selectin (reflecting alpha granule secretion); (ii) exposure of the fibrinogen binding site on the glycoprotein (GP) IIb-IIIa complex; and (iii) fibrinogen binding to the activated GPIIb-IIIa complex. As determined by experiments with inhibitors [prostaglandin I2, staurosporine, wortmannin, the endothelium-derived relaxing factor congener S-nitroso-N-acetylcysteine (SNAC), EDTA, EGTA, and genestein], and with Bernard-Soulier or Glanzmann's platelets, we demonstrated that TR1-41-induced platelet activation is: (i) inhibited by cyclic AMP; (ii) mediated by protein kinase C, phosphatidyl inositol-3-kinase, myosin light chain kinase, and intracellular protein tyrosine kinases; (iii) dependent on extracellular calcium; and (iv) independent of the GPIb-IX and GPIIb-IIIa complexes. TR1-41-induced platelet activation was synergistic with TR42-55. In summary, the cleaved peptide of the seven-transmembrane domain TR (TR1-41) is a strong platelet agonist.

Increased platelet reactivity and circulating monocyte-platelet aggregates in patients with stable coronary artery disease

OBJECTIVES

We sought to examine whether patients with stable coronary artery disease (CAD) have increased platelet reactivity and an enhanced propensity to form monocyte-platelet aggregates.

BACKGROUND

Platelet-dependent thrombosis and leukocyte infiltration into the vessel wall are characteristic cellular events seen in atherosclerosis.

METHODS

Anticoagulated peripheral venous blood from 19 patients with stable CAD and 19 normal control subjects was incubated with or without various platelet agonists and analyzed by whole blood flow cytometry.

RESULTS

Circulating degranulated platelets were increased in patients with CAD compared with control subjects (mean [+/- SEM] percent P-selectin-positive platelets: 2.1 +/- 0.2 vs. 1.5 +/- 0.2, p < 0.01) and were more reactive to stimulation with 1 micromol/liter of adenosine diphosphate (ADP) (28.7 +/- 3.9 vs. 16.1 +/- 2.2, p < 0.01), 1 micromol/liter of ADP/epinephrine (51.4 +/- 4.6 vs. 37.5 +/- 3.8, p < 0.05) or 5 micromol/liter of thrombin receptor agonist peptide (TRAP) (65.7 +/- 6.8 vs. 20.2 +/- 5.1, p < 0.01). Patients with stable CAD also had increased circulating monocyte-platelet aggregates compared with control subjects (percent platelet-positive monocytes: 15.3 +/- 3.0 vs. 6.3 +/- 0.9, p < 0.01). Furthermore, patients with stable CAD formed more monocyte-platelet aggregates than did control subjects when their whole blood was stimulated with 1 micromol/liter of ADP (50.4 +/- 4.5 vs. 28.1 +/- 5.3, p < 0.01), 1 micromol/liter of ADP/epinephrine (60.7 +/- 4.3 vs. 48.0 +/- 4.8, p < 0.05) or 5 micromol/liter of TRAP (67.6 +/- 5.7 vs. 34.3 +/- 7.0, p < 0.01).

CONCLUSIONS

Patients with stable CAD have circulating activated platelets, circulating monocyte-platelet aggregates, increased platelet reactivity and an increased propensity to form monocyte-platelet aggregates.

Effect of elevated leukocyte count on in-hospital mortality following acute myocardial infarction

The peripheral white blood cell count on presentation with acute myocardial infarction directly correlates with short-term in-hospital mortality. This association is independent of other prognostic factors, including extent and size of the acute myocardial infarction.

Effects of nitric oxide/EDRF on platelet surface glycoproteins

We examined the effects of nitric oxide (NO)/endothelium-derived relaxing factor (EDRF) on platelet surface glycoproteins (GP). As determined by flow cytometry, in both a washed platelet system and platelet-rich plasma, the EDRF congener (S-nitroso-N-acetylcysteine) markedly inhibited both the thrombin-induced and the (stable thromboxane A2 analogue) U-46619-induced upregulation of P-selectin (alpha-granule protein), CD63 (lysosomal protein), and the GPIIb-IIIa complex (fibrinogen receptor) but minimally inhibited downregulation of the GPIb-IX complex (von Willebrand factor receptor). The inhibitory effects of EDRF were markedly reduced in whole blood or by the addition of washed erythrocytes. Platelets in whole blood were still responsive to guanosine 3',5'-cyclic monophosphate (cGMP), as shown by complete inhibition of P-selectin upregulation by the stable analogue N6,2'-O dibutyryl cGMP. These data suggests that 1) cGMP negatively regulates the platelet surface expression of P-selectin, CD63, and the GPIIb-IIIa complex but not the platelet surface expression of the GPIb-IX complex and 2) hemoglobin within erythrocytes inhibits the effects of EDRF/NO on platelet surface glycoproteins.

