Describe how distribution to adipose tissue can affect the pharmacokinetics of a lipophilic drug and its half-life?

• A. Partitioning into fat decreases Vd and shortens terminal half-life.
• B. Partitioning into fat increases Vd and prolongs terminal half-life due to slow release.
• C. Adipose distribution doesn't affect half-life or Vd.
• D. Adipose tissue serves as a rapid elimination site, shortening half-life.

Answer: (B)

When a drug is highly lipophilic, adipose tissue becomes a large reservoir where the drug can partition and be stored. This increases the apparent volume of distribution because more drug sits outside the plasma and tissues that are readily accessible to elimination. Since the terminal half-life is proportional to the volume of distribution divided by clearance (t1/2 ≈ 0.693 × Vd / Cl), a larger Vd means a longer terminal half-life if clearance remains similar. The fat acts as a slow-release source: the drug slowly diffuses back into the bloodstream and is then eliminated, so the concentration in plasma declines more gradually, extending the drug’s duration of action.

Fat’s lower perfusion also makes the exchange between fat and blood slower, reinforcing the prolonged terminal phase. This is not a rapid elimination site; elimination still depends on metabolism and excretion after the drug re-enters circulation. In contrast, choices that say Vd decreases, or that fat is a rapid elimination site, or that adipose distribution has no effect, don’t fit the observed pharmacokinetic behavior of lipophilic drugs.