© 1995 American Heart Association, Inc.
Articles |
Aurintricarboxylic Acid Prevents Acute Rethrombosis in a Canine Model of Arterial Thrombosis
From the Department of Medicine, Division of Cardiology, Case Western Reserve University Hospitals, and the Department of Veterans Affairs Medical Center, Cleveland, Ohio (J.S., L.R.); the Department of Medicine, Medical College of Virginia, Richmond (A.S.); and the Department of Medicine, Division of Hematology, Medical College of Virginia, and the Department of Veterans Affairs Medical Center, Richmond (B.A.).
Correspondence to John Strony, MD, Case Western Reserve University Hospitals of Cleveland, 11100 Euclid Ave, Cleveland, OH 44106.
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Abstract |
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Abstract Acute rethrombosis following thrombolytic therapy occurs in 5% to 25% of patients. We evaluated the effect of aurintricarboxylic acid (ATA), a triphenyl dye that blocks von Willebrand factor (vWF) binding to platelet glycoprotein Ib, on arterial reperfusion and acute rethrombosis following fibrinolytic therapy. Primary thrombosis was induced in the femoral arteries of anesthetized dogs by application of anodal current and partial arterial constriction. Blood flow was monitored with an electromagnetic flow probe, and primary thrombosis was considered to have occurred when blood flow was reduced to and remained at zero. Reperfusion was induced by intravenous streptokinase 30 minutes after thrombosis. Streptokinase reduced plasma fibrinogen levels from an average of 144 mg/dL to <5 mg/dL resulting in inhibition of ADP- and epinephrine-induced platelet aggregation ex vivo. Acute rethrombosis following reperfusion occurred within 37±18 (mean±SD) minutes in 89% (16/18) of animals receiving thrombolytic activator treatment. Histological examination of reoccluding thrombi revealed densely aggregated platelets and erythrocytes with no detectable fibrin. In the two other study groups, ATA was infused in conjunction with thrombolytic therapy (10 arteries) or at near completion of acute rethrombosis following fibrinolytic activator treatment (6 arteries). In each case ATA prevented rethrombosis. However, concomitant administration of ATA and thrombolytic therapy delayed restoration of blood flow. ATA had no direct effect on hemodynamics, thrombin time, platelet count, or platelet aggregation response to ADP, epinephrine, or collagen. These data indicate that inhibition of vWF–platelet glycoprotein Ib interaction is effective in preventing acute rethrombosis following thrombolytic therapy. However, the complex paradoxical effects of ATA on platelet activity should be considered when it, or agents of its class, are used as antithrombotics.
Key Words: fibrinolysis • platelets • thrombosis • acute rethrombosis
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Introduction |
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Fibrinolytic therapy is highly effective in the treatment of acutely thrombosed arteries.1 2 3 Although reperfusion occurs rapidly in a majority of patients, there is a 5% to 25% incidence of acute rethrombosis associated with thrombolytic therapy.1 4 Clinically, rethrombosis occurs despite depletion of fibrinogen and anticoagulation with heparin. Experimental models of acute rethrombosis and histological examination of thrombi indicate that platelets are a significant component of reoccluding thrombi.5 6 Activated platelets can cause vessel obstruction by extensive aggregation and may promote fibrin gel formation by providing a surface for assembly of the factor Va/Xa–prothrombinase complex.7 Various substances and mechanisms have been proposed as possible mediators of platelet activation after fibrinolytic therapy. These include a highly thrombogenic ulcerated plaque, persistence of a high-grade stenosis resulting in shear stress–induced platelet activation, thrombin generation secondary to fibrinolytic activator action, and direct platelet activation by plasmin.8 9 10 11 12 13 14
Platelet thrombus formation is the result of platelet adhesion and aggregation. Platelet adhesion is mediated by binding of von Willebrand factor (vWF) to both the vessel surface and the platelet receptor glycoprotein Ib (GPIb). Platelet aggregation occurs when metabolic processes capacitate the GPIIb/IIIa receptor complex. Fibrinogen and/or vWF then binds to GPIIb/IIIa, thereby cross-linking the platelets to form an aggregate. Agents that interrupt each of these processes have been proposed as potential inhibitors of acute rethrombosis.
The primary antiplatelet agents currently available for clinical use are metabolic inhibitors of platelet activation. With long-term use, drugs such as aspirin and ticlopidine are effective in preventing thrombosis, but their effect is unpredictable when administered during acute thrombosis.15 Prostacyclin and its analogues, when administered during acute thrombus formation, rapidly inhibit platelet activity. However, significant adverse effects, particularly hypotension, limit their usefulness.
