What is the primary determinant of the apparent volume of distribution for a drug that partitions strongly into tissues?

• A. Half-life
• B. Tissue binding and distribution
• C. Absorption rate constant
• D. Renal filtration rate

Answer: (B)

The amount of drug in the body relative to its concentration in plasma is set by how far and how strongly the drug spreads into tissues. When a drug partitions strongly into tissues, a large portion of the total amount resides outside the plasma, and plasma levels drop. Because Vd = amount in the body / plasma concentration, extensive tissue distribution and binding push the apparent volume of distribution to a large value. In other words, tissue uptake and binding determine how widely the drug is distributed beyond the blood, which is why this factor dominates Vd for tissue-partitioning drugs.

Absorption rate affects how quickly the drug appears in systemic circulation, not how far it distributes once inside the body. Renal filtration rate influences clearance and elimination, and while this can alter overall pharmacokinetics, it does not define the extent of tissue distribution that sets the apparent Vd.