DrugTheGuide
Pharmacokinetics

Nonlinear Pharmacokinetics

Pharmacokinetics in which clearance, half-life, or AUC change with dose or concentration — caused by saturable elimination (Michaelis-Menten kinetics) or target-mediated drug disposition (TMDD) in biologics — making dose-proportional predictions unreliable.

Definition and Mechanism

In linear (first-order) pharmacokinetics, the rate of elimination is proportional to concentration — doubling the dose doubles AUC, half-life remains constant. Nonlinear PK deviates from this: a disproportionate relationship exists between dose and AUC, and apparent clearance and half-life change with concentration.

The most common cause is saturable enzymatic elimination described by the Michaelis-Menten equation:

Rate = (Vmax × C) / (Km + C)

Where Vmax is the maximum metabolic rate and Km is the Michaelis constant (the concentration at which rate = ½Vmax). At low C (<<Km): first-order kinetics dominate. At high C (>>Km): zero-order kinetics — elimination rate is constant at Vmax regardless of further concentration increases. See our Michaelis-Menten kinetics glossary and zero-order kinetics glossary.

Clinical Examples

  • Phenytoin: CYP2C9/CYP2C19-mediated hydroxylation is saturated within the therapeutic range (10–20 mg/L). A dose increase of 50 mg/day can raise levels from 15 to 25 mg/L (toxicity). Standard 10% dose increments rule: above 300 mg/day, increment by no more than 25–30 mg/day. TDM is mandatory.
  • Ethanol: Alcohol dehydrogenase is saturated at blood alcohol concentrations seen in social drinking. Elimination is approximately zero-order at ~7–10 g/h (one standard drink). Time to sobriety scales linearly with drinks consumed, not via half-life.
  • High-dose salicylate: Glycine conjugation to salicylurate and glucuronidation are both saturable. At therapeutic analgesic doses (~600 mg), kinetics are linear; at anti-inflammatory doses (3–5 g/day) or toxic doses, nonlinear.
  • Monoclonal antibodies (TMDD): See target-mediated drug disposition below. Examples: cetuximab (anti-EGFR), trastuzumab (anti-HER2). Discussed in our mAb therapy guide.

Target-Mediated Drug Disposition (TMDD)

TMDD is the dominant source of nonlinear PK for biologics. When a drug binds a high-affinity, saturable pharmacological target:

Low concentration (below Kd)

Drug is bound to target → receptor-mediated endocytosis → rapid elimination. Apparent clearance is high. Apparent half-life is short. PK is dose-dependent and non-linear.

High concentration (above Kd)

Target is saturated → drug is cleared by non-specific pathways (FcRn recycling, pinocytosis). Apparent clearance is lower. Half-life approaches the intrinsic IgG half-life (~21 days). PK becomes linear.

TMDD has dosing implications: subtherapeutic doses that fail to saturate the target are eliminated faster — exposure is disproportionately low. This is why mAb dosing strategies often aim to achieve concentrations well above the Kd at trough. Reference: Mager & Jusko 2001 TMDD model (PubMed) ↗.

FAQ

What are nonlinear pharmacokinetics?
Nonlinear pharmacokinetics describes a situation where a drug's pharmacokinetic parameters — clearance, half-life, and AUC — change with dose or concentration. Unlike linear (first-order) kinetics, where doubling the dose doubles the AUC proportionally, nonlinear drugs show disproportionate changes. The most common cause is saturable (capacity-limited) elimination — when drug concentrations exceed the Km of the eliminating enzyme or transporter, the enzyme operates near maximum velocity (Vmax) and the rate of elimination no longer increases proportionally with concentration.
What is Michaelis-Menten elimination and which drugs show it?
Michaelis-Menten elimination describes saturable enzymatic metabolism where the elimination rate = (Vmax × C) / (Km + C). At low concentrations (C << Km), the rate approximates first-order kinetics. At high concentrations (C >> Km), the rate approaches Vmax and becomes zero-order (constant rate, not concentration-dependent). Classic examples: phenytoin (CYP2C9 is saturated at therapeutic levels — small dose increments cause large, unpredictable concentration rises); ethanol (alcohol dehydrogenase saturated at typical social drinking concentrations, ~10 g/h elimination); high-dose salicylate; and omeprazole at higher doses. Phenytoin is the most clinically significant: the therapeutic range is narrow (10–20 mg/L) and TDM is essential.
What is target-mediated drug disposition (TMDD)?
Target-mediated drug disposition (TMDD) is a form of nonlinear pharmacokinetics seen with monoclonal antibodies and other biologics that bind high-affinity, saturable targets (receptors, antigens). At low drug concentrations, the drug is largely bound to its pharmacological target and eliminated via receptor-mediated endocytosis — this is concentration-dependent and non-linear (apparent clearance is high). At higher concentrations, the target becomes saturated, and the drug is cleared by non-specific pathways (FcRn recycling, fluid-phase pinocytosis) at a slower, linear rate. This produces a characteristic PK profile: short apparent half-life at low concentrations, long half-life at higher (saturating) concentrations.
What are the clinical implications of nonlinear pharmacokinetics?
Nonlinear PK has important clinical consequences: (1) Dose proportionality is lost — a small dose increase can cause a disproportionately large rise in plasma concentration, increasing toxicity risk. (2) Half-life and clearance are not constant — they change with concentration, making standard PK predictions unreliable. (3) Time to steady state is not predictable from half-life alone. (4) TDM is essential for drugs like phenytoin — empirical dose adjustments without level monitoring are dangerous. (5) For biologics with TMDD, dosing at sub-saturating concentrations leads to rapid apparent clearance; once above the Km for the target, PK shifts to linear and half-life extends substantially.