A clinical trial evaluates a new monoclonal antibody. The volume of distribution () is calculated to be in a adult subject. This value suggests that the drug is distributed primarily in which physiologic fluid compartment?

Answer and explanation

Correct answer: A. Plasma compartment only

  1. Step 1: Review physiological fluid volumes for a standard adult: plasma volume is approximately ( body weight), extracellular fluid (ECF) volume is (), and total body water (TBW) is ().
  2. Step 2: Large macromolecular therapeutics like monoclonal antibodies (MW ) and highly plasma protein-bound drugs cannot readily cross capillary fenestrations. A of approximates total plasma volume, indicating near-complete intravascular confinement.
Answer
Plasma compartment only.
Why not the others: Extracellular fluid compartment (plasma + interstitial fluid) corresponds to a of approximately , typical of small, hydrophilic molecules (e.g., aminoglycosides); Total body water corresponds to a , typical of small lipophilic uncharged molecules (e.g., ethanol, phenytoin); Sequestration in adipose and deep tissues yields very high apparent values exceeding total body weight (e.g., chloroquine, digoxin, amiodarone, where ).

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