Source: FDA, National Drug Code (US) Revision Year: 2021
CEREBYX is contraindicated in patients with:
Do not confuse the amount of drug to be given in PE with the concentration of the drug in the vial.
Doses of CEREBYX are always expressed in terms of milligrams of phenytoin sodium equivalents (mg PE). 1 mg PE is equivalent to 1 mg phenytoin sodium.
Do not, therefore, make any adjustment in the recommended doses when substituting CEREBYX for phenytoin sodium or vice versa. For example, if a patient is receiving 1000 mg PE of CEREBYX, that is equivalent to 1000 mg of phenytoin sodium.
Medication errors associated with CEREBYX have resulted in patients receiving the wrong dose of fosphenytoin. CEREBYX 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. Errors 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 vial actually contains a total of 100 mg PE or 500 mg PE. In some cases, ten-fold overdoses were associated with fatal outcomes. To help minimize confusion, the prescribed dose of CEREBYX should always be expressed in milligrams of phenytoin equivalents (mg PE) [see Dosage and Administration (2.1)]. Additionally, when ordering and storing CEREBYX, consider displaying the total drug content (i.e., 100 mg PE/2 mL or 500 mg PE/10 mL) instead of concentration in computer systems, pre-printed orders, and automated dispensing cabinet databases to help ensure that total drug content can be clearly identified. Care should be taken to ensure the appropriate volume of CEREBYX is withdrawn from the vial when preparing the drug for administration. Attention to these details may prevent some CEREBYX medication errors from occurring.
Rapid intravenous administration of CEREBYX increases the risk of adverse cardiovascular reactions, including severe hypotension and cardiac arrhythmias. Cardiac arrhythmias have included bradycardia, heart block, QT interval prolongation, ventricular tachycardia, and ventricular fibrillation which have resulted in asystole, cardiac arrest, and death. Severe complications are most commonly encountered in critically ill patients, elderly patients, and patients with hypotension and severe myocardial insufficiency. However, cardiac events have also been reported in adults and children without underlying cardiac disease or comorbidities and at recommended doses and 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 [see Dosage and Administration (2.3, 2.4)].
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.
As non-emergency therapy, intravenous CEREBYX should be administered more slowly. Because of the risks of cardiac and local toxicity associated with IV CEREBYX, oral phenytoin should be used whenever possible.
Because adverse cardiovascular reactions have occurred during and after infusions, careful cardiac and respiratory monitoring is needed during and after the administration of intravenous CEREBYX. Reduction in rate of administration or discontinuation of dosing may be needed.
Antiepileptic drugs should not be abruptly discontinued because of the possibility of increased seizure frequency, including status epilepticus. When, in the judgment of the clinician, the need for dosage reduction, discontinuation, or substitution of alternative antiepileptic medication arises, this should be done gradually. However, in the event of an allergic or hypersensitivity reaction, rapid substitution of alternative therapy may be necessary. In this case, alternative therapy should be an antiepileptic drug not belonging to the hydantoin chemical class.
CEREBYX can cause severe cutaneous adverse reactions (SCARs), which may be fatal. Reported reactions in phenytoin (the active metabolite of CEREBYX)-treated patients have included toxic epidermal necrolysis (TEN), Stevens-Johnson syndrome (SJS), acute generalized exanthematous pustulosis (AGEP), and Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) [see Warnings and Precautions (5.5)]. The onset of symptoms is usually within 28 days, but can occur later. CEREBYX should be discontinued at the first sign of a rash, unless the rash is clearly not drug-related. If signs or symptoms suggest a severe cutaneous adverse reaction, use of this drug should not be resumed and alternative therapy should be considered. If a rash occurs, the patient should be evaluated for signs and symptoms of SCARs.
Studies in patients of Chinese ancestry have found a strong association between the risk of developing SJS/TEN and the presence of HLA-B*1502, an inherited allelic variant of the HLA B gene, in patients using carbamazepine. Limited evidence suggests that HLA-B*1502 may be a risk factor for the development of SJS/TEN in patients of Asian ancestry taking other antiepileptic drugs associated with SJS/TEN, including phenytoin. In addition, retrospective, case-control, genome-wide association studies in patients of southeast Asian ancestry have also identified an increased risk of SCARs in carriers of the decreased function CYP2C9*3 variant, which has also been associated with decreased clearance of phenytoin. Consider avoiding CEREBYX as an alternative to carbamazepine in patients who are positive for HLA-B*1502 or in CYP2C9*3 carriers.
