Fosphenytoin Sodium

FDA Drug Information • Also known as: Cerebyx, Fosphenytoin, Fosphenytoin Sodium

Brand Names: Cerebyx, Fosphenytoin, Fosphenytoin Sodium
Route: INTRAMUSCULAR, INTRAVENOUS
Dosage Form: INJECTION, SOLUTION
Product Type: HUMAN PRESCRIPTION DRUG

⚠ Boxed Warning (Black Box)

WARNING: CARDIOVASCULAR RISK ASSOCIATED WITH RAPID INFUSION RATES The rate of intravenous CEREBYX administration should not exceed 150 mg phenytoin sodium equivalents (PE) per minute in adults and 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients because of the risk of severe hypotension and cardiac arrhythmias. Careful cardiac monitoring is needed during and after administering intravenous CEREBYX. 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 Dosage and Administration (2.3 , 2.4) and Warnings and Precautions (5.2) ]. WARNING: CARDIOVASCULAR RISK ASSOCIATED WITH RAPID INFUSION RATES See full prescribing information for complete boxed warning .

The rate of intravenous CEREBYX administration should not exceed 150 mg phenytoin sodium equivalents (PE) per minute in adults and 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients because of the risk of severe hypotension and cardiac arrhythmias.

Careful cardiac monitoring is needed during and after administering intravenous CEREBYX.

Reduction in rate of administration or discontinuation of dosing may be needed ( 2.3 , 2.4 , 5.2 ) .

Description

11 DESCRIPTION CEREBYX ® (fosphenytoin sodium injection) 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 . The pharmacological class of the fosphenytoin sodium is hydantoin derivative, and the therapeutic class is anticonvulsant. CEREBYX is marketed in 2 mL vials containing a total of 100 mg PE/2 mL (50 mg PE/mL) and 10 mL vials containing a total of 500 mg PE/10 mL (50 mg PE/mL), for intravenous or intramuscular administration. The concentration of each vial is 50 mg PE/mL. CEREBYX is supplied in vials as a sterile solution in Water for Injection, USP, and Tromethamine, USP (TRIS) (12.11 mg/mL), buffer adjusted to pH 8.6 to 9.0 with either Hydrochloric Acid, NF, or Sodium Hydroxide, NF. CEREBYX 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: The molecular weight of fosphenytoin is 406.24. Chemical Structure

What Is Fosphenytoin Sodium Used For?

1 INDICATIONS AND USAGE CEREBYX is indicated for the treatment of generalized tonic-clonic status epilepticus and prevention and treatment of seizures occurring during neurosurgery. CEREBYX can also be substituted, short-term, for oral phenytoin. CEREBYX should be used only when oral phenytoin administration is not possible [see Dosage and Administration (2.4) and Warnings and Precautions (5.2) ] . CEREBYX is indicated for the treatment of generalized tonic-clonic status epilepticus and prevention and treatment of seizures occurring during neurosurgery. CEREBYX can also be substituted, as short-term use, for oral phenytoin. CEREBYX should be used only when oral phenytoin administration is not possible. ( 1 )

Dosage and Administration

2 DOSAGE AND ADMINISTRATION

The dose, concentration, and infusion rate of CEREBYX should always be expressed as phenytoin sodium equivalents (PE) ( 2.1 )

For Status Epilepticus: o Adult loading dose is 15 to 20 mg PE/kg at a rate of 100 to 150 mg PE/min ( 2.3 ) o Pediatric loading dose is 15 to 20 mg PE/kg at a rate of 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) ( 2.3 )

For Non-emergent Loading and Maintenance Dosing: o Adult loading dose is 10 to 20 mg PE/kg given IV or IM; initial maintenance dose is 4 to 6 mg PE/kg/day in divided doses ( 2.4 ) o Pediatric loading dose is 10 to 15 mg PE/kg at a rate of 1 to 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower); initial maintenance dose is 2 to 4 mg PE/kg every 12 hours at a rate of 1 to 2 mg PE/kg/min (or 100 mg PE/min, whichever is slower) ( 2.4 )

