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Clopidogrel Inhibition of Stent, Graft, and Vascular Thrombogenesis With Antithrombotic Enhancement by Aspirin in Nonhuman Primates
From the Division of Hematology and Oncology, Department of Medicine, and Yerkes Regional Primate Research Center, Emory University School of Medicine, Atlanta, Ga, and Sanofi Recherche, Toulouse, France (J.-M.H.).
Correspondence to Laurence A. Harker, MD, Blomeyer Professor and Director, Division of Hematology and Oncology, Emory University School of Medicine, 1639 Pierce Dr, WMB Room 1003, Atlanta, GA 30322. E-mail lharker{at}emory.edu
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Abstract |
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 Top Abstract IntroductionMethodsResultsDiscussionReferences |
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Background—A recent study showed that clopidogrel reduces thrombo-occlusive complications in patients with symptomatic atherosclerosis more effectively than aspirin.
Methods and Results—The effects of clopidogrel and aspirin have
been compared, singly and in combination, for measurements of
111In-labeled platelets and 125I-labeled
fibrin deposition in baboon models of arterial thrombosis
and related to platelet aggregation and expression of activation
epitopes induced by ADP, collagen, and thrombin receptor agonist
peptide (TRAP) and to template bleeding times (BTs). Low-dose oral
clopidogrel (0.2 mg · kg-1 ·
d-1) produced cumulative (1) intermediate decreases in
111In-platelet and 125I-fibrin deposition
for segments of prosthetic vascular graft, deployed
endovascular metallic stents, and endarterectomized aorta
(P<0.009 in all cases); (2) elimination of ADP-induced
platelet aggregation (P<0.001); (3) modest
inhibition of collagen-induced platelet aggregation
(P<0.01); (4) no reduction in TRAP-induced platelet
aggregation; and (5) minimal prolongation of BTs
(P=0.03). High-dose oral clopidogrel (
2 mg/kg)
produced the same effects within 3 hours. The effects of clopidogrel
dissipated over 5 to 6 days. Aspirin 10 mg ·
kg-1 · d-1 alone did not decrease
111In-platelet and 125I-fibrin deposition
on segments of vascular graft but detectably decreased
111In-platelet and 125I-fibrin accumulation
on stents (P<0.01), minimally inhibited ADP- and
collagen-induced platelet aggregation (P<0.05 in
both cases), and minimally prolonged BTs (P=0.004).
Within 3 hours of aspirin administration, the antithrombotic effects of
acute high-dose or chronic low-dose clopidogrel were substantially
enhanced, and BTs were modestly prolonged without inhibiting
platelet aggregation induced by TRAP (P<0.001 in
all cases compared with clopidogrel alone).
Conclusions—Clopidogrel produces irreversible, dose-dependent, intermediate reduction in thrombosis that is substantially enhanced by the addition of aspirin. The effects of combining aspirin and clopidogrel need to be evaluated in patients at risk of vascular thrombosis.
Key Words: clopidogrel • thrombus • stents • aspirin
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Introduction |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Heart attacks, strokes, and peripheral arterial occlusion are generally caused by thrombo-occlusive episodes in stenotic atherosclerotic arteries.1 Ruptured atherosclerotic plaques initiate complex interactions among damaged atheromatous vascular structures and highly reactive platelets and coagulation proteins, resulting in the formation of tissue factor–dependent vascular thrombosis. Tissue factor–initiated thrombotic occlusion also complicates interventional procedures used in the management of patients with symptomatic atherosclerotic vascular disease, including thrombolytic reperfusion for acute coronary thrombosis, angioplasty, various types of atherectomy, deployment of endovascular stents, endarterectomy, and implantation of small-caliber vascular grafts.2 3
Large-scale, randomized, placebo-controlled trials have established that daily oral aspirin therapy decreases the relative risk of vaso-occlusive episodes in patients with symptomatic atherosclerotic disease by 20% to 25%.1 4 The benefits of aspirin are attributable to its irreversible interruption of thromboxane A2 (TxA2) generation by platelets.5 Controlled clinical trials have also established that oral ticlopidine therapy decreases the relative risk of vaso-occlusive events in symptomatic atherosclerotic patients by 30% to 35%.1 4 6 7 8 9 Ticlopidine, a thienopyridine requiring hepatic modification in vivo to exhibit antiplatelet effects, selectively inhibits ADP-dependent platelet aggregation that is cumulative over 8 to 10 days.10 11 12 Recent clinical studies demonstrate that adding aspirin to ticlopidine therapy markedly reduces thrombo-occlusive events associated with the deployment of coronary stents.13 14 15 16 Unfortunately, ticlopidine therapy has a number of troublesome adverse effects, including reversible neutropenia, diarrhea, and idiosyncratic cutaneous rashes.
