A monoclonal antibody with target-mediated drug disposition shows nonlinear PK at low concentrations but linear PK at high concentrations. Which explains this behavior?

• A. At low concentrations, receptor binding dominates clearance; at high concentrations the receptor saturates and clearance becomes linear.
• B. Nonlinear PK occurs only at high concentrations.
• C. TMDD affects distribution but not clearance.
• D. TMDD has no impact on pharmacokinetics.

Answer: (A)

Target-mediated drug disposition arises when a drug binds with high affinity to a pharmacologic target that contributes to its clearance. At low concentrations this target-mediated clearance is a major elimination route and is saturable, so the rate of clearance does not scale proportionally with concentration. This makes the pharmacokinetics nonlinear, with exposure changing more than expected as dose changes. As the dose (and concentration) increases, the target becomes saturated, so the receptor-mediated clearance cannot increase further. What remains is clearance through non-specific, linear pathways, which leads to linear (first-order) pharmacokinetics at higher concentrations. That’s why the observed pattern—nonlinear PK at low concentrations and linear PK at high concentrations—is explained by receptor binding dominating clearance when unsaturated, and saturation of the receptor-mediated pathway causing a shift to linear clearance.