Which statement best describes how plasma protein binding can lead to a pharmacokinetic drug-drug interaction?

Which statement best describes how plasma protein binding can lead to a pharmacokinetic drug-drug interaction?

• A. A drug displaces another from plasma protein binding, increasing the free concentration.
• B. A drug increases hepatic metabolism of another.
• C. A drug decreases absorption of another.
• D. A drug enhances receptor affinity.

Answer: (C)

Plasma protein binding controls how much drug is freely circulating. Only the unbound fraction can move into tissues, be metabolized, or be excreted. If another drug competes for the same binding sites on the proteins, it can displace the first drug, increasing its unbound (free) concentration. That rise in free drug changes the drug’s distribution and clearance, altering overall exposure in a way that is pharmacokinetic in nature. Absorption and receptor interactions are separate concepts, so a statement about decreased absorption doesn’t describe the consequence of binding-based displacement, and a statement about enhanced receptor affinity describes pharmacodynamics rather than a PK interaction.

Plasma protein binding controls how much drug is freely circulating. Only the unbound fraction can move into tissues, be metabolized, or be excreted. If another drug competes for the same binding sites on the proteins, it can displace the first drug, increasing its unbound (free) concentration. That rise in free drug changes the drug’s distribution and clearance, altering overall exposure in a way that is pharmacokinetic in nature. Absorption and receptor interactions are separate concepts, so a statement about decreased absorption doesn’t describe the consequence of binding-based displacement, and a statement about enhanced receptor affinity describes pharmacodynamics rather than a PK interaction.