Antithrombotic and Bleeding Effects of a New Synthetic Direct Thrombin Inhibitor and of Standard Heparin in the Rabbit

This study reports on (he anticoagulant, antithrombotic and bleeding effects of a new synthetic direct thrombin inhibitor (SDTI) in comparison with standard heparin (SH). The anticoagulant effect was determined with the thrombin clotting time (TCI) and the activated partial thromboplastin time (APTT). SDTI was more potent than SII in prolonging the TCT, but as potent as SH in prolonging the APTT. The antithrombotic effect was determined using a modified Wessler model in the rabbit, either 30 min after a continuous IV infusion of increasing doses or at various times after a single SC injection (20 mg/kg). After continuous IV infusion of 187 μg/kg/h of SDTI and of 60 μg/kg/h of SH, significant thrombus prevention effects were obtained (59 and 57% respectively). Increasing the dose of SDTI up to 3000 μg/kg/h did not significantly impiove the antithrombotic effect. After SC injection, a significant antihrombotic effect was observed for 12 h with SDTI but for more than 24 h with SH. The bleeding effect was studied using the rabbit ear model 15 min after a continuous infusion of 7.5 and 15 mg/kg/h: the amounts of blood loss were dose-dependent and comparable for SDTI and SH. These studies also indicated that SDTI possesses a considerable shorter half-life in comparison with SH. Accordingly, the ex vivoconcentrations generated after continuous IV infusion or SC injection of the same dose were higher for SH than for the SDTI.

Heparin Reverses the Procoagulant Properties of Stimulated Endothelial Cells

We examined the ability of unfractionated heparin to modulate the procoagulant activities of stimulated endothelial cells (EC). Confluent human venous umbilical EC were incubated with heparin before or after stimulation, then rinsed extensively to eliminate any heparin in the solution. EC, stimulated for 4 h with endotoxin and interleukin 1β, expressed tissue factor and prothrombinase activities. When EC were treated with heparin (6 and 60 μg/ml) during the last 10 min of the stimulation period, EC-related procoagulant activities were inhibited in a dose-dependent manner (80-90% inhibition at 60 μg/ml). The inhibition was antithrombin-dependent and it disappeared after heparin removal in less than 15 min at 37° C but persisted at 4° C.

When EC were treated with heparin (60 μg/ml) for 24 h then extensively washed before stimulation, the anticoagulant effect was more modest (50% inhibition). The effect was antithrombin-dependent. Inhibition was maximum after 18-24 h of pretreatment of EC with heparin and was stable for at least 7 h. The cell surface displayed a “heparin-like” activity: treatment by heparin doubled the rate of thrombin-antithrombin complex formation and this effect was heparinase sensitive and chondroitinase ABC insensitive.

Thus, heparin modulates the procoagulant properties of stimulated EC according to two distinct mechanisms. The first one is rapid and transient, probably related to the presence of heparin molecules bound at the membrane surface. The second is delayed and persistent, and our results suggest that it is mediated by an increase in the membrane heparan sulfate molecules.

Further Studies on the Mechanisms for the Antithrombotic Effects of Sulfated Polysaccharides in Rabbits

A recent study (Fernandez et al., Thromb. Haemostas. 1987; 57: 286-93) demonstrated that when rabbits were injected with the minimum weight of a variety of glycosaminoglycans required to inhibit tissue factor-induced thrombus formation by —80%, exogenous thrombin was inactivated —twice as fast in the post-treatment plasmas as the pre-treatment plasmas. In this study, we investigated the relationship between inhibition of thrombus formation and the extent of thrombin inhibition ex vivo. We also investigated the relationship between inhibition of thrombus formation and inhibition of prothrombin activation ex vivo. Four sulfated polysaccharides (SPS) which influence coagulation in a variety of ways were used in this study. Unfractionated heparin and the fraction of heparin with high affinity to antithrombin III potentiate the antiproteinase activity of antithrombin III. Pentosan polysulfate potentiates the activity of heparin cofactor II. At less than 10 pg/ml of plasma, all three SPS also inhibit intrinsic prothrombin activation. The fourth agent, dermatan sulfate, potentiates the activity of heparin cofactor II but fails to inhibit intrinsic prothrombin activation even at concentrations which exceed 60 pg/ml of plasma. Inhibition of thrombus formation by each sulfated polysaccharides was linearly related to the extent of thrombin inhibition achieved ex vivo. These observations confirm the utility of catalysis of thrombin inhibition as an index for assessing antithrombotic potential of glycosaminoglycans and other sulfated polysaccharides in rabbits. With the exception of pentosan polysulfate, there was no clear relationship between inhibition of thrombus formation and inhibition of prothrombin activation ex vivo.

Pharmacokinetics of Dermatan Sulfate in the Rabbit After Intravenous Injection

To investigate the pharmacokinetic properties of dermatan sulfate (DS), a new potential antithrombotic agent, two different approaches were used. In the first one, DS was derivatized with 3-4 hydroxyphenyl propionic acid N hydroxysuccinimide ester (SHPP) and iodinated. The labelled derivative was injected by IV route to rabbits with increasing doses of unlabelled compound ranging from 20 to 4000 μg/kg. The disappearance of DS from plasma was characterized by measuring both the circulating radioactivity and the biological activity using an original assay based upon the catalysis of heparin cofactor II – thrombin formation. The radioactivity data indicated that the volume of distribution, the clearance and the half life of the tracer were independent of the dose of DS injected. DS concentrations measured by the bioassay indicated that more than 90% were cleared with half lives close to those calculated from the radioactivity data; the remaining biological activity was cleared at a slower rate. Experiments performed with bi-nephrectomized animals indicated that the kidneys play a major role in the elimination of DS or of its metabolites which may have a residual biological activity. In the second set of experiments, unlabelled DS was delivered under continuous intravenous infusion for 5 hours at 5 increasing doses ranging from 160 to 4200 μg/kg/h. The biological activities were used to measure the plateau concentration of DS: there was a linear relationship between the dose delivered and the plasma concentration. These data indicate that the pharmacokinetic profile of DS is very close to that of low molecular weight heparin, and quite different from that of SH.

Standard Heparin Enhances the Antithrombotic Activity of Dermatan Sulfate in the Rabbit but CY 216 Does Not

Standard heparin (SH) and dermatan sulfate (DS) two gly- cosaminoglycans with different pharmacological targets are effective antithrombotic agents in the rabbit. We have investigated the antithrombotic activity of the association DS plus SH. It was found that doses as low as 25 µg/kg for DS and 10 µg/kg for SH were ineffective when injected separately but generated a high and significant antithrombotic activity when injected together. These results were confirmed when higher doses of each compound were delivered in association. Further experiments were performed to determine if the enhancement of the antithrombotic activity of DS by HS resulted from its anti-factor Ha or antifactor Xa activity or from its moiety without affinity to AT III. A low molecular weight heparin (CY 216) with an anti-factor Xa/ anti-factor Ha ratio of 5, the synthetic pentasaccharide bearing the minimum binding sequence to antithrombin III, and a low affinity fraction of SH to AT III did not increase the antithrombotic activity of DS; in contrast a high affinity fraction of SH to AT III had the same effect than SH. We conclude that the enhancement of the antithrombotic activity of DS by SH mainly results from its anti-factor IIa activity.

A New Method for Quantifying Platelet Deposition in Flowing Native Blood in an Ex Vivo Model of Human Thrombogenesis

No quantitative, simple and non-radioactive method has been described for measuring the platelet content of experimental thrombi. The aim of the present study was to develop a simple method for quantifying platelets in thrombi formed on thrombogenic surfaces in flowing native human blood. To test the relevance of this new method, the effect of unfractionated heparin on arterial thrombus formation was investigated. Tissue factor (TF)- and collagen-coated coverslips were exposed to non-anticoagulated blood at an arterial wall shear rate (2,600 s–1) for 1 to 4 min. Platelet deposition was quantified by measuring the P-selectin (PS) and β-thromboglobulin (βTG) content of dissolved plasmin-digested thrombi using immunoenzymoassays; fibrin deposition was determined by measuring the D-dimer levels. These results were compared to those established by morphometrical analysis.

