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Plavix Response Testing

Platelet activation and aggregation play a pivotal role in the pathogenesis of cardiovascular disease. Inhibition of platelet function has become a primary therapeutic goal. Several large clinical trials have demonstrated improved risk reduction by dual antiplatelet therapy with aspirin and clopidogrel (Plavix).  The latter is a thienopyridine that irreversibly blocks binding of ADP to the platelet P2Y12 receptor. The degree of platelet inhibition by clopidogrel varies from patient to patient in a normal or bell-shaped distribution. Between 4 to 30 percent of patients treated with clopidogrel do not have an adequate antiplatelet response as measured by platelet aggregation. Between 5 and 10% of patients treated with dual antiplatelet therapy still have thrombosis after stent implantation.

Five factors may contribute to this variable response; patient noncompliance, malabsorption, drug to drug interactions, platelet ADP receptor heterogeneity, and differences in the rate of drug metabolism. The last factor is particularly important for clopidogrel because it is a prodrug that is converted in the liver by the cytochrome P450 enzyme system to its active thiol metabolite. A single nucleotide polymorphism in the gene encoding the CYP2C19 enzyme is associated with decreased clopidogrel metabolism. Patients with one or more copies of the variant CYP2C19*2 and/or CYP2C19*3 alleles have reduced conversion of clopidogrel to its active thiol metabolite.

Allele Frequencies of CYP2C19 Variants

 

CYP2C19*2 is the most common variant associated with reduced metabolism in all ethnic groups. Patients of Chinese ancestry have a higher incidence of both the CYP2C19*2 and CYP2C19*3 alleles compared to Caucasian or African-American patients.  

Lower active metabolite exposure may result in reduced platelet inhibition and, thus, a higher rate of cardiovascular events following myocardial infarction or percutaneous coronary intervention (NEJM 2009;360:1-9). Because of this risk, the FDA added a black box warning to clopidogrel, which informs physicians that tests are available to determine an individual's CYP2C19 genotype and that this knowledge can be used to determine the best therapeutic strategy.

Although the black box warning specifically mentions genotyping, this type of testing is too laborious and expensive to perform on a routine basis. A reliable alternative is to measure platelet response to clopidogrel using the VerifyNow PRUTest. This system consists of an instrument and a disposable assay cartridge.

Patient whole blood is added to a single-use cartridge that contains fibrinogen-coated polystyrene beads and a standard dose of the platelet agonist ADP. ADP-activated patient platelets bind to the fibrinogen-coated beads, which then promotes platelet aggregation. The instrument uses an optical detection system to measure the increase in light transmittance that accompanies platelet aggregation. The rate of aggregation is proportional to the number of activated P2Y12 receptors.

The number of activated P2Y12 receptors is reported in P2Y12 Reaction Units (PRU), which is the amount of residual P2Y12 receptor mediated aggregation. The reference range is 180 to 335 PRU, which was determined in patients not taking ADP receptor antagonist drugs. Platelet inhibition by clopidogrel results in a lower PRU. Responders usually have PRU <180. PRU values >180 are often referred to as high on-treatment platelet reactivity (HPR) and have been associated with an increased risk of ischemic events after PCI including death, MI and stent thrombosis. These patients may benefit from either a higher dose of clopidogrel or alternative antiplatelet therapy such as prasugrel, ticagrelor or cangrelor. In contrast to patients with HPR, patients with low platelet reactivity (PRU <180) have a higher risk of bleeding, especially after PCI.

Inhibition of platelet aggregation can usually be detected 2 hours after a 300 to 600 mg loading dose of clopidogrel. Peak effect is usually detected 6 hours after treatment. Platelets blocked by clopidrogrel are affected for the remainder of their lifespan, which is 7 to10 days.

Preoperative management of patients who are taking Plavix and require an invasive procedure or surgery can be problematic. Operating on patients who are taking Plavix is associated with excessive bleeding but discontinuing Plavix therapy prior to surgery has been associated with a 20% incidence of ischemic events.

At least 3 clinical practice guidelines provide direction regarding the timing of surgery in patients receiving Plavix. Each of these guidelines recommends discontinuing Plavix for at least 3 to 7 days prior to surgery to allow platelet function to return to normal and decrease the risk of bleeding.

• The American College of Chest Physicians Evidence Based Clinical Practice Guidelines from 2008 state that platelet function returns to normal 7 days after the last dose of clopidogrel (Chest 2008;133:71S-105S, specifically page 217S)

• The Society of Thoracic Surgeons Clinical Practice Guidelines, which were updated in 2011, state that previous reports recommended a 5 to 7 day delay after discontinuation of clopidogrel in patients requiring coronary artery bypass graft surgery to lessen bleeding (Ann Thorac Surg 2011;91:944-82, specifically page 951).

• In between these two sets of guidelines is an update from May 2011 by the American College of Cardiology Foundation/American Heart Association for management of patients with unstable angina/non-ST elevation myocardial infarction. This document recommended withdrawing clopidogrel for at least 5 days prior to coronary artery bypass graft surgery (JACC 2011;57:1920-59).

All of these recommendations are based on the pharmacokinetics of clopidogrel and do not take into account individualized response to drugs. Approximately 40% of patients have suboptimal antiplatelet response to clopidogrel. Patients who are hyporesponders or nonresponders would be expected to normalize platelet function even sooner than 5 to 7 days after discontinuing clopidogrel. An individual’s response to clopidogrel can only be determined by performing platelet function testing. In our experience more than 50% of patients have <30% platelet inhibition within 3 days after discontinuing clopidogrel.

This test has been named the Platelet P2Y12 Response Assay to avoid confusion with other tests such as the platelet function assay (PFA-100) and platelet aggregation. Also, the use of this test is not limited to clopidogrel. It can be used to measure the effect of other P2Y12 drugs, such as prasugrel (Effient), ticagrelor (Brilanta, Brilique) and cangrelor (Kengreal).

Blood must be collected in a 2 mL Greiner Bio-One light blue capped vacuette tube containing 3.2% sodium citrate using a 21 gauge or larger needle. Two tubes must be collected. The first tube should be discarded and the second tube sent to the laboratory for testing. The tube should be gently inverted 5 times to avoid clotting. If a CBC is ordered at the same time, it should be collected last to avoid contamination with EDTA. Specimens that are clotted, hemolyzed, collected in the wrong tube, incompletely filled or centrifuged are unacceptable for testing. Specimens must be transported at ambient temperature and tested within 4 hours after collection.

This test should not be ordered on patients with platelet counts below 100,000/uL or treated with glycoprotein IIb/IIIa inhibitors until platelet glycoprotein IIb/IIIa function has recovered. The recovery period after the drug is discontinued is 14 days for abciximab (ReoPro), and 48 hours for eptifibatide (Integrillin) and tirofiban (Aggrastat).

References

Orme R. et al. Monitoring antiplatelet therapy. Semin Thromb Hemost 2017;43:311-9.

Michelson AD, Bhatt DL.  How I use laboratory monitoring of antiplatelet therapy.  Blood. 2017; 130(6): 713-721.