Collagen receptors as potential targets for novel anti-platelet agents

Platelets have important roles in atherosclerosis and thrombosis and their inhibition reduces the risk of these disorders. There is still a need for platelet inhibitors affecting pathways that reduce thrombosis and atherosclerosis while leaving normal hemostasis relatively unaffected, thus reducing possible bleeding complications. Although combinations show progress in achieving these goals none of the present inhibitors completely fulfill these requirements. Collagen receptors offer attractive possibilities as alternative targets at early stages in platelet activation. Three major collagen receptors are assessed in this review; the alpha2beta1 integrin, responsible primarily for platelet adhesion to collagen; GPVI, the major signaling receptor for collagen; and GPIb-V-IX, which is indirectly a collagen receptor via von Willebrand factor. Several thrombosis models and experimental approaches suggest that all three are interesting targets and merit further investigation.     

Novel mechanistic concept of platelet inhibition

Background: Activation of circulating platelets by exposed vessel wall collagen is a primary step in the pathogenesis of thrombotic diseases such as heart attack and stroke. Drugs that are capable of blocking platelet activation successfully reduce cardiovascular mortality and morbidity. However, despite intensive research efforts in antithrombotic drug discovery and development, uncontrolled hemorrhage still remains the most common side effect associated with antithrombotic drugs that are currently in use. Objective: The selective inhibition of glycoprotein VI (GPVI), the central platelet collagen receptor, and/or its signaling may inhibit thrombosis without affecting hemostatic plug formation. However, the mechanism of GPVI signaling is not known, hindering the further development of this promising antithrombotic strategy. Methods: This review focuses on an innovative mechanistic concept of platelet inhibition. Results/conclusion: A novel model of GPVI signaling, the signaling chain homooligomerization (SCHOOL) model, has revealed new therapeutic targets for GPVI inhibition, resulting in the development of novel antithrombotic pharmacological approaches and the invention of new platelet inhibitors.     

New perspectives in antiplatelet therapy

Platelet activation is a complex mechanism of response to vascular injury and atherothrombotic disease, leading to thrombus formation. A wide variety of surface receptors -integrins, leucine-rich family receptors, G protein coupled receptors, tyrosine kinase receptors- and intraplatelet molecules support and regulate platelet activation. They are potential targets of antiplatelet therapy for the prevention and treatment of arterial thrombosis. Despite the overall clinical benefit of established antiplatelet drugs targeting cyclooxigenase-1 (COX-1), glycoprotein integrin αIIbβ3, and the purinergic P2Y(12) receptor of adenosine diphosphate, a significant proportion of treated patients continue to experience recurrent ischaemic events. This may be in partly attributed to insufficient inhibition of platelet activation. In addition, it should not be underestimated that these drugs are not immune from bleeding complications. The substantial progress in understating the regulation of platelet activation has played a key role in the development of novel antiplatelet agents. Current examples of drug under development and evaluation include: novel P2Y(12) receptor inhibitors (prasugrel, ticagrelor, cangrelor, and elinogrel), thrombin receptor PAR-1 antagonists (vorapaxar, atopaxar), new integrin glycoprotein IIb/IIIa inhibitors, and inhibitors targeting the thromboxane receptor (TP), phosphodiesterases, the collagen receptor glycoprotein VI, and intraplatelet signalling molecules. This review summarizes the mechanisms of action and current clinical evaluation of these novel antiplatelet agents.     

Understanding the mechanism of platelet thrombus formation under blood flow conditions and the effect of new antiplatelet agents

