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Difficulty: MediumPharmacokinetic and Pharmacodynamic Principles

Match the following clinically significant drug combinations with the primary pharmacokinetic or pharmacodynamic mechanism underlying their interaction.

  • Simvastatin and ClarithromycinInhibition of cytochrome P450 3A4 (CYP3A4), decreasing hepatic metabolism and increasing the risk of systemic toxicity (e.g., myopathy).
  • Lithium and HydrochlorothiazideInduction of volume depletion causing compensatory enhancement of proximal tubular reabsorption, leading to decreased renal clearance.
  • Clopidogrel and OmeprazoleInhibition of cytochrome P450 2C19 (CYP2C19), preventing the biotransformation of a prodrug into its active therapeutic metabolite.
  • Digoxin and VerapamilInhibition of P-glycoprotein (P-gp) efflux transporters, increasing intestinal absorption and decreasing renal and biliary clearance.

Answer

Simvastatin and Clarithromycin matches CYP3A4 inhibition; Lithium and Hydrochlorothiazide matches enhanced proximal tubular reabsorption; Clopidogrel and Omeprazole matches CYP2C19 inhibition; Digoxin and Verapamil matches P-glycoprotein inhibition.
Clinically significant drug interactions frequently arise from specific, predictable pharmacokinetic mechanisms. Clarithromycin inhibits CYP3A4, preventing the metabolism of simvastatin and leading to toxic systemic levels. Hydrochlorothiazide induces mild volume depletion, prompting a compensatory increase in proximal tubular sodium reabsorption; lithium is reabsorbed alongside sodium, increasing the risk of toxicity. Omeprazole inhibits CYP2C19, the enzyme strictly required to convert clopidogrel from an inactive prodrug into its active form, leading to potential therapeutic failure (e.g., stent thrombosis). Verapamil inhibits P-glycoprotein, a vital efflux transporter for digoxin, thereby drastically decreasing digoxin's renal and biliary clearance and precipitating toxicity.

Step-by-Step Solution

1
Analyze the primary pharmacokinetic pathway for clearance or activation of the first drug in the pair.
Simvastatin relies on CYP3A4 for clearance; Clopidogrel requires CYP2C19 for activation; Digoxin relies on P-gp for excretion; Lithium relies on renal filtration and proximal tubular handling.
Understanding the baseline pharmacokinetics of the affected drug is necessary to identify how it can be disrupted.
2
Identify the pharmacological effect of the interacting drug on metabolic enzymes, transporters, or renal hemodynamics.
Clarithromycin inhibits CYP3A4; Omeprazole inhibits CYP2C19; Verapamil inhibits P-gp; Hydrochlorothiazide causes volume depletion, leading to compensatory proximal tubular reabsorption.
The interacting drug acts as the precipitant that alters the pharmacokinetics of the primary drug.
3
Pair the specific mechanism of the precipitant drug with the corresponding consequence on the affected drug.
The correct matches align the precise molecular or physiological interference with the resulting clinical vulnerability (e.g., toxicity vs. therapeutic failure).
Accurate pairing distinguishes between different types of pharmacokinetic disruptions (enzymatic, transporter-mediated, or renal).

Key Concept

Drug-drug interactions often manifest through specific pharmacokinetic alterations, primarily involving inhibition of cytochrome P450 enzymes (e.g., CYP3A4, CYP2C19), inhibition of efflux transporters (e.g., P-glycoprotein), or changes in renal tubular handling.
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Topics

Pharmacokinetic and Pharmacodynamic Principles

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