Why is the unbound drug fraction the one that can be distributed and cleared?

Why is the unbound drug fraction the one that can be distributed and cleared?

• A. Only unbound drug can cross membranes, be metabolized, and be filtered by the kidneys; protein-bound drug is usually pharmacologically inactive and not readily cleared.
• B. Protein-bound drug crosses membranes more readily than unbound.
• C. Bound drug is the primary substrate for metabolism.
• D. Unbound drug does not cross membranes.

Answer: (A)

Only the unbound drug fraction can move through membranes, reach tissues, and be processed by the body. When a drug is bound to plasma proteins like albumin, that portion is largely pharmacologically inactive and is not readily cleared. For distribution, the drug must diffuse across capillary and cell membranes to equilibrate between blood and tissues; the bound portion is effectively trapped by the protein, so it does not participate in this movement, making the unbound fraction the determiner of how widely the drug distributes. For clearance, the kidneys can filter only the free drug, and metabolic enzymes in the liver access and act on free drug; thus the rate at which a drug is cleared depends on how much is unbound. This explains why increasing the unbound fraction can increase both distribution and clearance, and why protein binding acts as a reservoir that can modulate overall drug exposure. The bound form is not primarily involved in crossing membranes or being metabolized, and the unbound form is the one with activity and clearance.

Only the unbound drug fraction can move through membranes, reach tissues, and be processed by the body. When a drug is bound to plasma proteins like albumin, that portion is largely pharmacologically inactive and is not readily cleared. For distribution, the drug must diffuse across capillary and cell membranes to equilibrate between blood and tissues; the bound portion is effectively trapped by the protein, so it does not participate in this movement, making the unbound fraction the determiner of how widely the drug distributes. For clearance, the kidneys can filter only the free drug, and metabolic enzymes in the liver access and act on free drug; thus the rate at which a drug is cleared depends on how much is unbound.

This explains why increasing the unbound fraction can increase both distribution and clearance, and why protein binding acts as a reservoir that can modulate overall drug exposure. The bound form is not primarily involved in crossing membranes or being metabolized, and the unbound form is the one with activity and clearance.