Inhibition of platelet aggregation by direct blockade of platelet–adhesive glycoprotein interaction is a potentially effective means of interrupting arterial thrombus formation. Drugs that act in this manner can exert their effects rapidly, and because their activity is independent of the mechanism of platelet activation, they can inhibit platelet aggregation irrespective of the agonist. In fact, agents that interfere with the binding of fibrinogen and vWF to GPIIb/IIIa or of vWF to GPIb have already demonstrated effective antithrombotic activity in in vivo studies.16 17 18 19 20 21
In a previous study we demonstrated that aurintricarboxylic acid (ATA), a triphenyl dye that blocks vWF binding to platelet GPIb, inhibited platelet-dependent thrombus formation in an in vivo model of coronary artery constriction and thrombosis.20 ATA was shown to be an effective antithrombotic agent even when shear stress was greatly elevated at the site of stenosis. The effectiveness of ATA was mediated by its binding to vWF and consequent inhibition of vWF interactions with platelet GPIb.22 Infusion of ATA blocked platelet-dependent thrombus formation without either inhibiting ADP or epinephrine-induced platelet aggregation or affecting bleeding time.
In the current study we evaluated the effect of blockade of vWF-GPIb interactions by ATA on reperfusion of acutely thrombosed arteries during fibrinolytic therapy. We also appraised the effect of ATA on platelet-dependent acute rethrombosis in an in vivo canine model of femoral artery thrombosis/rethrombosis.
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Methods |
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Materials
Streptokinase was purchased from Hoechst Roussel Pharmaceuticals, Inc. ATA was obtained from Aldrich Chemical Co. To ensure consistency throughout the study, all ATA was obtained from a single lot. All other reagents were reagent grade or better and obtained from standard commercial suppliers.
Surgical Preparation of Experimental Model
Conditioned mongrel dogs (n=23) of either sex weighing 20 to 25
kg were anesthetized with pentobarbital sodium (30 mg/kg body wt IV),
intubated with a cuffed endotracheal tube, and ventilated on room air
(Harvard Apparatus). Additional anesthetic agent (10 mg/kg body wt IV)
was administered as indicated to maintain deep anesthesia. These
studies were approved by the animal use committees at both Case Western
Reserve University and the McGuire Department of Veterans Affairs
Medical Center and were conducted in compliance with institutional
guidelines. Body temperature was maintained at 37 °C to 39°C with
radiant heat and a heating pad. Arterial and venous catheters were
inserted in the right jugular artery and vein to enable continuous
blood pressure monitoring and fluid administration. All fluids were
warmed to 37°C prior to infusion. Right and left femoral arteries
were exposed and dissected from the surrounding tissue, with ligation
of all branches. An adjustable renal artery occluder (Harvard
Apparatus) was placed on the proximal portion of the dissected vessels,
followed by a calibrated electromagnetic flow probe (model 501B,
Carolina Medical Equipment) and a loosely fitted cylindrical plastic
constrictor modeled after that described by Folts et al.23
To maintain consistent conditions between animals, femoral arterial
blood flow was adjusted to 40 to 50 mL/min with the renal artery
occluder. A stimulation electrode, constructed from a 25-gauge
stainless steel hypodermic needle tip attached to a 30-gauge
polytetrafluoroethylene (Teflon)-insulated copper wire (Fig 1
), was inserted into the artery proximal to the
constrictor.
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Continuous recordings of systemic blood pressure, mean and phasic femoral arterial blood flow, and mean and phasic Doppler flow were displayed and recorded on an eight-channel recorder (model 2800, Gould). On completion of the study, all animals were killed with a lethal intravenous injection of pentobarbital sodium (85 mg/kg body wt).
Induction of Femoral Artery Thrombosis, Thrombolysis, and
Inhibition of Acute Rethrombosis
Following surgical preparation and a 20-minute baseline
monitoring period, 200 µA of continuous, anodal, DC stimulation was
delivered through the electrode already inserted in the femoral artery
by use of a constant-current unit (model LM 5A, Grass Medical
Instrument Co). The current was maintained until blood flow decreased
to and remained at zero. Vessels that failed to reach total primary
occlusion were excluded from the study.
Thirty minutes after primary occlusion was attained, a single bolus infusion of streptokinase (3 750 000 IU, IV) was administered. Because of marked variability in the interanimal response to streptokinase and rapid metabolism of the canine plasmin-streptokinase complex, this bolus dose had been determined from previous experience to optimize fibrinolysis by reliably producing profound and prolonged fibrinogen depletion.24 Once blood flow was restored, anodal DC stimulation was resumed until rethrombosis occurred.
