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.

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

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)

Antithrombotic and bleeding effects of a new synthetic direct thrombin inhibitor and of standard heparin in the rabbit.

This study reports on the 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 (TCT) and the activated partial thromboplastin time (APTT). SDTI was more potent than SH 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 micrograms/kg/h of SDTI and of 60 micrograms/kg/h of SH, significant thrombus prevention effects were obtained (59 and 57% respectively). Increasing the dose of SDTI up to 3000 micrograms/kg/h did not significantly improve the antithrombotic effect. After SC injection, a significant antithrombotic 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 vivo concentrations generated after continuous IV infusion or SC injection of the same dose were higher for SH than for the SDTI.

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.

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.

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.

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.

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.

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.

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 micrograms/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 micrograms/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.

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)

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.

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.

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.

[Genetic predisposition to arterial and venous thrombosis]. / La prédisposition génétique à la thrombose artérielle et veineuse.

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.

[Association of acquired Willebrand's disease, monoclonal gammopathy and angiodysplasia of the small bowel: a rare indication of high-dose intravenous immunoglobulins]. / Association maladie de Willebrand acquise--gammapathie monoclonale--angiodysplasie du grêle: une indication rare des immunoglobulines intraveineuses á forte dose.

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.