Morphometric evaluation showed that fibrin deposition was maximum on TF by 1 min perfusion time. Platelets deposited subsequently and reached a maximum at 3 min. On collagen, platelets deposited directly on the collagen fibrils without detectable fibrin deposit. Platelet deposition increased from 1 to 4 min. Platelet deposition quantified by PS was correlated to the values obtained by morphometry (r = 0.72, r = 0.67, p <0.001, on TF and collagen, respectively). As compared to PS, βTG measurements gave an underestimation of the size of the thrombus platelet number. Unfractionated heparin infused through a mixing device proximal to the perfusion chamber to obtain plasma concentrations of 0.5, 1 and 3 IU/ml, reduced fibrin deposition on TF-coated coverslips in a dose-dependent manner (77% reduction at 3 IU/ml, p <0.01), but had no significant effect on platelet deposition (33% at 3 IU/ml, p >0.05). In contrast, heparin had no effect on fibrin or platelet deposition on collagen-coated coverslips.

Thus, a new quantitative and simple method for measuring platelet deposition in flowing blood has been developed and characterized. Utilizing this system, we have demonstrated that unfractionated heparin did not inhibit arterial thrombus formation either on procoagulant or on proaggregant surface.

Role of 4 platelet membrane glycoprotein polymorphisms on experimental arterial thrombus formation in men

This study investigates whether the polymorphisms of 3 important platelet receptors affected experimental thrombus formation in men. Forty healthy male volunteers randomly recruited were genotyped for the variable number of tandem repeat (VNTR) of GPIbalpha, the -5T/C polymorphism in the Kozak sequence of GPIbalpha, the 807C/T polymorphism of GPIa, and the PI(A1)/PI(A2) polymorphism of GPIIb/IIIa. Platelet thrombus formation was induced ex vivo by exposing a collagen-coated coverslip in a parallel plate perfusion chamber to native blood for 4 minutes. The shear rates at the collagen surface were 650 and 2600 x s(-1). At 2600 x s(-1) platelet thrombus formation was significantly related only to the 807C/T polymorphism. In contrast, at 650 x s(-1) thrombus formation was significantly altered only by the Kozak sequence polymorphism. The VNTR and the PI(A1)/PI(A2) polymorphisms did not influence thrombus formation. Thus, platelet thrombus formation is significantly influenced by genetic variations of the GPIbalpha and GPIa receptors. The effect of these polymorphisms was dependent on the blood flow rate.

[Genetic predisposition to arterial and venous thrombosis]

Thrombosis is a multifactorial pathology. In addition to the environmental factors, there are genetic components which either predispose or protect against its occurrence. Some are common to only a small number of subjects: these are the high risk families to thrombotic disease. Others are present in a larger percentage of the general population: this is called polymorphism and its clinical expression is usually modest. The role of these genetic factors in the development of thrombosis is not easy to demonstrate, especially as there are many gene-gene and gene-environment interactions. Nevertheless, the genetic basis of venous thrombosis is well established and the value of this knowledge in the management of these patients is becoming recognised. On the other hand, there are many genetic factors in arterial pathology and the role of each taken in isolation is small. Well-targeted large scale trials are required to determine their effects.

Methods and models to evaluate shear-dependent and surface reactivity-dependent antithrombotic efficacy

The purpose of the present communication is to evaluate the importance of blood flow and surface reactivity for measurement of antithrombotic drug activity or efficacy in selected model systems of thrombus formation. Such information is essential for proper evaluation of antithrombotic drug profiles. The continuous development of flow-dependent thrombosis models for in vitro (anticoagulated blood) and ex vivo (native blood) studies and their application in in vivo animal models from the early 1970s and onwards are briefly considered. Central to this process was the development of various types of perfusion chambers in which a thrombogenic surface is exposed to flowing blood. Such perfusion chambers have been inserted into arteriovenous (AV) shunts in baboon, pig, dog, and rabbit. These approaches have allowed reproducible testing of traditional and novel experimental antithrombotic drugs, and studies on novel drug strategies under well-defined shear conditions and surface reactivity. Shear-dependent antithrombotic efficacy in these models is observed with anticoagulants such as unfractionated heparin, low-molecular weight heparins, or selective inhibitors of thrombin, Factor Xa, or Factor VIIa. However, the degree of shear dependency depends on the nature of the thrombogenic surface, e.g., the inhibition is more pronounced on a tissue factor (TF)-rich surface than on a collagen-rich surface, particularly at venous or low arterial shear. Platelet antagonists such as the COX-1 inhibitor aspirin, inhibitors of thromboxane A2 (TxA2) synthetase, the TxA2 platelet receptor, and of von Willebrand factor (vWf) are shear dependent also, being more efficient at high arterial shear. In contrast, the platelet ADP antagonist clopidogrel, or antagonists to the active platelet membrane glycoprotein IIb-IIIa complex (GPIIb-IIIa) are shear independent. At extremely high arterial shear, which activates platelets and elicit aggregates of circulating platelets, aspirin looses its antithrombotic effect, whereas ADP and GPIIb-IIIa antagonists still interrupt thrombus formation. In general, results obtained with these models mimic and predict antithrombotic efficacy in man when comparison is possible. Information on antithrombotic efficacy in flow devices with various thrombogenic surfaces is now sufficiently available to suggest recommendations for experimental conditions, particularly with regard to blood flow and reactive surfaces.

The effect of platelet PlA polymorphism on experimental thrombus formation in man depends on blood flow and thrombogenic substrate

A number of studies have reported conflicting data on the association of the PlA1/PlA2 polymorphism of the GPIIIa gene and coronary syndromes. We have investigated the effect of this polymorphism on experimental platelet thrombus formation in man. Forty healthy male volunteers were genotyped for the PlA1/PlA2 polymorphism. Thrombus formation was induced ex vivo by exposing a tissue factor (TF) or a collagen-coated coverslip in a parallel plate perfusion chamber to native blood for 2 and 4 min. The shear rates at these surfaces were 650 and 2,600 s(-1). Platelet and fibrin deposition was quantified by immunoenzymatic methods. The frequencies of PlA1/PlA1 and PlA1/PlA2 genotypes were 52.5% and 47.5%, respectively. Ex vivo deposition of fibrin on TF was not affected by the PlA1/PlA2 polymorphism. However, the ex vivo platelet deposition at 650 s(-1) was higher in blood from PlA1/PlA1 individuals than in PlA1/PlA2 individuals (P= 0.008 at 4 min). On collagen, neither fibrin nor platelet deposition was significantly affected by the PlA1/PlA2 polymorphism. Platelet thrombus formation is significantly influenced by genetic variations in the GPIIIa platelet receptor. This effect depends on the blood flow properties and the nature of the thrombogenic stimulus.

Early potent antithrombotic effect with combined aspirin and a loading dose of clopidogrel on experimental arterial thrombogenesis in humans

BACKGROUND

We conducted a double-blind, randomized, crossover study to assess the antithrombotic effects of the combination of aspirin (acetylsalicylic acid, ASA) and clopidogrel, with or without a loading dose, versus ASA alone in a model of arterial thrombosis in humans.

METHODS AND RESULTS

Eighteen male volunteers received the following 3 regimens for 10 days separated by a 1-month period: (1) 325 mg ASA daily, (2) 325 mg ASA+75 mg clopidogrel daily, (3) 325 mg ASA daily+300-mg clopidogrel loading dose on day 1 and +75 mg clopidogrel per day on days 2 to 10. The antithrombotic effect was measured 1.5, 6, and 24 hours after drug intake on day 1 and 6 hours after drug intake on day 10. Arterial thrombus formation was induced ex vivo by exposing a collagen-coated coverslip in a parallel-plate perfusion chamber to native blood for 3 minutes at an arterial wall shear rate. Without a loading dose, clopidogrel+ASA developed an antithrombotic effect within 6 hours after the first intake. It was superior to that produced by ASA, but it was moderate (P</=0.03). However, with the loading dose, the antithrombotic effect of clopidogrel+ASA appeared within 90 minutes, and after 6 hours it was comparable to that on day 10. On day 10, clopidogrel+ASA decreased platelet thrombus formation by approximately 70%, and the effect was significantly more potent than that produced by ASA alone (P<0.001).

