Fosphenytoin sodium injection (or phenytoin) doses are usually selected to attain therapeutic serum total phenytoin concentrations of 10 to 20 mcg/mL (unbound phenytoin concentrations of 1 to 2 mcg/mL). Following fosphenytoin sodium injection administration, it is recommended that phenytoin concentrations not be monitored until conversion to phenytoin is essentially complete. This occurs within approximately 2 hours after the end of IV infusion and 4 hours after intramuscular (IM) injection. Prior to complete conversion, commonly used immunoanalytical techniques, such as TDx® /TDxFLx™ (fluorescence polarization) and Emit® 2000 (enzyme multiplied), may significantly overestimate serum phenytoin concentrations because of cross-reactivity with fosphenytoin. The error is dependent on serum phenytoin and fosphenytoin concentration (influenced by fosphenytoin sodium injection dose, route and rate of administration, and time of sampling relative to dosing), and analytical method. Chromatographic assay methods accurately quantitate phenytoin concentrations in biological fluids in the presence of fosphenytoin. Prior to complete conversion, blood samples for phenytoin monitoring should be collected in tubes containing EDTA as an anticoagulant to minimize ex vivo conversion of fosphenytoin to phenytoin. However, even with specific assay methods, phenytoin concentrations measured before conversion of fosphenytoin is complete will not reflect phenytoin concentrations ultimately achieved.
Trough levels provide information about clinically effective serum level range and are obtained just prior to the patient’s next scheduled dose. Peak levels indicate an individual’s threshold for emergence of dose-related side effects and are obtained at the time of expected peak concentration. Therapeutic effect without clinical signs of toxicity occurs more often with serum total phenytoin concentrations between 10 and 20 mcg/mL (unbound phenytoin concentrations of 1 to 2 mcg/mL), although some mild cases of tonic-clonic (grand mal) epilepsy may be controlled with lower serum levels of phenytoin. In patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of unbound phenytoin concentrations may be more relevant [see Dosage and Administration (2.7)].
When treatment with oral phenytoin is not possible, fosphenytoin sodium injection can be substituted for oral phenytoin at the same total daily phenytoin sodium equivalents (PE) dose. Dilantin capsules are approximately 90% bioavailable by the oral route. Phenytoin, derived from administration of fosphenytoin sodium injection, is 100% bioavailable by both the IM and IV routes. For this reason, serum phenytoin concentrations may increase modestly when IM or IV fosphenytoin sodium injection is substituted for oral phenytoin sodium therapy. The rate of administration for IV fosphenytoin sodium injection should be no greater than 150 mg PE/min in adults and 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients. In controlled trials, IM fosphenytoin sodium injection was administered as a single daily dose utilizing either 1 or 2 injection sites. Some patients may require more frequent dosing. Intramuscular administration of fosphenytoin sodium injection should ordinarily not be used in pediatric patients.
Because the fraction of unbound phenytoin (the active metabolite of fosphenytoin sodium injection) is increased in patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of phenytoin serum levels should be based on the unbound fraction in those patients. After IV fosphenytoin sodium injection administration to patients with renal and/or hepatic disease, or in those with hypoalbuminemia, fosphenytoin clearance to phenytoin may be increased without a similar increase in phenytoin clearance. This has the potential to increase the frequency and severity of adverse events [see Warnings and Precautions (5.13)].
The clearance of phenytoin (the active metabolite of fosphenytoin sodium injection) is decreased slightly in elderly patients and lower or less frequent dosing may be required [see Clinical Pharmacology (12.3)].
Decreased serum concentrations of phenytoin (the active metabolite of fosphenytoin sodium injection) may occur during pregnancy because of altered phenytoin pharmacokinetics [see Clinical Pharmacology (12.3)]. Periodic measurement of serum phenytoin concentrations should be performed during pregnancy, and the fosphenytoin sodium injection dosage should be adjusted as necessary. Postpartum restoration of the original dosage will probably be indicated [see Use in Specific Populations (8.1)]. Because of potential changes in protein binding during pregnancy, the monitoring of phenytoin serum levels should be based on the unbound fraction.
Fosphenytoin sodium injection is a clear, colorless to pale yellow solution available as 50 mg phenytoin sodium equivalents (PE) per mL in:
- 10 mL single-dose injection vials, each containing 500 mg PE/10 mL (50 mg PE/mL)
- 2 mL single-dose injection vials, each containing 100 mg PE/2 mL (50 mg PE/mL)
Fosphenytoin sodium injection is contraindicated in patients with:
- A history of hypersensitivity to fosphenytoin sodium injection or its inactive ingredients, or to phenytoin or other hydantoins [see Warnings and Precautions (5.6)]. Reactions have included angioedema.
- Sinus bradycardia, sino-atrial block, second and third degree A-V block, or Adams-Stokes syndrome because of the effect of parenteral phenytoin or fosphenytoin sodium injection on ventricular automaticity.
- A history of prior acute hepatotoxicity attributable to fosphenytoin sodium injection or 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.
Phenytoin Sodium Equivalents (PE)
Do not confuse the amount of drug to be given in PE with the concentration of the drug in the vial.
Doses of fosphenytoin sodium injection are always expressed in terms of milligrams of phenytoin sodium equivalents (mg PE). 1 mg PE is equivalent to 1 mg phenytoin sodium.
Do not, therefore, make any adjustment in the recommended doses when substituting fosphenytoin sodium injection for phenytoin sodium or vice versa. For example, if a patient is receiving 1000 mg PE of fosphenytoin sodium injection, that is equivalent to 1000 mg of phenytoin sodium.
Concentration of 50 mg PE/mL
Medication errors associated with fosphenytoin sodium injection have resulted in patients receiving the wrong dose of fosphenytoin. Fosphenytoin sodium injection is marketed in 2 mL vials containing a total of 100 mg PE and 10 mL vials containing a total of 500 mg PE. The concentration of each vial is 50 mg PE/mL. Errors have occurred when the concentration of the vial (50 mg PE/mL) was misinterpreted to mean that the total content of the vial was 50 mg PE. These errors have resulted in two- or ten-fold overdoses of fosphenytoin sodium injection since each vial actually contains a total of 100 mg PE or 500 mg PE. In some cases, ten-fold overdoses were associated with fatal outcomes. To help minimize confusion, the prescribed dose of fosphenytoin sodium injection should always be expressed in milligrams of phenytoin equivalents (mg PE) [see Dosage and Administration (2.1)]. Additionally, when ordering and storing fosphenytoin sodium injection, consider displaying the total drug content (i.e., 100 mg PE/2 mL or 500 mg PE/10 mL) instead of concentration in computer systems, pre-printed orders, and automated dispensing cabinet databases to help ensure that total drug content can be clearly identified. Care should be taken to ensure the appropriate volume of fosphenytoin sodium injection is withdrawn from the vial when preparing the drug for administration. Attention to these details may prevent some fosphenytoin sodium injection medication errors from occurring.
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