The animals were divided into three treatment groups (Fig 2). Following successful thrombolysis, group I received
0.9% saline at the onset of rethrombosis, group II received ATA (10
mg/kg body wt IV) when rethrombosis was almost completed, and group III
received ATA (10 mg/kg body wt IV) in conjunction with streptokinase
after primary thrombosis. The bolus dose of ATA (10 mg/kg body wt IV)
had been determined in previous studies to consistently cause total
inhibition of platelet-dependent thrombus formation in a canine model
of coronary thrombosis without affecting hemodynamics, platelet count,
thrombin time, or ex vivo platelet aggregation in response to ADP or
epinephrine.20 ATA was dissolved in sterile PBS (pH 7.4)
and sterilized by filtration prior to infusion. All studies were
conducted for a minimum of 120 minutes following restoration of blood
flow.
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Criteria Used to Judge the Time to Thrombosis, Reperfusion, and
Rethrombosis
Thrombosis was judged to be complete and anodal current was
discontinued when femoral arterial blood flow decreased to and remained
at zero for 5 minutes. Reperfusion following administration of
fibrinolytic activator was defined as the time when blood flow returned
to at least 50% of the baseline value. Rethrombosis was defined as
sustained zero blood flow after successful reperfusion. Near completion
of rethrombosis was judged to have occurred when blood flow ranged from
1 to 10 mL/min.
Coagulation and Aggregation Studies
Blood samples were obtained at baseline and at 2, 5, 8, 13, 15,
30, 60, and 120 minutes following administration of ATA, fibrinolytic
activator, or both. These samples were drawn by a double-syringe
technique into trisodium citrate anticoagulant (final anticoagulant
concentration, 0.38%) containing aprotinin (2000 IU/mL) to inhibit
plasmin. Thrombin time and fibrinogen determinations were performed on
plasma in a fibrometer with the method of Clauss (BBL Microbiology
Systems).25
The effects of ATA on ADP- and collagen-induced platelet aggregation ex vivo and in vitro were evaluated turbidimetrically using an aggregometer (Sienco). Platelet aggregation response after ATA infusion was compared with the pretreatment response and with the in vitro platelet response to ATA at concentrations ranging from 20 to 180 µg/mL. Platelet response was measured following the addition of 12.5 µmol ADP and 0.25 µmol epinephrine and reported as the maximal change in light transmission relative to baseline. Collagen-induced aggregation was performed with equine tendon type I collagen (Chrono-Log) in doses ranging from 0.6 µg/mL to 40 µg/mL. Platelet response was reported as the concentration of collagen necessary to induce aggregation to 50% of the maximum baseline value.
ATA Plasma Concentrations
Plasma levels of ATA after bolus injection were measured
indirectly with an ex vivo agglutination method. Platelet-rich plasma,
adjusted to 2.0x104 platelets/µL, was prepared from
blood samples obtained at baseline and at 2, 5, 8, 13, 15, 30, 60, and
120 minutes following bolus injection. Platelet agglutination was
induced with botrocetin (final concentration, 1.5 µg/mL) and measured
turbidimetrically in an aggregometer.
A standard calibration curve was generated for each study animal. ATA was added to aliquots obtained from baseline platelet-rich plasma. Final concentrations of ATA ranged from 1 µg/mL to 250 µg/mL. Following a 30-minute incubation at 37°C, platelet agglutination was performed with 1.5 µg/mL botrocetin (final concentration). A standard curve was generated by plotting the percent inhibition of agglutination versus the final concentration of ATA. In vivo plasma concentrations of ATA for the times stated above were calculated by using the standard curve.
Pathological Examination
Microscopic examination of thrombi was performed on
femoral arteries of selected animals harvested 30 minutes following
primary thrombosis or 5 minutes following reocclusion. Thrombosed
segments were removed intact, rinsed in saline, and immersed in 10%
formaldehyde. Following overnight fixation, specimens were embedded in
paraffin, sectioned at 4-µm intervals, and stained with hematoxylin
and eosin or Carstairs' stain for identification of platelets and
fibrin.26 Serial sections from each artery were examined.
Photomicrographs were taken of the stained sections with a Nikon
Optiphot photomicroscope.
