Why would protein binding decrease a drug's distribution?

Why would protein binding decrease a drug's distribution?

• A. Because proteins are large molecules and cannot pass through endothelial junctions and regular transport mechanisms, causing the drug to stay in the blood
• B. Because bound drug is actively pumped into the cerebrospinal fluid
• C. Because binding accelerates renal clearance
• D. Because binding enhances diffusion into tissues

Answer: (C)

Protein binding keeps most of the drug in the bloodstream because only the unbound fraction can cross capillary walls and enter tissues. When a drug is bound to plasma proteins, the complex is too large to pass through endothelial gaps, so the amount available to distribute to tissues is reduced. This lowers the apparent volume of distribution. In terms of elimination, binding usually slows renal clearance because only free drug is filtered by the kidneys, so high protein binding can prolong the drug’s presence in the body. The notion that binding would accelerate renal clearance or actively push drug into the cerebrospinal fluid or tissues contradicts the basic idea that only the free, unbound drug distributes.

Protein binding keeps most of the drug in the bloodstream because only the unbound fraction can cross capillary walls and enter tissues. When a drug is bound to plasma proteins, the complex is too large to pass through endothelial gaps, so the amount available to distribute to tissues is reduced. This lowers the apparent volume of distribution. In terms of elimination, binding usually slows renal clearance because only free drug is filtered by the kidneys, so high protein binding can prolong the drug’s presence in the body. The notion that binding would accelerate renal clearance or actively push drug into the cerebrospinal fluid or tissues contradicts the basic idea that only the free, unbound drug distributes.