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Zorluk: OrtaPharmacokinetic and Pharmacodynamic Principles

A 55-year-old man comes to the clinic for a follow-up visit. He has a history of major depressive disorder and hypertension. His medications include paroxetine, which he has been taking for 2 years with excellent symptom control. Two weeks ago, his primary care physician started him on metoprolol succinate for poorly controlled blood pressure. Today, the patient reports severe fatigue, lightheadedness, and episodes of feeling "close to passing out" when standing up quickly. Vital signs show a blood pressure of 98/60 mm Hg and a pulse of 44/min. Physical examination is unremarkable except for bradycardia. An electrocardiogram shows sinus bradycardia with no other abnormalities. Which of the following pharmacokinetic or pharmacodynamic interactions best explains this patient's current presentation?

  1. Inhibition of a specific cytochrome P450 isoenzymeCevap
  2. B
    Synergistic pharmacodynamic effect at the receptor level
  3. C
    Induction of a specific cytochrome P450 isoenzyme
  4. D
    Displacement from plasma protein binding sites
  5. E
    Alteration of renal tubular secretion

Cevap

Inhibition of a specific cytochrome P450 isoenzyme
The patient is experiencing beta-blocker toxicity (bradycardia and hypotension) following the initiation of metoprolol. This occurred because he was already taking paroxetine, which is a potent inhibitor of the cytochrome P450 2D6 (CYP2D6) enzyme. Metoprolol relies heavily on CYP2D6 for its hepatic metabolism. The concurrent use of a CYP2D6 inhibitor decreases the clearance of metoprolol, leading to elevated plasma concentrations and an exaggerated pharmacological effect. Other SSRIs that strongly inhibit CYP2D6 include fluoxetine.

Adım Adım Çözüm

1
Identify the patient's new symptoms and vital signs.
The patient has symptomatic bradycardia (pulse 44/min) and hypotension (98/60 mm Hg).
Recognizing the clinical manifestation of beta-blocker toxicity is the first step in diagnosing the problem.
2
Review the patient's medication list for potential interactions.
The patient is concurrently taking paroxetine (an SSRI) and recently started metoprolol (a beta-blocker).
Identifying the specific drugs involved is necessary to determine the mechanism of the adverse event.
3
Determine the pharmacokinetic relationship between the two medications.
Paroxetine is a potent inhibitor of CYP2D6, the exact enzyme that extensively metabolizes metoprolol.
Understanding the specific metabolic pathways and inhibitory properties explains the elevated metoprolol levels causing the toxicity.

Anahtar Kavram

Paroxetine strongly inhibits CYP2D6, leading to decreased clearance and potential toxicity of drugs metabolized by this pathway, such as metoprolol.
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Pharmacokinetic and Pharmacodynamic Principles

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