Experimental Thrombosis on a Collagen Coated Arterioarterial Shunt in Rats: A Pharmacological Model to Study Antithrombotic Agents inhibiting Thrombin Formation and Platelet Deposition

Arterial antithrombotic effects of aspirin, heparin, enoxaparin and clopidogrel alone, or in combination, in the rat

INTRODUCTION

Many antithrombotic drugs may have a deleterious effect on normal haemostasis leading to bleeding complications. The aim of this study was to determine if sub-therapeutic (low) doses of antithrombotic agents, when administered in combination, have enhanced efficacy without augmentation of bleeding time.

MATERIALS AND METHODS

The antithrombotic effects of i.v. aspirin (4-30 mg/kg), heparin (100-500 U/kg), enoxaparin (4-30 mg/kg) and clopidogrel (10-20 mg/kg) were studied in a rat Folts-like preparation of carotid arterial thrombosis. The frequency of cyclic flow reductions (CFRs; indicating occlusive thrombus formation) and bleeding time were measured. Drug doses that were singly ineffective at preventing occlusive thrombus formation were tested in the following combinations: aspirin (10 mg/kg) with heparin (250 U/kg); aspirin (4 mg/kg) with enoxaparin (4 mg/kg); and aspirin (10 mg/kg) with clopidogrel (10 mg/kg).

RESULTS

Control period (pretreatment) CFRs were not significantly different between groups; average 7.0+/-0.3 CFRs/30 min (n=64). Tail bleeding time before drug(s) was 3.1+/-0.1 min (n=86). When administered alone, aspirin (4-30 mg/kg), heparin (250 U/kg) or enoxaparin (4 mg/kg) had no effect on CFRs or bleeding time. Heparin (500 U/kg), enoxaparin (10 and 30 mg/kg) and clopidogrel (20 mg/kg) significantly decreased CFRs. Single administration of heparin (500 U/kg) or enoxaparin (30 mg/kg) increased bleeding time by 4- or 11-fold. When co-administered, aspirin 10 mg/kg and heparin 250 U/kg decreased CFRs, but also increased bleeding time by 11-fold. However, combination of aspirin and enoxaparin (4 mg/kg each), or aspirin and clopidogrel (10 mg/kg each), decreased CFRs with no effect on bleeding.

CONCLUSIONS

In a preparation of arterial thrombosis in the rat, combinations of sub-efficacious (low) doses of aspirin with enoxaparin or clopidogrel inhibited thrombus formation without augmenting bleeding time. However, low-dose aspirin combined with heparin, whilst inhibiting thrombus formation, exacerbated bleeding time. If these findings translate into the clinic, the use of effective low-dose combinations may have therapeutic advantages.

The platelet P2 receptors as molecular targets for old and new antiplatelet drugs

Platelet activation by ADP and ATP plays a crucial role in haemostasis and thrombosis, and their so-called P2 receptors are potential targets for antithrombotic drugs. The ATP-gated channel P2X1 and the 2 G protein-coupled P2Y1 and P2Y12 ADP receptors selectively contribute to platelet aggregation. The P2Y1 receptor is responsible for ADP-induced shape change and weak and transient aggregation, while the P2Y12 receptor is responsible for the completion and amplification of the response to ADP and to all platelet agonists, including thromboxane A2 (TXA2), thrombin, and collagen. The P2X1 receptor is involved in platelet shape change and in activation by collagen under shear conditions. Due to its central role in the formation and stabilization of a thrombus, the P2Y12 receptor is a well-established target of antithrombotic drugs like ticlopidine or clopidogrel, which have proved efficacy in many clinical trials and experimental models of thrombosis. Competitive P2Y12 antagonists have also been shown to be effective in experimental thrombosis as well as in several clinical trials. Studies in P2Y1 and P2X1 knockout mice and experimental thrombosis models using selective P2Y1 and P2X1 antagonists have shown that, depending on the conditions, these receptors could also be potential targets for new antithrombotic drugs.

