Fosphenytoin is approved in the United States for the short term (five days or less) 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.
In April of 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.
One mmol (millimole) of phenytoin is produced for every mmol 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 are similar to phenytoin, except that fosphenytoin causes less hypotension and more paresthesia. Fosphenytoin can cause hyperphosphatemia in end-stage renal failure patients.
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.
^ ab McCleane GJ (2002). "Intravenous infusion of fosphenytoin produces prolonged pain relief: a case report". The Journal of Pain3 (2): 156-8. PMID 14622802. List of Library Holdings Worldwide
^ Browne TR. (1997). "Fosphenytoin (Cerebyx)". Clinical Neuropharmacology20 (1): 1-12. PMID 9037568. List of Library Holdings Worldwide
^ McBryde KD, Wilcox J, Kher KK (2005). "Hyperphosphatemia due to fosphenytoin in a pediatric ESRD patient" (PDF). Pediatric Nephrology (Berlin, Germany)20 (8): 1182-5. PMID 15965770.
^ Yamaoka Y, Roberts RD, Stella VJ. "Low-melting phenytoin prodrugs as alternative oral delivery modes for phenytoin: a model for other high-melting sparingly water-soluble drugs." Journal of Pharmaceutical Science. 1983 Apr;72(4):400-5. PMID 6864479