CONCLUSIONS

This study confirms the synergistic antithrombotic effects of a combined ASA and clopidogrel therapy and shows the early benefit obtained with a loading dose of clopidogrel.

Polymorphonuclear leukocytes modulate tissue factor production by mononuclear cells: role of reactive oxygen species

To determine whether polymorphonuclear leukocytes (PMN) modulate the production of tissue factor (TF) by monocytes, PBMC were incubated with increasing concentrations of PMN. PMN did not express any procoagulant activity. After 20-h cocultures, PMN enhanced or inhibited the TF production of PBMC, and this effect depended on the PMN/PBMC ratio. When the ratio increased from 1/1000 to 1/5, without or with LPS, the TF activity of PBMC increased to peak at 2.5-fold the baseline value (p < 0.01). The TF Ag and TF mRNA also increased. This potentiating effect was mediated by reactive oxygen species (ROS) released by PMN during the coculture; it did not require direct cell contact between PMN and PBMC, it was enhanced when PMN were stimulated by fMLP (a chemotactic peptide), and it was inhibited by two antioxidants, N-acetyl cysteine and pyrrolidine dithiocarbamate. In contrast, when the PMN/PBMC ratio was further increased from 1/2 to 2/1, the PBMC TF activity, Ag, and mRNA decreased and were inhibited compared with those of PBMC cultured alone (p < 0.01). This inhibitory effect required direct cell contact between PMN and PBMC, and it was not due to a PMN-mediated cytotoxicity. To confirm the role of ROS, H2O2 enhanced then inhibited the TF activity of PBMC in a dose-dependent manner, similarly to PMN. Thus, PMN may play an important role in the pathogenesis of thrombosis and atherosclerosis by exerting concentration-dependent regulatory effects on the TF production by PBMC via the release of ROS.

Antithrombotic efficacy of the vitamin K antagonist fluindione in a human Ex vivo model of arterial thrombosis : effect of anticoagulation level and combination therapy with aspirin

Thrombin is a main mediator of arterial thrombus formation, and its inhibition is an important antithrombotic strategy. However, the place of vitamin K antagonists among the different therapeutic strategies for preventing arterial thrombus formation is still debated. We studied the antithrombotic efficacy of the vitamin K antagonist fluindione in a human ex vivo model of arterial thrombosis and determined whether aspirin enhances fluindione efficacy. Ten healthy male volunteers were randomly assigned to receive fluindione, alone or in combination with aspirin (325 mg/d). Fluindione was given at increasing doses to give a stable international normalized ratio (INR) between 1.5 and 2.0 and between 2.1 and 3.0. We induced arterial thrombus formation ex vivo by exposing collagen- or tissue factor (TF)-coated coverslips in a parallel-plate perfusion chamber to native blood for 3 minutes at an arterial wall shear rate of 2600 s(-1). Platelet and fibrin deposition were measured by immunoenzymatic methods. Fluindione inhibited thrombus formation on TF-coated coverslips in a dose-dependent manner by 50% and 80% at INR 1.5 to 2.0 and INR 2.1 to 3.0, respectively (P<0.05). Fluindione in combination with aspirin inhibited TF-induced thrombus formation in a comparable manner. Collagen-induced thrombus formation was not reduced in subjects treated by fluindione. It was reduced by 50% to 60% in those treated with fluindione plus aspirin, regardless of the level of anticoagulation (P<0.05). Thus, the effectiveness of fluindione for preventing arterial thrombosis is dependent on the nature of the thrombogenic trigger. Fluindione is very effective in preventing TF- but not collagen-triggered thrombus formation. Aspirin enhances the antithrombotic effectiveness of fluindione, because combined treatment interrupts both TF- and collagen-induced thrombus formation.

Nonfractionated heparin fails to inhibit arterial thrombosis in a human ex vivo thrombosis model

The effect of nonfractionated heparin on the formation and composition of arterial thrombus is unclear. The purpose of this study in a human ex vivo model was to analyze fibrinoplatelet thrombi and test the inhibitory effect of nonfractionated heparin on arterial thrombus formation. Experiments were carried out in Sakariassen perfusion chambers. Strips coated with either tissue factor (TF) or collagen were exposed to human blood collected from healthy volunteers at an arterial shear stress rate of 2600 s-1 for 1 to 4 min. Platelet deposition was determined using immunoenzymatic techniques to quantify P-selectine, a platelet membrane receptor, in thrombi. Fibrin deposition was determined by quantifying fibrin degradation products released after application of plasmin (D-dimers). Heparin was injected into the blood flow through a blender port system located between the venous puncture site and perfusion chamber. The results of the study showed that in a human ex vivo model, formation of arterial thrombus on two thrombogenic surfaces (tissue factor and collagen) is not inhibited by nonfractionated heparin.

Comparison of the antithrombotic effect of PEG-hirudin and heparin in a human ex vivo model of arterial thrombosis

Polyethylene glycol (PEG)-hirudin is a derivative of hirudin with a long plasma half-life. We have compared the efficacy of PEG-hirudin with unfractionated heparin (UH) in preventing arterial thrombosis. Arterial thrombus formation was induced ex vivo in 12 healthy human volunteers by exposing a tissue factor-coated coverslip positioned in a parallel-plate perfusion chamber to flowing nonanticoagulated human blood drawn directly from an antecubital vein at an arterial wall shear rate of 2600 s-1 for 3.5 minutes. PEG-hirudin, UH, or saline (as control) were administered ex vivo through a heparin-coated mixing device positioned proximal to the perfusion chamber. Platelet and fibrin deposition was quantified by immunoenzymatic measure of the P-selectin and D-dimer content of dissolved plasmin-digested thrombi, respectively. UH was administered to a plasma concentration of 0.35 IU/mL. This concentration prolonged the activated partial thromboplastin time from 32+/-1 seconds to 79+/-4 seconds (P<0.01). UH did not significantly prevent platelet deposition. However, fibrin deposition was reduced by 39% (P<0.05). PEG-hirudin in plasma concentrations of 0.5, 2.5, and 5 microg/mL prolonged the activated partial thromboplastin time to 48+/-2, 87+/-4, and 118+/-4 seconds, respectively. In contrast to UH, PEG-hirudin prevented both platelet and fibrin deposition in a dose-dependent manner with a >80% reduction at 5 microg/mL (P<0.01). Furthermore, the plasma level of PEG-hirudin required to significantly prevent fibrin deposition (0.5 microg/mL) corresponded to a much shorter prolongation of activated partial thromboplastin time (48+/-2 seconds) than that needed for UH (79+/-4 seconds). Thus, our results are compatible with the view that thrombin is greatly involved in recruitment of platelets in evolving thrombi, and that PEG-hirudin is an effective agent for preventing arterial thrombosis in a human ex vivo experimental model.

A double-blind randomized comparison of combined aspirin and ticlopidine therapy versus aspirin or ticlopidine alone on experimental arterial thrombogenesis in humans

No randomized study comparing the effect of combined ticlopidine and aspirin therapy versus each drug alone in reducing poststenting thrombotic complications has been performed. To compare these three antiplatelet regimens versus placebo, we conducted a double-blind randomized study using an ex vivo model of thrombosis. Sixteen healthy male volunteers were assigned to receive for 8 days the following four regimens separated by a 1-month period: aspirin 325 mg/d, ticlopidine 500 mg/d, aspirin 325 mg/d + ticlopidine 500 mg/d, and placebo. At the end of each treatment period, native nonanticoagulated blood was drawn directly from an antecubital vein over collagen- or tissue factor (TF)-coated coverslips positioned in a parallel-plate perfusion chamber at an arterial wall shear rate (2, 600 s-1 ) for 3 minutes. Thrombus, which formed on collagen in volunteers treated by placebo, were rich in platelets and poor in fibrin. As compared with placebo, aspirin and ticlopidine alone reduced platelet thrombus formation by only 29% and 15%, respectively (P > .2). In contrast, platelet thrombus formation was blocked by more than 90% in volunteers treated by aspirin + ticlopidine (P < .01 v placebo or each treatment alone). Furthermore, the effect of the drug combination therapy was significantly larger than the sum of the two active treatments (P < .05). Thrombus, which formed on TF-coated coverslips in volunteers treated by placebo, were rich in fibrin and platelets. Neither of the three antiplatelet treatments significantly inhibited fibrin deposition and platelet thrombus formation on this surface (P > .2). Thus, the present study shows that combined aspirin and ticlopidine therapy dramatically potentiates the antithrombotic effect of each drug alone, but that the antithrombotic effect of the combined treatment depends on the nature of the thrombogenic surface.

