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Zorluk: OrtaPharmacokinetic and Pharmacodynamic Principles

A 62-year-old man comes to the emergency department complaining of severe, diffuse muscle pain and dark urine for the past 2 days. He has a history of hypertension, hyperlipidemia, and type 2 diabetes mellitus. His daily medications include lisinopril, amlodipine, metformin, and atorvastatin. Ten days ago, he was evaluated at an urgent care clinic for community-acquired pneumonia and was prescribed a 7-day course of clarithromycin, which he completed 3 days ago. His temperature is 37.2°C (99.0°F), blood pressure is 134/82 mm Hg, and pulse is 78/min. Physical examination shows diffuse muscle tenderness on palpation. Laboratory studies show a serum creatine kinase level of 8,500 U/L and an elevated serum creatinine of 1.9 mg/dL (baseline 1.0 mg/dL). Urinalysis is positive for blood but shows no red blood cells on microscopy. Which of the following mechanisms best explains this patient's current presentation?

  1. Inhibition of hepatic enzyme metabolismCevap
  2. B
    Displacement of medication from plasma proteins
  3. C
    Induction of hepatic enzyme metabolism
  4. D
    Decreased renal tubular secretion
  5. E
    Pharmacodynamic synergism at the tissue level

Cevap

Inhibition of hepatic enzyme metabolism
The correct mechanism is the inhibition of hepatic enzyme metabolism. The patient is presenting with classic signs of rhabdomyolysis (muscle pain, markedly elevated CK, and myoglobinuria causing dark urine). This adverse event was triggered by the recent addition of clarithromycin to his chronic atorvastatin therapy. Atorvastatin is heavily metabolized by the hepatic cytochrome P450 3A4 (CYP3A4) isoenzyme. Clarithromycin (along with erythromycin, but not azithromycin) is a potent inhibitor of CYP3A4. By inhibiting this enzyme, clarithromycin significantly decreases the hepatic clearance of atorvastatin, leading to a rapid and massive accumulation of the statin in the systemic circulation, which precipitates severe dose-dependent toxicity (myopathy and rhabdomyolysis).

Adım Adım Çözüm

1
Identify the primary clinical syndrome from the patient's presentation and laboratory findings.
The patient has severe muscle pain, extremely high creatine kinase (8,500 U/L), and dark urine positive for heme without red blood cells, which is highly diagnostic of rhabdomyolysis.
Recognizing the adverse event is the first step in identifying its pharmacologic trigger.
2
Correlate the adverse event with the patient's medication history to find the inciting drug interaction.
The patient is on atorvastatin (a known cause of myopathy/rhabdomyolysis) and recently completed a course of clarithromycin.
Rhabdomyolysis in a patient chronically stable on a statin strongly suggests a new drug interaction.
3
Determine the specific pharmacokinetic mechanism between the two identified drugs.
Clarithromycin is a potent inhibitor of the cytochrome P450 3A4 (CYP3A4) enzyme, which is responsible for the hepatic clearance of atorvastatin. This leads to toxic accumulation of the statin.
Understanding the metabolism pathway (CYP3A4) of the specific statin involved is necessary to explain the toxicity.

Anahtar Kavram

Pharmacokinetic drug-drug interactions involving cytochrome P450 enzyme inhibition, specifically the interaction between CYP3A4 inhibitors (macrolides) and statins.
Tahmini Süre:1m 15s
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Pharmacokinetic and Pharmacodynamic Principles

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