Clopidogrel is a ticlopidine-like thienopyridine that is severalfold more potent and is free of the adverse effects plaguing ticlopidine therapy.17 18 Like ticlopidine, clopidogrel is devoid of direct antiplatelet effects and must undergo hepatic metabolic modification to exhibit selective inhibition of ADP-induced platelet aggregation.22 Clopidogrel acts by irreversibly inactivating platelet ADP receptor–initiated signaling in a dose-dependent manner.10 20 21 In a recently reported large-scale, randomized, controlled clinical trial, clopidogrel was shown to be significantly more effective than and at least as safe as aspirin in decreasing arterial thrombo-occlusive episodes in patients with symptomatic atherosclerotic disease.22 Clinical trials reporting that ticlopidine plus aspirin markedly reduces thrombotic occlusion of coronary stents suggest that adding aspirin to clopidogrel therapy may enhance its antithrombotic efficacy.13 14 15 16
Clopidogrel and aspirin each inhibit platelet recruitment by interrupting ADP- and TxA2-mediated platelet activation, respectively. Presumably, the relative contributions of ADP- and TxA2-dependent platelet recruitment varies with different clinical thrombotic processes. Accordingly, the present study was designed to measure the relative antithrombotic effects of administering clopidogrel and aspirin, singly and in combination, for 3 different thrombosis models in baboons, ie, graft, stent, and vascular thrombosis models. The relative antihemostatic and antithrombotic effects of oral clopidogrel, clopidogrel with aspirin, clopidogrel with heparin, or the combination of clopidogrel, aspirin, and heparin were determined.23
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Methods |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Baboon Model of Thrombosis and Hemostasis
Twenty-four normal juvenile male baboons weighing 9 to 11.5 kg and bearing chronic exteriorized arteriovenous (AV) femoral shunts were used in these studies. Before experimentation, all animals were observed to be disease-free for at least 3 months. All procedures were approved by the Institutional Animal Care and Use Committee (Emory University) in compliance with National Institutes of Health guidelines (Guide for the Care and Use of Laboratory Animals, 1985), Public Health Service policy, the Animal Welfare Act, and related university polices. In these models, quantitative, reproducible, nonoccluding, platelet-rich thrombi were formed on deployed endovascular metallic stents, segments of vascular graft, and segments of endarterectomized baboon aorta interposed in the AV shunts under physiological flow conditions for 60 minutes without systemic anticoagulation. The forming thrombus incorporated circulating prelabeled autologous 111In-labeled platelets and homologous 125I-labeled fibrin(ogen). These chronic AV shunts, per se, do not detectably activate platelets or coagulation.24 25
Thrombogenic Devices
Nonocclusive thrombi were formed over 60 minutes in the
exteriorized AV shunts of awake animals by deployment of metallic
endovascular stents, interposing 2-cm-long, 4-mm-ID thrombogenic
segments of uncrimped Dacron vascular grafts or incorporating segments
of endarterectomized homologous aorta in established AV shunts while
blood flow was controlled at 100 mL/min.
Stent thrombosis was produced by deployment of stainless steel endovascular stents (3.5 mm) in the exteriorized chronic AV shunt. The stainless steel stents, a gift from Johnson & Johnson Interventional Systems, Warren, NJ, were mounted on sterile water–filled noncompliant Duralyn coronary angioplasty balloons (Cordis Corp). The stents were manually crimped onto the deflated balloon and inserted into a 3.3-mm-ID, 20-cm-long segment of silicone rubber tubing (Technical Products Inc). The balloon was inflated 3 times to a pressure of 10 atm to achieve maximal apposition of the stent struts with the tubing wall. The shunt tubing was then filled with sterile saline to remove potentially confounding air bubbles from the surface of the stent and to facilitate interpositioning of stent-containing segments in the exteriorized chronic AV femoral shunt.
