What characterizes nonlinear (saturable) pharmacokinetics, and can you name a classic drug example?

• A. Clearance decreases with rising dose due to saturation of metabolic pathways, leading to disproportionately higher exposure; example: phenytoin
• B. Clearance increases with dose due to induction
• C. Clearance remains constant; first-order kinetics
• D. Half-life decreases with dose due to saturable distribution

Answer: (A)

Nonlinear pharmacokinetics occurs when the processes that eliminate or distribute a drug become saturated at higher concentrations. When metabolism is saturable, the enzymes responsible have a finite capacity (think Michaelis-Menten kinetics). As plasma levels rise, these enzymes approach their maximum rate, so clearance decreases rather than staying constant. With less clearance, the drug stays in the body longer and exposure (the AUC) increases disproportionately for small increases in dose. The half-life often lengthens as dose increases because the body’s ability to clear the drug is overwhelmed.

Phenytoin is a classic example. Its hepatic metabolism is saturable, so at higher concentrations, clearance drops and drug exposure climbs more than proportionally. This makes dosing tricky and is a hallmark of nonlinear, saturable PK.

Other statements describe different scenarios. Induction can increase clearance, but that’s a time-dependent change in capacity rather than a direct, dose-related saturation of a single pathway. First-order kinetics imply linear, not nonlinear, clearance. Saturable distribution can cause unusual PK in some cases, but the characteristic feature of nonlinear saturable PK is decreasing clearance with increasing dose leading to disproportionately higher exposure, as seen with phenytoin.