Platelet GPIIb/IIIa antagonist, XV459, in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a serious, immune-related complication of heparin therapy. One of the most severe manifestations of HIT is the development of thromboembolic events, which is based on platelet activation and aggregation caused by HIT-associated antibodies. Therapeutic options for patients with HIT are limited despite advancement toward the development of alternative (nonheparin) anticoagulants, such as direct thrombin inhibitors and indirect anti-factor Xa agents. Platelet GPIIb/IIIa receptor antagonists have been shown to be the final common pathway for platelet aggregation regardless of the use of activator or anticoagulant. In this study, the ability of a novel platelet GPIIb/IIIa antagonist, a free acid form of roxifiban (XV459), to block platelet activation/aggregation in response to highly characterized heparin-PF4 antibody-positive plasma/heparin was examined using light transmittance aggregometry, serotonin release, and (125)I-fibrinogen binding assays to human platelets. XV459 at 20 nM maximally inhibited (P < 0.001) the platelet-activation/aggregation responses as mediated by the HIT antibody-positive plasma (in the presence of therapeutic heparin concentrations). Compared with controls, both HIT antibodies/heparin and TEAC (a mixture of thrombin [0.1 IU/ml], epinephrine [1 microg/ml], arachidonate [0.1 mM], and collagen [10 microml]) resulted in significantly higher levels of fibrinogen binding to human platelets (5-7-fold increase; P < 0.001). Concentration-dependent profiles of XV459 on the mean percent inhibition of (125)I-fibrinogen binding in the presence of HIT antibodies and TEAC were achieved ( approximately 50% inhibition at 10 nM XV459). The platelet GPIIb/IIIa receptor antagonist (XV459) might be of potential benefit in the management of thrombotic thrombocytopenia produced by heparin and/or related glycosaminoglycans.     

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.     

In vitro measurement of platelet glycoprotein IIb/IIIa receptor blockade by abciximab: interindividual variation and increased platelet secretion

BACKGROUND AND OBJECTIVES: Inhibition of soluble fibrinogen binding to activated platelets represents the target of pharmacologic approach with antagonists of the glycoprotein IIb/IIIa (GPIIb/IIIa) complex. In this study we assessed the effects of abciximab, a recombinant chimeric Fab fraction of the antibody against GPIIb/IIIa, on several markers of platelet activation. DESIGN AND METHODS: The platelet surface expression of GPIIb/IIIa was measured by a flow cytometry technique using a two-color assay. GPIIb/IIIa was detected by FITC-conjugated antibodies in whole blood, either unstimulated or exposed to platelet stimuli. The following antibodies were used: CD41, which recognizes the IIb/IIIa complex both in activated and non-activated conformers, and PAC-1, which is directed toward the activated conformer of GPIIb/IIIa. In addition, the same blood sample was incubated with CD62 antibody to measure P-selectin, as a marker of a-granule degranulation. The effect of abciximab was also assessed by experiments carried out on shear stress-induced platelet aggregation, a test that appears to be a predictor of platelet hemostatic function. RESULTS: Abciximab inhibited CD41 binding to glycoprotein IIb (GPIIb) in a concentration-dependent manner and also inhibited the binding of PAC-1 to active GPIIb/IIIa. In contrast, membrane-associated P-selectin was significantly increased by the drug, which suggests that blockade of GPIIb/IIIa receptors results in an increased platelet degranulation in response to agonists. Shear stress-induced platelet aggregation was inhibited by abciximab, with a more pronounced effect on blood filtration, which represents an index of platelet aggregate formation. INTERPRETATION AND CONCLUSIONS: Our results indicate that GPIIb/IIIa blockade by abciximab is accompanied by an increase of a-granule secretion, suggesting that different mechanisms regulate these aspects of platelet activation. The described flow cytometry technique, that allows the simultaneous in vitro detection of several platelet markers, is a suitable method for assessing the effects of agents which interfere with platelet function.     

