Phenytoin is effective in preventing focal onset seizures and also tonic-clonic seizures, whether they are focal-to-bilateral tonicclonic (secondarily generalized) or occurring in the setting of an idiopathic generalized epilepsy syndrome. Phenytoin may worsen other seizure types in primary generalized epilepsies, including absence epilepsy, juvenile myoclonic epilepsy, and Dravet’s syndrome.
Absorption of phenytoin is highly dependent on the formulation. Particle size and pharmaceutical additives affect both the rate and the extent of absorption. Therefore, while absorption from the gastrointestinal tract is nearly complete in most patients, the time to peak may range from 3 to 12 hours. Phenytoin is extensively (~90%) bound to serum albumin and is prone to displacement in response to a variety of factors (eg, hyperbilirubinemia or drugs such as warfarin or valproic acid), which can lead to toxicity. Also, low plasma albumin (such as in liver disease or nephrotic syndrome) can result in abnormally high free concentrations and toxicity. Small changes in the bound fraction dramatically affect the amount of free (active) drug. Increased proportions of free drug are also present in the neonate and in the elderly. Some agents such as valproic acid, phenylbutazone, and sulfonamides can compete with phenytoin for binding to plasma proteins. Valproic acid also inhibits phenytoin metabolism. The combined effect can result in marked increases in free phenytoin. In all of these situations, patients may exhibit signs of toxicity when total drug levels are within the therapeutic range. Because of its high protein binding, phenytoin has a low volume of distribution (0.6–0.7 L/kg in adults).
Phenytoin is metabolized by CYP2C9 and CYP2C19 to inactive metabolites that are excreted in the urine. Only a small proportion of the dose is excreted unchanged. The elimination of phenytoin depends on the dose. At low blood levels, phenytoin metabolism follows first-order kinetics. However, as blood levels rise within the therapeutic range, the maximum capacity of the liver to metabolize the drug is approached (saturation kinetics). Even small increases in dose may be associated with large changes
FIGURE 24–4 Relationship between dose and exposure for antiseizure drugs (ASDs). Most antiseizure drugs follow linear (firstorder) kinetics, in which a constant fraction per unit time of the drug is eliminated (elimination is proportional to drug concentration). In the case of phenytoin, as the dose increases, there is saturation of metabolism and a shift from first-order to zero-order kinetics, in which a constant quantity per unit time is metabolized. A small increase in dose can result in a large increase in concentration. Orally administered gabapentin also exhibits zero-order kinetics, but in contrast to phenytoin where metabolism can be saturated, in the case of gabapentin, gut absorption, which is mediated by the large neutral amino acid system L transporter, is susceptible to saturation. The bioavailability of gabapentin falls at high doses as the transporter is saturated so that increases in blood levels do not keep pace with increases in dose.
in phenytoin serum concentrations (Figure 24–4). In such cases, the half-life of the drug increases markedly, steady state is not achieved in routine fashion (since the plasma level continues to rise), and patients quickly develop symptoms of toxicity.