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Difficulty: MediumPharmacokinetic and Pharmacodynamic Principles

A 6868-year-old woman is evaluated in the cardiology clinic for symptomatic palpitations. She has a history of ischemic cardiomyopathy, heart failure with reduced ejection fraction, and chronic atrial fibrillation. Her current daily medications include lisinopril, metoprolol succinate, furosemide, and digoxin. Her vital signs are stable, but an electrocardiogram reveals frequent premature ventricular contractions and runs of non-sustained ventricular tachycardia. The physician decides to initiate oral amiodarone therapy. Recognizing a critical drug-drug interaction, the physician simultaneously reduces the patient's digoxin dose by 50%50\%. This empiric dose reduction is primarily required to compensate for which of the following amiodarone-induced pharmacokinetic changes?

  1. Decreased P-glycoprotein-mediated drug effluxAnswer
  2. B
    Inhibition of cytochrome P450 3A4 hepatic metabolism
  3. C
    Displacement of the drug from plasma albumin
  4. D
    Decreased glomerular filtration rate
  5. E
    Enhanced enterohepatic recirculation

Answer

Decreased P-glycoprotein-mediated drug efflux
The correct answer accurately identifies that amiodarone inhibits P-glycoprotein. P-glycoprotein is an ATP-dependent efflux pump located on the luminal membrane of enterocytes and the apical membrane of proximal renal tubule cells. Digoxin is a classic substrate for this transporter. When amiodarone blocks P-glycoprotein, digoxin cannot be efficiently secreted into the urine or pumped back into the intestinal lumen, causing its systemic levels to rise dramatically. Consequently, a 50%50\% dose reduction of digoxin is the standard of care when amiodarone is initiated.

Step-by-Step Solution

1
Analyze the clinical intervention described in the vignette.
A patient stabilized on digoxin is being started on amiodarone, prompting the physician to proactively reduce the digoxin dose by 50%50\%.
Identifying the two interacting drugs is the first step in determining the pharmacokinetic mechanism of their interaction.
2
Identify the pharmacokinetic properties of digoxin.
Digoxin is not extensively metabolized by the liver; rather, it relies heavily on P-glycoprotein (P-gp) for active tubular secretion in the kidneys and efflux in the gastrointestinal tract.
Understanding the baseline clearance mechanism of the affected drug establishes how the perpetrator drug alters its kinetics.
3
Determine the effect of amiodarone on digoxin clearance.
Amiodarone is a potent inhibitor of P-glycoprotein. This inhibition blocks the renal secretion and intestinal efflux of digoxin, leading to a substantial increase in serum digoxin concentrations.
Connecting the perpetrator's mechanism (P-gp inhibition) to the substrate's clearance pathway confirms the rationale for the 50%50\% dose reduction.

Key Concept

Amiodarone profoundly increases serum digoxin concentrations by inhibiting P-glycoprotein (P-gp), an efflux pump responsible for the renal tubular secretion and intestinal clearance of digoxin. When initiating amiodarone, the digoxin dose must typically be reduced by 30%30\% to 50%50\% to prevent toxicity.
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Topics

Pharmacokinetic and Pharmacodynamic Principles

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