MK-383 (tirofiban) induces a GPIIb/IIIa receptor conformation which differs from the resting and activated receptor

The platelet integrin alphaIIb beta3 (GPIIb/IIIa) acts as a receptor for fibrinogen, playing a critical role in platelet aggregation. GPIIb/IIIa antagonists, which block the receptor-ligand interaction, have been accused of causing occasional thrombocytopenia, probably via drug-induced platelet activation or immunogenic neoepitopes. We, therefore, analyzed the effects of the GPIIb/IIIa antagonist MK-383 (tirofiban) on platelet activation and GpIIb/IIIa conformation. At a concentration of 10(-7) mol/l, MK-383 completely inhibited fibrinogen binding to in vitro stimulated platelets. Simultaneously, the GPIIb/IIIa expression density increased, similar to that on activated platelets, but no effect on P-selectin expression or the formation of platelet-leukocyte aggregates could be observed, indicating that MK-383 binding did not induce general platelet activation. The GPIIb/IIIa receptor conformation was further analyzed by fluorescence resonance energy transfer analysis between fluorochrome-labeled antibodies against different GpIIb/IIIa epitopes. As a result, MK-383 induced a receptor conformation that differed from the resting as well as the activated receptor as induced by ADP or TRAP-6. This conformational modulation of GPIIb/IIIa presents an interesting mechanism which may be linked to receptor recruitment without inducing general platelet activation.     

Glycoprotein IIb/IIIa and P2Y12 receptor antagonists yield additive inhibition of platelet aggregation, granule secretion, soluble CD40L release and procoagulant responses

Glycoprotein IIb/IIIa (GPIIb/IIIa) antagonists, including abciximab and tirofiban, are administered concurrently with clopidogrel, a P2Y12 antagonist, and aspirin in some patients undergoing percutaneous coronary intervention. We studied the effects of, and interactions between, abciximab, tirofiban, aspirin and the P2Y12 antagonist cangrelor on platelet aggregation, alpha and dense granule secretion and procoagulant responses in vitro. Blood was obtained from healthy volunteers. Platelet aggregation, dense granule secretion, alpha granule secretion (PAI-1 and soluble CD40 ligand levels) and procoagulant responses (annexin-V and microparticle formation) were assessed using collagen and thrombin receptor activating peptide (TRAP) as agonists. All the antagonists used singularly inhibited collagen-induced responses. Combinations of abciximab or tirofiban with aspirin and/or cangrelor gave additive inhibition with the greatest effect seen when abciximab or tirofiban was combined with both aspirin and cangrelor. Cangrelor inhibited TRAP-induced responses and, again, there was additive inhibition of these parameters when abciximab or tirofiban were combined with cangrelor. The GPIIb/IIIa receptor plays an important role in amplification of platelet activation such that there are important interactions between GPIIb/IIIa antagonists and inhibitors of both P2Y12 receptor activation and, to a lesser extent, thromboxane A2 generation. These interactions are likely to have important influences on the safety and efficacy of combination anti-platelet therapies.     

Differential inhibition of adenosine diphosphate- versus thrombin receptor-activating peptide-stimulated platelet fibrinogen binding by abciximab due to different glycoprotein IIb/IIIa activation kinetics

The exposure of internal glycoprotein (GP) IIb/IIIa receptors has been proposed to explain the incomplete inhibition of aggregation of thrombin receptor-activating peptide (TRAP)-stimulated platelets by abciximab. However, a marked and rapid externalization of GPIIb/IIIa was also observed upon stimulation with 30 microM adenosine diphosphate (ADP). ADP-induced fibrinogen binding was completely inhibited by 10 microg/mL abciximab, 30 nM tirofiban, or 3 microg/mL eptifibatide, while fibrinogen binding induced by 100 microM TRAP was inhibited only by 50%. Interestingly, striking differences in fibrinogen binding kinetics in ADP- versus TRAP-stimulated platelets were observed. ADP-induced fibrinogen binding was much slower than that of abciximab. These differences in the fibrinogen binding rate were due to differential GPIIb/IIIa activation kinetics because the actual fibrinogen binding rate (measured by adding fibrinogen after platelet activation) was similar in ADP- and TRAP-stimulated platelets. Thus, the TRAP-induced GPIIb/IIIa activation rate would allow significant amounts of fibrinogen to occupy externalized GPIIb/IIIa receptors even in the presence of the inhibitor.     