Graft thrombosis was produced by segments of Dacron vascular grafts (Bioknit, C.R. Bard, Inc) rendered impervious to blood leakage by external wrapping in Parafilm (American Can Co) and 5.3-mm-ID "heat-shrinkable" Teflon tubing. Connections were constructed to ensure that the devices were isodiametric and suitable for incorporation into the AV shunts.
Endarterectomy thrombosis was produced by endarterectomizing fresh baboon aorta (5- to 6-mm ID) obtained from other donor animals, flushed with saline, and divided into 4-cm lengths. Branches were ligated, and specimens were stored in normal saline. For endarterectomies, the aortic segments were inverted and the intima and inner media were removed for a distance of 1 cm in the central portion of the vessel by sharp dissection. After completion of the endarterectomy, each segment was returned to its normal configuration and cannulated with 1-cm lengths of heat-shrinkable Teflon tubing (Small Parts, Inc) attached to segments of 4-mm-ID silicone rubber medical tubing (Dow Corning, Inc). The aortic segments were encased with heat-shrinkable Teflon tubing and each end was carefully sealed by heating, but direct heat to tissues was avoided. The resultant configuration maintained stable geometry, with a smooth transition from vessel to tubing.
Clopidogrel Dosing
Clopidogrel was administered orally at 7 doses spanning more
than 2 orders of magnitude, ie, 0.1, 0.2, 0.5, 2, 5, 10, and 20 mg
· kg-1 · d-1.
Antithrombotic and antihemostatic effects of high-dose clopidogrel for
stent and graft thrombosis were obtained 3 hours after dosing with 2 to
20 mg/kg. Because the effects of low-dose clopidogrel were cumulative,
doses of 0.1, 0.2, and 0.5 mg/kg clopidogrel were administered daily
for 6 days to determine full dose-response effects (see below).
Thrombus formation was measured in the following sequence on different
days during the subsequent 2 weeks, assessing the effects of (1)
clopidogrel 0.2 mg · kg-1 ·
d-1 alone, (2) clopidogrel 0.2 mg ·
kg-1 · d-1 with
heparin 100 IU/kg bolus and 100 IU/kg infused over 1 hour, (3)
clopidogrel 0.2 mg · kg-1 ·
d-1 plus aspirin 10 mg/kg administered 2 hours
previously, and (4) clopidogrel 0.2 mg ·
kg-1 · d-1 in
combination with aspirin 10 mg/kg and heparin 100 IU/kg infused over 1
hour. Treatments were discontinued for 1 week to permit the
antiplatelet effects of clopidogrel and the enhancing effects of
aspirin to dissipate before other studies were begun.
Measurements of Thrombus Formation
Autologous platelets were labeled with 1 mCi
[111In]indium oxine
(111In) as previously
described24 26 and reinjected at least 1 hour
before the thrombogenic devices were interposed.
125I-labeled baboon fibrinogen (5 µCi),
purified and labeled as described previously,27
was injected intravenously 10 minutes before thrombogenic
devices were introduced. Images of the segments containing vascular
graft, deployed stents, or endarterectomized aorta were acquired
separately with a General Electric 400T MaxiCamera, stored, and
analyzed with a Medical Data Systems A3
image processing system (Medtronic) interfaced with the camera, by
routines already described.27 The total numbers
of deposited platelets in regions of interest were calculated by
dividing the deposited platelet activity (counts/min) by the
circulating blood activity (counts ·
min-1 · mL-1) and
multiplying by the circulating platelet count (platelets/mL).
Fibrin was determined after completion of the experiments by removal of
the thrombogenic segments for counting
125I-fibrin radioactivity 30 days later when the
111In activity had decayed. Total fibrin
accumulated was calculated by dividing the deposited
125I activity (counts/min) by the clottable
fibrinogen activity (counts · min-1
· mL-1) and multiplying by the plasma
fibrinogen level (mg/mL).26 27
Laboratory Studies
Platelet counts, erythrocyte counts, and total leukocyte
counts were performed on whole blood collected in 2 mg/mL disodium EDTA
with a Serono Baker model 9000 whole-blood
analyzer.28 Blood samples for testing
platelet hemostatic function were collected in citrate (3.2% for
platelet aggregation studies and 3.8% for flow cytometric
determination of P-selectin and ligand-induced binding site [LIBS]
expression).
