Which pharmacokinetic parameter is related to binding to plasma or tissue proteins?

Which pharmacokinetic parameter is related to binding to plasma or tissue proteins?

• A. Absorption
• B. Excretion
• C. Distribution
• D. Metabolism

Answer: (C)

Binding to plasma or tissue proteins directly shapes how a drug moves between blood and tissues. Only the unbound portion of a drug is free to cross capillary walls and enter or leave tissues, so the amount bound to proteins acts like a reservoir that slows distribution and can prolong the drug’s presence in the body. This is exactly what the distribution aspect of pharmacokinetics captures: how the drug partitions between plasma and various tissues, and how protein binding governs that partitioning. When a drug is highly bound in plasma, the free fraction is small, so distribution into tissues is slower; if tissue binding also sequesters drug, the apparent volume of distribution can appear large because much of the drug is stored in tissues rather than circulating freely. In short, protein binding governs how readily a drug disperses from plasma into tissues and how long it stays around, which is why distribution is the parameter tied to binding to plasma or tissue proteins.

Binding to plasma or tissue proteins directly shapes how a drug moves between blood and tissues. Only the unbound portion of a drug is free to cross capillary walls and enter or leave tissues, so the amount bound to proteins acts like a reservoir that slows distribution and can prolong the drug’s presence in the body. This is exactly what the distribution aspect of pharmacokinetics captures: how the drug partitions between plasma and various tissues, and how protein binding governs that partitioning. When a drug is highly bound in plasma, the free fraction is small, so distribution into tissues is slower; if tissue binding also sequesters drug, the apparent volume of distribution can appear large because much of the drug is stored in tissues rather than circulating freely. In short, protein binding governs how readily a drug disperses from plasma into tissues and how long it stays around, which is why distribution is the parameter tied to binding to plasma or tissue proteins.