UR‐3216: A Manageable Oral GPIIb/IIIa Antagonist

UR‐3216, a prodrug, is a novel, selective, and orally active platelet surface glycoprotein GPIIb/IIIa) receptor antagonist. The most important property of UR‐3216 is the very tight binding of its active metabolite to platelets (K_i for resting platelets is <1 nM). UR‐2992, the active form of UR‐3216, binds to platelets for a long period of time, while the unbound drug is rapidly cleared. Therefore, after an initial loading dose of 0.1 mg/kg, only once daily repeated low maintenance doses of UR‐3216 (<0.05 mg/kg p.o.) are required. This regimen maintains a high level of inhibition of platelet aggregation and, due to a small peak‐to‐trough ratio, severe bleeding is avoided. The therapy with UR‐3216 is easy to manage, because it has low peak‐to‐trough ratio and high efficacy (>80% inhibition of platelet aggregation). In addition, UR‐3216 does not produce excessive bleeding or thrombocytopenia and does not interact with abciximab. UR‐3216 is excreted mostly in bile, so that it will not accumulate in patients with chronic renal dysfunction. UR‐2316 has the following abciximab‐like features: (a) its half‐lives for residence on platelets, inhibition of platelets aggregation and bleeding time prolongation are 60 to 80 h, 24, and 2 h, respectively; (b) its receptor binding occupancy is similar to that of abciximab (Mab1 is inhibited and Mab2 is unaltered). In conclusion, UR‐3216 is a promising, orally active GPIIb/IIIa antagonist for the treatment of cardiovascular diseases.     

MK-383 (Tirofiban) induces a GPIIb/IIIa receptor conformation which differs from the resting and activated receptor

The platelet integrin α_IIb β₃ (GPIIb/IIIa) acts as a receptor for fibrinogen, playing a critical role in platelet aggregation. GPIIb/IIIa antagonists, which block the receptor-ligand interaction, have been accused of causing occasional thrombocytopenia, probably via drug-induced platelet activation or immunogenic neoepitopes. We, therefore, analyzed the effects of the GPIIb/IIIa antagonist MK-383 (tirofiban) on platelet activation and GpIIb/IIIa conformation. At a concentration of 10^-7 mol/l, MK-383 completely inhibited fibrinogen binding to in vitro stimulated platelets. Simultaneously, the GPIIb/IIIa expression density increased, similar to that on activated platelets, but no effect on P-selectin expression or the formation of platelet-leukocyte aggregates could be observed, indicating that MK-383 binding did not induce general platelet activation. The GPIIb/IIIa receptor conformation was further analyzed by fluorescence resonance energy transfer analysis between fluorochrome-labeled antibodies against different GpIIb/IIIa epitopes. As a result, MK-383 induced a receptor conformation that differed from the resting as well as the activated receptor as induced by ADP or TRAP-6. This conformational modulation of GPIIb/IIIa presents an interesting mechanism which may be linked to receptor recruitment without inducing general platelet activation.     

Different inhibiting effects of abciximab and tirofiban on platelet thrombus formation on a collagen surface under flow conditions

OBJECTIVES: Recently developed anti-GPIIb/IIIa agents effectively inhibit acute thrombotic occlusion of coronary arteries after interventional treatment, and have similar inhibiting effects on plasma ligand binding to GPIIb/IIIa. Clinical investigation has revealed that abciximab, the chimeric monoclonal antibody against GPIIb/IIIa, has superior in vivo antithrombotic effects to other agents. The inhibiting effects of abciximab and another anti-GPIIb/IIIa agent, tirofiban, were investigated on platelet thrombus formation on a collagen surface under flow conditions. METHODS: Blood was drawn from 6 normal volunteers and anticoagulated with a specific inhibitor of thrombin, argatroban, at a final concentration of 100 microM. Platelets were rendered fluorescent by addition of mepacrine to a final concentration of 10 microM. Mepacrine is concentrated in the dense granules of platelets and leukocytes, but does not accumulate in red blood cells, so platelet thrombi can be detected by fluorescence microscopy even in the presence of red blood cells. Horizontal glass slips covered with fibrillar type I collagen were assembled in a Hele-Shaw type flow chamber. RESULTS: Platelet thrombi were developed on the collagen surface even in the absence of platelet activating agents. Both abciximab and tirofiban inhibited the platelet thrombus formation. Single platelet adhesion on the collagen surface was inhibited only by abciximab and not by tirofiban. CONCLUSIONS: The superior in vivo antithrombotic effects of abciximab may be partially explained by its inhibiting effects on the platelet adhesion on exposed subendothelial matrix.     