Should CEREBYX be utilized for CYP2C9*3 carriers, consider starting at the lower end of the dosage range [see Use in Specific Populations (8.7)].
The use of HLA-B*1502 or CYP2C9 genotyping has important limitations and must never substitute for appropriate clinical vigilance and patient management. The role of other possible factors in the development of, and morbidity from, SJS/TEN, such as antiepileptic drug (AED) dose, compliance, concomitant medications, comorbidities, and the level of dermatologic monitoring have not been studied.
Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS), also known as Multiorgan hypersensitivity, has been reported in patients taking antiepileptic drugs, including phenytoin and CEREBYX. Some of these events have been fatal or life-threatening. DRESS typically, although not exclusively, presents with fever, rash, lymphadenopathy, and/or facial swelling, in association with other organ system involvement, such as hepatitis, nephritis, hematological abnormalities, myocarditis, or myositis sometimes resembling an acute viral infection. Eosinophilia is often present. Because this disorder is variable in its expression, other organ systems not noted here may be involved. It is important to note that early manifestations of hypersensitivity, such as fever or lymphadenopathy, may be present even though rash is not evident. If such signs or symptoms are present, the patient should be evaluated immediately. CEREBYX should be discontinued if an alternative etiology for the signs or symptoms cannot be established.
CEREBYX and other hydantoins are contraindicated in patients who have experienced phenytoin hypersensitivity [see Contraindications (4) and Warnings and Precautions (5.7)]. Additionally, consider alternatives to structurally similar drugs such as carboxamides (e.g., carbamazepine), barbiturates, succinimides, and oxazolidinediones (e.g., trimethadione) in these same patients. Similarly, if there is a history of hypersensitivity reactions to these structurally similar drugs in the patient or immediate family members, consider alternatives to CEREBYX.
Angioedema has been reported in patients treated with phenytoin and CEREBYX in the postmarketing setting. CEREBYX should be discontinued immediately if symptoms of angioedema, such as facial, perioral, or upper airway swelling occur. CEREBYX should be discontinued permanently if a clear alternative etiology for the reaction cannot be established.
Cases of acute hepatotoxicity, including infrequent cases of acute hepatic failure, have been reported with phenytoin (the active metabolite of CEREBYX). These events may be part of the spectrum of DRESS or may occur in isolation [see Warnings and Precautions (5.5)]. Other common manifestations include jaundice, hepatomegaly, elevated serum transaminase levels, leukocytosis, and eosinophilia. The clinical course of acute phenytoin hepatotoxicity ranges from prompt recovery to fatal outcomes. In these patients with acute hepatotoxicity, CEREBYX should be immediately discontinued and not re-administered.
Hematopoietic complications, some fatal, have occasionally been reported in association with administration of phenytoin (the active metabolite of CEREBYX). These have included thrombocytopenia, leukopenia, granulocytopenia, agranulocytosis, and pancytopenia with or without bone marrow suppression.
There have been a number of reports that have suggested a relationship between phenytoin and the development of lymphadenopathy (local or generalized), including benign lymph node hyperplasia, pseudolymphoma, lymphoma, and Hodgkin’s disease. Although a cause and effect relationship has not been established, the occurrence of lymphadenopathy indicates the need to differentiate such a condition from other types of lymph node pathology. Lymph node involvement may occur with or without symptoms and signs resembling DRESS [see Warnings and Precautions (5.5)].
In all cases of lymphadenopathy, follow-up observation for an extended period is indicated and every effort should be made to achieve seizure control using alternative antiepileptic drugs.
