Measurement of Clopidogrel Resistance Measurement of Clopidogrel Resistance

by ADP-Inhibition Does Not Reflect the by ADP-Inhibition Does Not Reflect the Benefit of Clopidogrel on Overall Benefit of Clopidogrel on Overall

Thrombotic StatusThrombotic StatusDr Diana A Gorog

Consultant Cardiologist & Honorary Clinical Senior Lecturer

E & N Hertfordshire NHS Trust &Imperial College, London, UK

Presented at

“Clopidogrel resistance” Definition

clinical ongoing thrombotic events despite medication laboratory results of platelet function tests

Does it exist? Clinical resistance infrequent (5-10%) Laboratory resistance frequent (up to 50%)

Relationship between clinical and lab resistance is unclear, if it exists at all

Is “laboratory resistance” clinically relevant? unsure, some evidence that it is only a laboratory

phenomenon

Presented at

Limitations of laboratory assessment of clopidogrel

resistance Most tests use citrated blood Platelet aggregation studies assess response to a specific

agonist or combination of agonists, e.g. PFA 100: Collagen/ADP VerifyNow: ADP/EpinephrineIn addition to physiological doses of ADP used, there is also

ADP release from aggregating plts (only in citrated blood) so overall level can be an order of magnitude higher

Clopidogrel effective at inhibiting low conc ADP but not high conc

Overall effect of clopidogrel on global thrombotic status unclear, as the most important contributor to platelet aggregation, thrombin generation, not assessed

Correlation between tests very poor

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Effect of ADP in native bloodADP 5-20 μM aggregation

[Ca2+] 1.0-1.3 mMThrombin generation(only in native blood)

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activation stabilization

Effect of ADP in citrated bloodADP 5-20 uM

aggregation

[Ca2+]10 – 30 uM

Thrombin generation

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ADPADPADP

ADP

ADP

up to mM

Platelets in native blood are more sensitive to inhibition of ADP-induced aggregation than platelets in citrated blood (Viigimaa M et. al.,1996)

Citraterelease of granule contents

Only in citrated blood

Limitations of platelet function tests

in measuring clopidogrel effect Measuring the effect of clopidogrel on ADP-

induced aggregation in citrated blood does not take into account inhibition of ADP-induced thrombin generation

Because of the individually variable release rxn, actual ADP concentration is much higher (mM?) than that used to induce aggregation (5-20uM).

Clopidogrel is less effective at inhibiting high conc of ADP than low concentration

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“Ideal” platelet function test for clopidogrel effect

test involves thrombin generation employs non-citrated blood assesses response to endogenous

ADP concentration

Global Thrombosis Test (GTT)

Native (non-anticoagulated) blood

Mechanism involves high shear, thrombin and ADP

“Global” test of platelet function

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Normal volunteers before and 12h after 300 mg clopidogrel

0

100

200

300

400

500

Pre[C] Post [C]

OT

(sec

)

P=0.001

Effect of clopidogrel on occlusion time (OT)

0

100

200

300

400

500

Pre[C] Post [C]

GTT – modified to cause ADP release

Wanted to ↑ actual ADP concentration (to assess specific ADP inhibitory effect of clopidogrel) but still using native blood

Small volume water (0.3 ml) added to cartridge prior to testing at the site where thrombus formation occurs

This caused lysis of red cells and thus ADP release, manifesting in more rapid occlusion (aOT)

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0

100

200

300

400

OT aOT

P=.0007

Normal vs. Accelerated Occlusion Time (aOT)

0

100

200

300

400

500

OT aOT

Pre [C]Post [C]

Normal vs. Accelerated Occlusion Time (aOT)

Acute coronary syndrome study

ACS patients (n=86)On aspirin and clopidogrel >48 h Platelet function and “clopidogrel

resistance” tested usingGTT and aOTVerifyNow P2Y12 cartridge

Accelerated Thrombosis Test • N=60• Criteria OT<150 sec (i.e. <50% of normal OT)• Non-responders 4 (6.7%)

VerifyNow P2Y12 assay• N=86• Criteria <10% inhibition: 19.8% non-responders• Criteria <30% inhibition: 32.6% non-responders• Criteria <50% inhibition: 46.5% non-responders

ACS patients on dual anti-platelet therapy

Conclusion Measurement of clopidogrel resistance by ADP-

inhibition in citrated blood does not reflect the benefit of clopidogrel on overall thrombotic status

Detection of clopidogrel “resistance” using citrated blood is likely to be misleading

When assessing efficacy of clopidogrel, should use a global test of platelet function, using native blood, where thrombin makes a significant contribution to the thrombus formation.

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