Platelet glycoprotein IIb/IIIa inhibition and its clinical use

The activation of platelets and the resultant aggregation have been shown to play important role in the pathogenesis of cardiovascular, cerebrovascular and peripheral vascular diseases and in acute coronary syndromes. Hence platelet adhesion and aggregation have been identified as promising targets for the development of anti-thrombotic drugs. Glycoprotein (GP) IIb/IIIa antagonism exerts a strong anti-platelet effect, because this interference inhibits the final common pathway of platelet aggregation and is not dependent on a single activation pathway. Three GPIIb/IIIa antagonists have been approved by the US Food and Drug administration. They include abciximab (the chimeric monoclonal antibody 7E3 Fab fragment), eptifibatide (the cyclic heptapeptide based on the KGD amino acid sequence) and tirofiban (a nonpeptide tyrosine derivative). In addition, nonpeptide oral GPIIb/IIIa antagonists are also in various stages of clinical development. This paper reviews the molecular biology of the GPIIb/IIIa receptor, history of development of GPIIb/IIIa antagonists, some issues about GPIIb/IIIb antagonists including their affinity, reversibility and receptor specificity, adverse effects including bleeding and thrombocytopenia, clinical trials and costs. Future direction in the development of GPIIb/IIIa antagonists is also discussed.     

A Val193Met mutation in GPIIIa results in a GPIIb/IIIa receptor with a constitutively high affinity for a small ligand

We have identified a patient designated as (GTa) with Glanzmann's Thrombasthenia (GT) diagnosed on the basis of a prolonged bleeding time and failure of the patient's platelets to aggregate. The number of glycoprotein (GP)IIb/IIIa receptors on the platelet surface was 37% of normal and those receptors displayed a defect in soluble fibrinogen binding. Nevertheless, GTa platelets showed increased adhesion to solid-phase fibrinogen and binding affinity for the RGD-mimetic (3)H-SC52012, a non-peptide GPIIb/IIIa antagonist. Dithiothreitol (DTT) and ADP enhanced the affinity for [(3)H]-SC52012 in normal platelets, but had little effect in GTa platelets. These findings suggested that GTa platelets were locked in an altered affinity state. Genetic analysis showed that GTa was a compound heterozygote for the GPIIIa gene. One allele showed a deletion at the 3' end of exon 3 resulting in a premature stop codon. The second GPIIIa allele had a G to A transition at nucleotide 577, resulting in a Val193Met substitution. HEK 293T cells transfected with mutant GPIIb/IIIaV193M bound [(3)H]-SC52012 with a higher affinity than wild-type GPIIb/IIIa, and this was not increased by DTT. The mutant receptor distinguishes between platelet adhesion and aggregation, and demonstrates the phenotype that may be expected when platelet aggregation alone is inhibited.     

The Membrane Glycoprotein IIb/IIIa Complex Mediates Deposition of Thrombin-stimulated Blood Platelets on Polystyrene Plastic Under Static Conditions

A major challenge in the use of artificial materials for implant devices, artificial organs, and extra-corporeal circulation systems, is the adhesion of platelets and the subsequent formation of platelet aggregates on the non-biological surface. The mechanism of platelet attachment to artificial surfaces is not completely understood. Using an enzyme immunoassay, we examined platelet deposition to the polystyrene plastic of microtiter plate wells under static conditions. Following thrombin stimulation, platelets adhered to the wells. This adhesion process was suppressible by the use of different substances known to interfere with the function of the platelet surface glycoprotein IIb/IIIa complex (GPIIb/IIIa). The substances we used were ethylenediaminetetraacetic acid (EDTA), tetrapeptide RGDS (Arg-Gly-Asp-Ser), and a monoclonal antibody directed against the IIIa moiety of the GPIIb/IIIa complex. Our results indicate that the GPIIb/IIIa complex is the platelet receptor which mediates platelet adhesion to polystyrene plastic under such static conditions. The GPIIb/IIIa complex should consequently be regarded as a multifunctional platelet regulator which, depending on the circumstances, may support platelet adhesion as well as platelet aggregation. By contrast, a monoclonal antibody directed against the platelet surface glycoprotein complex Ib/IX (GPIb/IX) did not under the same static conditions inhibit platelet deposition to the polystyrene plastic. In the microtiter wells, platelet alpha-granular proteins were detected either on the surface of adherent platelets or, when platelet deposition was inhibited by EDTA directly on the polystyrene plastic. In the latter case, fibrinogen and thrombospondin were definitely the dominating proteins. The presence of platelet-derived proteins in the microtiter wells significantly enhanced the adhesion of thrombin-stimulated platelets but not of non-stimulated platelets.     

