Fosphenytoin Sodium

Warnings and Precautions ( 5.9 ) 1/2023

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

2 DOSAGE AND ADMINISTRATION The dose, concentration, and infusion rate of fosphenytoin sodium injection should always be expressed as phenytoin sodium equivalents (PE) ( 2.1 ) For Status Epilepticus: Adult loading dose is 15 to 20 mg PE/kg at a rate of 100 to 150 mg PE/min ( 2.3 ) 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: 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 ) 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 : Fosphenytoin sodium injection should ordinarily not be given intramuscularly ( 2.3 , 2.4 ) 2.1 Important Administration Instructions to Avoid Dosing Errors Use caution when administering fosphenytoin sodium injection because of the risk of dosing errors [see Warnings and Precautions (5.1) ]. Phenytoin Sodium Equivalents (PE) The dose, concentration, and infusion rate of fosphenytoin sodium injection 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. Fosphenytoin sodium injection 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 fosphenytoin sodium injection 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 fosphenytoin sodium injection 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 fosphenytoin sodium injection is withdrawn from the vial when preparing the dose for administration. Attention to these details may prevent some fosphenytoin sodium injection medication errors from occurring. 2.2 Preparation Prior to intravenous (IV) infusion, dilute fosphenytoin sodium injection 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 fosphenytoin sodium injection in any solution should be 25 mg PE/mL. When fosphenytoin sodium injection is given as an IV infusion, fosphenytoin sodium injection 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 administration for IV fosphenytoin sodium injection should be no greater than 150 mg PE/min in adults and 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients [see Warnings and Precautions (5.2) ] . Continuous monitoring of the electrocardiogram, blood pressure, and respiratory function is essential, and the patient should be observed throughout the period where maximal serum phenytoin concentrations occur, approximately 10 to 20 minutes after the end of fosphenytoin sodium injection infusions. Because the full antiepileptic effect of phenytoin, whether given as fosphenytoin sodium injection or parenteral phenytoin, is not immediate, other measures, including concomitant administration of an IV benzodiazep …

3 DOSAGE FORMS AND STRENGTHS Fosphenytoin sodium injection is a clear, colorless to pale yellow solution available as 50 mg phenytoin sodium equivalents (PE) per mL in: 10 mL single-dose injection vials, each containing 500 mg PE/10 mL (50 mg PE/mL) 2 mL single-dose injection vials, each containing 100 mg PE/2 mL (50 mg PE/mL) Injection: 50 mg phenytoin sodium equivalents (PE)/mL available as: 10 mL single-dose injection vials, each containing 500 mg PE/10 mL (50 mg PE/mL) ( 3 ) 2 mL single-dose injection vials, each containing 100 mg PE/2 mL (50 mg PE/mL) ( 3 )

4 CONTRAINDICATIONS Fosphenytoin sodium injection is contraindicated in patients with: A history of hypersensitivity to fosphenytoin sodium injection 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 fosphenytoin sodium injection on ventricular automaticity. A history of prior acute hepatotoxicity attributable to fosphenytoin sodium injection 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 fosphenytoin sodium injection, 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 fosphenytoin sodium injection or phenytoin ( 4 , 5.8 ) Coadministration with delavirdine ( 4 )

ADVERSE REACTIONS The more important adverse clinical events caused by the IV use of fosphenytoin or phenytoin are cardiovascular collapse and/or central nervous system depression. Hypotension can occur when either drug is administered rapidly by the IV route. The rate of administration is very important; for fosphenytoin, it should not exceed 150 mg PE/min. The adverse clinical events most commonly observed with the use of fosphenytoin in clinical trials were nystagmus, dizziness, pruritus, paresthesia, headache, somnolence, and ataxia. With two exceptions, these events are commonly associated with the administration of IV phenytoin. Paresthesia and pruritus, however, were seen much more often following fosphenytoin administration and occurred more often with IV fosphenytoin administration than with IM fosphenytoin administration. These events 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 fosphenytoin infusion. Some patients experienced symptoms for hours. These events did not increase in severity with repeated administration. Concurrent adverse events or clinical laboratory change suggesting an allergic process were not seen (see PRECAUTIONS, Sensory Disturbances ). Approximately 2% of the 859 individuals who received fosphenytoin 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 Events Following IV Fosphenytoin : The incidence of adverse events tended to increase as both dose and infusion rate increased. In particular, at doses of ≥15 mg 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 trials by the clinical investigators using terminology of their own choosing. Similar types of events were grouped into standardized categories using modified COSTART dictionary terminology. These categories are used in the tables and listings below with the frequencies representing the proportion of individuals exposed to fosphenytoin or comparative therapy. The prescriber should be aware that these figures cannot be used to predict the frequency of adverse events in the course of usual medical practice where patient characteristics and other factors may differ from those prevailing during clinical studies. Similarly, the cited frequencies cannot be directly compared with figures obtained from other clinical investigations involving different treatments, uses or investigators. An inspection of these frequencies, however, does provide the prescribing physician with one basis to estimate the relative contribution of drug and nondrug factors to the adverse event incidences in the population studied. Incidence in Controlled Clinical Trials - IV Administration To Patients With Epilepsy or Neurosurgical Patients : Table 2 lists treatment-emergent adverse events that occurred in at least 2% of patients treated with IV fosphenytoin at the maximum dose and rate in a randomized, double-blind, controlled clinical trial where the rates for phenytoin and fosphenytoin administration would have resulted in equivalent systemic exposure to phenytoin. TABLE 2. Treatment-Emergent Adverse Event Incidence Following IV Administration at the Maximum Dose and Rate to Patients with Epilepsy or …

