Overview
11 DESCRIPTION Phenytoin sodium is related to the barbiturates in chemical structure, but has a five-membered ring. The chemical name is 5,5-Diphenylhydantoin sodium salt, having the following structural formula: Each 200 mg or 300 mg PHENYTEK ® capsule (extended phenytoin sodium capsule, USP) for oral administration contains 200 mg or 300 mg phenytoin sodium, USP.
Mechanism of Action
12 CLINICAL PHARMACOLOGY 12.1 Mechanism of Action The precise mechanism by which phenytoin exerts its therapeutic effect has not been established but is thought to involve the voltage-dependent blockade of membrane sodium channels resulting in a reduction in sustained high-frequency neuronal discharges. 12.3 Pharmacokinetics Absorption For PHENYTEK ® capsules, peak serum levels occur 4 to 12 hours after administration. Steady-state therapeutic levels are achieved at least 7 to 10 days (5-7 half-lives) after initiation of therapy with recommended doses of 300 mg/day.
Indications
- 1 INDICATIONS AND USAGE PHENYTEK ® capsules (extended phenytoin sodium capsules) are indicated for the treatment of tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures and prevention and treatment of seizures occurring during or following neurosurgery. PHENYTEK ® CAPSULES are indicated for the treatment of tonic-clonic (grand mal) and psychomotor (temporal lobe) seizures and prevention and treatment of seizures occurring during or following neurosurgery. ( 1 )
Common Doses
- 100 mg
- 300 mg
- 5 mg
- 8 mg
Dosage
2 DOSAGE AND ADMINISTRATION • Adult starting dose in patients who have received no previous treatment is one 100 mg extended phenytoin sodium capsule three times a day, with dose adjustments as necessary. For most adults, the satisfactory maintenance dose will be one capsule three to four times a day. An increase, up to two capsules three times a day may be made, if necessary. ( 2.1 ) • Adult once-a-day dose: If seizure control is established with divided doses of three 100 mg extended phenytoin sodium capsules daily, once-a-day dosage with 300 mg PHENYTEK ® capsules may be considered. ( 2.
Contraindications
- 4 CONTRAINDICATIONS PHENYTEK ® capsules are contraindicated in patients with: • A history of hypersensitivity to phenytoin, its inactive ingredients, or other hydantoins [see Warnings and Precautions (5.5) ] . Reactions have included angioedema. • A history of prior acute hepatotoxicity attributable to phenytoin [see Warnings and Precautions (5.8) ] . • Coadministration with delavirdine because of the potential for loss of virologic response and possible resistance to delavirdine or to the class of non-nucleoside reverse transcriptase inhibitors.
Side Effects
- 6 ADVERSE REACTIONS The following serious adverse reactions are described elsewhere in the labeling: • Withdrawal Precipitated Seizure, Status Epilepticus [see Warnings and Precautions (5.1) ] • Suicidal Behavior and Ideation [see Warnings and Precautions (5.2) ] • Serious Dermatologic Reactions [see Warnings and Precautions (5.3) ] • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)/Multiorgan Hypersensitivity [see Warnings and Precautions (5.4) ] • Hypersensitivity [see Warnings and Precautions (5.5) ] • Cardiac Effects [see Warnings and Precautions (5.
Interactions
- 7 DRUG INTERACTIONS Phenytoin is extensively bound to plasma proteins and is prone to competitive displacement. Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19, and is particularly susceptible to inhibitory drug interactions because it is subject to saturable metabolism. Inhibition of metabolism may produce significant increases in circulating phenytoin concentrations and enhance the risk of drug toxicity. Monitoring of phenytoin serum levels is recommended when a drug interaction is suspected.