Platelet aggregation was determined within 1 hour of drawing blood
with a Chrono-Log aggregometer by recording the increase in
light transmission through a stirred suspension of platelet-rich
plasma (PRP) maintained at 37°C. PRP and platelet-poor plasma
(PPP) were prepared by differential centrifugation, as
previously described.29 The platelet count in
the PRP was adjusted to 300x103/µL.
Percent aggregation was calculated linearly between the optical
densities of PPP and PRP. ADP (Sigma Chemical Co), collagen (Nycomed
Arzenmittel), and thrombin receptor agonist peptide
(TRAP1–6) (Peninsula Laboratories) were added at
doses spanning the range of responsiveness. The results were plotted
and expressed as the agonist concentration that induced half-maximal
aggregation (AC50).29 The
appearance of activated platelets in the
peripheral blood was evaluated by flow cytometry using
fluoresceinated monoclonal antibodies against neoantigens
expressed on membrane surfaces of activated platelets,
including conformationally altered integrin
IIbß3 (or
glycoprotein IIb/IIIa), LIBS (a gift from Dr E. Plow,
Cleveland, Ohio),30 and the secretory granule
membrane, P-selectin, a gift from Biogen Inc, Cambridge,
Mass.30
Template bleeding time (BT) measurements were performed at baseline and at 60 minutes. BT testing was carried out on the shaved volar surface of the forearm as previously described in nonhuman primates.28 31
Statistical Analysis of Data
Data were presented as mean±SD. Student's t
test for paired or unpaired data was used when data were normally
distributed. Otherwise, Mann-Whitney nonparametric
analysis was used. Factorial ANOVA and ANCOVA were used. A
value of P
0.05 was considered to be the estimate of
statistical significance.
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Results |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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Baboon Models of Graft, Stent, and Vascular Thrombosis
Segments of Dacron vascular graft interposed in exteriorized AV shunts flowing at 100 mL/min induced rapid platelet deposition that reached plateau levels by 60 minutes (Figure 1
, top). Neither aspirin 10 mg/kg PO 2
hours previously nor heparin 100 IU/kg IV bolus and 100 IU/kg infused
over 60 minutes significantly reduced thrombus formation (Figure 1, top; P>0.2).
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Deployed metallic endovascular stents in AV shunts flowing at 100
mL/min also produced thrombus that reached plateau values by 
60
minutes (Figure 1
, bottom). Whereas heparin had no effect on stent
thrombosis (P>0.2), oral aspirin 10 mg/kg modestly
decreased platelet accumulation on stents, ie,
111In-platelet deposition decreased from
2.56±0.96x109 to
1.76±0.57x109 platelets
(P<0.01; Figure 1, bottom).
Control endarterectomized aortic segments produced substantial thrombus by 60 minutes, ie, platelet deposition averaged 1.6±0.39x109 platelets/cm, and mean fibrin accumulation was 1.1±0.3 mg/cm.
Baseline platelet concentrations averaged 285±60x103/µL, and control mean plasma fibrinogen concentration was 2.95±0.27 mg/mL. Baseline template BT averaged 3.3±0.8 minutes. During baseline thrombus formation on thrombogenic devices, platelets remaining in the circulation did not change aggregatory responsiveness or express activation epitopes, ie, ex vivo platelet aggregation induced by ADP, collagen, or TRAP was unaltered, and there was no significant flow cytometric expression of P-selectin or LIBS.
Antithrombotic and Antihemostatic Effects of Clopidogrel
In vitro, clopidogrel exhibited no direct inhibitory
effects on baboon platelet aggregation or LIBS expression induced
by ADP, collagen, or TRAP (P>0.5).
