FOSPHENYTOIN � fosphenytoin sodium injection, solution
Fresenius Kabi USA, LLC
Rx only
WARNING: CARDIOVASCULAR RISK ASSOCIATED WITH RAPID INFUSION RATES
The rate of intravenous fosphenytoin sodium injection administration should not exceed 150 mg phenytoin sodium equivalents (PE) per minute because of the risk of severe hypotension and cardiac arrhythmias. Careful cardiac monitoring is needed during and after administering intravenous fosphenytoin sodium. Although the risk of cardiovascular toxicity increases with infusion rates above the recommended infusion rate, these events have also been reported at or below the recommended infusion rate. Reduction in rate of administration or discontinuation of dosing may be needed (see WARNINGS and DOSAGE AND ADMINISTRATION).
DESCRIPTION:
Fosphenytoin Sodium Injection, USP is a prodrug intended for parenteral administration; its active metabolite is phenytoin. 1.5 mg of fosphenytoin sodium is equivalent to 1 mg phenytoin sodium, and is referred to as 1 mg phenytoin sodium equivalents (PE). The amount and concentration of fosphenytoin is always expressed in terms of mg PE.
Fosphenytoin Sodium Injection, USP 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. Fosphenytoin Sodium Injection, USP, is supplied in vials as a ready-mixed solution in water for injection, and tromethamine (TRIS), buffer adjusted to pH 8.6 to 9.0 with either hydrochloric acid, or sodium hydroxide. Fosphenytoin Sodium Injection, USP, is a clear, colorless to pale yellow, sterile solution.
The chemical name of fosphenytoin is 5,5-diphenyl-3-[(phosphonooxy)methyl]-2,4- imidazolidinedione disodium salt. The molecular structure of fosphenytoin is:
structure
(click image for full-size original)
M.W. 406.24
IMPORTANT NOTE: Throughout all fosphenytoin sodium injection product labeling, the amount and concentration of fosphenytoin are always expressed in terms of phenytoin sodium equivalents (PE). Fosphenytoin�s weight is expressed as phenytoin sodium equivalents to avoid the need to perform molecular weight-based adjustments when substituting fosphenytoin for phenytoin or vice versa. Care should be taken to ensure that fosphenytoin sodium injection is always prescribed and dispensed in phenytoin sodium equivalents (PE) (see DOSAGE AND ADMINISTRATION).
CLINICAL PHARMACOLOGY:
Introduction
Following parenteral administration of fosphenytoin sodium injection, fosphenytoin is converted to the anticonvulsant phenytoin. For every mmol of fosphenytoin administered, one mmol of phenytoin is produced. The pharmacological and toxicological effects of fosphenytoin include those of phenytoin. However, the hydrolysis of fosphenytoin to phenytoin yields two metabolites, phosphate and formaldehyde. Formaldehyde is subsequently converted to formate, which is in turn metabolized via a folate dependent mechanism. Although phosphate and formaldehyde (formate) have potentially important biological effects, these effects typically occur at concentrations considerably in excess of those obtained when fosphenytoin sodium injection is administered under conditions of use recommended in this labeling.
Mechanism of Action
Fosphenytoin is a prodrug of phenytoin and accordingly, its anticonvulsant effects are attributable to phenytoin. After IV administration to mice, fosphenytoin blocked the tonic phase of maximal electroshock seizures at doses equivalent to those effective for phenytoin. In addition to its ability to suppress maximal electroshock seizures in mice and rats, phenytoin exhibits anticonvulsant activity against kindled seizures in rats, audiogenic seizures in mice, and seizures produced by electrical stimulation of the brainstem in rats. The cellular mechanisms of phenytoin thought to be responsible for its anticonvulsant actions include modulation of voltage-dependent sodium channels of neurons, inhibition of calcium flux across neuronal membranes, modulation of voltage-dependent calcium channels of neurons, and enhancement of the sodium-potassium ATPase activity of neurons and glial cells. The modulation of sodium channels may be a primary anticonvulsant mechanism because this property is shared with several other anticonvulsants in addition to phenytoin.