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Results |
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Induction of Femoral Artery Thrombosis
Thirty-eight femoral arteries in 19 animals were instrumented to produce acute thrombosis. Thrombosis was initiated with a combination of flow reduction, partial occlusion, and anodal current stimulation. Thrombosis occurred in 79% (30/38) of study arteries within 59±22 minutes of starting the current. Hemodynamic parameters, heart rate, and blood pressure remained stable throughout each experiment.
Thrombolysis and the Effect of ATA on Acute Rethrombosis
Animals in groups I and II (20 thrombosed arteries) received
thrombolytic activator 30 minutes after primary thrombosis occurred.
Reperfusion was observed in 90% (18/20) of the arteries 33±26 minutes
after streptokinase administration (Fig 3). Animals in
group III (10 thrombosed arteries) received both streptokinase and 10
mg/kg body wt ATA 30 minutes after primary thrombosis. Time to
reperfusion required an average of 52±11 minutes after combined drug
infusion (Fig 3). The prolonged time to reperfusion observed in group
III is statistically significant (P=.036).
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In groups I and II acute rethrombosis occurred within 37±18 minutes
(range, 5 to 65 minutes) after reperfusion in 89% (16/18) of femoral
arteries. In group I, 12 of 13 arteries reperfused and 10 rethrombosed.
These animals received only a saline infusion following rethrombosis,
and all vessels remained occluded. In group II, 6 of 7 arteries
reperfused, and each animal received ATA (10 mg/kg body wt IV) when
rethrombosis was nearly complete (1 to 10 mL/min blood flow). In all
group II animals, ATA reversed the process of thrombosis, prevented
acute rethrombosis, and maintained blood flow (6 reperfused arteries).
In group III (10 arteries), reperfusion occurred and rethrombosis was
prevented in all arteries. Representative tracings of arterial
blood flow from all three study groups are presented in Fig 4. Data from all groups are summarized in the Table and
Fig 3
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Thrombolytic infusion, either alone or with adjunct ATA, produced profound fibrinogen depletion and inhibition of ADP-, epinephrine-, and collagen-induced ex vivo platelet aggregation in all animals. Initial fibrinogen concentrations averaged 128, 127, and 144 mg/dL for groups I, II, and III, respectively. Fifteen minutes after fibrinolytic infusion, fibrinogen levels fell to <1 mg/dL for groups I and II and to <5 mg/dL for group III. At 1 hour after infusion, fibrinogen levels were still markedly reduced, as they were at the time of rethrombosis (<1, <1, and 4 mg/dL for groups I, II, and III, respectively).
Effect of ATA on Coagulation Parameters
When ATA was administered alone, no significant change in platelet
count, thrombin time, or ADP- or epinephrine-induced ex vivo platelet
aggregation was noted. These parameters were monitored in four animals
after infusion of ATA alone (10 mg/kg body wt IV) (no fibrinolytic
activator). Platelet count prior to infusion averaged
272 000±42 000/µL and at 1 hour after infusion was
242 000±13 000/µL. Baseline thrombin times averaged 15.2±2.3
seconds, reached a maximum of 18.9±4.9 seconds at 15 minutes
(P=NS), and declined to 11.8±3.2 seconds after 1 hour.
These findings validate those reported previously on the effect of ATA
on platelet count and thrombin time.20
Infusion of ATA alone resulted in significant inhibition of collagen-induced ex vivo platelet aggregation. A twofold to threefold rise in collagen concentration was necessary to produce an aggregatory response that was 50% of the baseline maximum. This response was noted immediately and for 60 minutes following ATA administration. Thereafter, incomplete normalization of aggregation was noted (data not shown). These findings parallel in vitro studies that have demonstrated that with ATA levels of 180 µg/mL, a mean 2.7-fold rise in collagen concentration is necessary to induce a half-maximal aggregation response.
Biological Assay of ATA Plasma Levels
Botrocetin-induced platelet agglutination studies were performed
in four animals that received ATA alone. Platelet agglutination was
maximally reduced to an average of 33±24% of the baseline value. The
peak inhibitory effect was noted 13 to 15 minutes following bolus
injection of ATA (Fig 5). When these data are compared
with the calibration curves for each study animal, peak ATA plasma
levels averaged 145±64 µg/mL and declined to 57±15 µg/mL at 60
minutes. These concentrations are well above the minimum levels
reported as necessary to inhibit vWF-mediated platelet
adhesion.27
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Histological Examination of Primary and Reoccluding Thrombi
Vessels were harvested from four additional animals for
histological examination of four primary and four reoccluding thrombi.