Oral dosing of clopidogrel at 0.1 mg/kg for 6 days abolished
ADP-induced platelet aggregation (undetectable aggregation despite
the addition of >100 µmol/L ADP; Table 1). This dosing regimen also
modestly inhibited platelet aggregation induced by collagen, as
shown by the increased concentration of collagen required to produce
half-maximal aggregation, ie, from 2.2±1.0 to 8.4±4.9 µg/mL
(P=0.008), and prolonged the BT to 6.9±5.3 minutes (Table 1; P<0.01). However, neither TRAP-induced platelet
aggregation nor thrombus formation on segments of vascular graft or
deployed endovascular stents was significantly decreased
(P>0.2 in all cases).
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Increasing the dose of clopidogrel to 0.2 mg/kg for 6 days
significantly reduced platelet and fibrin accumulation on vascular
grafts and stents (Table 2;
P<0.01 in all cases), prolonged the BT to 7.7±7.4 minutes
(Tables 1, 2, and 3; P<0.01
compared with baseline values) without additional inhibition of
platelet aggregation and expression of activation epitopes induced
by ADP or collagen, and produced no reduction in TRAP-induced
platelet activation (Tables 1 and 3).
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Within 3 hours, increasing clopidogrel dosing 10-fold to 2 mg/kg
significantly decreased platelet deposition (Figure 2; Table 2; P<0.01 in both
cases) but not fibrin accumulation (Table 2; P>0.1 in both
cases) for both vascular grafts and stents. ADP-induced aggregation was
markedly inhibited (AC50 20±13 µmol/L;
P<0.0001 compared with baseline), collagen-induced
aggregation was inhibited beyond that produced after 6 days of 0.1
mg · kg-1 ·
d-1 clopidogrel (Table 1), and BT was prolonged
to 24±9.5 minutes (P<0.0001 compared with baseline).
TRAP-induced platelet aggregation was not reduced (Table 1).
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When the dose of clopidogrel was increased another 10-fold to 20 mg/kg,
the accumulation of both platelets and fibrin on segments of
vascular graft and stents was significantly decreased within 3 hours of
oral administration (Table 2; Figure 2; P<0.02 in all
cases). ADP-induced aggregation was abolished, and BT was increased to
>30 minutes. TRAP-induced platelet aggregation was minimally
inhibited (Table 1; P<0.06 compared with baseline), and no
additional inhibition of collagen-induced aggregation was produced
(Table 1; P>0.4). The reduction in thrombosis and
prolongation of the BT resulting from high-dose clopidogrel gradually
dissipated over 6 days, after therapy was discontinued. Thus, high-dose
clopidogrel selectively and irreversibly abolished ADP-dependent
platelet activation within 3 hours (Tables 1 and 3).
The dose-response effects of clopidogrel for platelet accumulation
in graft and stent thrombosis are displayed in Figure 3 and documented quantitatively in Table 2. Although clopidogrel has a steep antithrombotic dose-response at
0.2 mg/kg, the dose-response relationship is relatively flat for 10-
to 100-fold increased dosing, indicating that the antithrombotic
effects of clopidogrel remained intermediate, despite large doses of
drug. By contrast, the BT was progressively prolonged to >30 minutes
by high-dose clopidogrel.
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The overall extent to which clopidogrel reduced thrombus formation was
substantially greater for stent thrombosis than for graft thrombosis
(Figure 3 and Table 2), implying that ADP-mediated platelet
recruitment was quantitatively more important for thrombus forming on
stents than for thrombus forming on segments of vascular graft.
The effects of clopidogrel on the formation of thrombus at sites
of arterial endarterectomy are shown in
Figure 4. Dosing clopidogrel at 0.2
mg · kg-1 ·
d-1 for 6 days substantially decreased
platelet and fibrin accumulation on segments of endarterectomized
aorta (P<0.001). Increasing the dose of clopidogrel to 5
mg/kg for 3 days abolished thrombus forming on endarterectomized aorta
(Figure 4; P<0.0001).
