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Difficulty: MediumPharmacokinetic and Pharmacodynamic Principles

Match each clinical drug interaction scenario to its primary underlying pharmacokinetic mechanism.

  • Oral levofloxacin administered concurrently with calcium carbonateFormation of insoluble chelates in the gastrointestinal tract, reducing systemic absorption
  • Simvastatin taken with a daily large glass of grapefruit juiceInhibition of intestinal CYP3A4, decreasing first-pass metabolism and increasing bioavailability
  • Oral contraceptive pills taken by a patient who recently started St. John's wortInduction of hepatic cytochrome P450 enzymes, accelerating clearance and reducing efficacy
  • Probenecid co-administered with penicillin GBlockade of organic anion transporters in the renal proximal tubule, decreasing excretion

Answer

The correct matching aligns the levofloxacin scenario with chelation in the GI tract, the simvastatin scenario with intestinal CYP3A4 inhibition, the oral contraceptive scenario with hepatic CYP450 induction, and the probenecid scenario with renal OAT blockade.
Each specific drug interaction corresponds to an alteration in a distinct phase of pharmacokinetics: absorption (chelation), first-pass metabolism (intestinal CYP inhibition), systemic clearance (hepatic CYP induction), and elimination (renal transporter blockade).

Step-by-Step Solution

1
Analyze the levofloxacin and calcium carbonate interaction.
Matches with formation of insoluble chelates.
Fluoroquinolones interact with polyvalent cations in the gut to form unabsorbable complexes, impacting the absorption phase of pharmacokinetics.
2
Analyze the simvastatin and grapefruit juice interaction.
Matches with inhibition of intestinal CYP3A4.
Furanocoumarins in grapefruit inhibit local intestinal enzymes, which decreases first-pass metabolism and significantly increases the drug's oral bioavailability.
3
Analyze the oral contraceptive and St. John's wort interaction.
Matches with induction of hepatic cytochrome P450 enzymes.
St. John's wort is a classic broad inducer of CYP enzymes, leading to accelerated systemic clearance and reduced clinical efficacy of the contraceptives.
4
Analyze the probenecid and penicillin G interaction.
Matches with blockade of organic anion transporters.
Probenecid specifically blocks active tubular secretion of organic anions in the kidney, which alters the elimination phase by decreasing renal excretion.

Key Concept

Pharmacokinetic mechanisms of drug-drug interactions (Absorption, Metabolism, Excretion)
Estimated Time:1m 30s
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Topics

Pharmacokinetic and Pharmacodynamic Principles

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