Which statement correctly describes saturable processes causing nonlinear PK?

• A. Saturable metabolism or transport can cause nonlinear PK.
• B. Linear metabolism is the main cause of nonlinear PK.
• C. Nonlinear PK occurs only during absorption.
• D. Nonlinear PK cannot occur in clinical pharmacology.

Answer: (C)

Nonlinear pharmacokinetics happens when the processes that handle a drug become capacity-limited, so increasing the dose doesn’t lead to proportional increases in drug exposure. This occurs most often when metabolism or transporters saturate. When enzymes or transporters reach their maximum rate, adding more drug doesn’t get cleared or moved as quickly, so the blood levels rise more than expected.

The statement that best captures this is that saturable metabolism or transport can cause nonlinear PK. That means nonlinear PK can arise from either metabolism or transport processes, not just from absorption. Absorption can be saturable, but nonlinear behavior is not confined to that step; it can also occur during metabolism, distribution, or excretion.

A helpful example is a drug whose hepatic metabolism becomes saturated: as the dose increases, clearance falls or becomes unpredictable, and AUC increases more than proportionally. This illustrates how capacity-limited processes drive nonlinear PK.