Biochemical evidence of platelet activation in patients with persistent unstable angina

The beneficial effect of sulforaphane on platelet responsiveness during caloric load: a single-intake, double-blind, placebo-controlled, crossover trial in healthy participants

Background and aims

As our understanding of platelet activation in response to infections and/or inflammatory conditions is growing, it is becoming clearer that safe, yet efficacious, platelet-targeted phytochemicals could improve public health beyond the field of cardiovascular diseases. The phytonutrient sulforaphane shows promise for clinical use due to its effect on inflammatory pathways, favorable pharmacokinetic profile, and high bioavailability. The potential of sulforaphane to improve platelet functionality in impaired metabolic processes has however hardly been studied in humans. This study investigated the effects of broccoli sprout consumption, as a source of sulforaphane, on urinary 11-dehydro-thromboxane B₂ (TXB₂), a stable thromboxane metabolite used to monitor eicosanoid biosynthesis and response to antithrombotic therapy, in healthy participants exposed to caloric overload.

Methods

In this double-blind, placebo-controlled, crossover trial 12 healthy participants were administered 16g of broccoli sprouts, or pea sprouts (placebo) followed by the standardized high-caloric drink PhenFlex given to challenge healthy homeostasis. Urine samples were collected during the study visits and analyzed for 11-dehydro-TXB₂, sulforaphane and its metabolites. Genotyping was performed using Illumina GSA v3.0 DTCBooster.

Results

Administration of broccoli sprouts before the caloric load reduced urinary 11-dehydro-TXB₂ levels by 50% (p = 0.018). The amount of sulforaphane excreted in the urine during the study visits correlated negatively with 11-dehydro-TXB₂ (r_s = -0.377, p = 0.025). Participants carrying the polymorphic variant NAD(P)H dehydrogenase quinone 1 (NQO1*2) showed decreased excretion of sulforaphane (p = 0.035).

Conclusion

Sulforaphane was shown to be effective in targeting platelet responsiveness after a single intake. Our results indicate an inverse causal relationship between sulforaphane and 11-dehydro-TXB₂, which is unaffected by the concomitant intake of the metabolic challenge. 11-Dehydro-TXB₂ shows promise as a non-invasive, sensitive, and suitable biomarker to investigate the effects of phytonutrients on platelet aggregation within hours.

Clinical trial registration

[https://clinicaltrials.gov/], identifier [NCT05146804].

Impairment of platelet NO signalling in coronary artery spasm: role of hydrogen sulphide

BACKGROUND AND PURPOSE

The pathophysiology of coronary artery spasm (CAS), with its associated ischaemic crises, is currently poorly understood and treatment is frequently ineffective. In view of increasing evidence that platelet-based defects may occur in CAS patients, we investigated platelet reactivity in CAS patients and whether symptomatic crises reflect activation of platelet-endothelial interactions.

EXPERIMENTAL APPROACH

CAS patients were evaluated during acute and/or chronic symptomatic phases and compared with healthy control subjects. Inhibition of ADP-induced platelet aggregation by the NO donor sodium nitroprusside (SNP) and plasma concentrations of syndecan 1 (glycocalyx shedding marker), tryptase (mast cell activation marker) and platelet microparticles were measured.

KEY RESULTS

Inhibition of platelet aggregation by SNP was diminished in chronic CAS, with further (non-significant) deterioration during symptomatic crises, whereas plasma concentrations of syndecan 1, tryptase and platelet microparticles increased. Treatment of patients with high-dose N-acetylcysteine (NAC) plus glyceryl trinitrate rapidly increased platelet responsiveness to SNP and decreased plasma syndecan 1 concentrations. The effect of NAC on platelet responsiveness to SNP was confirmed in vitro and mimicked by the H₂ S donor NaHS. Conversely, inhibition of enzymatic production of H₂ S attenuated NAC effect.

CONCLUSION AND IMPLICATIONS

CAS is associated with substantial impairment of platelet NO signalling. During acute symptomatic exacerbations, platelet resistance to NO is aggravated, together with mast cell activation and damage to both vasculature and platelets. NAC, via release of H₂ S, reverses platelet resistance to NO and terminates glycocalyx shedding during symptomatic crises: This suggests that H₂ S donors may correct the pathophysiological anomalies underlying CAS.

Significance of urinary 11-dehydro-thromboxane B2 in age-related diseases: Focus on atherothrombosis

Platelet activation plays a key role in atherogenesis and atherothrombosis. Biochemical evidence of increased platelet activation in vivo can be reliably obtained through non-invasive measurement of thromboxane metabolite (TXM) excretion. Persistent biosynthesis of TXA₂ has been associated with several ageing-related diseases, including acute and chronic cardio-cerebrovascular diseases and cardiovascular risk factors, such as cigarette smoking, type 1 and type 2 diabetes mellitus, obesity, hypercholesterolemia, hyperhomocysteinemia, hypertension, chronic kidney disease, chronic inflammatory diseases. Given the systemic nature of TX excretion, involving predominantly platelet but also extraplatelet sources, urinary TXM may reflect either platelet cyclooxygenase-1 (COX-1)-dependent TX generation or COX-2-dependent biosynthesis by inflammatory cells and/or platelets, or a combination of the two, especially in clinical settings characterized by low-grade inflammation or enhanced platelet turnover. Although urinary 11-dehydro-TXB₂ levels are largely suppressed with low-dose aspirin, incomplete TXM suppression by aspirin predicts the future risk of vascular events and death in high-risk patients and may identify individuals who might benefit from treatments that more effectively block in vivo TX production or activity. Several disease-modifying agents, including lifestyle intervention, antidiabetic drugs and antiplatelet agents besides aspirin have been shown to reduce TX biosynthesis. Taken together, these aspects may contribute to the development of promising mechanism-based therapeutic strategies to reduce the progression of atherothrombosis. We intended to critically review current knowledge on both the pathophysiological significance of urinary TXM excretion in clinical settings related to ageing and atherothrombosis, as well as its prognostic value as a biomarker of vascular events.

Platelets and Cardiovascular Disease

Platelets play an important, but often under-recognized role in cardiovascular disease. For example, the normal response of the platelet can be altered, either by increased pro-aggregatory stimuli or by diminished anti-aggregatory substances to produce conditions of increased platelet activation/aggregation and occur in active cardiovascular disease states both on a chronic (e.g. stable angina pectoris) and acute basis (e.g. acute myocardial infarction). In addition, platelet hyperaggregability is also associated with the risk factors for coronary artery disease (e.g. smoking, hypertension, and hypercholesterolaemia). Finally, the utility of an increasing range of anti-platelet therapies in the management of the above disease states further emphasizes the pivotal role platelets play in the pathogenesis of cardiovascular disease. This paper provides a comprehensive overview of the normal physiologic role of platelets in maintain homeostasis, the pathophysiologic processes that contribute to platelet dysfunction in cardiovascular disease and the associated role and benefits of anti-platelet therapies.

Synthesis and pharmacological evaluation of 2-aryloxy/arylamino-5-cyanobenzenesulfonylureas as novel thromboxane A₂ receptor antagonists

New series of original 2-aryloxy/arylamino-5-cyanobenzenesulfonylureas were synthesized and evaluated as thromboxane A2 receptor (TP receptor) antagonists. A functional pharmacological test was used, which consisted of measuring the inhibition of intracellular calcium mobilization in a model of mammalian cell line that specifically over-expressed the individual TPα or TPβ isoforms. 2-Arylamino-5-cyanobenzenesulfonylureas exhibited virtually identical affinity and/or functional activity than 2-aryloxy-5-cyanobenzenesulfonylureas for both TPα and TPβ, but some 2-aryloxy-substituted compounds showed increased selectivity for TPβ relative to TPα. Several compounds were found to be as potent as the 2-arylamino-5-nitrobenzenesulfonylurea reference compound BM-573, supporting the view that the bioisosteric replacement of the nitro group by a cyano group was tolerated. TP receptor antagonist activity of the most promising molecules was confirmed in a platelet aggregation assay using the TP receptor agonist U-46619 as a proaggregant. Three compounds (7e, 7h and 8h) were identified as leads for further non-clinical pharmacological and toxicological studies.

