Aspirin Resistance
Clinical Questions
How do we define aspirin resistance?
 Is this concept clinically relevant?
 Why aren’t we testing for it?
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Aspirin Basics
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Antithrombotic Trialists’ Collaboration
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Major meta-analysis reporting 25% reduction in
significant vascular events as secondary prevention
Prevents at least 10-20 fatal and nonfatal vascular
events for every 1000 patients treated for one year.
American Heart Association recommends
prophylaxis for: 10yr risk of CHD >10%
Pharmacology
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Consistent absorption via passive diffusion in GI
tract with peak plasma levels in 30min
Enteric coated peak in 3-4 hours with variable
bioavailability
Platelet Basics
Wide acceptance that
platelet activity plays a major
role in atherothrombosis
Adhesion: Platelet membrane
receptors bind to endothelium
and subendothelium at sites of
damage
Activation: Transmembrane
signaling increases surface
receptors, granule release, and
exposure of membrane
phospholipid
Aggregation: Procoagulant
surface of the membrane serves
as basis for coagulation
cascade, amplification of platelet
response, and production of
fibrin.
Aspirin Inhibition of Platelets
Aspirin inhibits platelet cyclooxygenase
(COX)-1 thereby preventing formation of
thromboxane A2 (TxA2), a potent
aggregating and vasocontrictor agent.
Acetylation serine 529 of COX-1
 Platelets – anucleate and unable to
regenerate COX-1
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• Platelet lifespan 7-10 days
• Maturation time from megakaryocytes: 4 days
NSAID Mechanism
Definition of
“Aspirin Resistance”
Clinical failure of prevention
 Biochemical resistance
 Laboratory Phenomenon
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Or….
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Patient just doesn’t
want to take it.
Clinical “Aspirin Resistance”
Perhaps more appropriate term would
be “treatment failure”
 Probabilities Game
 Failure is to be expected in the
treatment/prevention of any multifactorial disease process
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Theories of Biochemical
Resistance
Endothelial cells recover COX-1 activity
shortly after aspirin exposure
 Role of COX-2 in thrombotic process
 Thromboxane-independent methods of
platelet activation
 Genetic polymorphisms
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GP IIb-IIIa receptor
 Platelet alloantigen 2 (PlA2)
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Laboratory Definition
Bleeding time
 Aggregometry
 Platelet function analyzer (PFA-100)
 Platelet Release Products
 VerifyNow Aspirin
 Platelet Membrane Expression
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Bleeding Time
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Should not be used
Low Sensitivity
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Invasive
Poorly reproducible
Multiple Variables
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Platelet function
Platelet count
Plasma factors
Red Blood Cells
Skin trophism
Aggregometry
Light Transmission Aggregometry
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Measures light transmission through a platelet
suspension in response to agonist
(arachodonic acid or ADP)
Reports: % residual platelet function
Clinical association with vascular events
Historic Reference Standard
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time-consuming and user-dependent
limited availability and requires rapid analysis
lack of standardization
Aggregometry
Whole Blood Aggegrometry
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Measures electrical impedance between two
electrodes immersed in whole blood 5min
after addition of a platelet agonist
Reports: impedance in Ω
Easy, sensitive, reproducible
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Large sample of blood required
Long preparation time
Expensive
Platelet Function Analyzer-100
Anticoagulated blood is aspirated through a
capillary and a 150μm aperture coated with
collagen and ADP or epinephrine.
Measures time needed until blood flow
interruption is recorded.
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Reports: closure time (<193s)
No association with vascular events
Point-of-Care test
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Must test within 4hrs of blood collection
Dependent on hematocrit and vWF
Platelet Release Products
Serum TXA2
Urinary 11-dehydrothromboxane B2
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Directly dependent on aspirin’s effect on
COX-1 inhibition
Simple and correlated with clinical events
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Indirect measurement
Not specific to platelets
Urinary test dependent on renal function
VerfiyNow Aspirin
Turbidometric-based optical system that
measures agglutination of fibrinogen-coated
beads by platelets stimulated in citrated whole
blood.
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Reports: Aspirin Reaction Units
Clinical association with vascular events
Point-of-care test
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Major limitation relates to the diagnostic criteria
set for definition of aspirin resistance
Compared to optical aggregometry with
epinephrine after one 325mg dose of aspirin
Platelet Membrane
Expression
Flow cytometry allows for the evaluation of
platelet reactivity and in vivo activation.
However, these techniques require
sophisticated and expensive instruments.
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Platelet bound P-selectin
Soluble plasma P-selectin
Platelet-leukocyte aggregates
Platelet-derived microparticles
A comparison of six major platelet
function tests to determine the
prevalence of aspirin resistance in
patients with stable coronary artery
disease.
Marie Lordkipanidze, Chantal Pharand, Erick
Shampaert, Jacques Turgeon, Donald Palisaitis, and
Jean G. Diodati.
