A drug has a volume of distribution (Vd) of 400 L in a 70 kg adult. This finding most likely indicates:
- A The drug is largely confined to plasma water with high protein binding
- B The drug extensively distributes into peripheral tissues or intracellular compartments ✓
- C The drug undergoes zero-order kinetics due to enzyme saturation
- D The drug has a short half-life requiring frequent dosing
Correct answer: B. The drug extensively distributes into peripheral tissues or intracellular compartments
Explanation
The total body water of a 70 kg adult is approximately 42 L. A Vd of 400 L far exceeds any physiological fluid compartment, indicating that the drug is highly concentrated in peripheral tissues (e.g., adipose tissue, muscle) or intracellular compartments relative to plasma. Such high Vd drugs (e.g., chloroquine, amiodarone) are not readily removed by haemodialysis because only a tiny fraction exists in plasma. High plasma protein binding would give a LOW Vd, not high.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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