Anti-thrombotic and haemorrhagic effects of active site-inhibited factor VIIa in rats

Active site-inhibited factor VIIa (FFR-rFVIIa) competes with factor VIIa (FVIIa) for binding to tissue factor (TF) and exerts an anti-thrombotic effect. We report an evaluation of the anti-thrombotic properties of FFR-rFVIIa in a model of thrombosis involving two thrombogenic surfaces. Uncoated glass capillaries or glass capillaries coated with TF were incorporated into an arterioarterial shunt in the rat and the occlusion time (OT) of the shunt was determined. An anti-thrombotic activity of FFR-rFVIIa was shown only on the TF-coated surface: the OT of the shunt was significantly prolonged, from 167 +/- 34 s in control animals to 312 +/- 42 s after i.v. bolus administration of 4 mg/kg FFR-rFVIIa. This OT was similar to those observed with the uncoated shunts in untreated animals (353 +/- 84 s). In vitro preincubation of the TF-coated shunt with FFR-rFVIIa significantly prolonged the OT to 245 +/- 45 s in the absence of detectable amounts of FFR-rFVIIa in the plasma. rFFR-rFVIIa weakly prolonged the tail template bleeding time by a factor of 1.5. This effect was more pronounced in animals pretreated with heparin. The anti-thrombotic and prohaemorrhagic effects of FFR-rFVIIa were totally reversed by administration of an equidose of rFVIIa. These results provide new information on the pharmacological properties of FFR-rFVIIa that will be useful for its clinical development.

Comparison of arterial and venous patency in a rat model of subendothelium-stimulated thrombosis

The effectiveness of anticoagulants and platelet aggregation inhibitors was compared using comparable rat models of arterial and venous thrombosis. A mechanical endothelium-denuding injury was created on the lumenal surface of donor Lewis rat carotid arteries. These were cut into 4-5 mm lengths and grafted into femoral veins and arteries of recipient syngeneic rats using microvascular anastomotic techniques. Recipients received either systemic heparin, or aspirin with dipyridamole, or saline (control). In the arteries, the 1-day patency rate was 94% in the heparin-treated rats, but only 50% in the aspirin/dipyridamole group and 44% in the control group. The venous patency rate was 56% in the heparin group, 31% in the aspirin/dipyridamole group, and 0% in the control group. This unique model for comparing thrombosis in arteries and veins shows that anticoagulation is more effective than inhibition of platelet aggregation in the rat arterial system, with less of a differential effect in the venous system.

A revised model of platelet aggregation

In this study we have examined the mechanism of platelet aggregation under physiological flow conditions using an in vitro flow-based platelet aggregation assay and an in vivo rat thrombosis model. Our studies demonstrate an unexpected complexity to the platelet aggregation process in which platelets in flowing blood continuously tether, translocate, and/or detach from the luminal surface of a growing platelet thrombus at both arterial and venous shear rates. Studies of platelets congenitally deficient in von Willebrand factor (vWf) or integrin alpha(IIb)beta(3) demonstrated a key role for platelet vWf in mediating platelet tethering and translocation, whereas integrin alpha(IIb)beta(3) mediated cell arrest. Platelet aggregation under flow appears to be a multistep process involving: (a) exposure of vWf on the surface of immobilized platelets; (b) a reversible phase of platelet aggregation mediated by the binding of GPIbalpha on the surface of free-flowing platelets to vWf on the surface of immobilized platelets; and (c) an irreversible phase of aggregation dependent on integrin alpha(IIb)beta(3). Studies of platelet thrombus formation in vivo demonstrate that this multistep adhesion mechanism is indispensable for platelet aggregation in arterioles and also appears to promote platelet aggregate formation in venules. Together, our studies demonstrate an important role for platelet vWf in initiating the platelet aggregation process under flow and challenge the currently accepted view that the vWf-GPIbalpha interaction is exclusively involved in initiating platelet aggregation at elevated shear rates.

Characteristics of rat platelets and relative contributions of platelets and blood coagulation to haemostasis

