Aspirin Resistance

Clinical Questions

How do we define aspirin resistance? Is this concept clinically relevant? Why aren’t we testing for it?

Aspirin Basics

Antithrombotic Trialists’ Collaboration 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

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

Adhesion: Platelet membrane receptors bind to endothelium and subendothelium at sites of damageActivation: Transmembrane signaling increases surface receptors, granule release, and exposure of membrane phospholipidAggregation: Procoagulant surface of the membrane serves as basis for coagulation cascade, amplification of platelet response, and production of fibrin.

Wide acceptance that platelet activity plays a major role in atherothrombosis

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-1Platelets – anucleate and unable to

regenerate COX-1• 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

Or….

Patient just doesn’twant 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 multi-factorial disease process

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

GP IIb-IIIa receptorPlatelet alloantigen 2 (PlA2)

Laboratory Definition

Bleeding time Aggregometry Platelet function analyzer (PFA-100) Platelet Release Products VerifyNow Aspirin Platelet Membrane Expression

Bleeding Time

Should not be used Low Sensitivity

Invasive Poorly reproducible Multiple Variables

• Platelet function• Platelet count• Plasma factors• Red Blood Cells• Skin trophism

Aggregometry

Light Transmission Aggregometry 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

time-consuming and user-dependent limited availability and requires rapid analysis lack of standardization

Aggregometry

Whole Blood Aggegrometry Measures electrical impedance between two

electrodes immersed in whole blood 5min after addition of a platelet agonist

Reports: impedance in Ω Easy, sensitive, reproducible

Large sample of blood required Long preparation time Expensive

Platelet Function Analyzer-100

Reports: closure time (<193s) No association with vascular events Point-of-Care test

Must test within 4hrs of blood collection Dependent on hematocrit and vWF

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.

Platelet Release Products

Serum TXA2

Urinary 11-dehydrothromboxane B2

Directly dependent on aspirin’s effect on COX-1 inhibition

Simple and correlated with clinical events 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.

Reports: Aspirin Reaction Units Clinical association with vascular events Point-of-care test

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

Platelet bound P-selectin Soluble plasma P-selectin Platelet-leukocyte aggregates Platelet-derived microparticles

Flow cytometry allows for the evaluation of platelet reactivity and in vivo activation. However, these techniques require sophisticated and expensive instruments.

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

201 patients with stable CAD on daily aspirin therapy (>80mg) Consecutive patients diagnosed with CAD Montreal, Canada Sample collection: morning urine and blood

Exclusions: 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

Patient Data

155 Male (71%) Mean Age: 66.5 ± 10.4 All with aspirin ≥ 1 month

80mg – 110 patients81mg – 10 patients162.5mg – 1 patient325mg – 79 patients1300mg – 1 patient

Results

κ Statistic Agreement0 - 0.2 Slight0.2 - 0.4 Fair0.4 - 0.6 Moderate0.6 - 0.8 Substantial

Prevalence of Resistance

Discussion

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.”

Perhaps it is more important to determine cut-off values with the intent of predicting clinical outcomes instead of focusing on a “gold standard” test.

Clinical Confounders

Drug Interactions: NSAIDs

may occupy nearby catalytic site

Block access of aspirin to serine 529

Ibuprofen, Indomethacin

PPIs? ACE Inhibitors?

Comorbid Conditions Isoprostanes

prostaglandin F2-like compounds

Produced from arachidonic acid in a non-COX process

Amplify platelet response Smoking Diabetes Hyperlipidemia, Unstable Angina

Variable Aspirin Response

Most studies have not evaluated aspirin resistance over time

Cohort study out of the University of Chicago published in 1994 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

Schwartz, et al. studied tested 190 patients using LTA and found 9% resistance Repeat testing after directly observed therapy

in all 9% Nonresponders responders

Tantry, et al. studied 223 patients with CHD All but 7 patients had aspirin response via LTA Those 7 patients were interviewed again and

admitted to noncompliance

No laboratory testing to monitor for clinical adherence

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

“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 Airee, et al. Aspirin resistance: disparities and clinical

implications. Pharmacotherapy 2008; 28(9): 999-1018. Antithrombotic Trialists Collaboration. Collaborative meta-

analysis 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 aspirin-resistance 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.