Which statement best explains how protein binding affects clearance and apparent volume of distribution?

• A. Only bound drug is cleared; high protein binding increases clearance.
• B. Clearance is unaffected by protein binding, but Vd increases.
• C. High protein binding reduces clearance and can increase Vd if the unbound distributes into tissues.
• D. Protein binding has no effect on PK parameters.

Answer: (C)

Protein binding controls how much drug is free to be cleared and to move into tissues. Only the unbound (free) fraction is accessible to metabolism and excretion, so when a drug binds tightly to plasma proteins, the amount that can be cleared is reduced. That’s why high protein binding tends to lower the overall clearance.

At the same time, distribution into tissues depends on the unbound drug. If the small, unbound portion readily leaves the blood and distributes into tissues, more drug resides outside the plasma. This makes the apparent volume of distribution larger, because the ratio of the total drug in the body to the plasma concentration increases as drug leaves the plasma pool. The bound drug in plasma serves as a reservoir that can continually replenish the free pool, further supporting a larger apparent Vd.

So, high protein binding reduces clearance (less free drug available for clearance) and can increase the apparent volume of distribution if the unbound drug distributes into tissues.