How can drug plasma protein binding be categorized?

• A. pharmacokinetic drug-drug interaction; drug may displace other drug and increase its plasma concentration.
• B. Pharmacodynamic synergy.
• C. A mechanism of receptor desensitization.
• D. A therapeutic effect.

Answer: (A)

Plasma protein binding determines how much drug is freely circulating and able to exert an effect. When two drugs compete for the same binding sites on proteins like albumin, one can displace the other, increasing the unbound fraction of the displaced drug. This is a pharmacokinetic drug-drug interaction because it changes the drug’s distribution and clearance by altering the amount that is free in the plasma. The unbound portion is the active portion that distributes to tissues and is cleared, so displacement can raise exposure and toxicity risk until a new equilibrium is reached.

Pharmacodynamic synergy would describe enhanced effects at the receptor level, not changes in protein binding. Receptor desensitization refers to reduced receptor responsiveness over time, not displacement at binding sites. A therapeutic effect is an outcome, not a mechanism categorization.