Isolation and study of insulin activated nitric oxide synthase inhibitory protein in acute myocardial infarction subjects

Insulin inhibits platelet aggregation through nitric oxide synthesis by stimulating platelet insulin activated nitric oxide synthase. Impaired platelet insulin activated nitric oxide synthase in acute myocardial infarction (AMI) patients had been reported and thus our aim was to identify and isolate the factors impairing insulin activated nitric oxide in acute myocardial infarction patients' plasma and study its effect on platelets aggregation in vitro. The insulin activated nitric oxide synthase inhibitor was identified as a protein and was purified from the plasma of AMI subjects using DEAE cellulose and Sephadex G-50 column, molecular weight determined by SDS-PAGE, nitric oxide quantified by methaemoglobin method, inhibitor protein quantified in plasma by immunoblot and ELISA, platelet aggregation studies done using an aggregometer, thromboxane-A2 in the platelets determined by radioimmunoassay, (125)I-insulin radioligand binding studies done using normal subject platelets. The purified nitric oxide synthase inhibitor protein was ~66 kDa, concentration in AMI subjects' plasma varied from 114 to 9,090 μM and was undetected in normal subjects' plasma. The inhibitor protein competes with insulin for insulin receptor binding sites. The Incubation of the normal subject PRP with 5.0 μM inhibitor for 30 min followed by 0.4 μM ADP addition caused platelet aggregation in vitro, 130 μM aspirin or 400 μU insulin/ml addition was able to abrogate 0.4 μM ADP induced platelet aggregation even in the presence of 5.0 μM inhibitor. A potent inhibitory protein against insulin activated nitric oxide synthase in platelets appears in circulation of AMI subjects impairing nitric oxide production, potentiating ADP induced platelet aggregation and increasing the thromboxane-A2 level in platelets.

Protein kinase C ε expression in platelets from patients with acute myocardial infarction

Objective

Platelets play crucial roles in the pathophysiology of thrombosis and myocardial infarction. Protein kinase C ε (PKCε) is virtually absent in human platelets and its expression is precisely regulated during human megakaryocytic differentiation. On the basis of what is known on the role of platelet PKCε in other species, we hypothesized that platelets from myocardial infarction patients might ectopically express PKCε with a pathophysiological role in the disease.

Methods and Results

We therefore studied platelet PKCε expression from 24 patients with myocardial infarction, 24 patients with stable coronary artery disease and 24 healthy subjects. Indeed, platelets from myocardial infarction patients expressed PKCε with a significant frequency as compared to both stable coronary artery disease and healthy subjects. PKCε returned negative during patient follow-up. The forced expression of PKCε in normal donor platelets significantly increased their response to adenosine diphosphate-induced activation and adhesion to subendothelial collagen.

Conclusions

Our data suggest that platelet generations produced before the acute event retain PKCε-mRNA that is not down-regulated during terminal megakaryocyte differentiation. Results are discussed in the perspective of peri-infarctual megakaryocytopoiesis as a critical component of myocardial infarction pathophysiology.

Thromboxane biosynthesis and metabolism in relation to cardiovascular risk factors

Enhanced platelet biosynthesis of thromboxane A(2) is associated with several cardiovascular risk factors, as a consequence of a direct effect on platelet biochemistry and/or some form of endothelial dysfunction. Moreover, episodic increases in thromboxane biosynthesis occur in acute coronary and cerebral ischemic syndromes. Thromboxane-dependent platelet activation represents an important mechanism that amplifies the consequences of acute vascular lesions as well as those of longstanding metabolic or hemodynamic disturbances, and results in increased risk of vascular occlusive events.

Original articles: increased platelet volume in patients with adult polycystic kidney disease

Platelet volume is a measure of platelet function. An increased platelet volume is found in acute vascular syndromes and may have a causative role and predict outcome. Patients with autosomal dominant polycystic kidney disease (APKD) are at increased risk of developing, and dying from, premature vascular disease. We hypothesized that platelet volume might be altered in patients with APKD. Platelet volume was measured in 16 normotensive APKD patients with normal renal function and 16 normal volunteers pair-matched for age, gender, race and body mass index. Mean platelet volume was increased by 0.4 fl (95% confidence limits 0.1 to 0.9, 2P=0.017) in patients with APKD as compared with normal subjects: median 8.4 fl (0.7) versus 8.0 fl (0.3) respectively. In contrast, platelet count was lower by 63 × 10(9)/1 (-5 to -108, 2P = 0.031): 225 (40) × 10(9)/1 versus 280 (35) × 10(9)/1 while the platelet mass was not different, -0.36 ml/l (-0.81 to 0.02, 2P=0.070): 1.82 (0.30) ml/l versus 2.28 (0.38) ml/l. Platelet volume and count were inversely correlated across the two groups, r(s)= -0.342 (2P=0.055). Serum levels of erythropoietin, a hormone that contributes to the regulation of thrombopoeisis, were not different between the APKD patients and normal controls, 35 U/1 (-71 to 120, 2P=0.42): 107 U/I (53) versus 84 U/I (57). Although erythropoietin did not correlate with platelet volume, R., = -0.185 (2P =0.36), it was positively associated with platelet count, R., = 0.465 (2P=0.029) and platelet mass, r(s) = 0.466 (2P= 0.028). Since an increased platelet volume is associated with platelet hyperactivity, the increased platelet volume in APKD may be a marker, or even contribute to the development, of premature vascular disease and sudden cardiac death in patients with APKD.

Biomarkers of platelet activation in acute coronary syndromes

The most convincing evidence for the participation of platelets in arterial thrombosis in humans comes from studies of platelet activation in patients with acute coronary syndromes (ACS) and from trials of antiplatelet drugs. Both strongly support the concept that repeated episodes of platelet activation over the thrombogenic surface of a vulnerable plaque may contribute to the risk of death from coronary causes. However, the relation of in vivo platelet activation and adverse clinical events to results of platelet function tests remains largely unknown. A valuable marker of in vivo platelet activation should be specific, unaltered by pre-analytical artefacts and reproducibly measured by easily performed methods. This article describes current biomarkers of platelet activation in ACS, reviews their advantages and disadvantages, discusses their potential pitfalls, and demonstrates emerging data supporting the positive clinical implications of monitoring in vivo platelet activation in the setting of ACS.

Inhibitory Effects of Resveratrol on Platelet Activation Induced by Thromboxane A2Receptor Agonist in Human Platelets

Resveratrol (RSVL), a polyphenolic compound found in red wine is believed to be a contributor in decreasing the incidence of coronary heart disease. Although its primary target is unknown, it blocks platelet aggregation by an ill-defined mechanism. Protein kinase C (PKC), which would redistribute from the cytosol to the platelet membrane upon platelet stimulation, plays a key role in the signal transduction system of platelets in human. In this study, we investigated the effect of RSVL and a PKC inhibitor (DL-erythro-1,3-Dihydroxy-2-aminooctadecane, PKCI) on platelet aggregation induced by a thromboxane A2receptor agonist (U46619, 9,11-Dideoxy-11α, 9α-epoxymethanoprostaglandin F2α) using a platelet aggregometer. We also studied the platelet membranebound fibrinogen (PFig) content and the activity of protein kinase C (PKC) in platelets from healthy volunteers using flow cytometry, and a phosphorimaging system, respectively. Our results showed that RSVL blocked platelet aggregation and PFig content induced by U46619 in a concentration-dependent manner. PKCI and RSVL had an additive effect in inhibiting platelet aggregation and PFig content. Furthermore, RSVL (final concentration 50 μM) remarkably depressed the activity of PKC in the membrane of platelets and the percentage of membrane PKC activity in total PKC activity. Taken together, these results suggested that RSVL suppressed U46619-induced platelet aggregation and PFig content partially through the inhibition of the activity of PKC in platelets.