Arachidonic acid enhances the tissue factor expression of mononuclear cells by the cyclo-oxygenase-1 pathway: beneficial effect of n-3 fatty acids

Monocytes express tissue factor (TF) upon stimulation by inflammatory agents. Dietary administration of fish oil rich in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) results in an impairment of TF expression by monocytes. EPA and DHA are metabolized differently from arachidonic acid (AA), the major fatty acid present in cell membranes. We examined the effects of AA on the TF expression of isolated human PBMC, and we determined whether EPA and DHA modulated this phenomenon differently. Nonstimulated PBMC had a low TF-dependent procoagulant activity. When PBMC were incubated with increasing concentrations of AA, the TF-dependent procoagulant activity increased in a dose-dependent manner to 190% at 7.5 microM. Indomethacin, a cyclo-oxygenase inhibitor, totally abolished the stimulating effect of AA, whereas specific pharmacologic inhibitors of cyclo-oxygenase-2 or of 5-lipoxygenase had no inhibitory effect. A thromboxane (TX)A2/endoperoxides receptor antagonist and a TX synthase inhibitor blocked the potentiating effect of AA. Purified PGG2 and carbocyclic TXA2, a TXA2 agonist, enhanced the procoagulant activity of PBMC in a dose-dependent manner whereas, in contrast, PGE2 inhibited it. Finally, contrary to AA, EPA or DHA did not increase TXB2 production or TF expression by PBMC. The TF-dependent procoagulant activity of isolated PBMC was increased by AA through the production of cyclo-oxygenase-1 metabolites; the combined action of PGG2 and TXA2, which potentiated it, was greater than that of PGE2, which inhibited it. Dietary n-3 fatty acids exert part of their beneficial effect by modulating this procoagulant activity differently from AA.

A new method for quantifying platelet deposition in flowing native blood in an ex vivo model of human thrombogenesis

No quantitative, simple and non-radioactive method has been described for measuring the platelet content of experimental thrombi. The aim of the present study was to develop a simple method for quantifying platelets in thrombi formed on thrombogenic surfaces in flowing native human blood. To test the relevance of this new method, the effect of unfractionated heparin on arterial thrombus formation was investigated. Tissue factor (TF)- and collagen-coated coverslips were exposed to non-anticoagulated blood at an arterial wall shear rate (2,600 s(-1)) for 1 to 4 min. Platelet deposition was quantified by measuring the P-selectin (PS) and beta-thromboglobulin (betaTG) content of dissolved plasmin-digested thrombi using immunoenzymoassays; fibrin deposition was determined by measuring the D-dimer levels. These results were compared to those established by morphometrical analysis. Morphometric evaluation showed that fibrin deposition was maximum on TF by 1 min perfusion time. Platelets deposited subsequently and reached a maximum at 3 min. On collagen, platelets deposited directly on the collagen fibrils without detectable fibrin deposit. Platelet deposition increased from 1 to 4 min. Platelet deposition quantified by PS was correlated to the values obtained by morphometry (r = 0.72, r = 0.67, p <0.001, on TF and collagen, respectively). As compared to PS, betaTG measurements gave an underestimation of the size of the thrombus platelet number. Unfractionated heparin infused through a mixing device proximal to the perfusion chamber to obtain plasma concentrations of 0.5, 1 and 3 IU/ml, reduced fibrin deposition on TF-coated coverslips in a dose-dependent manner (77% reduction at 3 IU/ml, p <0.01), but had no significant effect on platelet deposition (33% at 3 IU/ml, p >0.05). In contrast, heparin had no effect on fibrin or platelet deposition on collagen-coated coverslips. Thus, a new quantitative and simple method for measuring platelet deposition in flowing blood has been developed and characterized. Utilizing this system, we have demonstrated that unfractionated heparin did not inhibit arterial thrombus formation either on procoagulant or on proaggregant surface.

The thrombomodulin/protein C/protein S anticoagulant pathway modulates the thrombogenic properties of the normal resting and stimulated endothelium

We investigated the role of the thrombomodulin (TM/protein C/protein S anticoagulant pathway in modulating the thrombogenic properties of the endothelium. Endothelial cells (ECs) were placed in parallel-plate flow chambers and exposed to nonanticoagulated human blood at a venous wall shear rate (50 s-1). Fibrin deposition on resting ECs treated with a control IgG1 was negligible. In contrast, a significant amount of fibrin deposited when TM expression was specifically suppressed by > 95% by preincubating ECs with an anti-TM IgG1. Similarly, fibrin deposited on interleukin 1-stimulated ECs, but the fibrin deposition was further increased threefold with anti-TM IgG1. Comparable results were found when ECs were perfused at 650 s-1. When TM surface activity was enhanced by 150% by treating ECs with active phorbol ester (4-phorbol 12-myristate 13-acetate; PMA), the deposition of fibrin was 30% lower than on ECs not pretreated with PMA. Finally, fibrin deposition on stimulated ECs was significantly higher in 11 untreated patients with well-characterized deficiencies of protein C or S or heterozygous factor V Leiden mutation than in 11 healthy individuals, and it was significantly correlated to basal plasma levels of thrombin-antithrombin complexes. Thus, this study underlines the central role of the TM/protein C/protein S pathway in modulating the thrombogenic status of resting and stimulated ECs and indicates that basal coagulation system markers may be helpful in monitoring patients presenting a disorder of this anticoagulant pathway.

[Association of acquired Willebrand's disease, monoclonal gammopathy and angiodysplasia of the small bowel: a rare indication of high-dose intravenous immunoglobulins]

Acquired von Willebrand's disease associated with a monoclonal gammopathy and angiodysplasia of the gut is a rare disorder. It is sometimes complicated by chronic intestinal bleeding and severe anemia, that is poorly responsive to usual treatments. We report such a new case that has been revealed by anemia, and characterised by the absence of the high-molecular weight multimers. The correction of the hemostasis defect and of anemia were related to the reappearance of the high-molecular weight multimers, that was achieved only after high-dose intravenous immunoglobulin courses. The perfusions were performed every 3 weeks for 2 years without loss of efficiency, that could be explained by the dissociation of immunoglobulin-von Willebrand's factor complex.

Heparin reverses the procoagulant properties of stimulated endothelial cells

We examined the ability of unfractionated heparin to modulate the procoagulant activities of stimulated endothelial cells (EC). Confluent human venous umbilical EC were incubated with heparin before or after stimulation, then rinsed extensively to eliminate any heparin in the solution. EC, stimulated for 4 h with endotoxin and interleukin 1 beta, expressed tissue factor and prothrombinase activities. When EC were treated with heparin (6 and 60 micrograms/ml) during the last 10 min of the stimulation period, EC-related procoagulant activities were inhibited in a dose-dependent manner (80-90% inhibition at 60 micrograms/ml). The inhibition was antithrombin-dependent and it disappeared after heparin removal in less than 15 min at 37 degrees C but persisted at 4 degrees C. When EC were treated with heparin (60 micrograms/ml) for 24 h then extensively washed before stimulation, the anticoagulant effect was more modest (50% inhibition). The effect was antithrombin-dependent. Inhibition was maximum after 18-24 h of pretreatment of EC with heparin and was stable for at least 7 h. The cell surface displayed a "heparin-like" activity: treatment by heparin doubled the rate of thrombin-antithrombin complex formation and this effect was heparinase sensitive and chondroitinase ABC insensitive. Thus, heparin modulates the procoagulant properties of stimulated EC according to two distinct mechanisms. The first one is rapid and transient, probably related to the presence of heparin molecules bound at the membrane surface. The second is delayed and persistent, and our results suggest that it is mediated by an increase in the membrane heparan sulfate molecules.