Intramuscular Administration: o CEREBYX should ordinarily not be given intramuscularly ( 2.3 , 2.4 ) 2.1 Important Administration Instructions to Avoid Dosing Errors Use caution when administering CEREBYX because of the risk of dosing errors [see Warnings and Precautions (5.1) ]. Phenytoin Sodium Equivalents (PE) The dose, concentration, and infusion rate of CEREBYX should always be expressed as phenytoin sodium equivalents (PE). There is no need to perform molecular weight-based adjustments when converting between fosphenytoin and phenytoin sodium doses. CEREBYX should always be prescribed and dispensed in phenytoin sodium equivalent units (PE). The amount and concentration of fosphenytoin is always expressed in terms of mg of phenytoin sodium equivalents (mg PE). Concentration of 50 mg PE/mL Do not confuse the concentration of CEREBYX with the total amount of drug in the vial. Errors, including fatal overdoses, have occurred when the concentration of the vial (50 mg PE/mL) was misinterpreted to mean that the total content of the vial was 50 mg PE. These errors have resulted in two- or ten-fold overdoses of CEREBYX since each of the vials actually contains a total of 100 mg PE (2 mL vial) or 500 mg PE (10 mL vial). Ensure the appropriate volume of CEREBYX is withdrawn from the vial when preparing the dose for administration. Attention to these details may prevent some CEREBYX medication errors from occurring. 2.2 Preparation Prior to intravenous (IV) infusion, dilute CEREBYX in 5% Dextrose Injection or 0.9% Sodium Chloride Injection to a concentration ranging from 1.5 to 25 mg PE/mL. The maximum concentration of CEREBYX in any solution should be 25 mg PE/mL. When CEREBYX is given as an IV infusion, CEREBYX needs to be diluted and should only be administered at a rate not exceeding 150 mg PE/min. Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit. For single-dose only. After opening, any unused product should be discarded. 2.3 Status Epilepticus

Because of the risk of hypotension and cardiac arrhythmias, the rate of...

Side Effects (Adverse Reactions)

6 ADVERSE REACTIONS The following serious adverse reactions are described elsewhere in the labeling:

Cardiovascular Risk Associated with Rapid Infusion [see Warnings and Precautions (5.2) ]

Withdrawal Precipitated Seizure, Status Epilepticus [see Warnings and Precautions (5.3) ]

Serious Dermatologic Reactions [see Warnings and Precautions (5.4) ]

Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)/Multiorgan Hypersensitivity [see Warnings and Precautions (5.5) ]

Hypersensitivity [see Warnings and Precautions (5.6) ]

Angioedema [see Warnings and Precautions (5.7) ]

Hepatic Injury [see Warnings and Precautions (5.8) ]

Hematopoietic Complications [see Warnings and Precautions (5.9) ]

Sensory Disturbances [see Warnings and Precautions (5.10) ]

Local Toxicity (Including Purple Glove Syndrome) [see Warnings and Precautions (5.11) ]

Exacerbation of Porphyria [see Warnings and Precautions (5.14) ]

Teratogenicity and Other Harm to the Newborn [see Warnings and Precautions (5.15) ]

Hyperglycemia [see Warnings and Precautions (5.16) ] Most common adverse reactions (incidence ≥10%) are:

Adults: pruritus, nystagmus, dizziness, somnolence, and ataxia

Pediatrics: vomiting, nystagmus, and ataxia ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Pfizer, Inc. at 1-800-438-1985 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch . 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice. The more important adverse clinical reactions caused by the IV use of CEREBYX or phenytoin are cardiovascular collapse and/or CNS depression. Hypotension can occur when either drug is administered rapidly by the IV route. The rate of administration is very important; for CEREBYX, it should not exceed 150 mg PE/min [see Warnings and Precautions (5.2) ] . The adverse reactions most commonly observed with the use of CEREBYX in clinical trials were nystagmus, dizziness, pruritus, somnolence, and ataxia. With one exception, these reactions are commonly associated with the administration of IV phenytoin. Pruritus, however, was seen much more often following CEREBYX administration and occurred more often with IV CEREBYX administration than with IM CEREBYX administration. These reactions were dose and rate related; most alert patients (41 of 64; 64%) administered doses of ≥15 mg PE/kg at 150 mg PE/min experienced discomfort of some degree. These sensations, generally described as itching, burning, or tingling, were usually not at the infusion site. The location of the discomfort varied with the groin mentioned most frequently as a site of involvement. The paresthesia and pruritus were transient events that occurred within several minutes of the start of infusion and generally resolved within 10 minutes after completion of CEREBYX infusion. Some patients experienced symptoms for hours. These reactions did not increase in severity with repeated administration. Concurrent adverse events or clinical laboratory change suggesting an allergic process were not seen [see Warnings and Precautions (5.10) ] . Approximately 2% of the 859 patients who received CEREBYX in premarketing clinical trials discontinued treatment because of an adverse event. The adverse events most commonly associated with withdrawal were pruritus (0.5%), hypotension (0.3%), and bradycardia (0.2%). Dose and Rate Dependency of Adverse Reactions Following IV CEREBYX: The incidence of adverse reactions tended to increase as both dose and infusion rate increased. In particular, at doses of ≥15mg PE/kg and rates ≥150 mg PE/min, transient pruritus, tinnitus, nystagmus, somnolence, and ataxia occurred 2 to 3 times more often than at lower doses or rates. Incidence in Controlled Clinical Trials All adverse events were recorded during the...

Drug Interactions

7 DRUG INTERACTIONS Fosphenytoin is extensively bound to human plasma proteins. Drugs highly bound to albumin could increase the unbound fraction of fosphenytoin. Although, it is unknown whether this could result in clinically significant effects, caution is advised when administering CEREBYX with other drugs that significantly bind to serum albumin. The most significant drug interactions following administration of CEREBYX are expected to occur with drugs that interact with phenytoin. Phenytoin is extensively bound to serum plasma proteins and is prone to competitive displacement. Phenytoin is primarily metabolized by the hepatic cytochrome P450 enzyme CYP2C9 and to a lesser extent by CYP2C19 and is particularly susceptible to inhibitory drug interactions because it is subject to saturable metabolism. Inhibition of metabolism may produce significant increases in circulating phenytoin concentrations and enhance the risk of drug toxicity. Monitoring of phenytoin serum levels is recommended when a drug interaction is suspected. Phenytoin or CEREBYX is a potent inducer of hepatic drug-metabolizing enzymes.

Multiple drug interactions because of extensive plasma protein binding, saturable metabolism, and potent induction of hepatic enzymes ( 7.1 , 7.2 ) 7.1 Drugs that Affect Phenytoin or CEREBYX Table 6 includes commonly occurring drug interactions that affect phenytoin (the active metabolite of CEREBYX) concentrations. However, this list is not intended to be inclusive or comprehensive. Individual prescribing information from relevant drugs should be consulted. The addition or withdrawal of these agents in patients on phenytoin therapy may require an adjustment of the phenytoin dose to achieve optimal clinical outcome. Table 6. Drugs That Affect Phenytoin Concentrations Interacting Agent Examples Drugs that may increase phenytoin serum levels Antiepileptic drugs Ethosuximide, felbamate, oxcarbazepine, methsuximide, topiramate Azoles Fluconazole, ketoconazole, itraconazole, miconazole, voriconazole Antineoplastic agents Capecitabine, fluorouracil Antidepressants Fluoxetine, fluvoxamine, sertraline Gastric acid reducing agents H 2 antagonists (cimetidine), omeprazole Sulfonamides Sulfamethizole, sulfaphenazole, sulfadiazine, sulfamethoxazole-trimethoprim Other Acute alcohol intake, amiodarone, chloramphenicol, chlordiazepoxide, disulfiram, estrogen, fluvastatin, isoniazid, methylphenidate, phenothiazines, salicylates, ticlopidine, tolbutamide, trazodone, warfarin Drugs that may decrease phenytoin serum levels Antineoplastic agents usually in combination Bleomycin, carboplatin, cisplatin, doxorubicin, methotrexate Antiviral agents Fosamprenavir, nelfinavir, ritonavir Antiepileptic drugs Carbamazepine, vigabatrin Other Chronic alcohol abuse, diazepam, diazoxide, folic acid, reserpine, rifampin, St. John's wort, The induction potency of St. John's wort may vary widely based on preparation. theophylline Drugs that may either increase or decrease...