Antiplatelet Therapy Prasugrel: A Novel Platelet ADP P2Y12 Receptor Antagonist

Novel adenosine diphosphate (ADP) P2Y12 antagonists, including prasugrel, ticagrelor, cangrelor and elinogrel, are in various phases of clinical development. These ADP P2Y12 antagonists have advantages over clopidogrel ranging from faster onset to greater and less variable inhibition of platelet function. Novel ADP P2Y12 antagonists are under investigation to determine whether their use can result in improved antiplatelet activity, faster onset of action, and/or greater antithrombotic effects than clopidogrel, without an unacceptable increase in hemorrhagic or other side effects. Prasugrel (CS-747; LY-640315), a novel third-generation oral thienopyridine, is a specific, irreversible antagonist of the platelet ADP P2Y12 receptor. Preclinical and early phase clinical studies have shown prasugrel to be characterized by more potent antiplatelet effects, lower interindividual variability in platelet response, and faster onset of activity compared to clopidogrel. Recent findings from large-scale phase III testing showed prasugrel to be more efficacious in preventing ischemic events in patients with acute coronary syndrome (ACS) undergoing percutaneous coronary intervention (PCI); however, this is achieved at the expense of an increased risk of bleeding. Prasugrel provides more rapid and consistent platelet inhibition than clopidogrel.

Antiplatelet therapies: drug interactions in the management of vascular disorders

Antiplatelet drugs represent a key class of drugs that are of proven value in arterial thromboembolic disorders. There is a need for effective, safe antiplatelet agents or their combinations to provide predictable therapeutic benefit, dosage flexibility, and unique pharmacologic profiles, such as rapid onset in acute thrombotic states, as well as sustained antiplatelet effects in chronic platelet-activating states (e.g., post-stent placement). Aspirin, clopidogrel, or their combination have shown improved clinical outcomes in certain unique settings, and the search for additional antiplatelet agents is ongoing. Current studies suggest that combination antiplatelet therapy with existing agents is best considered a use-adapted strategy, with the greatest clinical benefit of combination therapy realized in acute, platelet-activating, and prothrombotic states.

Regulation of Coronary Blood Flow During Exercise

Exercise is the most important physiological stimulus for increased myocardial oxygen demand. The requirement of exercising muscle for increased blood flow necessitates an increase in cardiac output that results in increases in the three main determinants of myocardial oxygen demand: heart rate, myocardial contractility, and ventricular work. The approximately sixfold increase in oxygen demands of the left ventricle during heavy exercise is met principally by augmenting coronary blood flow (∼5-fold), as hemoglobin concentration and oxygen extraction (which is already 70–80% at rest) increase only modestly in most species. In contrast, in the right ventricle, oxygen extraction is lower at rest and increases substantially during exercise, similar to skeletal muscle, suggesting fundamental differences in blood flow regulation between these two cardiac chambers. The increase in heart rate also increases the relative time spent in systole, thereby increasing the net extravascular compressive forces acting on the microvasculature within the wall of the left ventricle, in particular in its subendocardial layers. Hence, appropriate adjustment of coronary vascular resistance is critical for the cardiac response to exercise. Coronary resistance vessel tone results from the culmination of myriad vasodilator and vasoconstrictors influences, including neurohormones and endothelial and myocardial factors. Unraveling of the integrative mechanisms controlling coronary vasodilation in response to exercise has been difficult, in part due to the redundancies in coronary vasomotor control and differences between animal species. Exercise training is associated with adaptations in the coronary microvasculature including increased arteriolar densities and/or diameters, which provide a morphometric basis for the observed increase in peak coronary blood flow rates in exercise-trained animals. In larger animals trained by treadmill exercise, the formation of new capillaries maintains capillary density at a level commensurate with the degree of exercise-induced physiological myocardial hypertrophy. Nevertheless, training alters the distribution of coronary vascular resistance so that more capillaries are recruited, resulting in an increase in the permeability-surface area product without a change in capillary numerical density. Maintenance of α- and ß-adrenergic tone in the presence of lower circulating catecholamine levels appears to be due to increased receptor responsiveness to adrenergic stimulation. Exercise training also alters local control of coronary resistance vessels. Thus arterioles exhibit increased myogenic tone, likely due to a calcium-dependent protein kinase C signaling-mediated alteration in voltage-gated calcium channel activity in response to stretch. Conversely, training augments endothelium-dependent vasodilation throughout the coronary microcirculation. This enhanced responsiveness appears to result principally from an increased expression of nitric oxide (NO) synthase. Finally, physical conditioning decreases extravascular compressive forces at rest and at comparable levels of exercise, mainly because of a decrease in heart rate. Impedance to coronary inflow due to an epicardial coronary artery stenosis results in marked redistribution of myocardial blood flow during exercise away from the subendocardium towards the subepicardium. However, in contrast to the traditional view that myocardial ischemia causes maximal microvascular dilation, more recent studies have shown that the coronary microvessels retain some degree of vasodilator reserve during exercise-induced ischemia and remain responsive to vasoconstrictor stimuli. These observations have required reassessment of the principal sites of resistance to blood flow in the microcirculation. A significant fraction of resistance is located in small arteries that are outside the metabolic control of the myocardium but are sensitive to shear and nitrovasodilators. The coronary collateral system embodies a dynamic network of interarterial vessels that can undergo both long- and short-term adjustments that can modulate blood flow to the dependent myocardium. Long-term adjustments including recruitment and growth of collateral vessels in response to arterial occlusion are time dependent and determine the maximum blood flow rates available to the collateral-dependent vascular bed during exercise. Rapid short-term adjustments result from active vasomotor activity of the collateral vessels. Mature coronary collateral vessels are responsive to vasodilators such as nitroglycerin and atrial natriuretic peptide, and to vasoconstrictors such as vasopressin, angiotensin II, and the platelet products serotonin and thromboxane A2. During exercise, ß-adrenergic activity and endothelium-derived NO and prostanoids exert vasodilator influences on coronary collateral vessels. Importantly, alterations in collateral vasomotor tone, e.g., by exogenous vasopressin, inhibition of endogenous NO or prostanoid production, or increasing local adenosine production can modify collateral conductance, thereby influencing the blood supply to the dependent myocardium. In addition, vasomotor activity in the resistance vessels of the collateral perfused vascular bed can influence the volume and distribution of blood flow within the collateral zone. Finally, there is evidence that vasomotor control of resistance vessels in the normally perfused regions of collateralized hearts is altered, indicating that the vascular adaptations in hearts with a flow-limiting coronary obstruction occur at a global as well as a regional level. Exercise training does not stimulate growth of coronary collateral vessels in the normal heart. However, if exercise produces ischemia, which would be absent or minimal under resting conditions, there is evidence that collateral growth can be enhanced. In addition to ischemia, the pressure gradient between vascular beds, which is a determinant of the flow rate and therefore the shear stress on the collateral vessel endothelium, may also be important in stimulating growth of collateral vessels.

Thromboxane A2: physiology/pathophysiology, cellular signal transduction and pharmacology

Thromboxane A(2) (TXA(2)), an unstable arachidonic acid metabolite, elicits diverse physiological/pathophysiological actions, including platelet aggregation and smooth muscle contraction. TXA(2) has been shown to be involved in allergies, modulation of acquired immunity, atherogenesis, neovascularization, and metastasis of cancer cells. The TXA(2) receptor (TP) communicates mainly with G(q) and G(13), resulting in phospholipase C activation and RhoGEF activation, respectively. In addition, TP couples with G(11), G(12), G(13), G(14), G(15), G(16), G(i), G(s) and G(h). TP is widely distributed in the body, and is expressed at high levels in thymus and spleen. The second extracellular loop of TP is an important ligand-binding site, and Asp(193) is a key amino acid. There are two alternatively spliced isoforms of TP, TPalpha and TPbeta, which differ only in their C-terminals. TPalpha and TPbeta communicate with different G proteins, and undergo hetero-dimerization, resulting in changes in intracellular traffic and receptor protein conformations. TP cross-talks with receptor tyrosine kinases, such as EGF receptor, to induce cell proliferation and differentiation. TP is glycosylated in the N-terminal region for recruitment to plasma membranes. Furthermore, TP conformation is changed by coupling to G proteins, showing several states of agonist binding. Finally, several drugs modify TP-mediated events; these include cyclooxygenase inhibitors, TXA(2) synthase inhibitors and TP antagonists. Some flavonoids of natural origin also have TP receptor antagonistic activity. Recent advances in TP research have clarified TXA(2)-mediated events in detail, and further study will supply more beneficial information about TXA(2) pathophysiology.