Platelet aggregation induced by stirring of a platelet suspension after activation by the exogenous addition of stimulants, such as ADP, epinephrine and thrombin, has long been used as a platelet function test, and for the screening of antiplatelet agents. Since platelet aggregation has been demonstrated to be mediated exclusively by the binding of plasma fibrinogen to the activated GP IIb/IIIa, anti-GP IIb/IIIa agents, which can completely inhibit platelet aggregation, were expected to become among the most potent of antithrombotic agents. The strong preventive effects of anti-GP IIb/IIIa agents against thrombotic complications after coronary intervention, and the weaker preventive effects of the same agents against the onset of coronary thrombosis in unstable angina pectoris patients point to both the efficacy and the limitations of anti-GP IIb/IIIa antithrombotic agents. Recently, many investigators have reported that platelets play a major role in thrombus formation at sites exposed to blood flow. Several platelet-function assay systems have been developed to elucidate the mechanism of thrombus formation under blood flow conditions. Numerous studies have demonstrated that von Willebrand factor (VWF) and its interaction with its receptors on platelets, including GP Ibalpha and GP IIb/IIIa, play essential roles not only in platelet activation, but also in platelet adhesion and possibly platelet cohesion. These findings prompted us to explore whether the VWF-mediated process of thrombus formation could be exploited as a target for potential antiplatelet agents. Moreover, recent studies have demonstrated that multiple synergistic signals, including those mediated by catecholamine receptors, purine nucleotide receptors, as well as some types of collagen receptors are involved in the process of VWF-mediated platelet thrombus formation. We now have new antiplatelet agents on the horizon, targeted against thrombus formation under blood flow conditions.     

Inhibition of platelet adhesion to collagen as a new target for antithrombotic drugs

Platelet adhesion to a damaged blood vessel is the initial trigger for arterial hemostasis and thrombosis. Platelets adhere to the subendothelium through an interaction with von Willebrand factor (VWF), which forms a bridge between collagen within the damaged vessel wall and the platelet receptor glycoprotein Ib/V/IX (GPIb), an interaction especially important under high shear conditions[1]. This reversible adhesion allows platelets to roll over the damaged area, which is then followed by a firm adhesion mediated by the collagen receptors (alpha(2)beta(1), GPVI, ) in addition[2] resulting in platelet activation. This leads to the conformational activation of the platelet alpha(IIb)beta3 receptor, fibrinogen binding and finally to platelet aggregation. Over the past decades, modulation of platelet function has been a strategy for the control of cardiovascular disease. Lately, drugs have been developed that target the fibrinogen receptor alphaIIbbeta3 or the ADP receptor and many of these promising compounds have been tested in clinical trials. However the development of products that interfere with the first step of hemostasis, i.e. the platelet adhesion, has lagged behind. In this review we want to discuss (i) the in vivo studies that were performed with compounds that target proteins involved in different adhesion steps i.e. the VWF-GPIb-axis, the collagen-VWF axis and the collagen-collagen receptor axis and (ii) the possible advantages these putative new drugs could have over the current antiplatelet agents.     

Small molecule inhibitors of integrin alpha2beta1

Interactions between the integrin, alpha2beta1, and extracellular matrix (ECM), particularly collagen, play a pivotal role in platelet adhesion and thrombus formation. Platelets interact with collagen in the subendothelial matrix that is exposed by vascular damage. To evaluate the potential of alpha2beta1 inhibitors for anticancer and antithrombotic applications, we have developed a series of small molecule inhibitors of this integrin based on a prolyl-2,3-diaminopropionic acid (DAP) scaffold using solid-phase parallel synthesis. A benzenesulfonamide substituent at the N-terminus of the dipepetide and a benzyl urea at the DAP side chain resulted in tight and highly selective inhibition of alpha2beta1-mediated adhesion of human platelets and other cells to collagen.     

The platelet ATP and ADP receptors

Adenine nucleotides, ADP and ATP, are coreleased from dense granules during platelet activation, as well as from endothelial cells and damaged red blood cells following vascular injury. Through autocrine and paracrine mechanisms, these extracellular signaling molecules interact with the platelet P2 receptors to amplify ongoing platelet activation. Two receptors for ADP, the G(q)-protein-coupled P2Y1 and G(i)-protein-coupled P2Y12 and one receptor for ATP, the P2X1 ion channel, have been identified on platelets. Due to distinct pharmacological properties and differential regulation, the P2Y and P2X receptors essentially operate on different scales of time and distance and trigger selective intracellular signaling cascades. Recent advances in the understanding of the P2Y receptor physiology have reinforced the concept of these receptors as useful targets for antithrombotic therapy. The function of P2X1 in platelet activation only recently started to be unraveled. This review focuses on recent findings on the physiology of these platelet ADP and ATP receptors, their distinct downstream intracellular signaling pathways as well as on the available agonists, antagonists and inhibitors that allow their pharmacological discrimination.     

Novel approaches to the treatment of thrombosis.

