Phenytoin pharmacokinetics is highlighted by which of the following characteristics ?
**Core Concept**
Phenytoin is an antiepileptic medication with a complex pharmacokinetic profile, characterized by nonlinear kinetics and a narrow therapeutic index. This is due to its saturable metabolism by the liver enzyme CYP2C9.
**Why the Correct Answer is Right**
Phenytoin's pharmacokinetics is unique because of its ability to autoinduce its own metabolism, leading to a decrease in plasma concentrations over time. This effect is particularly pronounced at concentrations above 20 mcg/mL, resulting in a steep dose-response curve. The metabolism of phenytoin is primarily mediated by CYP2C9, which is also involved in the metabolism of other drugs, including warfarin and tolbutamide.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because phenytoin's pharmacokinetics is not primarily characterized by first-order kinetics. While it is true that phenytoin's metabolism is influenced by CYP2C9, its nonlinear kinetics and autoinduction make it distinct from other drugs.
**Option B:** This option is incorrect because phenytoin's volume of distribution is not particularly large, and it is not primarily eliminated by the kidneys. While phenytoin does undergo some renal excretion, its metabolism is the primary route of elimination.
**Option C:** This option is incorrect because phenytoin's protein binding is not the primary factor influencing its pharmacokinetics. While phenytoin is highly protein-bound, its narrow therapeutic index and nonlinear kinetics are more relevant to its pharmacokinetic profile.
**Clinical Pearl / High-Yield Fact**
When prescribing phenytoin, it is essential to monitor plasma concentrations regularly to avoid toxicity. Additionally, phenytoin's autoinduction of its own metabolism can lead to a decrease in plasma concentrations over time, necessitating dose adjustments.
**Correct Answer: A. Nonlinear kinetics and autoinduction of its own metabolism.**