What is fu and its impact on pharmacodynamics and PK parameters?

• A. Fu is the unbound drug fraction; it determines how much drug is available for clearance and distribution and is tied to pharmacologic activity.
• B. Fu is the bound drug fraction; it determines clearance rate.
• C. Fu has no impact on PK or PD.
• D. Fu equals 1 for all drugs.

Answer: (A)

Fu is the unbound drug fraction in plasma. This free portion is the portion not bound to plasma proteins, and it is the portion that can cross membranes, distribute into tissues, be metabolized, and be cleared from the body. In pharmacodynamics, the pharmacologic effect is driven by the free drug concentration, because only the unbound drug can interact with receptors or targets. In pharmacokinetics, fu governs how much drug is available for clearance and distribution: higher fu means more drug is available to be cleared by the liver or kidneys and more can move into tissues, which typically increases both clearance and volume of distribution. Conversely, strong protein binding lowers fu, reducing immediate clearance and tissue exposure, though the overall outcome also depends on factors like intrinsic clearance and organ blood flow. Fu is not fixed for all drugs; it changes with the drug’s binding affinity and patient factors such as protein levels, disease states, and potential drug interactions that displace binding.