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Action/Kinetics:
Fosphenytoin is a prodrug of phenytoin; thus, its anticonvulsant effects are due to phenytoin. For every millimole of fosphenytoin administered, 1 mmol of phenytoin is produced.
t
1/2, fosphenytoin: 15 min after IV infusion.
Peak plasma levels, after IM: 30 min. Significantly bound (95% to 99%) to plasma protein. Fosphenytoin displaces phenytoin from plasma protein binding sites. Fosphenytoin is better tolerated at the infusion site than is phenytoin (i.e., pain and burning associated with IV phenytoin is decreased). The IV infusion rate for fosphenytoin is three times faster than for IV phenytoin. IM use results in systemic phenytoin concentrations that are similar to PO phenytoin, thus allowing interchangeable use. Phenytoin derived from fosphenytoin is extensively metabolized in the liver and excreted in the urine.
Uses:
Short-term parenteral use for the control of generalized convulsive status epilepticus and prophylaxis and treatment of seizures occurring during neurosurgery. It can be substituted, short term, for PO phenytoin when PO administration is not possible.
Contraindications:
Hypersensitivity to fosphenytoin, phenytoin, or other hydantoins. Use in clients with sinus bradycardia, SA block, second- and third-degree AV block, and Adams-Stokes syndrome. Use to treat absence seizures. Use during lactation.
Special Concerns:
The safety and efficacy of fosphenytoin have not been determined for longer than 5 days. Safety has not been determined in pediatric clients. After administration of fosphenytoin to those with renal and/or hepatic dysfunction or in those with hypoalbuminemia, fosphenytoin clearance to phenytoin may be increased without a similar increase in phenytoin clearance, thus increasing the potential for serious side effects. Do not confuse Cerebyx (fosphenytoin sodium injection) with Celebrex (celecoxib) or Celexa (Citalopram hydrobromide) -- read label carefully.
Side Effects:
See
Phenytoin. The most common side effects include ataxia, dizziness, headache, nystagmus, paresthesia, pruritus, and somnolence.
Laboratory Test Alterations:
See Phenytoin.
Drug Interactions:
See Phenytoin.
How Supplied:
Injection: 75 mg/mL
Dosage
NOTE: Doses of fosphenytoin are expressed as phenytoin sodium equivalents (PE = phenytoin sodium equivalent). Thus, adjustments in the recommended doses should not be made when substituting fosphenytoin for phenytoin sodium or vice versa. Do not confuse the amount of fosphenytoin equivalents per mL with the total amount of equivalents in the vial.
?IV
Status epilepticus.
Loading dose: 15-20 mg PE/kg given at a rate of 100-150 mg PE/min. The loading dose is followed by maintenance doses of either fosphenytoin or phenytoin, either PO or parenterally.
?IM, IV
Nonemergency loading and maintenance dosing.
Loading dose: 10-20 mg PE/kg given at a rate of 100-150 mg PE/min.
Maintenance: 4-6 mg PE/kg/day.
Temporary substitution for PO phenytoin.
Use the same daily PO dose of phenytoin in milligrams given at a rate not to exceed 150 mg PE/min.