Serial sections obtained throughout the length of each thrombus
(average length, 1 cm) were composed of two distinct zones: a proximal
region that had surrounded the stimulating electrode and a distal
region. Examination of the proximal regions of primary thrombi revealed
an outer layer of densely packed platelets attached to both the vessel
wall and a fibrin band that surrounded an eccentric core of
erythrocytes that had been in contact with the electrode. The distal
regions of primary thrombi consisted of platelets and occasional
erythrocytes and no detectable fibrin (Fig 6A).
Reoccluding thrombi (Fig 6B) were composed of an outer layer of
platelets and a core of erythrocytes. In contrast to primary thrombi,
little fibrin was detected in reoccluding thrombi, even in the area
where the electrode had been placed.
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Discussion |
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Recent studies have demonstrated the benefit of thrombolytic therapy in the treatment of acute myocardial infarction, with successful thrombolysis occurring in up to 80% of patients.2 3 28 Unfortunately, this benefit is offset by acute thrombotic reocclusion in as many as 25% of patients,4 28 which occurs despite fibrinogen depletion and full heparinization. Therefore, development of adjunct treatment strategies to enhance thrombolysis and prevent acute thrombotic reocclusion following fibrinolytic activator therapy has become an area of considerable interest.
Acute reocclusion following thrombolysis is the result of multiple factors. In vivo studies demonstrating platelet activation following fibrinolytic activator therapy suggest that platelets are important mediators of acute reocclusion.9 10 29 This notion is supported by results from histological studies of reoccluding thrombi, which contain platelets.30 31 Platelet activation following successful thrombolysis is caused by (1) the presence of a highly thrombogenic, ulcerated plaque; (2) a significant residual stenosis, resulting in elevated shear stress; (3) generation of thrombin and arachidonic acid; (4) direct platelet activation by plasmin; and (5) other, as yet unrecognized, factors.8 9 10 12 13 14 32 33
In this study we have described a model of arterial thrombosis, reperfusion by thrombolytic infusion, and acute rethrombosis. Thrombus formation was initiated by flow restriction and vessel wall damage induced by application of anodal current. It is important to note that clinically, acute reocclusion occurs spontaneously for unknown reasons. In this study we used a specific stimulus—anodal current—to initiate rethrombosis. Differences between our model and the clinical state must be kept in mind when interpreting these results.
The infusion of ATA in vivo blocks platelet-dependent thrombus formation within a constricted coronary artery under conditions of greatly elevated shear stress.20 In this study, rethrombosis was reversed when ATA was administered during thrombus formation and was prevented by concurrent administration of ATA and fibrinolytic activator. The effect of ATA in these experiments was pronounced. Acute reocclusion occurred in 88% of control vessels but in none of the vessels when combined ATA/fibrinolytic activator treatment was administered. Moreover, acute reocclusion was prevented and blood flow normalized in all cases on infusion of ATA during rethrombosis.
Histological examination of reoccluding thrombi demonstrated a large mass of platelets surrounding a core of erythrocytes but little to no fibrin. This finding may have resulted from thrombolytic activation and disruption of the primary thrombus by dissolution of either the fibrin in the area distal to the electrode or possibly of the platelet component of the thrombus. This event might have occurred if plasmin digested the platelet-bridging fibrinogen that was attached via GPIIb/IIIa. Although formation of fibrin gel was not apparent after fibrinogen depletion, it is possible that platelet-derived fibrinogen and thrombin were present in sufficient quantities to promote platelet aggregation via binding to GPIIb/IIIa. In addition, other adhesive glycoproteins, specifically vWF, could bind aggregated platelets within the reformed thrombus. vWF bridges platelets by its attachment to both GPIb and GPIIb/IIIa. Further immunochemical or histochemical analysis of reoccluding thrombi is necessary to precisely define their composition.
Phillips et al22 first observed the antiplatelet activity
of ATA and determined that it acted by binding directly to and
inhibiting attachment of vWF to GPIb. Girma et al34
demonstrated in vitro that a 50% reduction in vWF binding to
botrocetin-treated platelets occurred with ATA concentrations of 49
µg/mL and that maximal reduction of vWF binding occurred at ATA
concentrations of 
75 µg/mL. Using a bioassay of plasma ATA
concentrations, we demonstrated that a 10 mg/kg body wt IV bolus
infusion of ATA resulted in ATA plasma levels that maximally inhibited
vWF-GPIb interactions, which remained inhibited by more than 50% even
at 1 hour. Our observations are the first to demonstrate that blockade
of vWF-GPIb interaction may be an effective means of inhibiting
rethrombosis.