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Antithrombotic and Antihemostatic Effects of Combining Aspirin
and Clopidogrel
Combining aspirin 10 mg/kg and clopidogrel significantly
enhanced the reduction in platelet and fibrin deposition produced
within 3 hours by high-dose clopidogrel (20 mg/kg), as shown in Figure 5 and Table 2 for stent thrombosis
(P<0.01 in both cases) and in Figure 6 and Table 2 for graft thrombosis
(P=0.001 in both cases). Although platelet deposition
was significantly reduced by 2 mg/kg clopidogrel after 3 hours, fibrin
accumulation was not significantly decreased (Table 2;
P>0.07), although the BT remained maximally prolonged at
>30 minutes
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Single-dose aspirin therapy (10 mg/kg) also enhanced the reduction in
platelet and fibrin deposition after 6 days of low-dose clopidogrel
(0.2 mg · kg-1 ·
d-1) for stent and graft thrombosis (Figures 7 and 8;
Table 2; P<0.01 for all cases), with a BT of 16±11 minutes
(Table 2). Thus, by reduction of the chronic clopidogrel dosing to 0.2
mg · kg-1 ·
d-1, antithrombotic efficacy was retained while
the prolongation of template BTs was minimized (Figure 9). When oral aspirin 10 mg ·
kg-1 · d-1 was
administered concurrently throughout the 6-day period of low-dose
clopidogrel therapy (0.2 mg · kg-1
· d-1), antithrombotic effects were not
observed earlier or more intensely than the effects produced by adding
a single dose of aspirin after 6 days of low-dose clopidogrel therapy
(Figure 8). Reductions in platelet and fibrin accumulation remained
incomplete after 3 days of concurrent clopidogrel and aspirin therapy,
compared with the effects on day 6 for stent thrombosis (Table 3 and
Figure 7; P<0.05 in both cases) and graft thrombosis
(Figure 8; Tables 2 and 3; P<0.05 in all cases). ADP- and
collagen-induced platelet aggregation and ADP-induced P-selectin
and LIBS expression were fully abnormal after 3 days of concurrent 10
mg · kg-1 ·
d-1 aspirin and 0.2 mg ·
kg-1 · d-1
clopidogrel administration (Table 3).
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) or concurrently
throughout 6-day dosing (
) produces comparable reduction in
platelet deposition.
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Effects of Combining Heparin With Clopidogrel
Intravenous heparin therapy (100 IU/kg bolus and 100
IU/kg over 60 minutes) after 6 days of oral clopidogrel did not reduce
111In-platelet deposition on stents or
vascular graft thromboses (Table 2; P>0.1 in both cases).
Although 125I-fibrin accumulation was not reduced
on vascular grafts by the addition of heparin to clopidogrel
(P=0.15), 125I-fibrin accumulation was
decreased on stents by the combination of clopidogrel plus heparin
(P=0.0002). Importantly, the enhanced antithrombotic effects
produced by addition of either aspirin or heparin to clopidogrel were
not further augmented for vascular graft thrombosis and stent
thrombosis by addition of both aspirin and heparin to clopidogrel
(Table 2; P>0.2 in all cases).
The addition of heparin after 6 days of clopidogrel 0.2 mg ·
kg-1 · d-1 did not
detectably prolong the BT beyond that produced by clopidogrel alone
(Table 3; P>0.4). Similarly, platelet aggregation and
expression of P-selectin and LIBS induced by either collagen or TRAP
were not increased by combining heparin with clopidogrel
(P<0.05 in all cases).
Antithrombotic Benefits Versus Prolongation of Template
BTs
Antithrombotic efficacy was compared with changes in the BT by
relating 111In-platelet deposition on
segments of vascular graft (a measure of thrombus formation) and
template BT (generally a measure of overall platelet hemostatic
function). The minimal regimen producing maximal interruption of graft
thrombosis achievable by this therapy, ie, 0.2 mg ·
kg-1 · d-1
clopidogrel plus 10 mg/kg aspirin, prolonged the BT to 16±11 minutes
(Figure 9 and Table 2; P=0.0002 compared with baseline
control of 3.1±0.7 minutes and P=0.03 compared with
7.7±7.4 minutes for clopidogrel alone at 0.2 mg ·
kg-1 · d-1).
Interestingly, high-dose clopidogrel alone prolonged BT without
significantly decreasing 111In-platelet
deposition (Figure 9).
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Discussion |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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This study in baboons demonstrates that clopidogrel inhibition of ADP-mediated platelet recruitment produces irreversible, dose-dependent, intermediate reduction in thrombus forming on segments of prosthetic vascular grafts, endovascular metallic stents, and endarterectomized aorta within 3 hours of oral dosing at
2 mg/kg,
and cumulative for chronic daily dosing at 0.2 mg/kg, with
corresponding inhibitory effects on ex vivo platelet
activation induced by ADP and prolongation of template BT measurements.