A comparative study of the anticoagulant and anti-thrombotic effects of unfractionated heparin and a low molecular weight heparin (Fraxiparine) in an experimental model of human venous thrombosis

We have compared the anticoagulant and the antithrombotic effects of unfractionated heparin (Calciparine) and low molecular weight heparin (Fraxiparine) in an experimental human venous thrombosis model. One single subcutaneous injection of Calciparine or Fraxiparine was administered to healthy male volunteers at one month interval in a randomised and cross-over design. Ten subjects received doses used in man for preventing venous thrombosis (5,000 IU and 3,075 IU, respectively), and seven other subjects received curative doses (12,500 IU and 6,150 IU, respectively). Thrombus formation was measured 3 h and 8 h after drug administration. Non-anticoagulated human blood was drawn for 5 min directly from an antecubital vein over confluent cultured endothelial cells positioned in a parallel-plate perfusion chamber. The cells were previously stimulated for 4 h with lipopolysaccharides (10 micrograms/ml) and interleukin 1 beta (50 U/ml), resulting in optimal expression of biological active tissue factor. The wall shear rate at the cell surface was 50 s-1 and mimicked venous blood flow conditions. Immunologically quantified fibrin deposition on the stimulated cells was reduced only by curative doses of Calciparine and Fraxiparine at 3 h (3.4 +/- 0.8 versus 1.0 +/- 0.2 micrograms/cm/ and 2.6 +/- 0.8 versus 1.0 +/- 0.1 micrograms/cm2, respectively, p < or = 0.05). The influence of Calciparine and Fraxiparine on the formation of thrombin and fibrin was determined by measuring the plasma levels of thrombin-antithrombin III complexes and fibrinopeptide A (FPA) in blood samples collected distally to the perfusion chamber. The generation of these markers was significantly inhibited (50-83%) by both prophylactic and curative doses of Calciparine and Fraxiparine (p < or = 0.05). However, Fraxiparine still significantly inhibited the thrombin and fibrin generation at 8 h (p < or = 0.05), whereas Calciparine did not. The antithrombotic effects of both heparins were correlated with their plasma activities as measured by the antifactor Xa or the antithrombin assays. Thus, it appears in this model that Calciparine and Fraxiparine produce comparable antithrombotic effects at clinically comparable doses. However Fraxiparine has a longer-lasting anticoagulant activity than Calciparine. These results are in good agreement with clinical observations in man, and thus in favour of our model of human venous thrombogenesis for further studies of antithrombotic molecules.

Relationship between endothelial tissue factor and thrombogenesis under blood flow conditions

We have evaluated the relationship between the level of tissue factor (TF) expression by stimulated endothelial cells and thrombus formation under blood flow conditions. Cultures of human umbilical venous endothelial cells (HUVECs) were treated in order to express different levels of TF activity. They were stimulated for 4 h with either I) lipopolysaccharides (LPS, 10 micrograms/ml), II) recombinant interleukin 1 beta (IL1 beta, 50 UI/ml) or III) simultaneously with LPS and IL1 beta (LPS+IL1 beta). TF activity was low on confluent HUVECs or on the corresponding extracellular-matrix (ECM prepared by exposure of HUVECs to 0.1 N NH4OH). In contrast, it was high when HUVECs were stimulated with LPS or IL1 beta, and significantly higher (p < 0.05) with LPS+IL1 beta. The TF activity associated with the stimulated ECM was 2-fold higher (p < 0.05) than that expressed on the luminal surface of the stimulated HUVECs, irrespective of the agonist or combination of agonists used. These surfaces were exposed to non-anticoagulated human blood at a venous (50 s-1) and an arterial (650 s-1) wall shear rate in parallel-plate perfusion chambers for 5 min. Thrombus formation was morphologically quantified by measuring the deposition of platelets and fibrin. Fibrin deposition was also immunologically quantified. Fibrin deposition was related to the level of TF expression. Non-stimulated HUVECs and corresponding ECMs were not thrombogenic. The luminal surface of HUVECs stimulated with LPS or IL1 beta alone expressed low levels of TF activity and was a poor inducer of platelet deposition and fibrin deposition (< 15%) at 50 s-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Cross-reactivity of human molecular markers for detection of prethrombotic states in various animal species

The aim of the present study was to investigate the reactivity of immunoreagents developed for clinical applications in humans in different animal species (hen, mouse, rat, rabbit, guinea-pig, dog, pig, sheep, baboon). Prothrombin fragment 1 + 2, thrombin-antithrombin III complex and fibrinopeptide A were tested for coagulation, platelet factor 4 and beta-thromboglobulin for platelet activation, glycoprotein IIb-IIIa, glycoprotein Ib and P-selectin for platelet membrane glycoproteins, D-dimers for fibrinolysis, thrombomodulin for activation of endothelial cells and thrombospondin and von Willebrand factor for adhesive proteins. Prothrombin fragment 1 + 2, platelet factor 4, beta-thromboglobulin and D-dimers were revealed only in baboons. Fibrinopeptide A was well detected in baboons but weakly in mice, dogs, pigs and sheep. Whereas glycoprotein IIb-IIIa was revealed on guinea-pig, dog and sheep platelets and glycoprotein Ib on rabbit and dog platelets, P-selectin and thrombomodulin were never detected. Thrombospondin was revealed in hens, mice, rats, guinea-pigs, pigs, sheep and baboons and von Willebrand factor in mice, rats, guinea-pigs, dogs, pigs, sheep and baboons. Interestingly, thrombin-antithrombin III complex (TAT) was detected in all species tested except the hen. A time- and dose-dependent increase in TAT was observed when rats, dogs or pigs were infused with thromboplastin (4.5-450 microliters/kg/h), while administration of hirudin (1 mg/kg) abolished this TAT generation. Thus, the TAT immunoassay could provide a tool for the screening of antithrombotic drugs in a number of animal species. However, the possibility of using a wider panel of human immunoreagents would appear to be restricted to baboons which display good species cross-reactivity.

Effect of blood flow on thrombin generation is dependent on the nature of the thrombogenic surface

We have investigated the influence of blood flow on thrombin generation, fibrin formation, and fibrin deposition on procoagulant and nonprocoagulant surfaces. Nonanticoagulated human blood was drawn for 5 minutes directly from an antecubital vein over stimulated endothelial cells expressing tissue factor and over human type III collagen fibrils, positioned in parallel-plate perfusion chambers. The shear rates at these surfaces were 50, 650, and 2,600 s-1. Deposition of platelets and fibrin was measured by morphometry. Thrombin and fibrin formation was determined by measuring prothrombin fragments 1 + 2 (F 1 + 2), thrombin-antithrombin III complexes, (T-AT) and fibrinopeptide A (FPA) in blood effluent from the perfusion chamber at the end of the 5-minute perfusion period. On procoagulant endothelial cells, the thrombi were primarily composed of fibrin. The fibrin deposition (81%, 21%, and 2% at 50, 650, and 2,600 s-1, respectively) and plasma levels of F 1 + 2, T-AT and FPA were shear rate dependent and highest at 50 s-1. There was a positive correlation between F 1 + 2 and T-AT and the fibrin deposition (P < .01). In contrast, the collagen surface triggered primarily thrombi that were composed of platelets. The platelet thrombi and plasma levels of F 1 + 2 and T-AT were also dependent on the shear rate, but highest at 650 and 2,600 s-1. F 1 + 2 and T-AT reached the same level as observed with procoagulant endothelial cells at the higher shear rates. There was a positive correlation between F 1 + 2 and T-AT and the platelet thrombus formation (P < .05), confirming the predominant role of platelets in thrombin generation. Thus, thrombin formation is strongly influenced by the blood flow, and this effect depends on the composition of the thrombogenic surface.

