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Difficulty: MediumPharmacokinetic and Pharmacodynamic Principles

During a routine follow-up visit, a physician evaluates a 62-year-old male with a history of hypertension, type 2 diabetes mellitus, and hyperlipidemia. The patient is currently managed with lisinopril, metformin, and simvastatin. Due to persistently elevated blood pressure readings averaging 158/94 mm Hg, the physician adds oral diltiazem to the regimen. Fourteen days later, the patient presents to the emergency department with profound bilateral thigh and shoulder pain, weakness, and dark urine. Serum laboratory analysis demonstrates a creatine kinase concentration of 12,500 U/L. Which of the following pharmacokinetic alterations of the lipid-lowering agent most likely explains this patient's acute clinical presentation?

  1. Decreased systemic clearance and increased elimination half-lifeAnswer
  2. B
    Increased volume of distribution and prolonged elimination half-life
  3. C
    Decreased oral bioavailability and decreased peak plasma concentration
  4. D
    Displacement from plasma proteins and increased unbound active fraction
  5. E
    Enhanced renal tubular reabsorption and decreased urinary excretion

Answer

Decreased systemic clearance and increased elimination half-life
Simvastatin is a lipid-lowering agent that undergoes extensive metabolism by the hepatic and intestinal CYP3A4 enzyme. Diltiazem, a non-dihydropyridine calcium channel blocker, is a well-known moderate inhibitor of CYP3A4. When diltiazem is added to the patient's regimen, it inhibits the metabolism of simvastatin. In pharmacokinetic terms, this metabolic inhibition directly decreases the systemic clearance (ClCl) of simvastatin. According to the half-life equation, t1/2=0.693×VdClt_{1/2} = \frac{0.693 \times V_d}{Cl}, a decrease in clearance results in a proportional increase in the elimination half-life (t1/2t_{1/2}), assuming the volume of distribution (VdV_d) remains constant. This leads to drug accumulation, elevated peak plasma concentrations, and an increased risk of dose-dependent adverse effects such as rhabdomyolysis.

Step-by-Step Solution

1
Identify the patient's new symptoms and laboratory findings
The patient has profound muscle pain, weakness, dark urine, and an elevated creatine kinase (12,500 U/L).
These clinical and laboratory findings are classic for rhabdomyolysis, a known dose-dependent adverse effect of statin toxicity.
2
Determine the cause of the statin toxicity based on the medication history
The recent addition of diltiazem precipitated the toxicity.
Diltiazem is a known moderate inhibitor of the CYP3A4 enzyme, which is the primary pathway for metabolizing simvastatin.
3
Apply pharmacokinetic principles to the enzyme inhibition
CYP3A4 inhibition decreases the metabolic breakdown of simvastatin, directly decreasing its systemic clearance.
Because half-life (t1/2t_{1/2}) is inversely proportional to clearance (t1/2=0.693×VdClt_{1/2} = \frac{0.693 \times V_d}{Cl}), a decrease in clearance leads to an increased half-life, resulting in toxic drug accumulation.

Key Concept

Inhibition of hepatic metabolic enzymes (e.g., CYP3A4) decreases the systemic clearance of substrate drugs, leading to an increased elimination half-life and potential dose-dependent toxicity.
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Topics

Pharmacokinetic and Pharmacodynamic Principles

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