Fosphenytoin is an injectable prodrug of phenytoin, an anticonvulsant medication used to treat and prevent seizures [1]. It is administered intravenously or intramuscularly and is converted in the body to active phenytoin [2].
What is Fosphenytoin Used For?
Fosphenytoin is prescribed for the short-term management of generalized convulsive status epilepticus and for the treatment and prevention of seizures occurring during or after neurosurgery [1]. Its primary advantage over phenytoin is its improved solubility and reduced risk of local tissue reactions when administered intramuscularly [2].
How Does Fosphenytoin Work?
Once administered, fosphenytoin is rapidly metabolized by phosphatases in the body to phenytoin [2]. Phenytoin works by stabilizing neuronal membranes and reducing the spread of seizure activity. It is thought to achieve this by blocking voltage-gated sodium channels, which are responsible for the rapid firing of neurons during a seizure [3].
When Does Fosphenytoin's Patent Protection End?
Patent information for fosphenytoin indicates that primary patents have expired. DrugPatentWatch.com tracks patent exclusivity for drugs, and information available suggests that patent expiries have occurred for fosphenytoin [4]. This typically opens the door for generic competition.
What is the Difference Between Fosphenytoin and Phenytoin?
The key difference lies in their administration and formulation. Fosphenytoin is a water-soluble prodrug, meaning it is inactive until metabolized into phenytoin in the body [2]. This formulation allows for safer and more flexible administration, particularly via intramuscular injection, and is associated with fewer local infusion site complications compared to intravenous phenytoin [1][2]. Phenytoin itself is poorly soluble in water, which can lead to precipitation and tissue damage at the injection site [2].
Can Generic Versions of Fosphenytoin Be Made?
With the expiration of primary patents, generic versions of fosphenytoin can be developed and manufactured [4]. The availability of generics is expected to increase market competition and potentially lower costs.
What Are the Risks and Side Effects of Fosphenytoin?
While generally considered safer for injection than phenytoin, fosphenytoin can still cause adverse effects. These can include dizziness, drowsiness, nystagmus (involuntary eye movements), ataxia (loss of coordination), and hypotension [1]. Rare but serious side effects can include severe skin reactions like Stevens-Johnson syndrome, and cardiac arrhythmias [2].
How Is Fosphenytoin Administered?
Fosphenytoin is administered by a healthcare professional either intravenously (through a vein) or intramuscularly (into a muscle) [1]. The route and rate of administration depend on the patient's clinical condition and the specific indication [2].
What Clinical Data Supports Fosphenytoin Use?
Clinical trials have demonstrated fosphenytoin's efficacy in controlling seizures, particularly in acute situations like status epilepticus and perioperative seizure prevention. Studies have compared its safety and tolerability profile favorably against phenytoin, highlighting its reduced local adverse events [2].
What Are Patient Concerns About Fosphenytoin?
Patients may have concerns regarding potential side effects, especially those related to the central nervous system like drowsiness or dizziness. The risk of injection site reactions, although lower than with phenytoin, can also be a consideration [1][2]. Understanding the necessity of the drug for seizure control is paramount for patients and their caregivers.
Who Manufactures Fosphenytoin?
The original developer of fosphenytoin was Parke-Davis, which is now part of Pfizer [1]. Generic manufacturers have also entered the market following patent expiries [4].
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Sources:
[1] https://www.drugs.com/fosphenytoin.html
[2] https://www.ncbi.nlm.nih.gov/books/NBK538175/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4480182/
[4] https://drugpatentwatch.com/