Phenytoin undergoes saturation kinetics at therapeutic doses. Which pharmacokinetic parameter becomes dose-dependent and non-linear at higher concentrations?

Explanation

## Phenytoin Pharmacokinetics: Saturation Kinetics **Key Point:** Phenytoin exhibits **zero-order (saturation) kinetics** at therapeutic concentrations because its hepatic metabolism via CYP2C9 becomes saturated. This means elimination half-life increases unpredictably with dose increments, violating the assumption of first-order kinetics. ### Why Saturation Occurs Phenytoin is metabolized by hepatic CYP2C9 to inactive metabolites (5-p-hydroxyphenyl-5-phenylhydantoin). At therapeutic plasma concentrations (10–20 μg/mL), the enzyme becomes saturated: 1. **At low doses:** Elimination follows first-order kinetics; t½ ≈ 24 hours (constant) 2. **At therapeutic/high doses:** Enzyme saturation → zero-order kinetics; t½ becomes dose-dependent and increases 3. **Clinical consequence:** Small dose increases can cause disproportionate plasma level elevations and toxicity **High-Yield:** The relationship is: $$t_{1/2} = \frac{V_m}{2 \times C_{ss}}$$ where V~m~ is maximum metabolic rate and C~ss~ is steady-state concentration. As C~ss~ rises, t½ increases non-linearly. ### Clinical Implications - **Narrow therapeutic index:** Therapeutic window is 10–20 μg/mL; toxicity (ataxia, nystagmus, confusion) occurs above 20 μg/mL - **Unpredictable kinetics:** A 100 mg dose increase may cause minimal change in level at 200 mg/day but severe toxicity at 400 mg/day - **Monitoring requirement:** Plasma level monitoring is essential; cannot rely on standard dosing nomograms - **Drug interactions:** Inhibitors (cimetidine, isoniazid) or inducers (rifampicin) dramatically alter kinetics ### Comparison: First-Order vs. Zero-Order Kinetics | Parameter | First-Order (Most Drugs) | Zero-Order (Phenytoin at Therapeutic Doses) | | --- | --- | --- | | Clearance | Constant (mL/min) | Decreases as concentration rises | | Half-life | Constant | Increases with dose | | Plasma level vs. dose | Linear relationship | Non-linear (saturable) | | Steady-state time | ~5 half-lives | Unpredictable; may take weeks | | Example | Warfarin, digoxin | Phenytoin, aspirin (high dose), alcohol | **Mnemonic:** **PHENYTOIN SATURATION** = **P**lasma levels **H**ighly **E**ratic **N**on-linear **Y**ield **T**oxicity **O**ver **I**ncremental **N**eed (saturation kinetics make small dose changes unpredictable) **Clinical Pearl:** Phenytoin is one of the few drugs in clinical use with saturation kinetics; this makes it a classic exam question because students must understand why standard pharmacokinetic assumptions fail.