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 fosphenytoin sodium injection with other drugs that significantly bind to serum albumin. The most significant drug interactions following administration of fosphenytoin sodium injection 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 fosphenytoin sodium injection 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 Fosphenytoin Sodium Injection Table 6 includes commonly occurring drug interactions that affect phenytoin (the active metabolite of fosphenytoin sodium injection) 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, a theophylline Drugs that may either increase or decrease phenytoin serum levels Antiepileptic drugs Phenobarbital, valproate sodium, valproic acid a The induction potency of St. John's wort may vary widely based on preparation. 7.2 Drugs Affected by Phenytoin or Fosphenytoin Sodium Injection Table 7 includes commonly occurring drug interactions affected by phenytoin (the active metabolite of fosphenytoin sodium injection). However, this list is not intended to be inclusive or comprehensive. Individual drug package inserts should be consulted. The addition or withdrawal of phenytoin during concomitant therapy with these agents may require adjustment of the dose of these agents to achieve optimal clinical outcome. Table 7: Drugs Affected by Phenytoin Interacting Agent Examples Drugs whose efficacy is impaired by phenytoin Azoles Fluconazole, ketoconazole, itraconazole, posaconazole, voriconazole Antineoplastic agents Irinotecan, paclitaxel, teniposide Delavirdine Phenytoin can substantially reduce the concentrations of d …

8 USE IN SPECIFIC POPULATIONS Pregnancy: Phenytoin (the active metabolite of fosphenytoin sodium injection) prenatal exposure may increase risks for congenital malformations and other adverse developmental outcomes ( 5.15 , 8.1 ) Renal and/or Hepatic Impairment or Hypoalbuminemia: Monitor unbound phenytoin concentrations in these patients ( 8.6 ) 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 fosphenytoin sodium injection, during pregnancy. Physicians are advised to recommend that pregnant patients taking fosphenytoin sodium injection 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 fosphenytoin sodium injection) 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 Considerations Disease-associated maternal risk An increase in seizure frequency may occur during pregnancy because of altered phenytoin pharmacokinetics. Periodic measurement of serum phenytoin concentrations may be valuable in the management of pregnant women as a guide to appropriate adjustment of dosage [see Dosage and Administration (2.5 , 2.9) ] . However, postpartum restoration of the original dosage will probably be indicated. Fetal/Neonatal adverse reactions A potentially life-threatening bleeding disorder related to decreased levels of vitamin K-dependent clotting factors may occur in newborns exposed to phenytoin in utero . This drug-induced condition can be prevented with vitamin K administration to the mother before delivery and to the neonate after birth. Data Human Data Meta-analyses using data from published observational studies and registries have estimated an approximately 2.4-fold increased risk for any major malformation in children with prenatal phenytoin exposure compared to controls. An increased risk of heart defects, facial clefts, and digital hypoplasia has been reported. The fetal hydantoin syndrome is a pattern of congenital anomalies including craniofacial anomalies, nail and digital hypoplasia, prenatal-onset growth deficiency, and neurodevelopmental deficiencies. Animal Data Administration of phenytoin to pregnant rats, rabbits, and mice during organogenesis resulted in embryofetal death, fetal malformations, and decreased fetal growth. Malformations (including craniofacial, cardiovascular, neural, limb, and digit abnormalities) were observ …

12.1 Mechanism of Action Fosphenytoin is a prodrug of phenytoin and accordingly, its anticonvulsant effects are attributable to phenytoin. The precise mechanism by which phenytoin exerts its therapeutic effect has not been established but is thought to involve the voltage-dependent blockade of membrane sodium channels resulting in a reduction in sustained high-frequency neuronal discharges.

DESCRIPTION Fosphenytoin sodium injection, USP is a prodrug intended for parenteral administration; its active metabolite is phenytoin. 1.5 mg fosphenytoin sodium, USP (hereafter referred to as fosphenytoin) equivalent to 1 mg phenytoin sodium and is referred to as 1 mg phenytoin equivalents (PE). The amount and concentration of fosphenytoin is always expressed in terms of mg PE. Fosphenytoin injection 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 is supplied in vials as a ready-mixed solution in Water for Injection, USP, and Tromethamine, USP (TRIS), buffer adjusted to pH 8.6 to 9.0 with either Hydrochloric Acid, NF, or Sodium Hydroxide, NF. Fosphenytoin injection 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. IMPORTANT NOTE: Throughout all fosphenytoin 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 is always prescribed and dispensed in phenytoin sodium equivalents (PE) (see DOSAGE AND ADMINISTRATION ). Structural Formula

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 fosphenytoin sodium injection. Because fosphenytoin sodium injection 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 fosphenytoin sodium injection 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 fosphenytoin sodium injection or phenytoin overdosage. The adequacy of the respiratory and circulatory systems should be carefully observed, and appropriate supportive measures employed. Hemodialysis can be considered since phenytoin (the active metabolite of fosphenytoin sodium injection) is not completely bound to plasma proteins. Total exchange transfusion has been used in the treatment of severe intoxication in children. In acute overdosage the possibility of other CNS depressants, including alcohol, should be borne in mind.

16 HOW SUPPLIED/STORAGE AND HANDLING 16.1 How Supplied Fosphenytoin Sodium Injection, USP 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 NDC 0641-6137-10 100 mg PE/2 mL vial 2 mL per vial Package contains 25 vials NDC 0641-6136-25 Both sizes of vials contain Tromethamine, USP (TRIS), Hydrochloric Acid, NF, or Sodium Hydroxide, NF, and Water for Injection, USP. Fosphenytoin sodium injection 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. Vials are single-dose only. After opening, any unused product should be discarded. Not made with natural rubber latex. Brands listed are the trademarks of their respective owners.