Association of platelet function in hypertensive patients with left ventricular hypertrophy, transient myocardial ischemia, and coronary artery disease

The aim of the study was to evaluate how platelet aggregation relates to left ventricular hypertrophy, presence of transient myocardial ischemia and coronary atherosclerosis in essential hypertensive patients. For that purpose platelet aggregation, symptom-limited treadmill-test, echocardiography and coronary angiography were performed in 65 male patients with uncomplicated essential hypertension. Aggregation was monitored both by a turbidometric method and by a method based on real-time estimation of mean aggregate size. Platelet function was also studied in 18 male normotensive subjects. Hypertensive patients with transient myocardial ischemia were characterized by increased platelet sensitivity to adenosine diphosphate, thromboxane A2 mimetic-U46619, serotonin, platelet activating factor and by increased spontaneous aggregability of platelets compared with normotensive subjects and to patients without myocardial ischemia. Platelet aggregation induced by low doses of agonists positively correlated with the degree of left ventricular hypertrophy. This study provides evidence that platelets play an important role in myocardial ischemia. Besides, it was found that platelet activity positively correlated with the degree of left ventricular hypertrophy. This may provide an additional link between left ventricular hypertrophy and coronary heart disease in hypertensive patients.

Assessment of platelet activation indices using the ADVIATM 120 amongst 'high-risk' patients with hypertension

BACKGROUND

Platelet activation is involved in the pathogenesis of the thrombotic complications of hypertension. Novel surrogate markers of platelet activation (mean platelet volume (MPV), mean platelet component (MPC, measure of platelet density), platelet component distribution width (PCDW, a marker of platelet shape change) and the number of platelet clumps) have been related to cardiovascular risk. We hypothesized a stepwise increase in these platelet activation indices between healthy controls (HC, n = 60), 'high-risk' essential hypertensive subjects (HBP, n = 45) and treated, previously diagnosed patients with malignant phase hypertension (MHT, n = 45).

METHODS

In a cross-sectional study, we measured comparative platelet counts and indices of platelet activation (MPV, MPC, PCDW and the number of platelet clumps) using the Bayer ADVIATM haematology system, in our three study groups.

RESULTS

There was a stepwise increase in MPV (P = 0.0002) and MPM (P = 0.03), and a stepwise decrease in the MPC (P = 0.03) and PCDW (P = 0.001) across the three study groups, despite similarities in platelet count. These differences were only significantly different (on post-hoc analysis) between the healthy controls and the MHT group. On multivariate analysis, there was a significant relationship (R2 = 66.5%; P<0.0001) between the MPV and the PCDW (P<0.0001), systolic blood pressure (P = 0.008) and platelet count (P<0.0001).

CONCLUSION

There is a stepwise increase in platelet activation indices, despite similar platelet counts, with increasing severity of hypertensive disease. This may contribute to the pathogenesis of thrombosis-related complications in hypertension.

In-vitro efficacy of different platelet glycoprotein IIb/IIIa antagonists and thrombolytics on platelet/fibrin-mediated clot dynamics in human whole blood using thrombelastography

Suppressing platelet activation improves efficacy of thrombolytic therapy for stroke and acute myocardial infarction. Combination treatment with recombinant tissue plasminogen activator (r-tPA) and glycoprotein IIb/IIIa (GPIIb/IIIa) inhibitor that binds with high affinity to platelets may therefore improve the efficacy of thrombolytic therapy. The effect of platelet GPIIb/IIIa antagonists and/or r-tPA on the dynamics of platelet/fibrin clot formation, strength, and lysis was determined using thrombelastography in human blood under thrombin or tissue factor stimulation. The study utilized platelet GPIIb/IIIa antagonists with high affinity and slow off-rate (Class I) from resting and activated platelets in comparison with Class II antagonists (lower affinity and fast off-rate from platelet GPIIb/IIIa receptors). The combination of the active form of roxifiban (XV459; Class I) or the active form of orbofiban (Class II) with a subeffective concentration of r-tPA resulted in a synergistic effect in clot lysis with roxifiban active form XV459 but not with that of orbofiban at therapeutically achievable concentrations that inhibit human platelet aggregation. These data indicate differential enhanced thrombolysis of low levels of r-tPA with high-affinity Class I but not with low-affinity Class II GPIIb/IIIa antagonists in the absence of anticoagulants.

Evaluation of platelet function in aspirin treated patients with CAD

BACKGROUND

Studies of platelet function in the acute coronary syndrome (ACS) have revealed both increased and unchanged platelet activation. To obtain a better understanding of platelet function in coronary artery disease in the setting of aspirin therapy, we performed platelet functional testing in patients with ACS and compared results to patients without CAD.

METHODS

We measured platelet aggregation and activation in response to ADP and epinephrine in 80 age and gender matched hospitalized patients (40 with ACS, 40 with non-cardiac chest pain (NCCP)). All subjects received ASA (81-325 mg). Platelet aggregation was performed using standard light transmission in platelet-rich plasma and activation was measured via flow cytometric analyses. We also studied platelet function under high shear rates measured by the platelet function analyzer (PFA-100).

RESULTS

ASA effect was found to be present in all subjects by blunted platelet aggregation in response to arachidonic acid. Patients with ACS showed significantly higher levels of platelet aggregation to epinephrine compared to patients with NCCP (p = 0.001). Other measures of platelet function including ADP aggregation, Pselectin, activated glycoprotein IIb/IIIa expression, and PFA-100 were unchanged between the two groups.

CONCLUSIONS

We have found conflicting results of platelet functional testing in ACS. Specifically aspirin therapy in patients with ACS is effective in suppressing the platelet release response and is effective in the partial suppression of platelet aggregation; however, it appears that ACS patients have increased platelet aggregation to adrenergic stimuli when compared to age and gender matched controls without CAD despite the use of aspirin. In some studies, because ACS patients have an accentuated response to adrenergic stimuli this might be interpreted as aspirin resistance. Our study suggests that depending on the assay used to determine aspirin resistance, not all patients with this label are resistant to the biological effects of aspirin but they may have higher than normal baseline platelet sensitivity to adrenergic stimuli.

Pharmacological profile and therapeutic potential of BM-573, a combined thromboxane receptor antagonist and synthase inhibitor

BM-573 (N-terbutyl-N'-[2-(4'-methylphenylamino)-5-nitro-benzenesulfonyl]urea), a torsemide derivative, is a novel non-carboxylic dual TXA2 synthase inhibitor and receptor antagonist. The pharmacological profile of the drug is characterized by a higher affinity for the thromboxane receptor than that of SQ-29548, one of the most powerful antagonists described to date, by a complete prevention of human platelet aggregation induced by arachidonic acid at a lower dose than either torsemide or sulotroban, and by a significantly prolonged closure time measured by the platelet function analyser (PFA-100). Moreover, at the concentrations of 1 and 10 microM, BM-573 completely prevented production of TXB2 by human platelets activated by 0.6 mM of arachidonic acid. BM-573 prevents rat fundus contraction induced by U-46619 but not by prostacyclin or other prostaglandins. Despite possessing a chemical structure very similar to that of a diuretic torsemide, BM-573 has no diuretic activity. BM-573 does not prolong bleeding time and, unlike some of the other sulfonylureas, has no effect on blood glucose levels. In vivo, BM-573 appears to have antiplatelet and antithrombotic activities since it reduced thrombus weight and prolonged the time to abdominal aorta occlusion induced by ferric chloride. BM-573 also relaxed rat aorta and guinea pig trachea precontracted with U-46619. In pigs, BM-573 completely antagonized pulmonary hypertensive effects of U-46619 and reduced the early phase of pulmonary hypertension in models of endotoxic shock and pulmonary embolism. Finally, BM-573 protected pigs from myocardial infarction induced by coronary thrombosis. These results suggest that BM-573 should be viewed as a promising therapeutic agent in the treatment of pulmonary hypertension and syndromes associated with platelet activation.

