Abstract

1
Aspirin Resistance andPoint-of-Care Testing
Daniel I. Simon, MD Associate Director,
Interventional Cardiology Brigham and Womens
Hospital Associate Professor of Medicine Harvard
Medical School Boston, MA
2
ASA Resistance/Nonresponsiveness

• ASA use associated with 23 reduction in the odds
  of vascular events
• Beneficial antithrombotic effect of ASA mediated
  by irreversible acetylation of COX-1
• Prevalence ASA resistance 5-60
• Mechanism of ASA resistance multi-factorial
• ASA resistance associated with adverse clinical
  outcomes

Bhatt DL. J Amer Coll Cardiol 2004
Antithrombotic Trialists Collaboration. BMJ
200232471. Gum et al. Am J Cardiol
200188230. Eikelboom et al. Circulation
20021051650. Eikelboom et al. J Amer Coll
Cardiol 200341966. Gum et al. J Amer Coll
Cardiol 200341961. Wang et al. Amer J Cardiol
2003921492.
3
ASA Resistance and Clinical Outcome in CAD
Patients
HOPE Trial nested case-control substudy (n976),
ASA 75-325 mg, F/U 5 years
Eikelboom et al. Circulation 2002 1051650.
4
ASA Resistance and Clinical Outcome in CVD
Patients
326 CVD patients on ASA 325 mg gt 7 days
P 0.03
ASA-R mean aggregation 70 with 10 µM ADP and
20 with 0.5 mg/ml AA
Gum et al. J Am Coll Cardiol 2003 41961.
5
Platelet Function Assays

• Antiplatelet agents are detected by their ability
  to impair platelet function
• Platelet function tests directly measure a
  physiological response of platelets in response
  to agonist (eg, aggregation) or a biochemical
  marker of this (eg, TXB2 production)

6
Why measure platelet inhibition in clinical
settings?

• Platelet inhibition central to efficacy, but not
  routinely measured
• Patient-to-patient variability with fixed dosing
• Pharmacodynamic variability between agents
• Correlation of measured inhibition with clinical
  efficacy

7
Thrombosis Platelet activation and aggregation
White HD. Am J Cardiol 1997
8
Platelet Function Tests in CV Disease
AD Michelson. Circulation 2004110e489.
9
The Gold Standard Light Transmission
Aggregometry

• Citrate or PPACK anticoagulated whole blood
• Platelet-rich plasma
• Agonist
• ADP, TRAP, collagen, arachidonic acid

eptifibatide
ADP pre-bolus
TRAP pre
ADP post
TRAP post
80 target
EPIC Investigators. N Engl J Med 1994 B Coller.
Circulation 1998
10
Problems with aggregometry

• Measure rate or extent?
• Maximum aggregation or fixed time point?
• Reversible aggregation
• Adjust platelet count?
• Anticoagulant?
• Not practical for cardiac cath lab, office

11
Accumetrics VerifyNow
VerifyNow Aspirin VerifyNow IIb/IIIa
VerifyNow P2Y12
Light measurement
12
Platelet Inhibition and MACE
16
14.4
55 ? P0.006
12
8
MACE
6.4
4
0
lt 95 N125
gt 95 N344
Platelet Inhibition at 10 Minutes
Steinhubl et al. Circulation 2001
gt95 Inhibition at 10 min RR0.44 (0.22-0.87)
P0.019
13
ASA Resistance in PCI
VerifyNow ASA, ASA/clopidogrel (n151), 19.2 ASA
resistant

• ASA resistant
• ? ASA sensitive

Chen et al. J Amer Coll Cardiol 2004431122.
14
Dual Resistance and PCI
VerifyNow ASA, ASA/clopidogrel (n150),
bivalirudin, non-urgent PCI
Aspirin Resistant
Clopidogrel Resistant
n36
n23
n16
n21
Defined as baseline aggregation minus
post-treatment aggregation 10 in response to 5
and 20 µM ADP
5µM ADP agg 70 and 1.5µM AA agg20
VerifyNow ASA score 550
At least 2 of 3 criteria
Lev et al. J Amer Coll Cardiol 20064727.
15
Post-PCI Thrombotic Complications
Lev et al. J Amer Coll Cardiol 20064727.
16
ASA-R and Long-term CV Events
VerifyNow ASA, ASA/clopidogrel (n464), 26.9 ASA
resistant
Endpoint CV Death MI CVA/TIA Hosp UA
Chen et al. AACC 2005
17
Adjusted hazard ratios of CV death, MI, CVA, TIA,
and hospitalization for unstable angina according
to baseline characteristics
Chen et al. AACC 2005
18
PFA-100