Thromboembolic diseases remain the main cause of death in Western societies despite current antithrombotic treatments. Recent advances in the molecular bases of haemostasis have highlighted new targets for novel antiplatelet or anticoagulant agents. Considering antiplatelet agents, selective antagonists of specific receptors (von Willebrand factor, collagen or thrombin receptors) are effective in thrombosis models and direct ADP receptor antagonists and nitric-oxide-releasing aspirin are in Phase I-II clinical trials. Concerning anticoagulants, inhibitors of tissue-factor-induced clotting activation, selective inhibitors of thrombin and factor Xa, and components of the anticoagulant protein C system (recombinant activated human protein C or human soluble thrombomodulin) have been studied. Some of these agents have had promising results in Phase III studies. Several achievements are anticipated from the development of new antithrombotics, including a further reduction of cardiovascular mortality and unwanted bleeding, and easier patient management.     

Antithrombotic agents

The morbidity and mortality associated with thromboembolic diseases are a worldwide problem. Deep venous thrombosis (DVT) and pulmonary embolism (PE) were estimated, some years ago, to be responsible for between 300,000 and 600,000 hospitalisations annually in the US. Arterial thrombosis is a major cause of myocardial infarction (MI), cerebral infarction and peripheral arterial diseases. Currently available antithrombotic drugs can be divided into three classes: anticoagulants (including vitamin K antagonists, unfractionated heparin and low molecular weight heparins [LMWHs]), antiplatelet agents (platelet aggregation inhibitors such as aspirin, ticlopidine and dipyridamole) and thrombolytics (streptokinase, urokinase and tissue plasminogen activator [t-PA]). All these drugs have demonstrated efficacy in the clinic, but each has its limitations. The risk of haemorrhage persists (though to varying degrees), and efficacy in the prevention, in addition to the treatment, of thromboembolic diseases needs to be improved. In addition, reocclusion following thrombolytic therapy remains a far from negligible risk. Current research goals are focused on identifying more potent and specific inhibitors of targets playing a crucial role in the coagulation pathway, such as thrombin and Factor Xa. Long-term goals are to develop thrombin receptor antagonists or analogues of natural occurring anticoagulants (e.g., tissue factor pathway inhibitor [TFPI] and protein C). The search for more effective anti-aggregating agents has resulted in the ongoing development of clopidogrel (a chemical analogue of ticlopidine), and thromboxane synthase inhibitors and receptor blockers. However, the main efforts have been focused on developing antagonists of the platelet receptor glycoprotein, Gp IIb/IIIa.Much work has been performed to improve the fibrin specificity of thrombolytic agents. In all these areas, new promising chemical structures have been identified, and a number of new antithrombotic agents are in development. The potential development issues remain the risk of bleeding at therapeutic doses, the possible lack of efficacy in sub-groups of treated patients, as well as dose adjustment and treatment duration.The considerable clinical progress in interventional cardiology (e.g., percutaneous transluminal coronary angioplasty [PTCA] and stent implantation) has increased the need for antithrombotic drugs with high local efficacy at the site of arterial injury. Recent progress in the knowledge of plaque rupture-induced disorders, particularly the major role of tissue factor, is suggesting new concepts for antithrombotic therapy. The experience from clinical trials with antithrombotic agents, the validation of biological parameters predictive of a prethrombotic state, and the improvement in non-invasive methods to control drug efficacy are likely to contribute to significant therapeutic progress in the prevention and treatment of venous and arterial thrombosis.     

Review Cardiovascular & Renal; Recent Developments in Glycoprotein llb/llla Antagonists

Platelet adhesion and aggregation, which have been identified as promising targets for the development of antithrombotic drugs, are mediated by a family of glycoprotein receptors. Antagonists of the most important of these receptors, Gpllb/llla, are currently under investigation for a range of both cardiovascular and cerebrovascular disorders. This article aims to compare data for existing Gpllb/llla antagonists in various stages of clinical development, with those compounds which appear in the recent patent literature.     