Using thrombelastography to determine the efficacy of the platelet glycoprotein IIb/IIIa antagonist, roxifiban, on platelet/fibrin-mediated clot dynamics in humans

The effect of platelet glycoprotein IIb/IIIa antagonists on the dynamics of platelet/fibrin clot formation and strength was determined using thrombelastography (TEG) under conditions of recalcification or tissue factor addition. In the present investigation, the effect of roxifiban (class I) on ex vivo clot dynamics using recalcified blood was tested in normal, healthy volunteers (n = 7) dosed with 1 mg BID roxifiban for 9 days. Roxifiban inhibited platelet aggregation induced by 20 mumol/l adenosine diphosphate by 60-90% but did not significantly affect any of the TEG parameters either at peak, trough, or subtrough drug levels. Addition of 30 nmol/l roxifiban free acid (XV459; which is ineffective by itself to modify TEG parameters) to human blood obtained from roxifiban-treated subjects resulted in 45-60% (P < 0.01) inhibition of clot strength (maximum amplitude), 90-100% (P < 0.01) inhibition of initial kinetic of clot development (angle alpha), and 50-70% (P < 0.01) inhibition of early clot initiation (K). These data suggest that a subthreshold blood level of 40-50 nmol/l roxifiban active form was achieved in those subjects, as estimated from an in vitro calibration with XV459. These data indicate (not studied) that roxifiban, at a targeted clinical dosing regimen, failed to achieve sufficient exposure to modulate platelet-mediated clot retraction.

Determination of Tirofiban in human serum by liquid chromatography–tandem mass spectrometry

A liquid chromatography-tandem mass spectrometric (LC-MS-MS) method with a rapid and simple sample preparation was developed and validated for the determination of Tirofiban in biological fluids. Tirofiban in serum samples was extracted and cleaned up by using an automated solid phase extraction method. An external calibration was used. The mass spectrometer was operated in the multiple reaction monitoring mode (MRM). A good linear response over the range of 2-200ng/ml was demonstrated. The accuracy for Tirofiban ranged from 94.8 to 110.8% within-day and from 103.0 to 104.7% between-day. The lower limit of quantification was 2ng/ml. This method is suitable for pharmacokinetic studies.

Silicon carbide-coated stents in patients with acute coronary syndrome

Silicon carbide (aSIC-C) is a stent coating with antithrombogenic as well as anti-inflammatory properties as compared with uncoated stainless steal based on in vitro and in vivo studies. This study investigated the potential of this coating in patients with unstable angina. At 38 study sites, 485 patients were randomized to an aSIC-C (n = 238) or a conventional stainless steal stent (n = 247). Patient were classified according to angina at rest within last 48 hr to Braunwald in class IIB (= 314) and IIIB (n = 171). The primary endpoint was a combination of death, myocardial infarction, or ischemia-driven target vessel revascularization at 6 months. Complications of procedures performed at 0.4 +/- 1.1 days after admission occurred at lower rates than previously reported in this high-risk population, but the primary endpoint was not different between the study groups. Only in Braunwald class IIIB patients did the primary endpoint occur less frequently in patients with an aSIC-C stent as compared to patients with a conventional stent (5.8% vs. 15.3%; P = 0.049). At 9-month follow-up, the level of difference was maintained, but statistical significance was lost. Quantitative angiography revealed no significant difference between the stents in the subgroups. This study suggests that aSIC-C stents exert clinically measurable effects in patients with unstable angina with recent symptoms at rest. This coating deserves further clinical investigation and may serve as platform for antiproliferative drugs.

Pharmacological Characterization of N-tert-Butyl-N′-[2-(4′-methylphenylamino)-5-nitrobenzenesulfonyl]urea (BM-573), a Novel Thromboxane A2 Receptor Antagonist and Thromboxane Synthase Inhibitor in a Rat Model of Arterial Thrombosis and Its Effects on Bleeding Time

The present study was undertaken to characterize the antiplatelet and antithrombotic effects of BM-573 [N-tert-butyl-N'-[2-(4'-methylphenylamino)-5-nitrobenzenesulfonyl]urea], an original combined thromboxane receptor antagonist and thromboxane synthase inhibitor in rats, and to determine its effects on mice bleeding time. Intraperitoneal injection of a single dose of 5 mg/kg BM-573 to rats inhibited U-46619 (9,11-dideoxy-9,11-methanoepoxy-prostaglandin F(2))-induced washed platelet aggregation 30 min and 1, 2, and 4 h after drug administration with a maximum antiplatelet effect observed after 1 and 2 h. In a rat model of thrombosis induced by ferric chloride application on the abdominal aorta, BM-573 significantly reduced the thrombus weight by 92.53, 80.20, 64.75, and 18.21% at doses of 5, 2, 0.5, and 0.2 mg/kg, respectively. Time to occlusion of abdominal aorta in the BM-573-treated group (41.50 +/- 5.21 min) was significantly prolonged compared with the vehicle-treated rats (16.16 +/- 0.79 min). Like furegrelate, seratrodast, and acetylsalicylic acid, BM-573 did not affect the tail bleeding time induced by tail transection in mice compared with vehicle-treated mice. Moreover, BM-573, a close derivative of the loop diuretic torasemide, failed to induce a significant increase in diuresis in rat and did not produce a decrease in blood glucose concentration as observed with the sulfonylurea glibenclamide. In conclusion, we have demonstrated that the nitrobenzenic sulfonylurea BM-573, an original combined thromboxane receptor antagonist and thromboxane synthase inhibitor, is a potent antithrombotic agent that does not affect bleeding time. Moreover, BM-573 lost the diuretic property of torasemide and has no impact on glycemia.

Enhanced platelet/endothelial activation in depressed patients with acute coronary syndromes: evidence from recent clinical trials

Platelets play a key role in the progression of acute coronary syndromes (ACS). Clinical depression alone is also associated with enhanced platelet activation. The purpose of this study was to compare concentrations of established biomarkers of enhanced platelet/endothelial activation in clinically depressed versus non-depressed patients enrolled in recent clinical trials for ACS. Two hundred and eighty-one baseline plasma samples from patients with acute myocardial infarction (ASSENT-2; n = 41), with ACS (PRONTO; n = 126) and with clinical depression plus previous acute coronary syndrome within 6 months (SADHART; n = 64), and from normal healthy controls (n = 50) were analyzed. Blood was drawn before applying any therapeutic strategies including interventions, thrombolytics, infusions, and selective serotonin re-uptake inhibitors. Platelet factor 4, beta-thromboglobulin, platelet/endothelial cell adhesion molecule-1, P-selectin, thromboxane, prostacyclin, vascular cell adhesion molecule-1, and E-selectin were measured by enzyme-linked immunosorbent assay by a single core laboratory. Patients with ACS exhibited a higher degree of platelet activation than controls independently of the presence of depression. Plasma levels of P-selectin, thromboxane, prostacyclin, and vascular cell adhesion molecule-1 were the highest in the acute myocardial infarction group when compared with ACS despite the presence or absence of clinical depression. Surprisingly, patients with ACS and depression exhibited the highest levels of platelet factor 4, beta-thromboglobulin, and platelet/endothelial cell adhesion molecule-1 when compared with myocardial infarction or angina patients without clinical depression. E-selectin plasma level was constantly elevated compared with controls but did not differ among the groups dependent on the incidence of depression. The depressed plus ACS group had higher plasma levels of all biomarkers compared with the non-depressed patients. Retrospective analysis of the data from several clinical trials reveals that clinical depression is associated with enhanced activation of platelet/endothelial biomarkers even above the level expected in ACS. These findings may contribute to the unfavorable outcome associated with clinical depression in patients with ACS.

