What explains non-linear distribution when tissue binding saturates at high drug concentrations?

• A. Non-linear distribution arises when clearance changes with concentration
• B. Tissue binding saturation decreases fu, reducing Vd
• C. At high concentrations, tissue binding may saturate, increasing fu and altering Vd
• D. Non-linear distribution cannot occur with saturable processes

Answer: (C)

Non-linear distribution at high drug concentrations often comes from saturable tissue binding. Drugs can bind reversibly to tissue components, but only up to a finite number of sites. At low concentrations, most sites are occupied enough that the unbound fraction (fu) is relatively constant, so the apparent volume of distribution (Vd) stays steady. As concentration rises and those binding sites become saturated, more of the drug remains unbound. This increase in fu means more drug can distribute into tissues, so the apparent Vd increases and distribution becomes concentration-dependent.

This is why the statement describing saturable tissue binding raising fu and altering Vd best explains the non-linear distribution. In contrast, changes in clearance with concentration explain non-linear pharmacokinetics but not the mechanism of distribution per se; and a decrease in fu with saturation would actually reduce distribution, which isn’t correct. Saturable processes can indeed produce non-linear distribution, so that option is not correct either.