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Difficulty: MediumPharmacokinetic and Pharmacodynamic Principles

A 61-year-old man comes to the emergency department complaining of sudden-onset, crushing substernal chest pain that radiates to his left jaw. The pain began 45 minutes ago while he was resting. His medical history is significant for hypertension, hyperlipidemia, and a myocardial infarction three months ago, for which he underwent placement of a drug-eluting stent in the left anterior descending artery. His medications include aspirin, clopidogrel, metoprolol, and atorvastatin. Two weeks ago, he was evaluated for persistent heartburn and was started on omeprazole. An electrocardiogram obtained upon arrival shows 3-mm ST-segment elevations in leads V1 through V4. Urgent coronary angiography reveals acute thrombosis of the previously placed stent. Which of the following mechanisms best explains the failure of this patient's antiplatelet therapy?

  1. Decreased biotransformation to an active metaboliteAnswer
  2. B
    Induction of hepatic cytochrome P450 enzymes
  3. C
    Competitive inhibition at the target platelet receptor
  4. D
    Reduced intestinal absorption secondary to increased gastric pH
  5. E
    Displacement from plasma protein binding sites

Answer

Decreased biotransformation to an active metabolite
Clopidogrel is a thienopyridine antiplatelet agent that irreversibly inhibits the P2Y12 ADP receptor on platelets. It is administered as a prodrug and requires hepatic biotransformation, primarily by the cytochrome P450 enzyme CYP2C19, to become an active metabolite. Omeprazole is a proton pump inhibitor that also acts as a potent competitive inhibitor of CYP2C19. When administered concurrently, omeprazole prevents the metabolic activation of clopidogrel. Consequently, the concentration of the active antiplatelet metabolite decreases, resulting in inadequate platelet inhibition and an increased risk of severe cardiovascular events, such as the acute stent thrombosis seen in this patient.

Step-by-Step Solution

1
Identify the critical drug-drug interaction in the patient's recent medication history.
The patient was recently started on omeprazole while taking clopidogrel.
Recognizing the timeline of new medications is essential for diagnosing adverse events or therapeutic failures.
2
Determine the pharmacokinetic properties of clopidogrel.
Clopidogrel is a prodrug that requires activation by the hepatic enzyme CYP2C19.
Understanding prodrug metabolism is necessary to predict how other drugs will affect their efficacy.
3
Evaluate the effect of omeprazole on CYP2C19.
Omeprazole is a potent competitive inhibitor of CYP2C19.
CYP450 inhibitors decrease the activity of the enzyme, altering the metabolism of substrate drugs.
4
Correlate the pharmacokinetic interaction with the clinical presentation.
Inhibition of CYP2C19 prevents clopidogrel from being converted to its active form, leading to inadequate platelet inhibition and subsequent stent thrombosis.
This explains the patient's acute ST-elevation myocardial infarction despite being prescribed dual antiplatelet therapy.

Key Concept

Clopidogrel is a prodrug requiring activation by CYP2C19; concomitant use of CYP2C19 inhibitors like omeprazole prevents the formation of the active metabolite, increasing the risk of adverse cardiovascular events such as stent thrombosis.
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Topics

Pharmacokinetic and Pharmacodynamic Principles

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