In order to understand some of the haemostatic mechanisms in rats for the interpretation of toxicological data, basic haemostatic parameters with a special emphasis on platelet functions were first measured in vitro. The results of reactions of rat platelets to many aggregating agents suggest that only ADP may be a consistently significant aggregator. The search for physiologic aggregators revealed ADP to be available from erythrocytes. Adhesion reaction also required ADP. Collagen was not considered to be essential for either reaction. Aggregation and adhesion were probably both reversible in flowing blood, while irreversible thrombi were formed in blood at rest ex vivo. Blood coagulation parameters determined revealed that the intrinsic pathway may be more important than the extrinsic one. The rate of intrinsic coagulation reaction was rapid, and plasma coagulation appeared to be of primary importance while the influence of platelet aggregation was minor. A simple model of rat haemostatic mechanism is proposed based on these results. Additionally, to define the relative contribution of platelets versus other cellular and plasma coagulation in vivo, rats were administered antiplatelet drugs (ticlopidine, suprofen and clopidogrel) and an anticoagulant (warfarin) intraperitoneally. Bleeding times (BTs) were significantly increased in all treated groups. ADP-induced platelet aggregations were significantly depressed by the administration of the three antiplatelet drugs, while kaolon-activated partial thromboplastin time and prothrombin time were greatly increased in the warfarin-treated rats. The increase in BT may be due to the inhibition of platelet activity or blood coagulation defect in rats given antiplatelet drugs or warfarin, respectively. These results suggest that platelets play a key role in haemostasis in the rat. Two possible explanations of the disparity between in vitro and in vivo results may be that functional tests used here are not adequate to cover the properties of rat platelets or that mechanisms leading to the formation of platelet thrombi in rats are ADP-dependent adhesion and ADP-induced aggregation.

Antithrombotic effects of (n-3) polyunsaturated fatty acids in rat models of arterial and venous thrombosis

The antithrombotic effects of dietary lipids were investigated in rat models of arterial and venous thrombosis. In the arterial model, thrombus formation was evaluated by determination of the occlusion time and the deposition of 111In-labeled platelets and 125I-labeled fibrinogen in a collagen-coated glass capillary inserted into an arterio-arterial shunt. Venous thrombosis was evaluated by measurement of the thrombus weight after administration of thromboplastin as a source of tissue factor and establishment of stasis in the vena cava. Diets were supplemented with saturated (SAT group) or (n-3) fatty acids, the latter being added either as MaxEPA oil (MaxEPA group), or as docosahexaenoic (DHA group) or eicosapentaenoic (EPA group) ethyl ester. Only the MaxEPA group displayed a prolonged occlusion time as compared with all other groups. Platelet accumulation, similar in the MaxEPA, EPA and DHA groups (13.3, 16.7 and 17.7 x 10(6) platelets/shunt, respectively), was significantly higher in the SAT group (25.3 x 10(6) platelets/shunt), while accumulation of fibrinogen-fibrin was similar whatever the group. There was a trend towards a lower venous thrombus weight in MaxEPA fed rats relative to those fed other diets. Our data indicate that the MaxEPA diet had antithrombotic effects in arterial and to a lesser extent venous thrombosis models, best attributed to its multiple targeting of platelets and coagulation.

Effect of selective factor Xa inhibition on arterial thrombus formation triggered by tissue factor/factor VIIa or collagen in an ex vivo model of shear-dependent human thrombogenesis

Tick anticoagulant peptide (TAP) is a potent and selective inhibitor of factor Xa. TAP has shown good antithrombotic efficacy in experimental animal models of disseminated intravascular coagulation and venous and arterial thrombogenesis. In the present study we evaluated the effect of recombinant TAP (rTAP) on acute thrombus formation in human nonanticoagulated blood triggered either by tissue factor (TF) or by collagen at arterial shear conditions. The main goal was to establish the role of factor Xa in thrombus formation by use of an optimal inhibitory concentration of rTAP. Blood was drawn directly from an antecubital vein by a pump over the respective thrombogenic surfaces, which were positioned in a parallel-plate perfusion chamber. rTAP was mixed homogeneously into the flowing blood by a heparin-coated device positioned proximal to the perfusion chamber. The passage of blood through this device caused minor activation of coagulation but little activation of platelets. Fibrinopeptide A and beta-thromboglobulin levels after 5 minutes of blood perfusion were, on average, 14 ng/mL and 45 IU/mL, respectively. rTAP at a plasma concentration of 0.90 mumol/L completely inhibited TF/factor VIIa-dependent thrombus formation at wall shear rates of 650 and 2600 s-1. These shear conditions are comparable to those in medium-sized arteries and in moderately stenosed small arteries, respectively. In contrast to the TF-coated surface, rTAP was less efficient in reducing collagen-induced thrombus formation. While a significant reduction of 53% was observed at 650 s-1, thrombus formation at 2600 s-1 was not affected by rTAP.(ABSTRACT TRUNCATED AT 250 WORDS)