ADP receptors--targets for developing antithrombotic agents

Platelet P2 receptors--P2Y1, P2Y12, and P2X1--constitute the means by which adenine nucleotides can activate platelets. Coactivation of the Galphaq-coupled P2Y1 and Galphai2-coupled P2Y12 receptors is necessary for ADP-mediated platelet activation, which forms the basis of using P2 antagonists as antithrombotic drugs. P2Y1 receptor antagonists inhibit platelet activation, while P2Y1 knockout mice show longer bleeding times than normal mice but few other problems; however, its ubiquitous expression in other tissues renders P2Y1 questionable as an antithrombotic target. The P2Y12 receptor is expressed nearly exclusively in platelets and brain, making it an attractive antithrombotic target. Antagonists for the P2Y12 receptor have been developed that either require metabolic activation to covalently inhibit P2Y12 and are irreversible, or simply are competitive in nature and thus reversible. Ticlopidine and clopidogrel are irreversible P2Y12 antagonists and have been repeatedly proven as clinical antithrombotic agents. In addition, a recently reported P2Y12 antagonist, CS-747, shows promise as a future antithrombotic drug. The AR-C series of compounds represent reversible P2Y12 antagonists and have been used extensively to characterize the function of P2Y12 in platelets. Clinical studies show that AR-C69931MX is as effective as clopidogrel; furthermore, the combination of AR-C69931MX (cangrelor) and clopidogrel confers greater antagonism of P2Y12 than either antagonist alone. The P2X1 receptor is a calcium channel that functions to potentiate agonist-induced platelet shape change, and its inhibition or loss has little if any effect on hemostasis. A combination of P2Y1 and P2Y12 antagonists may represent an additional course of antithrombotic treatment.     

脑缺血性中风抗血栓治疗的研究进展

缺血性中风是严重威胁人类健康的常见病。该文从中风的抗血栓治疗出发,介绍近期抗血栓治疗的研发热点,并由此阐明颅内血栓形成的分子通路。通过阻断血小板表面受体糖蛋白Ibα（GPIbα）或糖蛋白VI（GPVI）受体以及抑制凝血因子FIX,FX和FXII等来抑制血栓形成与发展,新一代抗血栓药物有望更好地治疗和预防脑缺血性中风。     

Platelet and soluble glycoprotein VI - novel applications in diagnosis and therapy

One of the key receptors involved in the prothrombotic stage of an acute coronary syndrome (ACS) is platelet glycoprotein VI (GPVI). This constitutively expressed collagen receptor is platelet-specific and has shown to be a useful biomarker tool for the early detection of atheroslerotic diseases such as ACS and ischemic stroke. In a multimarker panel of several biomarkers, platelet GPVI may contribute to risk stratification and prediction of clinical outcome in patients with symptomatic atherosclerotic diseases. Moreover, the soluble receptor of GPVI may also be an interesting and prospective target for molecular imaging to identify sites of vulnerable plaques as the results of preclinical studies suggest. Apart from the diagnostic use of platelet as well as soluble, plasmatic GPVI, therapeutical implications may be the blockade of collagen binding sides of GPVI, as platelet GPVI is not able to bind to collagen where the binding sides were already saturated with soluble GPVI. This therapeutic aspect could serve as a promising strategy for antithrombotic and antiatherosclerotic therapy in future.     

The Role of the Platelet in the Pathogenesis of Atherothrombosis

Platelet adhesion, activation, and aggregation at sites of vascular endothelial disruption caused by atherosclerosis are key events in arterial thrombus formation. Platelet tethering and adhesion to the arterial wall, particularly under high shear forces, are achieved through multiple high-affinity interactions between platelet membrane receptors (integrins) and ligands within the exposed subendothelium, most notably collagen and von Willebrand factor (vWF). Platelet adhesion to collagen occurs both indirectly, via binding of the platelet glycoprotein (GP) Ib-V-IX receptor to circulating vWF, which binds to exposed collagen, and directly, via interaction with the platelet receptors GP VI and GP Ia/IIb. Platelet activation, initiated by exposed collagen and locally generated soluble platelet agonists (primarily thrombin, ADP, and thromboxane A2), provides the stimulus for the release of platelet-derived growth factors, adhesion molecules and coagulation factors, activation of adjacent platelets, and conformational changes in the platelet alpha(IIb)beta3 integrin (GP IIb/IIIa receptor). Platelet aggregation, mediated primarily by interaction between the activated platelet GP IIb/IIIa receptor and its ligands, fibrinogen and vWF, results in the formation of a platelet-rich thrombus. Currently available antiplatelet drugs (aspirin [acetylsalicylic acid], dipyridamole, clopidogrel, ticlopidine, abciximab, eptifibatide, tirofiban) act on specific targets to inhibit platelet activation and aggregation. Elucidation of the multiple mechanisms involved in platelet thrombus formation provides opportunities for selectively inhibiting the pathways most relevant to the pathophysiology of atherothrombosis.     

