How can high plasma protein binding influence drug clearance?

• A. High plasma protein binding always increases clearance.
• B. High plasma protein binding can reduce clearance by limiting metabolism for bound drug.
• C. Plasma protein binding has no effect on clearance.
• D. High plasma protein binding increases tissue distribution.

Answer: (B)

Only the unbound drug is cleared, so high plasma protein binding reduces clearance by limiting metabolism for the bound drug. When a drug is bound to plasma proteins like albumin, the bound portion is not available for enzymes to metabolize or for the kidneys to filter. Clearance is driven by the removal of the unbound fraction, so as the bound fraction increases and the free concentration drops, the rate at which the drug is eliminated decreases. This often leads to a longer duration in the body (a longer half-life) because the system must wait for reversible binding to release drug into the unbound form that can be eliminated.

In practice, this means a highly protein-bound drug tends to have lower clearance, not higher. It also means tissue distribution is limited by how much is free to cross membranes, since bound drug largely stays in the plasma. Displacement by other drugs can temporarily raise the free fraction and momentarily increase clearance, but the typical pattern is that high binding slows overall elimination.