Relative antithrombotic effects of monoclonal antibodies targeting different platelet glycoprotein-adhesive molecule interactions in nonhuman primates

The relative antithrombotic effectiveness of targeting glycoprotein (GP) Ib-dependent versus GPIIb-IIIa-dependent platelet interactions has been determined in baboons by measuring thrombus formation after infusing comparable antihemostatic doses of anti-von Willebrand factor (vWF) monoclonal antibody (MoAb) BB3-BD5, anti-GPIb MoAb AP1, and anti-GPIIb-IIIa MoAb LJ-CP8 under conditions of arterial and venous flow (shear rates of 750 to 1,000 seconds-1 and 100 seconds-1, respectively). Thrombus formation was quantified as 111In-platelet deposition and 125I-fibrin accumulation on segments of collagen-coated tubing interposed in chronic exteriorized arteriovenous (AV) shunts for 40 minutes. In vitro, anti-vWF MoAb BB3 BD5 (IgG) and anti-GPIb MoAb AP1 [IgG or F(ab)2 fragments] inhibited ristocetin-induced platelet aggregation (IC50 50 nmol/L and 1 mumol/L, respectively), but neither of these MoAbs blocked platelet aggregation induced by adenosine diphosphate (ADP) (P > .5). Conversely, anti-GPIIb-IIIa MoAb LJ-CP8 inhibited platelet aggregation induced by ADP (IC50 1 mumol/L, but failed to block ristocetin-induced platelet aggregation (P > .5). In vivo, the intravenous infusion of anti-vWF MoAb BB3 BD5 or anti-GPIIb-IIIa MoAb LJ-CP8 into baboons at doses that abolished corresponding agonist-induced aggregation ex vivo (bolus injections of 0.5 mg/kg and 10 mg/kg, respectively) prolonged template bleeding times from baseline values of 4.0 +/- 0.3 minutes to > 27 +/- 4 minutes, and to > 26 +/- 4 minutes, respectively (P < .001 in both cases), without affecting the peripheral platelet count (P > .5). However, injection of anti-GPIb MoAb AP1 [10 mg/kg as IgG or 1 mg/kg as F(ab)2 fragments] produced immediate irreversible thrombocytopenia (< 40,000 platelets/microL). Anti-GPIIb-IIIa MoAb LJ-CP8 abolished platelet deposition and fibrin accumulation on collagen segments under both arterial and venous flow conditions (P < .01 in all cases), whereas MoAb BB3 BD5 produced minimal inhibition of platelet deposition and no decrease in fibrin accumulation at arterial shear rates and undetectable antithrombotic outcomes at low shear. Thus, inhibiting GPIIb-IIIa-dependent platelet recruitment abrogates both thrombus formation and platelet hemostatic function at both venous and arterial shear rates. By contrast, interfering with GPIb-vWF-dependent platelet interactions abolishes platelet hemostatic function without producing corresponding antithrombotic effects.

Human type II hyperlipoproteinemia enhances platelet-collagen adhesion in flowing nonanticoagulated blood

We investigated the effects of high plasma lipid levels on platelet adhesion and platelet thrombus formation in nonanticoagulated human blood on collagen fibrils at an arterial wall shear rate of 2600 seconds-1. Nonanticoagulated blood was drawn directly at a flow rate of 10 mL/min for 3 minutes from an antecubital vein of patients with type IIa (n = 5) and type IIb (n = 4) hyperlipoproteinemia over purified human type III collagen fibrils that were positioned on a plastic coverslip in a parallel-plate perfusion chamber. Results were compared with those obtained in healthy individuals with normal lipid plasma levels (n = 9). Blood-collagen interactions were quantified by morphometry as platelet-collagen adhesion, thrombus volume, and fibrin deposition. Platelet-collagen adhesion in the two groups of patients was significantly higher than in healthy individuals (70.7 [61.2 to 82.0] and 70.3 [66.4 to 81.0] in types IIa and IIb patients, respectively, versus 51.2 [44.5 to 68.6] in control subjects; P < .05. All values are percent median [range]). In contrast, the thrombus volume was similar in the three groups (11.3 [8.0 to 13.0], 9.6 [6.4 to 15.3], and 10.2 [6.8 to 16.1] microns3/microns2 [range], respectively). Differences in fibrin deposition were not observed. Thus, it appears that platelet-collagen adhesion is augmented in patients with type IIa and IIb hyperlipoproteinemia, indicating that the process of thrombogenesis is hastened in these patients.

Antithrombotic effects of synthetic pentasaccharide with high affinity for plasma antithrombin III in non-human primates

The pentasaccharide (PS) comprising the minimal heparin structure capable of binding with antithrombin III (ATIII) and exhibiting anti-factor Xa (anti-fXa) activity in plasma without producing detectable antithrombin activity, has been evaluated for its relative antithrombotic and antihemostatic effects in a baboon model combining both platelet-rich and fibrin-rich thrombosis. Thrombosis was produced in a two-component thrombogenic device incorporated into an exteriorized femoral arteriovenous (AV) shunt in baboons; the proximal component constituted a segment of collagen-coated tubing and induced platelet-rich arterial-type thrombus, while the distal component consisted of an expanded chamber producing static and disturbed flow and initiated fibrin-rich venous-type thrombosis. Thrombus formation was measured as the deposition of 111In-platelets and the accumulation of 125I-fibrin. PS was administered intravenously to maintain plasma anti-fXa activity at three different levels: a) low dose (LD) 0.3 +/- 0.1 U/ml; b) intermediate dose (ID) 0.6 +/- 0.1 U/ml; and c) high dose (HD) 5.6 +/- 0.4 U/ml. In untreated controls, thrombus formed rapidly, reaching a plateau by 40 min of 2.3 +/- 0.2 x 10(9) platelets and 0.62 +/- 0.04 mg fibrin deposited on the collagen segments, and 1.9 +/- 0.4 x 10(9) platelets and 3.3 +/- 0.4 mg fibrin accumulated in the chambers.(ABSTRACT TRUNCATED AT 250 WORDS)

Dermatan sulfate inhibition of fibrin-rich thrombus formation in nonhuman primates

Dermatan sulfate (DS), a factor that amplifies plasma heparin cofactor II antithrombin (HCII) activity, has been evaluated in baboons for its relative antithrombotic and antihemostatic effects by use of a model that combines both platelet-rich and fibrin-rich thrombus formation. Thrombus was generated in a two-component thrombogenic device incorporated into exteriorized femoral arteriovenous shunts, in which a proximal segment of collagen-coated tubing induces platelet-rich arterial-type thrombus and distal expanded chambers with disturbed and static flow produce fibrin-rich venous-type thrombus. Thrombus formation was measured as the deposition of autologous 111In-platelets by imaging analysis and by the accumulation of 125I-fibrin. Intravenous infusion of DS at 0.83, 8.3, and 42 mg/kg maintained plasma levels at approximately 7, 70, and 400 micrograms/mL, respectively, throughout the period of study. By enhancing HCII-dependent inactivation of soluble thrombin, DS prolonged the coagulation times, reduced plasma fibrinopeptide A levels, and decreased fibrin-rich thrombus formation in the chamber portion of the device in a dose-dependent manner, ie, the intermediate dose reduced fibrin accumulation by approximately 70% (P < .05). By contrast, neither platelet deposition on collagen nor platelet hemostatic function, assessed with bleeding time determinations, was significantly affected by DS at any dose studied (P > .2 and P > .1, respectively, for the high dose), a finding presumably explained by the resistance of immobilized thrombin to inactivation by DS.

Evaluation of six markers of haemostatic system in normal pregnancy and pregnancy complicated by hypertension or pre-eclampsia

OBJECTIVE

To establish the plasma evolution of prothrombin fragments 1+2 (F 1+2), thrombin-antithrombin III complexes (TAT), fibrin fragment D-Dimers (DD), von Willebrand factor antigen (vWf), Type 1 plasminogen activator inhibitor antigen (PAI) and blood platelet count during normal pregnancy and to compare these values with those obtained in hypertensive or pre-eclamptic pregnancies.