Development of monoclonal antibodies that inhibit platelet adhesion or aggregation as potential anti-thrombotic drugs

Cardiovascular disease is the major cause of mortality in Western countries. Platelets play a crucial role in the development of arterial thrombosis and other pathophysiologies leading to clinical ischemic events. In the damaged vessel wall, platelets adhere to the subendothelium through an interaction with von Willebrand factor (VWF), which forms a bridge between subendothelial collagen and the platelet receptor glycoprotein (GP) Ib/IX/V. This reversible adhesion allows platelets to roll over the damaged area, decreasing their velocity and resulting in strong platelet activation. This leads to the conformational activation of the platelet GPIIb/IIIa receptor, fibrinogen binding and finally to platelet aggregation. As each interaction (collagen-VWF, VWF-GPIb and GPIIb/IIIa-fibrinogen) plays an essential role in primary haemostasis, loss of either of these interactions results in a bleeding diathesis, implying that interfering with these interactions might result in an anti-thrombotic effect. Whereas GPIIb/IIIa antagonists indeed are effective anti-thrombotics, it has been suggested that drugs which block the initial steps of thrombus formation (collagen-VWF or VWF-GPIb interaction) might have advantages over the ones that merely inhibit platelet aggregation. In this review we will discuss and compare the development of monoclonal antibodies (moAbs) that inhibit platelet adhesion or platelet aggregation. The effect of the moAbs in in vitro experiments, in in vivo models and in clinical trials will be described. Benefits, limitations, current applications and the future perspectives in the development of antibodies for each target will be discussed.     

P2Y receptor antagonists in thrombosis

The dual role of P2Y1 and P2Y12 receptors in platelet aggregation by ADP has been firmly established, based on the action of selective inhibitors, gene targeting in mice and human genetic evidence. Both of these receptor subtypes constitute targets for antithrombotic agents, and compounds with a dual action might also be of interest. However, the agents currently on the market (ticlopidine and clopidogrel), or known to be in development (cangrelor, AZD-6140 and prasugrel), all target the P2Y12 receptor. The thienopyridines (ticlopidine, clopidogrel and prasugrel) irreversibly inactivate the P2Y12 receptor via the covalent binding of an active metabolite generated in the liver, while the other compounds are competitive antagonists. Cangrelor, an ATP derivative, is suitable for intravenous perfusion, whereas AZD-6140 is in clinical development as an orally active agent.     

Contribution of in vivo models of thrombosis to the discovery and development of novel antithrombotic agents

Cardiovascular and cerebrovascular diseases continue to be the leading cause of death throughout the world. Over the past two decades, great advances have been made in the pharmacological treatment and prevention of thrombotic disorders (e.g., tissue plasminogen activators, platelet GPIIb/IIIa antagonists, ADP receptor antagonists such as clopidogrel, low-molecular weight heparins, and direct thrombin inhibitors). New research is leading to the next generation of antithrombotic compounds such as direct coagulation FVIIa inhibitors, tissue factor pathway inhibitors, gene therapy, and orally active direct thrombin inhibitors and coagulation Factor Xa (FXa) inhibitors. Animal models of thrombosis have played a crucial role in discovering and validiting novel drug targets, selecting new agents for clinical evaluation, and providing dosing and safety information for clinical trials. In addition, these models have provided valuable information regarding the mechanisms of these new agents and the interactions between antithrombotic agents that work by different mechanisms. This review briefly presents the pivitol preclinical studies that led to the development of drugs that have proven to be effective clinicallly. The role that animal models of thrombosis are playing in the discovery and development of novel antithrombotic agents is also described, with specific emphasis on FXa inhibitors. The major issues regarding the use of animal models of thrombosis, such as the use of positive controls, appropriate pharmacodynamic markers of activity, safety evaluation, species-specificity, and pharmacokinetics, are highlighted. Finally, the use of genetic models in thrombosis/hemostasis research and pharmacology is presented using gene-therapy for hemophilia as an example of how animal models have aided in the development of these therapies that are now being evaluated clinically. In summary, animal models have contributed greatly to the discovery of currently available antithrombotic agents and will play a primary role in the discovery and characterization of the novel antithrombotic agents that will provide safe and effective pharmacological treatment for life-threatening thrombotic diseases.     

