Fosphenytoin

Fosphenytoin (fosphenytoin sodium injection, previously Cerebyx, Parke-Davis; Prodilantin, Pfizer Holding France ) is a water-soluble phenytoin prodrug that is administered intravenously to deliver phenytoin, potentially more safely than intravenous phenytoin. It is most commonly used in the acute treatment convulsive status epilepticus.

On 18 November 2004, Sicor (a subsidiary of Teva) received a tentative approval letter from the United States Food and Drug Administration for a generic version of fosphenytoin.

Approved
Fosphenytoin is approved in the United States for the short term (five days or fewer) treatment of epilepsy when more widely used means of phenytoin administration are not possible or are ill-advised, such as endotracheal intubation, status epilepticus or some other type of repeated seizures; vomiting, and/or the patient is unalert or not awake or both.

Unapproved/off-label/investigational
In April 2003, Applebaum and colleagues at the Ben-Gurion University of the Negev in Beersheba reported that even though anticonvulsants are often very effective in mania, and acute mania requires rapid treatment, fosphenytoin had no antimanic effect even 60 minutes after administration of doses used in status epilepticus.

Fosphenytoin was more successfully used to relieve pain refractory to opiates in a 37-year-old woman with neuroma, according to Dr. Gary J. McCleane of the Rampark Pain Center in Lurgan, Northern Ireland. She was given 1,500 phenytoin equivalent units of fosphenytoin over a 24 hour period, producing pain relief that last three to fourteen weeks after each infusion, allowing her to use less opiates.

Metabolism
One millimole of phenytoin is produced for every millimole of fosphenytoin administered; the hydrolysis of fosphenytoin also yields phosphate and formaldehyde, the latter of which is subsequently metabolized to formate, which is in turn metabolized by a folate dependent mechanism.

Side effects
Side effects are similar to intravenous phenytoin and include hypotension, cardiac arrhythmias, CNS adverse events (nystagmus, dizziness, sedation/somnolence, ataxia and stupor), and local dermatological reactions. Purple glove syndrome probably occurs with fosphenytoin but possibly at lower frequency than with intravenous phenytoin. Fosphenytoin can cause hyperphosphatemia in end-stage renal failure patients.

History
Phenytoin, in both its acidic and sodium salt forms, is erratically bioavailable whether it is injected or taken orally due to its high melting point, weak acidity, and its being only sparingly soluble in water. Simply putting patients on other drugs is not always an option; this was especially true before 1993, when the number of anticonvulsants available was much more limited. One solution was to develop a prodrug that did not have these drawbacks.

Fosphenytoin was approved by the Food and Drug Administration (FDA) on August 5, 1996 for use in epilepsy.