Severe burning, itching, and/or paresthesia were reported by 7 of 16 normal volunteers administered IV CEREBYX at a dose of 1200 mg PE at the maximum rate of administration (150 mg PE/min). The severe sensory disturbance lasted from 3 to 50 minutes in 6 of these subjects and for 14 hours in the seventh subject. In some cases, milder sensory disturbances persisted for as long as 24 hours. The location of the discomfort varied among subjects with the groin mentioned most frequently as an area of discomfort. In a separate cohort of 16 normal volunteers (taken from 2 other studies) who were administered IV CEREBYX at a dose of 1200 mg PE at the maximum rate of administration (150 mg PE/min), none experienced severe disturbances, but most experienced mild to moderate itching or tingling. Patients administered CEREBYX at doses of 20 mg PE/kg at 150 mg PE/min are expected to experience discomfort of some degree. The occurrence and intensity of the discomfort can be lessened by slowing or temporarily stopping the infusion. The effect of continuing infusion unaltered in the presence of these sensations is unknown. No permanent sequelae have been reported thus far. The pharmacologic basis for these positive sensory phenomena is unknown, but other phosphate ester drugs, which deliver smaller phosphate loads, have been associated with burning, itching, and/or tingling predominantly in the groin area.
Edema, discoloration, and pain distal to the site of injection (described as “purple glove syndrome”) have also been reported following peripheral intravenous CEREBYX injection. This may or may not be associated with extravasation. The syndrome may not develop for several days after injection.
The phosphate load provided by CEREBYX (0.0037 mmol phosphate/mg PE CEREBYX) should be considered when treating patients who require phosphate restriction, such as those with severe renal impairment.
Because the fraction of unbound phenytoin (the active metabolite of CEREBYX) is increased in patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of phenytoin serum levels should be based on the unbound fraction in those patients. After IV administration to patients with renal and/or hepatic disease, or in those with hypoalbuminemia, fosphenytoin clearance to phenytoin may be increased without a similar increase in phenytoin clearance. This has the potential to increase the frequency and severity of adverse events.
In view of isolated reports associating phenytoin (the active metabolite of CEREBYX) with exacerbation of porphyria, caution should be exercised in using CEREBYX in patients suffering from this disease.
CEREBYX may cause fetal harm when administered to a pregnant woman. Prenatal exposure to phenytoin (the active metabolite of CEREBYX) may increase the risks for congenital malformations and other adverse developmental outcomes [see Use in Specific Populations (8.1)].
Increased frequencies of major malformations (such as orofacial clefts and cardiac defects), and abnormalities characteristic of fetal hydantoin syndrome, including dysmorphic skull and facial features, nail and digit hypoplasia, growth abnormalities (including microcephaly), and cognitive deficits, have been reported among children born to epileptic women who took phenytoin alone or in combination with other antiepileptic drugs during pregnancy. There have been several reported cases of malignancies, including neuroblastoma.
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.
Hyperglycemia, resulting from the inhibitory effect of phenytoin (the active metabolite of CEREBYX) on insulin release, has been reported. Phenytoin may also raise the serum glucose concentrations in diabetic patients.
Serum levels of phenytoin (the active metabolite of CEREBYX) sustained above the therapeutic range may produce confusional states referred to as “delirium,” “psychosis,” or “encephalopathy,” or rarely, irreversible cerebellar dysfunction and/or cerebellar atrophy. Accordingly, at the first sign of acute toxicity, serum levels should be immediately checked. CEREBYX dose reduction is indicated if serum levels are excessive; if symptoms persist, administration of CEREBYX should be discontinued.
The following serious adverse reactions are described elsewhere in the labeling:
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.
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 CEREBYX or comparative therapy.
Incidence in Controlled Clinical Trials – IV Administration to Adult Patients with Epilepsy or Neurosurgical Patients: Table 4 lists adverse reactions that occurred in at least 2% of patients treated with IV CEREBYX at the maximum dose and rate in a randomized, double-blind, controlled clinical trial where the rates for phenytoin and CEREBYX administration would have resulted in equivalent systemic exposure to phenytoin.