ATA has been demonstrated to inhibit thrombin-induced platelet
aggregation and to potentiate platelet response to several agonists,
including collagen, thromboxane A2, and the
arachidonate analogues U46619 and A23187.27 ATA
concentrations as high as 244 µg/mL inhibited collagen-mediated
platelet activation, whereas ATA concentrations of as little as 122
µg/mL or less potentiated platelet aggregation and release. In
addition, ATA has been found to have no effect on fibrinogen or
vWF–GPIIb/IIIa binding (B. Adelman and I.F. Charo, unpublished data,
1991). In vitro and ex vivo collagen-mediated aggregation in our
present canine model demonstrated no ATA-induced potentiation of
platelet aggregation during the entire 120-minute study period at ATA
concentrations ranging from 20 to 180 µg/mL. These findings may be
due in part to the high plasma concentrations achieved by bolus
administration of ATA and the effects of ATA on canine platelets. In
vitro studies performed in our laboratory on human platelet-rich plasma
demonstrated that ATA had a neutral effect on collagen-induced
aggregation at ATA concentrations of 
120 µg/mL, with inhibition of
aggregation at higher concentrations (J.S. et al, unpublished data,
1994). Additionally, inhibition of thrombin by ATA was minimal, with
only a modest prolongation of thrombin time.
The effect of ATA on platelet activation and adhesion is dependent on its total plasma concentration and the composition of its individual isomers.27 35 Weinstein et al35 reported that both low- and high-molecular-weight polymers of ATA (Mr, 500 to 6400) inhibited plasmin-catalyzed fibrinolysis, whereas high-molecular-weight ATA polymers (Mr, 6400) produced a paradoxical activation of platelets. Only the intermediate-molecular-weight fractions of ATA (Mr, 2500) were found to be most effective in inhibiting platelet aggregation.36 37
The antithrombotic action of ATA in vivo appears to be effective primarily as an adjunct in the prevention of acute reocclusion following fibrinolysis; its beneficial action is primarily limited to inhibition of vWF–platelet GPIb interactions. We theorize that the prolonged time to reperfusion in group III animals, ie, those that received ATA in conjunction with thrombolytic therapy, was a result of the inhibition of plasmin-mediated fibrinolysis influenced predominantly by the high-molecular-weight isomers of ATA (Mr, 6400). This prothrombotic effect was later superseded by the potent antiplatelet effect of the 2500-D ATA fraction, as demonstrated by the sustained patency associated with combination infusion (group III) and restoration of blood flow when ATA was given during reocclusion (group II). We also suspect that the 13- to 15-minute delay seen in our biological assay of ATA plasma levels reflected the proaggregatory properties of the 6400-D ATA fraction that masked the true plasma concentration.
Inhibition of platelet aggregation by direct blockade of platelet interaction with adhesive glycoproteins is potentially an effective means to interrupt arterial thrombus formation. Drugs that act in this manner exert their effects rapidly, and because their activity is independent of the mechanism of platelet activation, they are able to inhibit platelet aggregation induced by any agonist. In vivo studies have demonstrated that agents that inhibit binding of fibrinogen and vWF to GPIIb/IIIa or of vWF to GPIb are effective antithrombotics.16 19 20 21 38 39 In addition, the direct thrombin inhibitors argatroban and hirudin prevent reocclusion in experimental models of coronary thrombosis.40 41 Because experimental models are limited representations of the human disease process, clinical trials are underway to identify those agents most effective in preventing acute reocclusion.
To date, most agents that have been designed to inhibit platelet–adhesive glycoprotein interactions have been directed at the GPIIb/IIIa complex. Both peptide and monoclonal antibodies have proven to be effective antithrombotics in experimental models of thrombosis and acute reocclusion.16 19 20 21 41 The study presented in this article provides initial in vivo evidence that ATA, an agent that is neither a peptide nor a monoclonal antibody, can inhibit platelet thrombi formation mediated by glycoprotein vWF. Our results suggest that this approach may be an effective antithrombotic strategy. Although this study demonstrated a significant effect of ATA in delaying reperfusion, the biological significance of this finding is unknown. This may be due to the unique effects of ATA and its derivatives, which may mitigate the benefits of its platelet-inhibitory properties. These effects of ATA must be considered prior to its use in future studies.
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Acknowledgments |
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Support for this project was provided by research grants from the US Department of Veterans Affairs and the American Heart Association.
Received May 11, 1994; accepted January 12, 1995.
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