Greater inhibition of platelet procoagulant activity by antibody-derived glycoprotein IIb--IIIa inhibitors than by peptide and peptidomimetic inhibitors

Key role of integrin α(IIb)β (3) signaling to Syk kinase in tissue factor-induced thrombin generation

The fibrin(ogen) receptor, integrin α_IIbβ₃, has a well-established role in platelet spreading, aggregation and clot retraction. How α_IIbβ₃ contributes to platelet-dependent coagulation is less well resolved. Here, we demonstrate that the potent suppressing effect of clinically used α_IIbβ₃ blockers on tissue factor-induced thrombin generation is linked to diminished platelet Ca²⁺ responses and phosphatidylserine (PS) exposure. The same blockers suppress these responses in platelets stimulated with collagen and thrombin receptor agonists, whereas added fibrinogen potentiates these responses. In platelets spreading on fibrinogen, outside-in α_IIbβ₃ signaling similarly enhances thrombin-induced Ca²⁺ rises and PS exposure. These responses are reduced in α_IIbβ₃-deficient platelets from patients with Glanzmann’s thrombasthenia. Furthermore, the contribution of α_IIbβ₃ to tissue factor-induced platelet Ca²⁺ rises, PS exposure and thrombin generation in plasma are fully dependent on Syk kinase activity. Tyrosine phosphorylation analysis confirms a key role of Syk activation, which is largely but not exclusively dependent on α_IIbβ₃ activation. It is concluded that the majority of tissue factor-induced procoagulant activity of platelets relies on Syk activation and ensuing Ca²⁺ signal generation, and furthermore that a considerable part of Syk activation relies on α_IIbβ₃ signaling. These results hence point to a novel role of Syk in integrin-dependent thrombin generation.

Differential inhibitory effects of platelet glycoprotein IIb/IIIa antagonists on aggregation induced by procoagulant agonists

INTRODUCTION

In addition to mediating the final common pathway of aggregation, the glycoprotein (GP) IIb/IIIa receptor participates in the activation of coagulation on the platelet surface. High-affinity conformation of GP IIb/IIIa in response to collagen-induced inside-out signalling seems to be mediated by GP VI(-FcRgamma) and reinforced by release of soluble mediators.

METHODS

We assessed the effects of the three currently available GP IIb/IIIa antagonists--abciximab, tirofiban and eptifibatide--on platelet aggregation induced by various procoagulant and GP VI-related agonists, i.e. collagen-related peptide (CRP), convulxin and collagen fibrils, in PPACK-anticoagulated platelet-rich plasma.

RESULTS

At concentrations that equally inhibited 80% of ADP-induced maximal aggregation abciximab-inhibited GP VI-mediated platelet responses to CRP or convulxin significantly more than the low-molecular-weight antagonists (CRP: abciximab 75+/-18%, tirofiban 41+/-7% and eptifibatide 41+/-6%; convulxin: abciximab 90+/-6%, tirofiban 64+/-20%, eptifibatide 61+/-14%, p<0.01 for all). In contrast, aggregation induced by collagen was equally abolished with all antagonists under the similar conditions. During CRP- or convulxin-triggered platelet activation, inhibition of fibrin polymerisation with GPRP potentiated the antiaggregatory effects of tirofiban and eptifibatide to reach that of abciximab. GPRP as such did not affect platelet aggregation.

CONCLUSIONS

GP IIb/IIIa antagonists exhibit distinct inhibition profiles in platelet aggregation, depending on fibrin polymerization and calcium. Specifically, the ability of procoagulant platelet agonists to expose pre-activated and ligand-bound GP IIb/IIIa from the internal pool seems important.

Circulating procoagulant microparticles and soluble GPV in myocardial infarction treated by primary percutaneous transluminal coronary angioplasty. A possible role for GPIIb-IIIa antagonists

Circulating procoagulant microparticles (MP) were measured as markers of vascular damage and prothrombotic risk in patients undergoing ST-segment myocardial infarction (STEMI) treated by primary percutaneous transluminal coronary angioplasty (PTCA) and additional GPIIb-IIIa antagonists. Cells possibly more responsive to GPIIb-IIIa (alpha(IIb)beta(3)) antagonists were evidenced through MP phenotypes by comparison with healthy volunteers (HV) and STEMI patients treated by PTCA without GPIIb-IIIa antagonist (CP). In 50 STEMI patients, blood samples were collected at day 1 and day 6. Circulating procoagulant MP were captured on annexin V and quantified by prothrombinase assay as nanomolar phosphatidylserine equivalents (nm PhtdSer). Platelet activation by thrombin was confirmed through independent measurement of soluble GPV (sGPV). With respect to HV, procoagulant MP levels were high in patients with STEMI or unstable angina, platelet-derived MP and elevated sGPV testifying to significant platelet activation. A substantial release of endothelial-derived MP was evidenced simultaneously. In abciximab-treated patients, procoagulant MP, mainly of platelet origin, decreased precociously at day 1 (4.2 +/- 0.6 vs. CP 15.5 +/- 2.1 nm PhtdSer; P = 0.001) together with sGPV (36 +/- 3 vs. CP 58 +/- 8 ng mL(-1); P = 0.02). Leukocyte-derived MP decreased at day 6 (0.12 +/- 0.04 vs. CP 0.56 +/- 0.12 nm PhtdSer; P = 0.01) suggesting a possible effect on underlying inflammatory status. In patients presenting cardiovascular events at 6-month follow-up, procoagulant MP levels at day 1 could be indicative of a worsened outcome. MP could constitute a relevant parameter for the follow-up of STEMI patients treated by GPIIb-IIIa antagonists.