Table 4. Adverse Reaction Incidence Following IV Administration at the Maximum Dose and Rate to Adult Patients with Epilepsy or Neurosurgical Patients (Events in at Least 2% of CEREBYX-Treated Patients):
| BODY SYSTEM | IV CEREBYX | IV Phenytoin* |
|---|---|---|
| Adverse Event | N=90 | N=22 |
| BODY AS A WHOLE | ||
| Pelvic Pain | 4 | 0 |
| Asthenia | 2 | 0 |
| Back Pain | 2 | 0 |
| Headache | 2 | 5 |
| CARDIOVASCULAR | ||
| Hypotension | 8 | 9 |
| Vasodilatation | 6 | 5 |
| Tachycardia | 2 | 0 |
| DIGESTIVE | ||
| Nausea | 9 | 14 |
| Tongue Disorder | 4 | 0 |
| Dry Mouth | 4 | 5 |
| Vomiting | 2 | 9 |
| NERVOUS | ||
| Nystagmus | 44 | 59 |
| Dizziness | 31 | 27 |
| Somnolence | 20 | 27 |
| Ataxia | 11 | 18 |
| Stupor | 8 | 5 |
| Incoordination | 4 | 5 |
| Paresthesia | 4 | 0 |
| Extrapyramidal Syndrome | 4 | 0 |
| Tremor | 3 | 9 |
| Agitation | 3 | 0 |
| Hypesthesia | 2 | 9 |
| Dysarthria | 2 | 0 |
| Vertigo | 2 | 0 |
| Brain Edema | 2 | 5 |
| SKIN AND APPENDAGES | ||
| Pruritus | 49 | 5 |
| SPECIAL SENSES | ||
| Tinnitus | 9 | 9 |
| Diplopia | 3 | 0 |
| Taste Perversion | 3 | 0 |
| Amblyopia | 2 | 9 |
| Deafness | 2 | 0 |
Incidence in Clinical Trials – IV Administration to Pediatric Patients with Epilepsy or Neurosurgical Patients: The overall incidence of adverse reactions and the types of adverse reactions seen were similar among children and adults treated with CEREBYX. In an open-label, safety, tolerability, and pharmacokinetic study of fosphenytoin in pediatric subjects (neonates through age 16), the following adverse reactions occurred at a frequency of at least 5% in 96 subjects treated with intravenous CEREBYX: vomiting (21%), nystagmus (18%), ataxia (10%), fever (8%), nervousness (7%), pruritus (6%), somnolence (6%), hypotension (5%), and rash (5%).
Incidence in Controlled Trials – IM Administration to Adult Patients with Epilepsy: Table 5 lists adverse reactions that occurred in at least 2% of CEREBYX-treated patients in a double-blind, randomized, controlled clinical trial of adult epilepsy patients receiving either IM CEREBYX substituted for oral phenytoin or continuing oral phenytoin. Both treatments were administered for 5 days.
Table 5. Adverse Reaction Incidence Following Substitution of IM CEREBYX for Oral Phenytoin in Adult Patients with Epilepsy (Events in at Least 2% of CEREBYX-Treated Patients):
| BODY SYSTEM | IM CEREBYX | Oral Phenytoin?footnote? |
|---|---|---|
| Adverse Event | N=179 | N=61 |
| BODY AS A WHOLE | ||
| Headache | 9 | 5 |
| Asthenia | 9 | 3 |
| DIGESTIVE | ||
| Nausea | 5 | 0 |
| Vomiting | 3 | 0 |
| HEMATOLOGIC AND LYMPHATIC | ||
| Ecchymosis | 7 | 5 |
| NERVOUS | ||
| Nystagmus | 15 | 8 |
| Tremor | 10 | 13 |
| Ataxia | 8 | 8 |
| Incoordination | 8 | 5 |
| Somnolence | 7 | 10 |
| Dizziness | 5 | 3 |
| Paresthesia | 4 | 3 |
| Reflexes Decreased | 3 | 5 |
| SKIN AND APPENDAGES | ||
| Pruritus | 3 | 0 |
CEREBYX has been administered to approximately 900 individuals during clinical trials. Adverse events seen at least twice are listed in the following, except those already included in previous tables and listings. Events are further classified within body system categories and enumerated in order of decreasing frequency using the following definitions: frequent adverse events are defined as those occurring in greater than 1/100 individuals; infrequent adverse events are those occurring in 1/100 to 1/1000 individuals.
Frequent: fever, injection-site reaction, infection, chills, face edema, injection-site pain
Infrequent: sepsis, injection-site inflammation, injection-site edema, injection-site hemorrhage, flu syndrome, malaise, generalized edema, shock, photosensitivity reaction, cachexia, cryptococcosis.