• Platelet function analyser-100 A point of
  care assay simulating bleeding time ex vivo
• Whole blood is drawn at high shear through a
  capillary and forced through a hole in a membrane
• Membrane is coated in collagen-ADP or
  collagen-epinephrine
• Closure time is time to occlusion of pinhole by
  platelet thrombus.

19
Lack of Correlation of PFA-100 With Clinical
Outcomes
Death, MI, CVA
Death, MI, CVA
20
P0.4
15
Clinical outcomes based on
15.1
PFA-100 results
12.9
10
5
0
800
800
ASA
ASA nonresponder
responder
N294
N32
Gum et al. J Amer Coll Cardiol 200341961.
20
Computerized Thromboelastography

• TEG relies on the measurement of clot strength
  to enable a quantitative analysis of platelet
  function.
• Reptilase and factor XIIIa (activator F) are
  used to generate a cross-linked fibrin clot to
  isolate the fibrin contribution to the clot
  strength. The contribution of P2Y12 receptor or
  COX pathways to the clot formation can be
  measured by the addition of an appropriate
  agonist, ADP, or AA.

21
TEG and Clinical Outcomes
PREPARE POST-STENTING Study. Follow-up duration 6
months post-PCI. ASA/clopidogrel (n192)
Gurbel et al. J Amer Coll Cardiol 2005461820.
22

• Activation of platelets induces aggregate
  formation.
• This causes a reduction in platelet count,
  allowing for calculation of aggregation.
• Uncertain utility for ASA resistance.
• Limited data linking to clinical outcomes.

23
AspirinWorks

• Determines in vivo platelet activation by
  measuring levels of 11-dehydrothromboxane B2
  (TXB2), a metabolite of thromboxane A2, in a
  random urine specimen
• The urine sample is assayed for TXB2 and
  creatinine levels of TXB2 are normalized to
  urine creatinine concentration
• Sample sent to central lab, turnaround time is
  one week
• Not currently FDA-approved

24
Limitations

• The number of patients studied in all these
  reports is small.
• The study designs are not adequate for
  controlling confounding variables.
• The definition of antiplatelet resistance is not
  uniform. In aspirin resistance studies, dosage
  varied and treatment compliance was not verified.
• Widespread clinical application of antiplatelet
  resistance will require additional studies on
  larger populations that define antiplatelet
  resistance in a standardized manner using assays
  with consistency and reproducibility, that
  correlate the measurements with clinical
  outcomes, and that provide strategies for
  modifying antiplatelet regimens to improve
  outcome (eg, increasing dose of antiplatelet
  agent, adding or substituting second antiplatelet
  agent).

25
Randomized Trial in ASA Resistance

• ASCET ASpirin nonresponsiveness and Clopidogrel
  Endpoint Trial
• Recruiting stable patients with angiographically
  documented CAD to evaluate whether switching to
  clopidogrel will be superior to continued aspirin
  therapy in improving clinical outcomes among
  aspirin-resistant (PFA-100) patients.
• RESISTOR Research Evaluation to Study
  Individuals who Show Thromboxane Or P2Y12
  Receptor Resistance
• Primary objective is to determine if the use of
  eptifibatide is associated with a significant
  difference in post-PCI myonecrosis (CK-MB ratio
  2 times ULN) within 24h of low-medium risk PCI in
  patients who are aspirin nonresponsive as
  determined by VerifyNow Aspirin testing.

26
Low to medium risk PCI Patient receives ASA (at
least 81 mg) 4h prior to screening
1st VerifyNow Assay for ASA testing
Aspirin sensitive
Aspirin nonresponder
Usual care PCI
Patients given 300-600 mg of clopidogrel at least
2h prior to PCI
2nd VerifyNow Assay for clopidogrel testing
RANDOMIZATION
(Interactive Voice Randomization System) Occurs
prior to crossing target lesion with guide
wire stratified on the basis of clopidogrel
responsiveness test
Eptifibatide plus UFH (N300)
UFH alone (N300)