DESIGN

Cross-sectional study.

SUBJECTS

Forty-seven healthy pregnant women with gestational age ranging between 5 and 40 weeks, and fourteen women with gestational age ranging between 25 and 38 weeks presenting with either gestational hypertension (n = 4) or pre-eclampsia (n = 10). Numbers of nulliparous women in the control, hypertension and pre-eclampsia groups were 13/47 (28%), 1/4 (25%) and 9/10 (90%), respectively.

RESULTS

All six markers increased with gestational age in normal pregnant women (P < 0.01). Using the upper limit of 95% prediction interval obtained from regression curves as normality threshold, TAT showed the best sensitivity (71% vs < 30% for F1+2, DD, vWf, PAI and platelet count).

CONCLUSION

TAT appears to be an interesting marker for detecting haemostatic system alterations in pregnancies complicated by hypertension or pre-eclampsia. A large prospective study to determine its clinical usefulness for such complicated pregnancies is currently in progress.

Antihemostatic and antithrombotic effects of monoclonal antibodies against von Willebrand factor in nonhuman primates

BACKGROUND

Because the adhesive glycoprotein von Willebrand Factor (vWF) mediates initial platelet attachment at sites of vascular injury and may also contribute to shear-dependent platelet thrombus formation, we have determined in vivo the relative antithrombotic efficacy and hemostatic safety of infusing murine monoclonal antibodies against vWF.

METHODS

In baboons with chronic arteriovenous shunts, thrombus formation was initiated by interposition of thrombogenic Dacron vascular grafts (VG) and endarterectomized baboon aortic segments (EAS). Thrombus formation on VG and EAS was assessed by use of real-time scintillation camera imaging of indium 111-labeled platelet deposition. In control and treated animals (anti-vWF antibody) platelet hemostatic competence was evaluated by means of serial measurements of platelet count, bleeding time, and ex vivo platelet aggregation in response to adenosine diphosphate and ristocetin.

RESULTS

Although bolus antibody infusions did not affect circulating platelet counts, bleeding times were immediately prolonged to 28 +/- 4 minutes (vs 4.7 +/- 0.4 minutes before treatment, p = 0.01). Bleeding times normalized within 24 hours after antibody administration. Platelet aggregation in response to adenosine diphosphate was unchanged by antibody therapy, whereas ristocetin-induced platelet aggregation was abolished acutely and remained impaired for 24 hours. Platelet deposition on VG after 60 minutes of exposure to flowing blood was 2.95 +/- 0.74 x 10(9) platelets/cm in six control animals as compared to 1.86 +/- 0.16 x 10(9) platelets/cm in five treated animals (p = 0.04). Similarly, platelet deposition on EAS averaged 4.40 +/- 0.89 x 10(9) platelets/cm in control studies and was reduced significantly by antibody therapy (1.52 +/- 0.50 x 10(9) platelets/cm, p = 0.02).

CONCLUSIONS

Despite profound interruption of platelet hemostatic functions, therapeutic targeting of vWF modestly inhibits platelet-dependent thrombosis.

Hirudin interruption of heparin-resistant arterial thrombus formation in baboons

To determine the role of thrombin in high blood flow, platelet-dependent thrombotic and hemostatic processes we measured the relative antithrombotic and antihemostatic effects in baboons of hirudin, a highly potent and specific antithrombin, and compared the effects of heparin, an antithrombin III-dependent inhibitor of thrombin. Thrombus formation was determined in vivo using three relevant models (homologous endarterectomized aorta, collagen-coated tubing, and Dacron vascular graft) by measuring: (1) platelet deposition, using gamma camera imaging of 111In-platelets; (2) fibrin deposition, as assessed by the incorporation of circulating 125I-fibrinogen; and (3) occlusion. The continuous intravenous infusion of 1, 5, and 20 nmol/kg per minute of recombinant hirudin (desulfatohirudin) maintained constant plasma levels of 0.16 +/- 0.03, 0.79 +/- 0.44, and 3.3 +/- 0.77 mumol/mL, respectively. Hirudin interrupted platelet and fibrin deposition in a dose-dependent manner that was profound at the highest dose for all three thrombogenic surfaces and significant at the lowest dose for thrombus formation on endarterectomized aorta. Thrombotic occlusion was prevented by all doses studied. In contrast, heparin did not inhibit either platelet or fibrin deposition when administered at a dose that maximally prolonged clotting times (100 U/kg) (P greater than .1), and only intermediate effects were produced at 10-fold that dose (1,000 U/kg). Moreover, heparin did not prevent occlusion of the test segments. Hirudin inhibited platelet hemostatic function in concert with its antithrombotic effects (bleeding times were prolonged by the intermediate and higher doses). By comparison, intravenous heparin failed to affect the bleeding time at the 100 U/kg dose (P greater than .5), and only minimally prolonged the bleeding time at the 1,000 U/kg dose (P less than .05). We conclude that platelet-dependent thrombotic and hemostatic processes are thrombin-mediated and that the biologic antithrombin hirudin produces a potent, dose-dependent inhibition of arterial thrombus formation that greatly exceeds the minimal antithrombotic effects produced by heparin.

Effects of red blood cell concentration on hemostasis and thrombus formation in a primate model

Because the effects of red blood cell (RBC) concentration on hemostasis and thrombus formation have not been studied experimentally under conditions of whole blood flow without anti-coagulation, normal baboons were bled or transfused to obtain three different groups: a low hematocrit (Ht) group (20% less than Ht less than 25%), a normal Ht group (35% less than Ht less than 40%), and a high Ht group (50% less than Ht less than 55%). Measurements of platelet count, bleeding time, platelet aggregation, fibrinogen level, and coagulation time (APTT) were equivalent to normal values in each group. Thrombus formation was induced using a device composed of collagen-coated tubing followed by two sequentially placed expansion chambers designed to exhibit flow recirculation and stasis. The device was exposed for up to 40 minutes in an arterio-venous shunt system. Wall shear rates in the tubular collagen segment were 100 seconds-1 and 500 to 750 seconds-1. The accumulation of 111In-platelets and 125I-fibrinogen/fibrin was measured radioisotopically; RBC incorporation was determined from measurements of total thrombus hemoglobin. Thrombus that formed on the collagen substrate was rich in platelets and poor in fibrin and RBCs. Under high flow conditions, thrombus composition showed no dependence on Ht. Surprisingly, under low flow conditions, platelet thrombus volume was negatively correlated with Ht (r = -.73, P = .005), and was increased by greater than twofold in the low Ht group as compared with the high Ht group. Thrombus that formed in the disturbed flow regions contained relatively few platelets but was rich in fibrin and RBCs. The predominant finding was a positive correlation between RBC incorporation and Ht at both high and low shear rates (r = .90, P = .00003; and r = .77, P = .002, respectively), with thrombus volume increasing three- to sixfold between the low and high Ht groups. Thus, in vivo variations in Ht ranging between 20% and 55% did not affect hemostasis, but were found either to promote or inhibit the net accumulation of thrombus, depending on local flow conditions.