Costs and cost effectiveness of low molecular weight heparins and platelet glycoprotein IIb/IIIa inhibitors: in the management of acute coronary syndromes

The most well established antithrombotic treatment for acute coronary syndromes (ACS) is unfractionated heparin (UFH) plus aspirin, but such treatment may not prevent arterial thrombotic events. Low molecular weight heparins (LMWHs) and platelet glycoprotein (GP) IIb/IIIa inhibitors offer alternative or adjunctive treatments. However, before these alternatives with higher acquisition costs are accepted in today's healthcare systems, their cost effectiveness must be proven. This paper reviews international pharmacoeconomic studies on the use of LMWHs and GP IIb/IIIa inhibitors in patients with ACS in an attempt to determine whether these therapies are cost effective. Most of the studies on LMWHs have been cost-minimisation analyses and have focused on enoxaparin sodium, because this is the only LMWH proven to be superior to UFH. Several analyses show that, compared with UFH plus aspirin, enoxaparin sodium provides cost savings both during hospitalisation (30 days) and 1-year follow-up. These cost savings are mainly attributable to fewer cardiac interventions, shorter hospital stays and lower administrative costs. Indeed, the clinical and economic advantages of enoxaparin sodium have led to its recommendation in recent guidelines as the antithrombotic agent of choice for coronary artery disease. Most of the economic analyses of GP IIb/IIIa inhibitors have been cost-effectiveness analyses. Such analyses indicate that the high acquisition costs of these drugs may be at least partially offset by reductions in other costs if a noninvasive approach to risk stratification is used. Furthermore, use of GP IIb/IIIa inhibitors appears to give favourable cost-effectiveness ratios compared with other accepted therapies, such as fibrin-specific thrombolytic therapy, in the cardiovascular field, particularly in high-risk patients and those undergoing percutaneous coronary intervention. However, more comprehensive economic data on the GP IIb/IIIa inhibitors are needed.

Differential efficacy of different platelet glycoprotein IIb/IIIa antagonists on platelet/fibrin-mediated clot dynamics under different conditions using thrombelastography: the critical need for anticoagulant

BACKGROUND

Intravenous GpIIb/IIIa antagonists demonstrate various significant clinical benefits depending on the agent used. In contrast, oral delivery of GpIIb/IIIa antagonists failed in achieving clinical benefits. This raises the question about the differences among different GpII/IIIa antagonists.

METHODS

The effect of various platelet glycoprotein (GP) IIb/IIIa antagonists on the dynamics of platelet/fibrin clot formation and strength was determined using thrombelastography under different conditions.

RESULTS

GPIIb/IIIa antagonists with high affinity for resting and activated platelets and with slow rates of dissociation from GPIIb/IIIa (Class I antagonists) demonstrated potent and comparable inhibition of platelet aggregation and platelet-mediated clot strength under different conditions. In contrast to antagonists that dissociate rapidly from GPIIb/IIIa (class II antagonists). Class I antagonists such as the free acid form of roxifiban inhibited platelet-mediated clot strength, with the inhibiting concentration required for 50% effect (IC50) = 70 n mol/l, whereas the IC50 of the class II antagonists such as the free acid forms of orbofiban, sibrafiban, lotrafiban, integrilin or aggrastat ranged from 1 to 15 micromol/l. The IC50s for class II antagonists in inhibiting platelet/fibrin clot formation and strength were substantially greater (10-15 fold) than their clinically achievable concentrations. The limited efficacy for class II antagonists in inhibiting platelet-mediated clot dynamics was enhanced by the combination with heparin.

CONCLUSIONS

Thus, these data indicated that there are differences in the efficacy of various GPIIb/IIIa antagonists in inhibiting platelet/fibrin clot formation and strength, which might be corrected by heparin. Data also suggest that inhibition of platelet aggregation may not be the sole determinant for the in-vivo efficacy of various GPIIb/IIIa antagonists.

Platelet alpha-granule secretion and its modification by SC-57101A, a GPIIb/IIIa antagonist

Platelet-agonist interaction results in aggregatory and secretory responses. While the activation of glycoprotein (GP) IIb/IIIa plays an essential role in platelet aggregation, its role in granule secretion is not clear. The present study was performed to examine the effect of 3-[[[[1-[4-(aminoiminomethyl) phenyl]-2-oxo-3S-pyrrolidinyl]amino]carbonyl]amino]-propanoate monohydrochloride salt (SC-57101A), a GPIIb/IIIa antagonist, on platelet alpha-granule secretion responses to collagen, ADP, and thrombin receptor activating peptide (TRAP). Both SC-57101A and prostaglandin E(1) (PGE(1)) inhibited collagen-, ADP-, and TRAP-induced platelet aggregation in a concentration-dependent manner. SC-57101A inhibited the collagen- and ADP-induced release of platelet-derived growth factor (PDGF) and beta-thromboglobulin (beta-TG) from platelets, but not TRAP-induced secretion of these granule contents. On the other hand, PGE(1) inhibited the release of PDGF and beta-TG from platelets activated with all the agonists used. ADP and TRAP elicited P-selectin expression in the absence of platelet aggregation, while collagen produced no such reaction. SC-57101A only moderately inhibited P-selectin expression induced by ADP and had no inhibitory effect on that induced by TRAP. The inhibition of ADP-induced secretion of alpha-granule contents by SC-57101A was abolished when platelets were pretreated with aspirin. These results suggest that GPIIb/IIIa activation plays a minor role, if any, in alpha-granule secretion in human platelets.

Comparison of the effect of different platelet GPIIb/IIa antagonists on the dynamics of platelet/fibrin-mediated clot strength induced using thromboelastography

The effect of various platelet glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists on the dynamics of platelet-fibrin clot formation and strength induced by various stimuli was measured by thromboelastography (TEG). GPIIb/IIIa antagonists with high affinity for resting and activated platelets and with slow rates of dissociation from GPIIb/IIIa (Class I antagonists) demonstrated potent and comparable inhibition of platelet aggregation and tissue factor (TF), lipopolysaccharide (LPS), Factor Xa, and thrombin-induced clot strength, in contrast to antagonists that dissociate rapidly from GPIIb/IIIa (Class II antagonists). For example, the Class I antagonist XV459 (the free acid form of roxifiban) inhibited TF, endotoxin, Factor Xa, and thrombin-induced maximal clot strength and platelet aggregation with an IC(50)=30-70 nM, whereas the IC(50) of the Class II antagonist YZ211 (the free acid form of sibrafiban) for altering clot formation and strength was 0.3-4.7 microM. Moreover, the IC(50)'s of sibrafiban, and another Class II antagonist, orbofiban, for inhibiting platelet-fibrin clot formation and strength were substantially greater than their clinically achievable concentrations. Further, although aspirin treatment improved the efficacy of all GPIIb/IIIa antagonists, it did not alter the differences between Classes I and II antagonists. Thus, these data indicate that there are differences in the efficacy of various GPIIb/IIIa antagonists in inhibiting platelet-fibrin clot formation and strength. They also suggest that inhibiting platelet aggregation may not be the sole determinant for the in vivo efficacy of various GPIIb/IIIa antagonists.

Haematological aspects of obstructive sleep apnoea

Obstructive sleep apnoea (OSA) is associated with increased cardiovascular morbidity and mortality. Several studies indicate an influence of OSA on haematological features. There is evidence of elevated platelet activation and increased haematocrit in OSA. Studies investigating the underlying cause of polycythaemia in OSA reveal different results: increased haematocrit in OSA might be due to enhanced erythropoietin excretion and/or plasma volume dysregulation based on altered release of volume-regulating hormones such as atrial natriuretic peptide (ANP). This review addresses haematological issues associated with OSA, especially platelet function, haematocrit and haemoconcentration. 2001 Harcourt Publishers Ltd

Spirocyclic nonpeptide glycoprotein IIb-IIIa antagonists. Part 1: design of potent and specific 3,9-diazaspiro[5.5]undecanes

The synthesis and biological activity of novel glycoprotein IIb-IIla antagonists containing the 3,9-diazaspiro[5.5]undecane nucleus are described. The potent activity of these compounds as platelet aggregation inhibitors demonstrates the utility of the spirocyclic structures as a central template for nonpeptide RGD mimics.