Contraindications

4 CONTRAINDICATIONS CEREBYX is contraindicated in patients with:

A history of hypersensitivity to CEREBYX or its inactive ingredients, or to phenytoin or other hydantoins [see Warnings and Precautions (5.6) ] . Reactions have included angioedema.

Sinus bradycardia, sino-atrial block, second and third degree A-V block, or Adams-Stokes syndrome because of the effect of parenteral phenytoin or CEREBYX on ventricular automaticity.

A history of prior acute hepatotoxicity attributable to CEREBYX or phenytoin [see Warnings and Precautions (5.8) ] .

Coadministration with delavirdine because of the potential for loss of virologic response and possible resistance to delavirdine or to the class of non-nucleoside reverse transcriptase inhibitors.

Hypersensitivity to CEREBYX, its ingredients, phenytoin, hydantoins ( 4 )

Sinus bradycardia, sino-atrial block, second and third degree A-V block, and Adams-Stokes syndrome ( 4 )

A history of prior acute hepatotoxicity attributable to CEREBYX or phenytoin ( 4 , 5.8 )

Coadministration with delavirdine ( 4 )

Pregnancy and Breastfeeding

8.1 Pregnancy Pregnancy Exposure Registry There is a pregnancy exposure registry that monitors pregnancy outcomes in women exposed to antiepileptic drugs (AEDs), such as CEREBYX, during pregnancy. Physicians are advised to recommend that pregnant patients taking CEREBYX enroll in the North American Antiepileptic Drug (NAAED) Pregnancy Registry. This can be done by calling the toll-free number 1-888-233-2334, and must be done by patients themselves. Information on the registry can also be found at the website http://www.aedpregnancyregistry.org/ . Risk Summary In humans, prenatal exposure to phenytoin (the active metabolite of CEREBYX) may increase the risks for congenital malformations and other adverse developmental outcomes. Prenatal phenytoin exposure is associated with an increased incidence of major malformations, including orofacial clefts and cardiac defects. In addition, the fetal hydantoin syndrome, a pattern of abnormalities including dysmorphic skull and facial features, nail and digit hypoplasia, growth abnormalities (including microcephaly), and cognitive deficits has been reported among children born to epileptic women who took phenytoin alone or in combination with other antiepileptic drugs during pregnancy [see Data ] . There have been several reported cases of malignancies, including neuroblastoma, in children whose mothers received phenytoin during pregnancy. Administration of phenytoin to pregnant animals resulted in an increased incidence of fetal malformations and other manifestations of developmental toxicity (including embryofetal death, growth impairment, and behavioral abnormalities) in multiple species at clinically relevant doses [see Data ]. In the U.S. general population, the estimated background risk of major birth defects and of miscarriage in clinically recognized pregnancies is 2 to 4% and 15 to 20%, respectively. The background risk of major birth defects and miscarriage for the indicated population is unknown. Clinical...