Clopidogrel in combination with aspirin markedly inhibits
platelet-dependent thrombus formation while modestly impairing
platelet hemostatic plug-forming capability.
Under arterial flow conditions, platelet recruitment
into formation of thrombus proceeds via 3 independent pathways:
TxA2 generation, ADP secretion, and thrombin
production.32 In patients with
symptomatic atherosclerotic vascular disease, selective
irreversible inactivation of platelet TxA2
generation by aspirin reduces the risk of thrombo-occlusive episodes by
20% to 25%,1 and selective irreversible
inactivation of platelet ADP-receptor–mediated platelet
recruitment by clopidogrel is more effective than aspirin and at least
as safe.22 The antiplatelet effects of
clopidogrel are attributable to hepatically modified product(s)
that irreversibly inactivate platelet ADP
receptor–induced signaling in a dose-dependent manner, although no
metabolites of clopidogrel have yet been isolated that directly and
irreversibly inhibit platelet ADP receptor
function.10 11 17 33 34 The present study
underscores the selectivity of clopidogrel for abolishing ADP-mediated
platelet activation and its lack of significant
inhibitory effects on TRAP-induced platelet activation
(Table 1). The inhibition of collagen-induced platelet aggregation
by clopidogrel is explained by the contribution made by ADP secretion
from platelets activated by collagen (Table 1). The
inhibition of collagen-induced platelet aggregation by aspirin is
similarly ascribed to the production of
TxA2 by collagen-stimulated platelets.
The present study demonstrates that high-dose clopidogrel (2
mg/kg) produces immediate, irreversible inactivation of platelet
ADP-receptor signaling that dissipates over 6 days, the life span of
baboon platelets. One-hundredth that dose (0.2 mg ·
kg-1 · d-1)
produces cumulative, irreversible inhibition of platelet
ADP-receptor function that is complete after 5 to 6 days. Aspirin,
which irreversibly inactivates the generation of
TxA2 by platelets within 1 to 2 hours after
oral dosing, significantly enhances the antithrombotic effects of
clopidogrel. It follows that enhancement by aspirin of the
antithrombotic effects of clopidogrel represents additive,
concurrent, irreversible interruption of the both
TxA2 and ADP pathways of platelet
recruitment.
The thrombotic models used in the present study constitute 3 different thrombotic processes composing variable proportions of the 3 platelet recruitment pathways. It seems likely that the most favorable antithrombotic versus antihemostatic strategy for complex resistant thrombotic processes involves tailoring combinations of pathway-specific inhibitors appropriate for the specific thrombotic process. Various thrombotic processes in humans also appear to exhibit differences in their relative dependence on ADP, TxA2, and thrombin pathways of platelet recruitment. Now that mechanism-specific platelet inhibitors are available, it will be possible to define the relative contribution of each pathway to specific thrombogenic processes and select the most favorable antiplatelet therapy.
In extrapolation of these preclinical findings in nonhuman primates to
patients, doses of clopidogrel 1 mg/d plus aspirin 1 mg ·
kg-1 · d-1 are
predicted to produce additive antithrombotic benefits when evaluated in
controlled clinical trials, ie, 50% to 60% risk reduction in
vaso-occlusive episodes in patients with symptomatic
atherosclerotic vascular disease. These preclinical results also
suggest that an initial loading dose of clopidogrel will produce full
antithrombotic effects within 3 hours.
Clopidogrel and aspirin have highly favorable therapeutic ratios when
antithrombotic benefit is compared with hemorrhagic burden. By
contrast, safety may be limiting in the proposed long-term use of
platelet integrin
IIbß3 fibrinogen
receptor antagonists.35 The
present demonstration that aspirin enhances the antithrombotic
efficacy of clopidogrel with little hemostatic impairment strengthens
the rationale for performing controlled clinical trials comparing
antithrombotic and hemorrhagic effects of clopidogrel 75 mg/d plus
aspirin 325 mg/d versus aspirin 325 mg/d alone in patients at risk of
vascular thrombo-occlusion.
Received February 9, 1998; revision received June 5, 1998; accepted July 2, 1998.
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References |
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