Pharmaceutics Distribution of Drugs Practice Exam

Plasma protein binding controls how much drug is free to move into tissues and be cleared. The bound portion is largely unavailable for distribution into tissues or for metabolism and excretion, so the free fraction determines both where the drug goes and how quickly it is eliminated. When a drug is highly bound, the free fraction is small, limiting tissue distribution and slowing clearance, with the bound drug acting as a reservoir that can release more drug as plasma levels fall. If the free fraction rises, such as when another substance displaces the drug from albumin, there is more unbound drug to distribute to tissues and to be metabolized or excreted, so clearance can increase. This change in clearance affects overall exposure (AUC) for a given dose. In short, binding dampens both distribution and clearance by limiting the free drug; displacement increases the free drug, potentially increasing clearance and altering exposure.