Immobilized Arg-Gly-Asp (RGD) peptides of varying lengths as structural probes of the platelet glycoprotein IIb/IIIa receptor

The interactions between ligands containing the recognition sequence arginine-glycine-aspartic acid (RGD) and integrin receptors are important in many cell-cell and cell-protein interactions. The platelet contains five integrin receptors and they contribute significantly to platelet adhesion and aggregation. To investigate the RGD binding domains on platelet integrins, we immobilized a series of RGD peptides containing variable numbers of glycine residues [(G)n-RGDF] on polyacrylonitrile beads and evaluated the ability of the beads to interact with platelets. With native platelets, virtually no interaction occurred with G1-RGDF beads, but the interactions increased as the number of glycine residues increased, plateauing with the G9- RGDF and G11-RGDF beads. ADP pretreatment enhanced the interactions with all of the beads, whereas prostaglandin E1 pretreatment eliminated the interactions with the shortest peptide beads, but only partially inhibited interactions with the longer peptide beads. Monoclonal antibodies to glycoprotein (GP) IIb/IIIa were most effective in inhibiting the interactions, but antibodies to GPIIb/IIIa with similar inhibitory effects on fibrinogen binding varied dramatically in their ability to inhibit the interaction between platelets and immobilized RGD peptides. Our data indicate that the majority of RGD binding sites on GPIIb/IIIa can be reached by peptides that extend out approximately 11 to 32 A from the surface of the bead, and these results are in accord with the dimensions of integrin receptors deduced from electron microscopy. Activation of GPIIb/IIIa facilitates the interactions, but platelet inhibition fails to eliminate the interactions with the longer peptide beads, suggesting that access to the RGD binding site on at least a fraction of the GPIIb/IIIa receptors is always possible for preferred ligands. Finally, we found that the G3-RGDF peptide beads were uniquely sensitive to the activation state of the GPIIb/IIIa receptor.     

Platelet interaction with subendothelial extracellular matrix: platelet- fibrinogen interactions are essential for platelet aggregation but not for the matrix-induced release reaction

Cultured endothelial cells produce an extracellular matrix (ECM) to which platelets adhere and spread, ultimately resulting in platelet aggregation, thromboxane B2 production, and serotonin release. We have investigated the role of fibrinogen binding to the platelet GPIIb/IIIa complex in these reactions by comparing normal platelet-rich plasma (PRP), PRP from patients with Glanzman's thrombasthenia (whose platelets lack the GPIIb/IIIa complex), PRP in the presence of a monoclonal antibody that blocks the binding of fibrinogen to the GPIIb/IIIa complex, platelets washed free of fibrinogen, and washed platelets to which fibrinogen was added. Although platelet aggregation was virtually completely inhibited in the samples in which the normal interaction between fibrinogen and GPIIb/IIIa was impaired, adhesion of platelets to the matrix, spreading, and release of [14C]-serotonin were not affected. All of the platelet preparations released significant amounts of T X B2 with time, but there was a decrease in the amount produced by both the thrombasthenic and antibody-treated platelets. We conclude that the interaction of fibrinogen with platelet GPIIb/IIIa is not required for platelet adhesion to ECM or for adhesion-induced shape change or serotonin release. On the other hand, the platelet-fibrinogen interaction may play some role in augmenting adhesion-induced T X B2 production, and it is absolutely required for adhesion-induced platelet aggregation.     

Conformation-specific blockade of the integrin GPIIb/IIIa: a novel antiplatelet strategy that selectively targets activated platelets