The platelet surface expression of glycoprotein V is regulated by two independent mechanisms: proteolysis and a reversible cytoskeletal-mediated redistribution to the surface-connected canalicular system

In this study, we show that the platelet surface expression of glycoprotein (GP) V is regulated by two independent mechanisms. While confirming that both thrombin and neutrophil elastase proteolyse GPV, we show that neutrophil cathepsin G, thrombin receptor activating peptide (TRAP), and a combination of ADP and epinephrine can each result in a decrease in the platelet surface expression of GPV by a nonproteolytic mechanism: a cytoskeletal-mediated redistribution of platelet surface GPV to the surface-connected canalicular system (SCCS). Four independent lines of evidence documented the nonproteolytic nature of this decrease in the platelet surface expression of GPV. First, flow cytometric studies showed that cathepsin G, TRAP, and ADP/epinephrine decreased the platelet surface expression of GPV without changing the total platelet content of GPV. Second, immunoelectron microscopy directly demonstrated translocation of GPV from the platelet surface to the SCCS. Third, the cathepsin G-, TRAP-, and ADP/epinephrine-induced decreases in platelet surface GPV were fully reversible. Fourth, cytochalasin B, an inhibitor of actin polymerization, completely inhibited the cathepsin G-, TRAP-, and ADP/epinephrine-induced decreases in platelet surface GPV. The cytoskeletal-mediated redistribution of GPV occurred in a whole blood milieu and at physiologic temperatures (37 degrees C) and extracellular calcium concentrations (2 mmol/L). This study also defines the diverse effects on GPV, GPIb, and GPIX of multiple important platelet agonists. Cathepsin G proteolysed platelet surface GPIb alpha, but redistributed platelet surface GPIX and GPV to the SCCS. Thrombin proteolysed platelet surface GPV, but redistributed platelet surface GPIb and GPIX to the SCCS. Both TRAP and ADP/epinephrine redistributed platelet surface GPIb, GPIX, and GPV to the SCCS. Elastase proteolysed platelet surface GPIb alpha and GPV, but, unlike the other agonists tested, neither proteolysed nor redistributed platelet surface GPIX. The experiments with TRAP showed that activation of the seven-transmembrane domain thrombin receptor can result in translocation of GPIb, GPIX, and GPV to the SCCS independently of the GPIb-mediated pathway of thrombin-induced platelet activation. This study also provides two additional lines of support for the recent report that GPV is noncovalently complexed with GPIb and GPIX in the platelet surface membrane. First, although only the GPIb alpha subunit of this putative complex is known to be directly linked to the platelet cytoskeleton via actin-binding protein, cytochalasin B inhibited the ADP/epinephrine-, cathepsin G-, and TRAP-induced decrease in platelet surface GPV. Second, triple labeling flow cytometric experiments showed that, on each individual platelet, the ADP/epinephrine-induced decrease and subsequent return of the platelet surface expression of GPV occurred simultaneously with the decrease and subsequent return of the platelet surface expression of GPIb. In summary, the platelet surface expression of GPV is regulated by two independent mechanisms: proteolysis and a reversible, cytoskeletal-mediated redistribution to the SCCS.

Platelet tyrosine kinases and fibrinogen receptor activation

Platelet adhesion and aggregation during hemostasis and thrombosis are usually limited to sites where the integrity of the vessel wall is disrupted. The high concentration of platelet agonists within these sites represents a putative control mechanism for targeting platelet activation. Although much has been learned about the intracellular signaling systems controlling platelet activation, our understanding of the connection between signaling molecules and platelet aggregation remains limited. Tyrosine kinases are important signaling enzymes in cells and are abundant in platelets. Previous reports indicate that binding of glycoprotein IIb-IIIa (GPIIb-IIIa) to fibrinogen can induce the tyrosine phosphorylation of specific substrates. We show that, in turn, protein tyrosine kinase activity is necessary for agonist-induced activation of GPIIb-IIIa. Genistein and the tyrphostin AG-18 are two specific tyrosine kinase inhibitors, and the former has been shown to inhibit platelet aggregation. We use genistein and AG-18 in the present study to demonstrate that aggregation inhibition is due to suppression of GPIIb-IIIa activation. In contrast, genistin, an isoflavone compound related to genistein, and acetylsalicylic acid do not affect the tyrosine kinase-signaling pathway, nor do they inhibit GPIIb-IIIa activation induced by strong agonists. On identifying prominent tyrosine kinase substrates in activated platelets, we confirm that several substrates correspond to proteins associated with the cytoskeleton: the 85-kD subunit of phosphatidylinositol 3-kinase, the SH3-containing and actin-associating p85, pp60Src, and pp125FAK.(ABSTRACT TRUNCATED AT 250 WORDS)

The control of actin nucleotide exchange by thymosin beta 4 and profilin. A potential regulatory mechanism for actin polymerization in cells

We present evidence for a new mechanism by which two major actin monomer binding proteins, thymosin beta 4 and profilin, may control the rate and the extent of actin polymerization in cells. Both proteins bind actin monomers transiently with a stoichiometry of 1:1. When bound to actin, thymosin beta 4 strongly inhibits the exchange of the nucleotide bound to actin by blocking its dissociation, while profilin catalytically promotes nucleotide exchange. Because both proteins exchange rapidly between actin molecules, low concentrations of profilin can overcome the inhibitory effects of high concentrations of thymosin beta 4 on the nucleotide exchange. These reactions may allow variations in profilin concentration (which may be regulated by membrane polyphosphoinositide metabolism) to control the ratio of ATP-actin to ADP-actin. Because ATP-actin subunits polymerize more readily than ADP-actin subunits, this ratio may play a key regulatory role in the assembly of cellular actin structures, particularly under circumstances of rapid filament turnover.

Aging and the complexity of cardiovascular dynamics

Biomedical signals often vary in a complex and irregular manner. Analysis of variability in such signals generally does not address directly their complexity, and so may miss potentially useful information. We analyze the complexity of heart rate and beat-to-beat blood pressure using two methods motivated by nonlinear dynamics (chaos theory). A comparison of a group of healthy elderly subjects with healthy young adults indicates that the complexity of cardiovascular dynamics is reduced with aging. This suggests that complexity of variability may be a useful physiological marker.