Inhibition of platelet-dependent thrombosis by low molecular weight heparin (CY222): comparison with standard heparin

We have compared the relative antithrombotic and antihemostatic effects of the very low molecular weight heparin CY222 with standard unfractionated heparin (SH) in a baboon model of platelet-dependent thrombosis. Thrombus formation was induced by placement of a thrombogenic device in an exteriorized femoral arteriovenous shunt under conditions of intermediate-shear blood flow. The device consisted of a collagen-coated cannular segment positioned proximal to two regions of expanded diameter exhibiting disturbed flow and stasis. Thrombus formation was measured in real time by indium 111-labeled platelet imaging. The collagen-coated surface accumulated thrombi composed largely of platelets, and the regions of disturbed flow were morphologically rich in fibrin and red cells. SH and CY222 were administered by continuous infusion for 1 hour. Although both heparin preparations abolished thrombus formation in the low-shear fibrin-rich regions at plasma levels less than 0.5 anti-Xa U/ml, platelet deposition onto the collagen surface was not reduced by either SH or CY222 at that dosage. These findings were consistent with previously observed therapeutic benefits of this level of anti-Xa activity in venous, but not arterial, thrombosis. Platelet deposition on the collagen was reduced in a dose-dependent manner by both SH and CY222 administered at doses between 1 and 5 anti-Xa U/ml. It is important to note that although heparin preparations produced profound and equivalent antithrombotic effects for platelet-dependent thrombus formation at comparable levels of anti-Xa activity, SH prolonged both the coagulation time and the bleeding time substantially more than did CY222.(ABSTRACT TRUNCATED AT 250 WORDS)

RGDV peptide selectively inhibits platelet-dependent thrombus formation in vivo. Studies using a baboon model

Since platelet hemostatic functions are mediated in part through the binding of adhesive proteins containing an RGD (Arg-Gly-Asp) recognition sequence, and since platelet reactions may be inhibited in vitro by RGD-containing peptides, we assessed in vivo the antithrombotic activity of RGDV (Arg-Gly-Asp-Val) tetrapeptide using a baboon thrombosis model. Thrombus formation was induced by a device consisting of a tubular segment coated with type I collagen, followed by two regions of expanded diameter exhibiting disturbed flow and stasis. The thrombogenic device was incorporated into femoral arteriovenous shunts under conditions of intermediate wall shear rate (100 s-1). Thrombus formation was measured by scintillation camera imaging of 111In-platelets and by counting of 125I-fibrinogen/fibrin. Thrombus that formed on the collagen substrate was rich in platelets, while thrombus formed in the disturbed flow regions was rich in fibrin and red cells. RGDV peptide was infused proximal to the thrombogenic device to maintain local plasma concentrations of 25, 50, and 100 microM. Infused RGDV decreased the accumulation of both platelets and fibrin on the collagen substrate in a dose-response manner. At the highest dose platelet and fibrin deposition after 40 min was reduced by greater than 80% (P less than 0.01). In the region of disturbed flow, RGDV (100 microM) reduced platelet deposition by 85% (P less than 0.01) but did not reduce the accumulation of fibrin (P less than 0.3). Similarly, the peptide inhibited the release of granular proteins from platelets associated with thrombus (platelet factor 4, beta-thromboglobulin; P less than 0.01), but did not prevent the appearance of fibrinopeptide A in circulating blood (P greater than 0.1). No systemic alterations in blood pressure, bleeding time, or platelet aggregation ex vivo were produced by locally infused RGDV. The antithrombotic effects of RGDV peptide disappeared within 5 min after discontinuing the infusion. In control studies infused RGEV (Arg-Gly-Glu-Val, 100 microM) showed no antithrombotic activity. Thus, RGDV selectively blocks platelet-dependent thrombus formation in vivo.

Discrimination between platelet-mediated and coagulation-mediated mechanisms in a model of complex thrombus formation in vivo

To study mechanisms of complex thrombus formation in vivo, and to compare the relative antithrombotic effects of anticoagulants and antiplatelet agents, a model was developed in baboons. Segments of collagen-coated tubing followed by two sequentially placed expansion chambers exhibiting disturbed flow patterns were exposed to native blood under laminar flow conditions. The device was incorporated for 1 hour into an exteriorized arteriovenous shunt in baboons under controlled blood flow (20 ml/min). Morphologic evaluation by scanning electron microscopy showed that thrombi associated with collagen were relatively rich in platelets but thrombi in the chambers were rich in fibrin and red cells. Deposition of indium 111-labeled platelets was continuously measured with a scintillation camera. Platelet deposition increased in a linear (collagen-coated segment) or exponential (chambers 1 and 2) fashion over time, with values after 40 minutes averaging 24.1 +/- 3.3 x 10(8) platelets (collagen segment), 16.7 +/- 3.4 x 10(8) platelets (chamber 1), and 8.4 +/- 2.4 x 10(8) platelets (chamber 2). Total fibrinogen deposition after 40 minutes was determined by using iodine 125-labeled baboon fibrinogen and averaged 0.58 +/- 0.14 mg in the collagen segment, 1.51 +/- 0.27 mg in chamber 1, and 0.95 +/- 0.25 mg in chamber 2. Plasma levels of beta-thromboglobulin (beta TG), platelet-factor 4 (PF4), and fibrinopeptide A (FPA) increased fourfold to fivefold after 60 minutes of blood exposure to the thrombotic device. Platelet deposition onto the collagen segment, chamber 1, and chamber 2 was linearly dependent on the circulating platelet count. Platelet accumulation in chamber 1 and chamber 2 was also dependent on the presence of the proximal collagen segment. An anticoagulating dose of standard heparin decreased platelet deposition in the chambers (p less than 0.05) but did not decrease deposition onto the collagen segment. Although beta TG and PF4 levels remained elevated after the administration of standard heparin, the elevation in plasma FPA was interrupted. Further evidence that the thrombotic process was dependent on platelets was provided by the finding that prostaglandin I2 at high concentration (35 ng/ml) decreased platelet deposition onto the collagen segment and in chambers 1 and 2, decreased beta TG and PF4 release, and reduced FPA formation. The combination of standard heparin and PGI2 produced the most potent inhibition of platelet thrombus formation and prevented the increases in plasma PF4, beta TG and FPA.

Further studies on the mechanisms for the antithrombotic effects of sulfated polysaccharides in rabbits

A recent study (Fernandez et al., Thromb. Haemostas. 1987; 57: 286-93) demonstrated that when rabbits were injected with the minimum weight of a variety of glycosaminoglycans required to inhibit tissue factor-induced thrombus formation by approximately 80%, exogenous thrombin was inactivated approximately twice as fast in the post-treatment plasmas as the pre-treatment plasmas. In this study, we investigated the relationship between inhibition of thrombus formation and the extent of thrombin inhibition ex vivo. We also investigated the relationship between inhibition of thrombus formation and inhibition of prothrombin activation ex vivo. Four sulfated polysaccharides (SPS) which influence coagulation in a variety of ways were used in this study. Unfractionated heparin and the fraction of heparin with high affinity to antithrombin III potentiate the antiproteinase activity of antithrombin III. Pentosan polysulfate potentiates the activity of heparin cofactor II. At less than 10 micrograms/ml of plasma, all three SPS also inhibit intrinsic prothrombin activation. The fourth agent, dermatan sulfate, potentiates the activity of heparin cofactor II but fails to inhibit intrinsic prothrombin activation even at concentrations which exceed 60 micrograms/ml of plasma. Inhibition of thrombus formation by each sulfated polysaccharides was linearly related to the extent of thrombin inhibition achieved ex vivo. These observations confirm the utility of catalysis of thrombin inhibition as an index for assessing antithrombotic potential of glycosaminoglycans and other sulfated polysaccharides in rabbits. With the exception of pentosan polysulfate, there was no clear relationship between inhibition of thrombus formation and inhibition of prothrombin activation ex vivo.

Standard heparin enhances the antithrombotic activity of dermatan sulfate in the rabbit but CY 216 does not

Standard heparin (SH) and dermatan sulfate (DS) two glycosaminoglycans with different pharmacological targets are effective antithrombotic agents in the rabbit. We have investigated the antithrombotic activity of the association DS plus SH. It was found that doses as low as 25 micrograms/kg for DS and 10 micrograms/kg for SH were ineffective when injected separately but generated a high and significant antithrombotic activity when injected together. These results were confirmed when higher doses of each compound were delivered in association. Further experiments were performed to determine if the enhancement of the antithrombotic activity of DS by HS resulted from its anti-factor IIa or anti-factor Xa activity or from its moiety without affinity to AT III. A low molecular weight heparin (CY 216) with an anti-factor Xa/anti-factor IIa ratio of 5, the synthetic pentasaccharide bearing the minimum binding sequence to antithrombin III, and a low affinity fraction of SH to AT III did not increase the antithrombotic activity of DS; in contrast a high affinity fraction of SH to AT III had the same effect than SH. We conclude that the enhancement of the antithrombotic activity of DS by SH mainly results from its anti-factor IIa activity.