Comparison of platelet activation in unstable and stable angina pectoris and correlation with coronary angiographic findings

We sought to investigate the relation between platelet activation and the angiographic evidence of ruptured plaque in patients presenting with unstable and stable angina pectoris. We prospectively enrolled 25 consecutive patients (5 women and 20 men, mean age 62 +/- 3 years), 17 with unstable angina and 8 with stable angina. Systemic venous blood samples were collected within 4 to 6 hours of admission for flow cytometry analysis. Activation-dependent epitope CD63 and glycoprotein IIb/IIIa on the platelet membrane were assayed. Fibrinogen levels were also measured. All patients with unstable angina underwent cardiac catheterization and had angiographic evidence of ruptured plaque. Of the patients with stable angina, 5 underwent coronary angiography with smooth noncomplex lesions and 3 had negative technetium-99m sestamibi stress tests. Patients with unstable angina were characterized by 39% higher levels of fibrinogen than patients with stable angina (423 +/- 304 vs 304 +/- 51 mg/dl, p = 0.004). The percentage of platelets positive for the activation-dependent epitope CD63 was 5 times higher in patients with unstable than stable angina (14.6 +/- 5.6% vs 2.75 +/- 1.6%, p = 0.0026). They also had a 15% higher expression of their glycoprotein IIb/IIIa (517 +/- 79 vs 449 +/- 50 mean fluorescence intensity, p = 0.038). Thus, this study establishes a direct relation between the morphology of ruptured plaque and platelet activation in patients with unstable angina. This may allow for further risk stratification. Patients with unstable complex lesions had a fivefold higher expression of the platelet activation epitope CD63 than patients with stable angina. Furthermore, they had 15% more glycoprotein IIb/IIIa aggregation sites expressed on their platelet membrane, thus indicating an intense thrombogenic potential.

Human platelet alphaIIbeta3 integrin binding affinity and specificity of SJ874: antiplatelet efficacy versus aspirin

OBJECTIVE

To define the affinity and specificity of SJ874, a nonpeptide antiplatelet agent for platelet glycoprotein Ilb/IIIa integrin, and to determine the antiplatelet efficacy of SJ874 relative to those of glycoprotein IIbIIIa antagonists and aspirin.

METHODS

Binding affinity and specificity of SJ874 for platelet glycoprotein IIb/IIIa integrin were determined using integrin-mediated binding and adhesion assays with human cells. Additionally, the antiplatelet efficacy of SJ874 was determined and compared with those of other glycoprotein IIb/IIIa antagonists and aspirin using light-transmittance and laser-scattering aggregometry.

RESULTS

SJ874 inhibited aggregation of human platelets induced by 10 micromol/l adenosine diphosphate (ADP) with a concentration for half-maximal effect of 0.046 +/- 0.005 micromol/l using light-transmittance aggregometry. Using laser-scattering aggregometry, SJ874 was found to totally inhibit formation both of micro-aggregates and of macro-aggregates induced either by ADP or by epinephrine. In contrast, administration of 325 mg aspirin to normal healthy volunteers attenuated formation of macro-aggregates but not micro-aggregates. SJ874 inhibited binding of [125I]-fibrinogen to activated (by ADP, epinephrine, and arachidonic acid at concentrations of 100 micromol/l each) gel-filtered human platelets with a concentration for half-maximal effect of 0.0012 +/- 0.0005 micromol/l. SJ874 was demonstrated to associate more tightly with resting human platelets than did DMP754 [1] and slightly less tightly than did DMP802 [2]. SJ874 was demonstrated to exhibit a high degree of specificity for platelet glycoprotein IIb/IIIa (alphaIIb/beta3) integrin compared with other known integrins, including alphavbeta3, alphavbeta5, and alpha5beta1 (concentration for half-maximal effect > 100 micromol/l).

CONCLUSION

SJ874 is a potent and specific platelet glycoprotein IIb/IIIa antagonist with high affinity for and tight association with human platelets. These data suggest that SJ874 might have good antiplatelet utility for inhibiting formation both of platelet micro-aggregates and of macro-aggregates of platelets and a long duration of action in humans due to its slow dissociation from human platelets.

Platelet activation in acute myocardial infarction and the rationale for combination therapy

Current fibrinolytic regimens fail to fully restore coronary blood flow in slightly less than 50% of patients with acute myocardial infarction. Platelet activation and aggregation may be responsible for a large proportion of these therapeutic failures. Therefore, platelet inhibition may enhance thrombolysis. Experimental and early clinical evidence suggest that glycoprotein IIb/IIIa antagonists may enhance reperfusion when combined with reduced doses of thrombolytic agents. However, the clinical benefit of combination therapy will depend on the outcomes of a number large clinical trials that are currently being performed.

Antiplatelet effects of MK-852, a platelet fibrinogen receptor antagonist, in healthy volunteers

MK-852, a cyclic heptapeptide, is a potent platelet fibrinogen receptor antagonist. When administered to normal healthy male subjects by 1- and 4-hour constant rate intravenous infusions, it provides a generally well-tolerated and reversible means of inhibition of platelet function. At infusion rates of 1 microgram/kg/min for 1 hour and 0.44 microgram/kg/min for 4 hours, respectively, MK-852 extended baseline bleeding time by greater than 2.2-fold and 2.6-fold, inhibited ADP-induced platelet aggregation by 76% and 69%, and inhibited collagen-induced platelet aggregation by 65% and 67%, respectively. The pharmacokinetics of MK-852 include an elimination half-life of approximately 2 hours, total clearance of about 150 ml/min, and volume of distribution of about 18 liters. Examination of the relationship between MK-852 whole-blood concentration in vitro and inhibition of platelet aggregation showed an EC50 of about 55 ng/ml and a Hill coefficient of 1.55. The infusions were generally well tolerated, with no study drug-related changes in blood counts or biochemical profiles.

Comparative in vitro efficacy of different platelet glycoprotein IIb/IIIa antagonists on platelet-mediated clot strength induced by tissue factor with use of thromboelastography: differentiation among glycoprotein IIb/IIIa antagonists

In the present study, the in vitro efficacy of different platelet glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists on platelet-fibrin-mediated clot strength under shear was compared with the antiaggregatory efficacy by using tissue factor (TF) thromboelastography (TEG). The ability of platelets to augment the elastic properties of blood clots under shear conditions was measured by computerized TEG under conditions of maximal platelet activation accelerated by recombinant TF. Under these conditions, platelets significantly enhance clot strength 8-fold (relative to platelet-free fibrin clots). This effect was inhibited to a different extent by various platelet GPIIb/IIIa receptor antagonists; this inhibition appears to be dependent on the transmission of platelet contractile force to fibrin via the GPIIb/IIIa receptors. The GPIIb/IIIa antagonists with high binding affinity for resting and activated platelets and slow platelet dissociation rates (class I) but not those with fast platelet dissociation rates (class II) demonstrated potent and comparable inhibition of platelet aggregation and TF-TEG clot strength. Platelet GPIIb/IIIa antagonists of class I, such as XV459 (free-acid form of roxifiban), DMP802, XV454, and c7E3, demonstrated comparable inhibitory dose responses of TF-TEG clot strength and platelet aggregation, with an IC(50) of 50 to 70 nmol/L. In contrast, platelet GPIIb/IIIa antagonists from class II, with comparable antiaggregatory efficacy, such as DMP728, YZ202 (free-acid form of orbofiban), YZ211 (free-acid form of sibrafiban), YZ751, and other antagonists, have a much lower efficacy in altering the strength of TF-mediated clot formation (IC(50) >1.0 micromol/L). These data suggest differential efficacy among different GPIIb/IIIa antagonists in inhibiting platelet-fibrin clot retraction despite of equivalent antiaggregatory potency.