Frequent: hypertension
Infrequent: cardiac arrest, migraine, syncope, cerebral hemorrhage, palpitation, sinus bradycardia, atrial flutter, bundle branch block, cardiomegaly, cerebral infarct, postural hypotension, pulmonary embolus, QT interval prolongation, thrombophlebitis, ventricular extrasystoles, congestive heart failure.
Frequent: constipation
Infrequent: dyspepsia, diarrhea, anorexia, gastrointestinal hemorrhage, increased salivation, liver function tests abnormal, tenesmus, tongue edema, dysphagia, flatulence, gastritis, ileus.
Infrequent: diabetes insipidus.
Infrequent: thrombocytopenia, anemia, leukocytosis, cyanosis, hypochromic anemia, leukopenia, lymphadenopathy, petechia.
Phenytoin (the active metabolite of CEREBYX) may cause increased serum levels of glucose and alkaline phosphatase.
Frequent: hypokalemia
Infrequent: hyperglycemia, hypophosphatemia, alkalosis, acidosis, dehydration, hyperkalemia, ketosis.
Frequent: myasthenia
Infrequent: myopathy, leg cramps, arthralgia, myalgia.
Frequent: reflexes increased, speech disorder, dysarthria, intracranial hypertension, thinking abnormal, nervousness
Infrequent: confusion, twitching, Babinski sign positive, circumoral paresthesia, hemiplegia, hypotonia, convulsion, extrapyramidal syndrome, insomnia, meningitis, depersonalization, CNS depression, depression, hypokinesia, hyperkinesia, paralysis, psychosis, aphasia, emotional lability, coma, hyperesthesia, myoclonus, personality disorder, acute brain syndrome, encephalitis, subdural hematoma, encephalopathy, hostility, akathisia, amnesia, neurosis.
Frequent: pneumonia
Infrequent: pharyngitis, sinusitis, hyperventilation, rhinitis, apnea, aspiration pneumonia, asthma, dyspnea, atelectasis, cough increased, sputum increased, epistaxis, hypoxia, pneumothorax, hemoptysis, bronchitis.
Frequent: rash
Infrequent: maculopapular rash, urticaria, sweating, skin discoloration, contact dermatitis, pustular rash, skin nodule.
Infrequent: visual field defect, eye pain, conjunctivitis, photophobia, hyperacusis, mydriasis, parosmia, ear pain, taste loss.
Infrequent: urinary retention, oliguria, dysuria, vaginitis, albuminuria, genital edema, kidney failure, polyuria, urethral pain, urinary incontinence, vaginal moniliasis.
The following adverse reactions have been identified during post-approval use of fosphenytoin. Because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure.
Body as a Whole: Anaphylaxis, angioedema [see Warnings and Precautions (5.7)]
Laboratory Test Abnormality: Phenytoin or CEREBYX may decrease serum concentrations of T4. It may also produce lower than normal values for dexamethasone or metyrapone tests. Phenytoin may also cause increased serum levels of gamma glutamyl transpeptidase (GGT).
Nervous System Disorders: Dyskinesia
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.
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 | H2 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*, theophylline |
| Drugs that may either increase or decrease phenytoin serum levels | |
| Antiepileptic drugs | Phenobarbital, valproate sodium, valproic acid |
* The induction potency of St. John’s wort may vary widely based on preparation.