Different effects of various anti-GPIIb-IIIa agents on shear-induced platelet activation and expression of procoagulant activity

Inhibitors of the platelet glycoprotein (GP)IIb-IIIa receptor (integrin alphaIIbbeta3) reduce acute thrombotic events in patients with coronary artery disease. To characterize the mechanism of action of these drugs, we evaluated the effects of different GPIIb-IIIa antagonists on shear-induced platelet aggregation, activation, and the expression of procoagulant activity. Samples of platelet-rich plasma from 16 volunteers were exposed to the shear rate of 10 800 s-1 for 6 min in an optically modified cone-plate viscometer. Abciximab, tirofiban and eptifibatide inhibited aggregation to a similar extent (mean +/- SD: 74.1 +/- 8.5%, 69.5 +/- 13.6%, 65.6 +/- 17.0%, respectively), but only abciximab inhibited significantly microparticle release associated with shear-induced platelet activation (64.4 +/- 13.6%, P = 2.2 x 10-7; tirofiban = 20.0 +/- 23.4%; eptifibatide = 23.9 +/- 17.4%). P-selectin platelet surface translocation was also strongly inhibited by abciximab, weakly by eptifibatide, but not by tirofiban. The addition of anti-alphavbeta3 to tirofiban enhanced the inhibiting effects on shear-induced P-selectin translocation and microparticle release. Shearing of platelet-rich plasma shortened the re-calcification clotting time after addition of kaolin from 106.9 +/- 14.3 to 94.2 +/- 10.7 s (mean +/- SD; P = 0.0013). This effect, which is mediated by the appearance of procoagulant phospholipids on the surface of sheared platelets and microparticles, was prevented by abciximab and by the combination of tirofiban and anti-alphavbeta3, but not by tirofiban alone or eptifibatide. The ability to inhibit shear-induced platelet activation, as evidenced by microparticle release and P-selectin surface translocation as well as the expression of procoagulant activity, differentiates the effects of anti-GPIIb-IIIa agents, which may explain the distinct antithrombotic efficacy of the agents.

Integrin alphaIIbbeta3 and shear-dependent action of glycoprotein Ibalpha stimulate platelet-dependent thrombin formation in stirred plasma

Under conditions of arterial-wall shear rates, platelets bind to von Willebrand factor (vWf) by way of the glycoprotein Ib (GP Ib) complex and integrin alpha(IIb)beta(3). Both adhesive receptors may also play roles in the development of procoagulant activity of platelets. Here, we investigated the effect of shear stress, as provided by a rotating cylinder, on GP Ib- and integrin alpha(IIb)beta(3)-dependent thrombin generation in coagulating platelet-rich plasma (PRP). We measured thrombin continuously with the use of fluorometry from the cleavage rate of a fluorescent low-affinity substrate. The integrin alpha(IIb)beta(3) antagonist abciximab progressively reduced the peak of thrombin formation up to 43% when rate of stirring and shear stress were increased (estimated shear rates of 105-420 s(-1)). Abciximab did not lower the peak of thrombin formation in stirred PRP from patients with Glanzmann's thrombasthenia lacking alpha(IIb)beta(3) but, surprisingly, shortened the time until onset. In PRP from control subjects, antibodies specifically directed against vWf-binding epitopes on GP Ibalpha reduced thrombin formation, with 25% to 30% at the high but not at the low stirring rate. In combination with the anti-GP Ib antibody, abciximab retained its strong inhibitory effect only at the high stirring rate. We conclude that thrombin formation and coagulation in stirred PRP depend, to a large extent, on platelet adhesion to integrin alpha(IIb)beta(3) and, in a shear-dependent way, on GP Ib.

The role of platelets in blood coagulation--effects of platelet agonists and GPIIb/IIIa inhibitors studied by free oscillation rheometry

We have studied the contribution of platelets to the coagulation of plasma and the effects of activation or inhibition of platelets on the coagulation process in unanticoagulated fresh whole blood (subsequently termed native blood). For this purpose, we have used a free oscillation rheometer (FOR), the ReoRox4, a new instrument that enables noninvasive viscoelastic measurements of clot formation in plasma and whole blood. Platelets appear essential for the initiation of coagulation if no activating surface is present. We prepared platelet-free plasma by quick centrifugation and filtration of native blood, which did not coagulate if stored in plastic containers at 37 degrees C but clotted if transferred to glass containers. Addition of platelet agonists, such as collagen or the thrombin receptor agonist peptide, SFLLRN, significantly accelerated the clotting of native blood and also changed the rheometer curve appearance, accelerating both onset and completion of clot formation (i.e. fibrin gel formation). Inhibition of platelet glycoprotein (GP) IIb/IIIa with the peptide-derived compound MK-852 or the antibody-derived abciximab (Reopro) prevented clot retraction and prolonged the clotting time with SFLLRN. In collagen-stimulated samples, MK-852 accelerated clotting but delayed completion of clotting while abciximab prolonged both clotting time and completion of clotting. To our knowledge, this is the first report showing that activation of platelets in native whole blood shortens the coagulation time ex vivo. It also describes a new instrument that enables studies of the viscoelastic properties of a forming whole blood clot.