The dopamine agonism on ADP-stimulated platelets is mediated through D2-like but not D1-like dopamine receptors

Monoamines such as serotonin and epinephrine are known to be involved in platelet activation and aggregation. Dopamine is another monoamine identified in platelets, but published data about its effect on platelets and the receptors involved are controversial. In the present study, we investigated the dopamine agonism in platelets and the receptors involved in these pathways. Platelet-rich plasma (PRP) of healthy individuals was treated with agonists (ADP, epinephrine, dopamine) and various dopamine receptor and transporter antagonists such as SCH-23390, raclopride, clozapine, methylphenidate, and cocaine. Platelet activation was investigated by flow cytometry (CD62P and CD63 surface expression), optical aggregometry, and microaggregate adhesion to collagen IV in a flow chamber system. In our study, dopamine on its own had no effect on platelet activation in the different assays. However, when used in combination with ADP (10 μM), dopamine in a range of 1 to 100 μM significantly potentiated platelet microaggregate formation and adhesion to collagen under low shear flow conditions. Specific antagonists for D2-like receptors (L-741,626, raclopride, and clozapine) completely diminished the dopamine effect at selective concentrations, but not the effect of epinephrine. Neither the D1-like receptor antagonist SCH-23390 nor dopamine transporter antagonists (methylphenidate, cocaine) showed inhibitory effects on the dopamine agonism. Thus, dopamine is an ADP-dependent platelet agonist which acts via D2-like but not D1-like receptors or adrenergic receptors. Because many psychopharmacological drugs are directed to D2-like receptors, platelet dysfunction in patients being treated with such drugs may be linked to these mechanisms.     

Bleeding complications with glycoprotein IIb/IIIa inhibitors

The new class of antiplatelet drugs, the GPIIb/IIIa inhibitors, has proven to be effective in acute coronary syndromes including unstable angina and myocardial infarction as well as adjunct therapy for coronary interventions for preventing morbidity and mortality. As these drugs inhibit the final common pathway of platelet activation, effectively blocking the platelet aggregation response, potential bleeding is a concern with their use. The risk of bleeding has been demonstrated to be higher in patients treated with combination drug therapy (heparin, aspirin, thienopyridines, thrombolytics, oral anticoagulants), when antithrombotic drugs are not given on an individual weight basis and with late removal of vascular access sheaths. The early clinical trials have defined modifications in patient management that have effectively reduced bleeding. Pooled data from the more recent clinical trials, mostly in coronary intervention enrolling over 27,000 patients, show a bleeding rate of 3.6% in the drug group and 2.3% in the placebo group. Although this is acceptable, several unresolved issues remain to be addressed regarding the GPIIb/IIIa inhibitors. Thrombocytopenia occurs infrequently with all GPIIb/IIIa inhibitors but can be severe. The use of these drugs by oral administration presents new challenges with determining optimal dosing, drug-drug interactions and long-term effects. Incorporating point-of-care monitoring may enable better titration of these drugs to avoid bleeding complications. GPIIb/IIIa inhibitors are destined to become a mainstay therapy for cardiovascular treatment and over time these issues should be resolved.     

New developments in thrombosis and coagulation therapy

The 15(th) International Congress on Thrombosis (Mediterranean League against Thromboembolic Disease), held October 16-21, 1998, in Antalya, Turkey, brought researchers and clinicians up to date on the latest developments in thrombosis, hemostasis, coagulation and hematology, including epidemiology, use of traditional anticoagulants, heparin-associated thrombocytopenia and novel antithrombotic and antiplatelet agents. The pharmacology and clinical use of different heparins present in the market were extensively reviewed, and the characteristics of new LMWHs being launched in the market were presented in numerous communications. Recent issues regarding the efficacy and safety of oral anticoagulation in various indications were also reviewed in several lectures and communications. An important number of the presentations during the meeting focused on pharmacological, experimental and clinical studies with novel antithrombotic drugs. The development of specific inhibitors for collagen motifs or platelet receptors may become a promising alternative for the modulation of platelet function. The ability to manipulate macrophage proliferation and death may have therapeutic implications. Pharmacological inhibition of protein kinases and protein phosphatases could also provide new therapeutic approaches for the control of excessive platelet interactions. Recent studies suggest that PMN integrins are a potential target for pharmacological intervention.