[Aspirin and prevention of cardiovascular risk]

PURPOSE

Aspirin, a potent platelet inhibitor, is widely used in patients with cardiovascular diseases. Platelet aggregation is the cornerstone of acute atherothrombotic complications.

ACTUALITIES

Aspirin showed significant benefits when administered in patients with acute myocardial infarction or unstable angina, and also when used for secondary prevention in patients with known coronary artery disease. Aspirin has been evaluated in primary prevention, with interesting results in high-risk patients. Finally, aspirin can be used in some patients with supraventricular arrhythmias or with mechanical valves.

PERSPECTIVES

Further investigation concerning the exact role of aspirin in primary prevention is currently being done. The association of aspirin with new antiplatelet agents in patients with acute coronary syndromes has shown interesting results.

Dynamic Nature of Thrombin Generation, Fibrin Formation, and Platelet Activation in Unstable Angina and Non-Q-Wave Myocardial Infarction

Thrombin and platelets are directly involved in arterial thrombosis, typically occurring at sites of atherosclerotic plaque rupture among patients with acute coronary syndromes. Understanding the dynamic nature of pathologic thrombosis has important clinical implications. Methods: Fibrinopeptide A (FPA), thrombin-antithrombin complexes (TAT), and prothrombin activation fragment 1.2 (F1.2), plasma markers of fibrin formation (thrombin activity) and thrombin generation, and platelet activation, determined by the recognition of a surface-expressed platelet alpha-granule protein, P-selectin, using flow cytometry, were measured in 36 consecutive patients with unstable angina and non-Q-wave myocardial infarction participating in the Thrombolysis In Myocardial Ischemia (TIMI) III B trial. Results: Thrombin generation (TAT 12.1 +/- 17.8 ng/ml vs. 3.4 +/- 1.0 ng/ml; F1.2 0.19 +/- 0.14 nmol/l vs. 0.12 +/- 0.8 nmol/l), fibrin formation (FPA 15.8 +/- 23.5 ng/ml vs. 7.5 +/- 2.3 ng/ml), and platelet activation) 10.6 +/- 2.4% vs. 2.5 +/- 2.0%) were increased significantly in patients compared with healthy, age-matched controls (p < 0.01). Fibrin formation, represented by plasma FPA levels, did not correlate with the percentage of activated platelets (r = -.10, p = 0.69). Thrombin generation and platelet activation also did not correlate. A statistically insignificant trend between TAT and platelet activation was observed (r =.42, p = 0.07); however, even with TAT levels in excess of 20 ng/ml (nearly sixfold greater than normal healthy controls) platelet activation was increased by only 1.7-fold. Conclusions: Thrombin generation, fibrin formation, and platelet activation are increased modestly among patients with unstable angina and non-Q-wave myocardial infarction. Despite the involvement of platelets and coagulation proteins in arterial thrombotic processes, their relative contributions may vary, providing a pathophysiologic basis for the dynamic expression of di sease and response to treatment observed commonly in clinical practice.

Platelet Activation Determined by Flow Cytometry Persists Despite Antithrombotic Therapy in Patients with Unstable Angina and Non-Q-Wave Myocardial Infarction

Background: Current strategies in the treatment of patients with acute coronary syndromes include antiplatelet agents and thrombin antagonists, most commonly aspirin and heparin, respectively. Cardiac events, however, occur despite what is considered to be maximal medical treatment. Methods: We determined the percentage of activated platelets in whole blood samples taken from 22 patients with unstable angina and non-Q-wave myocardial infarection participating in the TIMI III B trial. Platelet activation was assessed using a monoclonal antibody to the surface-expressed alpha-granule protein, P-selectin, and flow cytometry. All patients received a full complement of antiischemic medications as well as intravenous heparin and oral aspirin, and were then randomized to tissue plasminogen activator or placebo. Results: Platelet activation prior to randomization was increased threefold to fourfold compared with healthy volunteers (11.4 +/- 11.4% vs. 2.0%; p < 0.01). Serial measurements performed 12, 24, 48, and 96 hours after treatment initiation revealed that platelet activation persisted. No differences in patients experiencing recurrent ischomic events (n = 9) or those randomized to a 90-minute, accelerated infusion of tissue plasminogen activator (n = 12) were observed. Conclusions: A modest degree of platelet activation is seen for at least 96 hours and possibly longer in patients with unstable angina and non-Q-wave myocardial infarction, despite being treated with intravenous heparin and oral aspirin. These findings support current efforts to identify more potent and selective antithrombotic treatment strategies.

Antiplatelet therapies: from aspirin to GPIIb/IIIa-receptor antagonists and beyond

This review discusses recent advances in antiplatelet therapies, comparative analysis between the antiplatelet/ antithrombotic efficacy of various antiplatelet strategies and that of platelet glycoprotein GPIIb/IIIa-receptor antagonists, issues in the development of chronic anti-GPIIb/IIIa-receptor therapy and potential adjunct strategies using GPIIb/IIIa-receptor antagonists. Acute coronary syndromes are secondary to unstable angina, ST-segment elevation, and acute myocardial infarction. These involve the rupture of a vulnerable atherosclerotic plaque, leading to platelet adhesion, activation and aggregation at the site of rupture. Several studies suggest that complex or ulcerated plaques, which might promote further thrombotic events, can persist for more than one month after the acute event. These data suggest the potential added benefit of chronic oral therapy with antiplatelet drugs beyond the well-documented benefit of acute intravenous use of various GPIIb/IIIa-receptor antagonists. However the efficacy-safety ratio or the risk-benefit ratio for chronic oral antiplatelet therapy needs to be defined. Both aspirin and clopidogrel are available for chronic oral use. By contrast, there are tremendous challenges ahead with the oral GPIIb/IIIa-receptor antagonists because of their lack of expected benefit over aspirin. However, much still remains to be defined with regard to the optimization of current and future antiplatelet therapies or their optimized combinations.

Isoxazoline GPIIb/IIIa antagonists bearing a phosphoramidate

Isoxazolinylacetamides bearing a phosphoramidate group alpha- to the carboxylate moiety (3) were prepared and evaluated for in vitro antiplatelet efficacy. They were found to bind GPIIb/IIIa with high affinity and were potent antagonists of ADP mediated platelet aggregation.

Platelet glycoprotein IIb/IIIa receptor antagonists derived from isoxazolidines

A series of isoxazolidines has been synthesized as mimetics of the RGD sequence and was evaluated as antagonists of the platelet glycoprotein IIb/IIIa receptor. These compounds were shown to be highly potent GPIIb/IIIa antagonists, exhibiting submicromolar potencies.

Unstable angina and non-Q wave myocardial infarction: does the clinical diagnosis have therapeutic implications?

OBJECTIVES

The goal of this review is to reevaluate the unstable coronary syndromes in the setting of new therapies and biochemical markers.

BACKGROUND

Patients with acute coronary syndromes comprise a large subset of many cardiology practices. Patients with unstable angina (UA) and non-Q wave myocardial infarction (NQMI) may sustain a small amount of myocardial loss but have significant amounts of viable, yet ischemic, myocardium, placing them at high risk for future cardiac events. In the past, enzyme differentiation of NQMI from UA was considered important to assess prognosis and direct therapy.

METHODS

Manuscripts published in peer-reviewed journals over the past three decades were reviewed and selected for this review. Recent abstracts were also considered and cited where appropriate.

RESULTS

In the late 1990's, although UA and NQMI remain parts of a spectrum, it is apparent that the distinction between these two entities is no longer sufficient to identify high risk patients; rather, specific electrocardiographic changes, aspects of the clinical history, newer biochemical markers, and angiographic findings help to better distinguish higher risk individuals from a large patient population with unstable coronary syndromes and these factors usually determine therapy.

CONCLUSIONS

Based on these results, it is likely that newer therapies such as glycoprotein IIb/IIIa receptor antagonists, low molecular weight heparins, and coronary stents will be directed toward these high risk patients.