Table 7 includes commonly occurring drug interactions affected by phenytoin (the active metabolite of CEREBYX). 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 delavirdine. This can lead to loss of virologic response and possible resistance [see Contraindications (4)]. |
| Neuromuscular blocking agents | Cisatracurium, pancuronium, rocuronium and vecuronium: resistance to the neuromuscular blocking action of the nondepolarizing neuromuscular blocking agents has occurred in patients chronically administered phenytoin. Whether or not phenytoin has the same effect on other non-depolarizing agents is unknown. Prevention or Management: Patients should be monitored closely for more rapid recovery from neuromuscular blockade than expected, and infusion rate requirements may be higher. |
| Warfarin | Increased and decreased PT/INR responses have been reported when phenytoin is coadministered with warfarin. |
| Other | Corticosteroids, doxycycline, estrogens, furosemide, oral contraceptives, paroxetine, quinidine, rifampin, sertraline, theophylline, and vitamin D |
| Drugs whose level is decreased by phenytoin | |
| Antiepileptic drugs* | Carbamazepine, felbamate, lamotrigine, topiramate, oxcarbazepine |
| Antilipidemic agents | Atorvastatin, fluvastatin, simvastatin |
| Antiviral agents | Efavirenz, lopinavir/ritonavir, indinavir, nelfinavir, ritonavir, saquinavir Fosamprenavir: phenytoin when given with fosamprenavir alone may decrease the concentration of amprenavir, the active metabolite. Phenytoin when given with the combination of fosamprenavir and ritonavir may increase the concentration of amprenavir |
| Calcium channel blockers | Nifedipine, nimodipine, nisoldipine, verapamil |
| Other | Albendazole (decreases active metabolite), chlorpropamide, clozapine, cyclosporine, digoxin, disopyramide, folic acid, methadone, mexiletine, praziquantel, quetiapine |
* The effect of phenytoin on phenobarbital, valproic acid and sodium valproate serum levels is unpredictable.
Care should be taken when using immunoanalytical methods to measure serum phenytoin concentrations following CEREBYX administration.
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/.
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.
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.
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.
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.
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 observed in rats, rabbits, and mice at doses as low as 100, 75, and 12.5 mg/kg, respectively.
It is not known whether fosphenytoin is secreted in human milk. Following administration of phenytoin (the active metabolite of CEREBYX), phenytoin is secreted in human milk. The developmental and health benefits of breastfeeding should be considered along with the mother’s clinical need for CEREBYX and any potential adverse effects on the breastfed infant from CEREBYX or from the underlying maternal condition.
CEREBYX is indicated for the treatment of generalized tonic-clonic status epilepticus and prevention and treatment of seizures occurring during neurosurgery in all pediatric age groups [see Indications and Usage (1) and Dosage and Administration (2.3, 2.4)]. Because rapid intravenous administration of CEREBYX increases the risk of adverse cardiovascular reactions, the rate of administration should not exceed 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients [see Dosage and Administration (2.3, 2.4) and Warnings and Precautions (5.2)].
No systematic studies in geriatric patients have been conducted. Phenytoin clearance tends to decrease with increasing age [see Clinical Pharmacology (12.3)]. Lower or less frequent dosing may be required [see Clinical Pharmacology (12.3) and Dosage and Administration (2.8)].
The liver is the site of biotransformation. Patients with impaired liver function, elderly patients, or those who are gravely ill may show early toxicity.
Because the fraction of unbound phenytoin (the active metabolite of CEREBYX) is increased in patients with renal or hepatic disease, or in those with hypoalbuminemia, the monitoring of phenytoin serum levels should be based on the unbound fraction in those patients.
After IV administration to patients with renal and/or hepatic disease, or in those with hypoalbuminemia, fosphenytoin clearance to phenytoin may be increased without a similar increase in phenytoin clearance. This has the potential to increase the frequency and severity of adverse events.
Patients who are intermediate or poor metabolizers of CYP2C9 substrates (e.g., *1/*3, *2/*2, *3/*3) may exhibit increased phenytoin serum concentrations compared to patients who are normal metabolizers (e.g., *1/*1). Thus, patients who are known to be intermediate or poor metabolizers may ultimately require lower doses to maintain similar steady-state concentrations compared to normal metabolizers. In patients who are known to be carriers of the decreased function CYP2C9*2 or *3 alleles (intermediate and poor metabolizers), consider starting at the low end of the dosage range and monitor serum concentrations to maintain total phenytoin concentrations of 10 to 20 mcg/mL. If early signs of dose-related central nervous system (CNS) toxicity develop, serum concentrations should be checked immediately [see Clinical Pharmacology (12.5)].
© All content on this website, including data entry, data processing, decision support tools, "RxReasoner" logo and graphics, is the intellectual property of RxReasoner and is protected by copyright laws. Unauthorized reproduction or distribution of any part of this content without explicit written permission from RxReasoner is strictly prohibited. Any third-party content used on this site is acknowledged and utilized under fair use principles.
