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Difficulty: MediumGenetic and Molecular Mechanisms

A 64-year-old man is evaluated in the emergency department for recurrent, crushing chest pain. Two weeks ago, he underwent percutaneous coronary intervention with the placement of a drug-eluting stent in the left anterior descending artery following an acute myocardial infarction. He was subsequently discharged on a dual antiplatelet regimen of aspirin and clopidogrel. An emergency coronary angiogram today reveals acute in-stent thrombosis. Genotyping of the patient's cytochrome P450 enzymes reveals a homozygous loss-of-function mutation in the CYP2C19 gene, classifying him as a poor metabolizer. Which of the following best describes the molecular mechanism underlying this patient's current presentation?

  1. Failure to enzymatically convert an administered prodrug into its active therapeutic metaboliteAnswer
  2. B
    Impaired systemic elimination of the active medication leading to ineffective plasma concentrations
  3. C
    Accelerated presystemic extraction leading to severely decreased bioavailability of the parent drug
  4. D
    Spontaneous conformational change in the target platelet receptor preventing effective binding
  5. E
    Upregulation of cellular efflux pumps preventing adequate intracellular accumulation of the agent

Answer

Failure to enzymatically convert an administered prodrug into its active therapeutic metabolite
The correct answer accurately identifies the molecular basis of this patient's treatment failure. Clopidogrel is a thienopyridine prodrug that has no intrinsic antiplatelet activity. It must undergo hepatic biotransformation, primarily via the cytochrome P450 isoenzyme CYP2C19, to generate its active thiol metabolite. This active metabolite irreversibly binds to the P2Y12 ADP receptor on platelets, inhibiting aggregation. Patients who are homozygous for loss-of-function alleles in the CYP2C19 gene are classified as poor metabolizers. Consequently, they are unable to convert clopidogrel into its active form at sufficient rates, leaving them vulnerable to life-threatening cardiovascular events such as acute in-stent thrombosis despite medication compliance.

Step-by-Step Solution

1
Analyze the patient's clinical outcome and genetic testing data.
The patient developed in-stent thrombosis while taking clopidogrel and has a genetic test showing he is a CYP2C19 poor metabolizer.
Establishing the link between the patient's genetic profile and the failure of his antiplatelet therapy is essential for diagnosis.
2
Identify the pharmacogenomic relationship between clopidogrel and CYP2C19.
Clopidogrel is an inactive prodrug that requires biotransformation by hepatic CYP2C19 to become an active thiol metabolite that inhibits P2Y12 receptors.
Understanding the unique pharmacokinetic properties of the drug explains why impaired metabolism leads to therapeutic failure rather than toxicity.

Key Concept

CYP2C19 polymorphisms and clopidogrel metabolism
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Topics

Genetic and Molecular Mechanisms

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