Platelet activation causes conformational changes of integrin GPIIb/IIIa (alpha(IIb)beta3), resulting in the exposure of its ligand-binding pocket. This provides the unique possibility to design agents that specifically block activated platelets only. We used phage display of single-chain antibody (scFv) libraries in combination with several rounds of depletion/selection to obtain human scFvs that bind specifically to the activated conformation of GPIIb/IIIa. Functional evaluation of these scFv clones revealed that fibrinogen binding to human platelets and platelet aggregation can be effectively inhibited by activation-specific scFvs. In contrast to clinically used GPIIb/IIIa blockers, which are all conformation unspecific, activation-specific GPIIb/IIIa blockers do not induce conformational changes in GPIIb/IIIa or outside-in signaling, as evaluated by ligand-induced binding-site (LIBS) exposure in flow cytometry or P-selectin expression in immunofluorescence microscopy, respectively. In contrast to the conformation-unspecific blocker abciximab, activation-specific scFvs permit cell adhesion and spreading on immobilized fibrinogen, which is mediated by nonactivated GPIIb/IIIa. Mutagenesis studies and computer modeling indicate that exclusive binding of activation-specific scFv is mediated by RXD motifs in the heavy-chain complementary-determining region (CDR) 3 of the antibodies, which in comparison with other antibodies forms an exceptionally extended loop. In vivo experiments in a ferric-chloride thrombosis model of the mouse carotid artery demonstrate similar antithrombotic potency of activation-specific scFv, when compared with the conformation-unspecific blockers tirofiban and eptifibatide. However, in contrast to tirofiban and eptifibatide, bleeding times are not prolonged with the activation-specific scFvs, suggesting lower bleeding risks. In conclusion, activation-specific GPIIb/IIIa blockade via human single-chain antibodies represents a promising novel strategy for antiplatelet therapy.     

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.     

G?r?g Thrombosis Test: a global in-vitro test of platelet function and thrombolysis.

This is the first laboratory evaluation of a new instrument, designed to test both platelet function and thrombolytic activity from a native blood sample, in vitro. The inventor assumed that the reduction and arrest of blood flow was due to activation, aggregation and stabilized thrombus formation by shear-activated platelets, and that re-establishment of flow was due to thrombolysis. Morphologic and functional studies presented here confirm these mechanisms. In vitro tests provided incontestable evidence for the principal role of platelets in the obstruction of flow (occlusion time) and for thrombolysis as the principal mechanism underlying the restoration of blood flow (lysis time). In addition to aggregation, it is the explosive generation of thrombin by shear-activated platelets that results in the formation of an occlusive haemostatic thrombus. Anticoagulation of blood completely prevented occlusion. Platelet-rich thrombus formation (occlusion time) was dose-dependently inhibited by monoclonal antibody against platelet glycoprotein (GP) Ib (6B4 and 12E4), aurin tricarboxylic acid, monoclonal antibody against platelet GPIIb/IIIa (MA-16N7C2 and abciximab), a synthetic GPIIb/IIIa antagonist (TAK-029), thrombin inhibitor (argatroban), and anti-von Willebrand factor, but not by anti-fibrinogen. Plasminogen activator streptokinase (Varidase) and tissue-type plasminogen activator (Monteplase) dose-dependently enhanced thrombolysis (lysis time) without affecting platelet function (occlusion time). The test is specific for thrombolysis. The plasmin inhibitor tranexamic acid prevented plasminogen activator-induced thrombolysis, while inhibition of clot retraction by cytochalasin B did not affect the lysis time. This rapid and sensitive global test of platelet function and thrombolytic activity could be of great value both in research and in clinical practice.     

Mechanisms underlying the inhibitory effects of lipopolysaccharide on human platelet adhesion

Alterations in platelet aggregation in septic conditions are well established. However, little is known about the effects of lipopolysaccharide (LPS) on platelet adhesion. We have therefore investigated the effects of LPS in human platelet adhesion, using an in vitro model of platelet adhesion to fibrinogen-coated wells. Microtiter plates were coated with human fibrinogen, after which washed platelets (6 × 10⁸ platelets/ml) were allowed to adhere. Adherent platelets were quantified through measurement of acid phosphatase activity. Calcium mobilization in Fura2-AM-loaded platelets was monitored with a spectrofluorimeter. Platelet flow cytometry in thrombin-stimulated platelets was performed using monoclonal mouse anti-platelet GPIIb/IIIa antibody (PAC-1). Prior incubation of washed platelets with LPS (0.01–300 µg/ml) for 5 to 60 min concentration- and time-dependently inhibited non-activated platelet adhesion. In thrombin-activated (50 mU/ml) platelets, LPS inhibited the adhesion to a significantly lesser extent than non-activated platelets. Cyclohexamide, superoxide dismutase polyethylene glycol (PEG-SOD) or catalase polyethylene glycol did not affect the LPS responses. No alterations in cyclic GMP levels were seen after platelet incubation with LPS, except with the highest concentration employed (300 µg/ml) where an increase of 36% (P < 0.05) was observed. Thrombin increased by 7.5-fold the internal Ca²⁺ platelet levels, an effect markedly inhibited by LPS. Thrombin induced concentration-dependent platelet GPIIb/IIIa activation, but LPS failed to affect the activation state of this membrane glycoprotein. In conclusion, LPS inhibits human platelet adhesion to fibrinogen by mechanisms involving blockade of external Ca²⁺, independently of cGMP generation and activation of GPIIb/IIIa complex.     