European Heart Journal in June 2007
Study Design
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201 patients with stable CAD on daily aspirin
therapy (>80mg)
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Consecutive patients diagnosed with CAD
Montreal, Canada
Sample collection: morning urine and blood
Exclusions:
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ACS or revascularization within 6 months
Concurrent use of NSAIDs, clopidogrel,
dipyridamole, warfarin, acenocoumarol
Major surgery within 1 month of enrollment
Thrombocytopenia, Hct <25%
Dialysis
Platelet Function Tests
LTA with arachidonic acid
 LTA after adenosine diphosphate
 Whole blood aggregometry
 PFA-100
 VerifyNow Aspirin
 Urinary 11-dehydrothrombaxane B2
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Patient Data
155 Male (71%)
 Mean Age: 66.5 ± 10.4
 All with aspirin ≥ 1 month
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80mg – 110 patients
 81mg – 10 patients
 162.5mg – 1 patient
 325mg – 79 patients
 1300mg – 1 patient
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Results
κ Statistic
0 - 0.2
0.2 - 0.4
0.4 - 0.6
0.6 - 0.8
Agreement
Slight
Fair
Moderate
Substantial
Prevalence of Resistance
Discussion
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Overall, poor correlation between different
platelet function tests
“Aspirin resistance” based on arbitrary,
clinically non-validated cut-off values.
“We believe the non-standardized use of these
assays and the absence of a formal definition
explains much of the disparity reported in the
literature in regards to the prevalance of aspirin
resistance.”
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Perhaps it is more important to determine cutoff values with the intent of predicting clinical
outcomes instead of focusing on a “gold
standard” test.
Clinical Confounders
Drug Interactions:
 NSAIDs
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Comorbid Conditions
 Isoprostanes
may occupy nearby
catalytic site
Block access of
aspirin to serine 529
Ibuprofen,
Indomethacin
PPIs?
ACE Inhibitors?
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prostaglandin F2-like
compounds
Produced from
arachidonic acid in a nonCOX process
Amplify platelet response
Smoking
Diabetes
Hyperlipidemia,
Unstable Angina
Variable Aspirin Response
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Most studies have not evaluated aspirin
resistance over time
Cohort study out of the University of
Chicago published in 1994
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Secondary prevention in 306 patients with
history of ischemic stroke
Repeated assessments of aspirin resistance
over a 6 month interval
Light Transmission Aggregometry
32.7% of patients did not maintain complete
inhibition with repeated testing
Reassessment of fixed-dose regimens?
Patient Compliance
No laboratory testing to monitor
for clinical adherence
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Schwartz, et al. studied tested 190 patients
using LTA and found 9% resistance
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Repeat testing after directly observed therapy
in all 9%
Nonresponders  responders
Tantry, et al. studied 223 patients with CHD
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All but 7 patients had aspirin response via LTA
Those 7 patients were interviewed again and
admitted to noncompliance
Working Group on Aspirin
Resistance in 2005
“Other than in research trials, it is not
currently appropriate to test for aspirin
‘resistance’ in patients or change
therapy based on these tests.”
Summary
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“Aspirin Resistance,” from biochemical and lab
perspective, probably has clinical relevance
There continues to be significant investigation and
discovery regarding aspirin therapy
Clinical application is on the distant horizon.
 Consider alternatives to NSAIDs
 Focus on patient compliance
 seek to dispel myths
 assess for and control side effects.
References
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Airee, et al. Aspirin resistance: disparities and clinical
implications. Pharmacotherapy 2008; 28(9): 999-1018.
Antithrombotic Trialists Collaboration. Collaborative metaanalysis of randomised trials of antiplatelet therapy for
prevention of death, myocardial infarction, and stroke in high
risk patients. BMJ 2002; 324: 71-86.
Cattaneo, M. Resistanc to antiplatelet drugs: molecular
mechanisms and laboratory detection. J Thromb Haemost
2007; 5 (Suppl 1): 230-7.
Christiaens, et al. Major clinical vascular events and aspirinresistance status determined by the PFA-100 method among
patients with stable coronary artery disease: a prospective
study. Blood Coag and Fib 2008. 19(3): 235-9.
Gasparyan, et al. The role of aspirin in cardiovascular
prevention. J Am Coll Cardiol 2008. 51; 19: 1829-43.
Gum, et al. A prospective, blinded determination of the natural
history of aspirin resistance among stable patients with
cardiovascular disease. J Am Coll Cardiol 2003; 41(6): 961-5.
Helgason, et al. Development of aspirin resistance in persons
with previous ischemic stroke. Stroke 1994. 25: 2331-2336
Lordkipanidze, et al. A comparison of six major platelet
function tests to determine the prevalence of aspirin resistance
in patients with stable coronary artery disease. Euro Heart J
2007; 28(14): 1673-5.
MacDonald, et al. Effect of ibuprofen on the cardioprotective
effect of aspirin. Lancet 2003. 15; 361(9357): 573-4.
Michelson, et al. Aspirin resistance: position paper of the
Working Group on Aspirin Resistance. J Thromb Haemost
2005; 3: 1309-11.
Patrono, et al. Low-dose aspirin for prevention of
atherothrombosis. N Engl J Med 2005. 353; 22: 2373-83.
Sanderson, et al. Narrative review: aspirin resistance and its
clinical implications. Annals of Int Med 2005. 142; 5: 370-80.
Viola et al. Aspirin resistance: is this term meaningful? Curr
Opin Hematol 2006. 13: 331-336.