Overdosage

10 OVERDOSAGE Nausea, vomiting, lethargy, tachycardia, bradycardia, asystole, cardiac arrest, hypotension, syncope, hypocalcemia, metabolic acidosis, and death have been reported in cases of overdosage with CEREBYX. Because CEREBYX is a prodrug of phenytoin, the following information about phenytoin overdosage may be helpful. Initial symptoms of acute phenytoin toxicity are nystagmus, ataxia, and dysarthria. Other signs include tremor, hyperreflexia, lethargy, slurred speech, nausea, vomiting, coma, and hypotension. Death is caused by respiratory and circulatory depression. The lethal dose of phenytoin in adults is estimated to be 2 to 5 grams. The lethal dose in pediatrics is not known. There are marked variations among individuals with respect to serum phenytoin concentrations where toxicity occurs. Lateral gaze nystagmus usually appears at 20 µg/mL, ataxia at 30 µg/mL, and dysarthria and lethargy appear when the serum concentration is over 40 µg/mL. However, phenytoin concentrations as high as 50 µg/mL have been reported without evidence of toxicity. As much as 25 times the therapeutic phenytoin dose has been taken, resulting in serum phenytoin concentrations over 100 µg/mL, with complete recovery. Irreversible cerebellar dysfunction and atrophy have been reported after overdosage. Formate and phosphate are metabolites of CEREBYX and therefore may contribute to signs of toxicity following overdosage. Signs of formate toxicity are similar to those of methanol toxicity and are associated with severe anion-gap metabolic acidosis. Large amounts of phosphate, delivered rapidly, could potentially cause hypocalcemia with paresthesia, muscle spasms, and seizures. Ionized free calcium levels can be measured and, if low, used to guide treatment. Treatment: Treatment is nonspecific since there is no known antidote to CEREBYX or phenytoin overdosage. The adequacy of the respiratory and circulatory systems should be carefully observed, and appropriate supportive measures...

How Supplied

16 HOW SUPPLIED/STORAGE AND HANDLING 16.1 How Supplied CEREBYX Injection is a clear, colorless to pale yellow solution supplied as follows: mg phenytoin sodium equivalents (PE) per vial Volume per vial (mL) Package Configuration NDC 500 mg PE/10 mL vial 10 mL per vial Package contains 10 vials of NDC 0069-6001-10 NDC 0069-6001-21 100 mg PE/2 mL vial 2 mL per vial Package contains 25 vials of NDC 0069-6001-02 NDC 0069-6001-25 Both sizes of vials contain Tromethamine, USP (TRIS), Hydrochloric Acid, NF, or Sodium Hydroxide, NF, and Water for Injection, USP. CEREBYX should always be prescribed in phenytoin sodium equivalents (PE) [see Dosage and Administration (2.1) and Warnings and Precautions (5.1) ] . 1.5 mg of fosphenytoin sodium is equivalent to 1 mg phenytoin sodium, and is referred to as 1 mg PE. The amount and concentration of fosphenytoin is always expressed in terms of mg 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. 16.2 Storage and Handling Store under refrigeration at 2°C to 8°C (36°F to 46°F). The product should not be stored at room temperature for more than 48 hours. Vials that develop particulate matter should not be used. Injection vials are single-dose only. After opening, any unused product should be discarded.

About This Information

This drug information is sourced from FDA-approved labeling via the openFDA database. It is intended for educational and reference purposes only. This is not medical advice. Always consult your healthcare provider before making decisions about medication. Drug information may be updated by the FDA; check with your pharmacist for the most current information.

What are side effects?

Side effects are unwanted reactions that can occur when taking a medication. They range from mild (headache, nausea) to severe (allergic reactions, organ damage). Not everyone experiences side effects, and severity varies. Report any concerning side effects to your doctor.

What are drug interactions?

Drug interactions occur when a medication is affected by another drug, food, or supplement. Interactions can make medications less effective or cause dangerous side effects. Always tell your doctor about all medications and supplements you take.