Collagen-platelet interactions: evidence for a direct interaction of collagen with platelet GPIa/IIa and an indirect interaction with platelet GPIIb/IIIa mediated by adhesive proteins

Using intact human platelets as the immunogen and a functional, collagen-coated bead agglutination assay, we have produced a murine monoclonal antibody (6F1) that blocks the interaction between platelets and collagen in the presence of Mg++. 6F1 affinity-purified the platelet glycoprotein Ia/IIa complex, and approximately 800 molecules of 6F1 bound per platelet at saturation. 6F1 nearly completely inhibited collagen-induced platelet aggregation and inhibited platelet adhesion to collagen by greater than 95% when plasma proteins were absent. Antibody 10E5, which blocks the binding of adhesive glycoproteins to GPIIb/IIIa, produced only minor inhibition (approximately 25%) of adhesion under the same circumstances. In contrast, when tested in platelet-rich plasma (PRP), 6F1 had only a minor effect on collagen-induced platelet aggregation, prolonging the lag phase but not the slope or maximum aggregation. Similarly, when collagen was precoated with plasma, 6F1 caused less inhibition of platelet adhesion (53%) than without the precoating (greater than 95%). Antibody 10E5 inhibited this adhesion by 32%, and the combination of 6F1 and 10E5 was more effective than either alone, inhibiting it by 90%. Time course studies of platelet agglutination of collagen-coated beads using PRP containing physiologic concentrations of divalent cations showed early inhibition by 6F1, indicating that the GPIa/IIa receptor operates in this environment. With more prolonged incubation, however, 6F1 was less effective; this later agglutination could be partially prevented by adding 10E5 or PGE1 to the 6F1. These data support a model wherein collagen can directly interact with GPIa/IIa and can indirectly interact with GPIIb/IIIa via intermediary adhesive proteins. The physiological significance of these interactions, and potential interactions with other receptors, remains to be established.     

Abciximab Pharmacodynamics Are Unaffected by Antecedent Therapy with Other GPIIb/IIIa Antagonists in Non-Human Primates

Background: Tirofiban and eptifibatide are currently approved for the medical stabilization of non-ST segment elevation acute coronary syndromes. In patients undergoing percutaneous coronary intervention (PCI) during infusion of these drugs, conversion to abciximab, which has long term proven clinical efficacy and cost-effectiveness, following PCI may be desirable. The purpose of this study was to determine if the binding or pharmacodynamics of abciximab is affected by a prior infusion of either tirofiban or eptifibatide. Methods: In vitro binding experiments were performed to determine if prior exposure to tirofiban or eptifibatide altered the affinity and extent of binding of abciximab to GPIIb/IIIa. For in vivo experiments, cynomolgus monkeys were pretreated with a bolus and 18 hour infusion of saline, tirofiban, or eptifibatide. At the end of the initial treatment, a bolus and 12 hr infusion of abciximab was started without delay. Inhibition of platelet aggregation, GPIIb/IIIa receptor blockade and abciximab pharmacokinetics were measured during and after both infusions. Results: Equilibrium binding of abciximab in vitro was unaffected by tirofiban or eptifibatide. The extent and duration of abciximab inhibition of ex vivo platelet aggregation, receptor blockade, and abciximab pharmacokinetics in monkeys during and after the abciximab infusion were not affected by prior infusion of the animals with tirofiban or eptifibatide. Conclusions: In vitro and in vivo studies revealed that the molecular interaction of abciximab with the platelet GPIIb/IIIa receptor is not altered by immediate prior exposure of platelets to small molecule GPIIb/IIIa antagonists. These preclinical studies suggest that the efficacy of abciximab should not be impaired if it is initiated following termination of therapy with small molecule GPIIb/IIIa antagonists.