Which statement about nonlinear pharmacokinetics due to saturable transporters is true?

• A. When transporters saturate, clearance can decrease at higher concentrations, leading to nonlinear PK.
• B. Saturable processes always produce linear PK.
• C. Nonlinear PK only occurs with zero-order elimination.
• D. Saturation increases clearance linearly with dose.

Answer: (A)

The main idea is that nonlinear PK from saturable transporters happens when the transport step can’t keep up as concentration rises. Transport processes often follow saturable kinetics: at low concentrations, the transport rate increases nearly linearly with concentration, but as concentrations approach the transporter’s capacity, the rate approaches a maximum (Vmax) and can’t rise further.

A helpful way to see the math is to think of the transport rate as v = (Vmax × C) / (Km + C). At low C, this looks like v ≈ (Vmax/Km) × C, so clearance appears relatively constant. At high C, when C is much larger than Km, v ≈ Vmax, meaning increasing C doesn’t increase the transport rate. Since clearance is roughly CL = v/C, a saturating transport makes clearance fall as concentration rises. The consequence is that drug exposure (AUC) increases more than proportionally with dose, and the pharmacokinetics become nonlinear.

In short, when transporters saturate, clearance can decrease at higher concentrations, leading to nonlinear PK.