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All about: Fosphenytoin

Big Image Pronouncation: (FOSS-FEN-ih-toe-in)
Class: Anticonvulsant, Hydantoin

Trade Names:
Cerebyx
- Injection 150 mg (100 mg phenytoin sodium)
- Injection 750 mg (500 mg phenytoin sodium)

Mechanism of Action

Pharmacology

Fosphenytoin is a prodrug, which is converted to the active metabolite phenytoin. Appears to act at motor cortex by inhibiting spread of seizure activity. Possibly works by promoting sodium efflux from neurons, thereby stabilizing threshold against hyperexcitability.

Pharmacokinetics

Absorption

T max is about 30 min. Bioavailability is 100% with IM dosing.

Distribution

Protein binding is 95% to 99%, about 88% for phenytoin. Vd is 4.3 to 10.8 L.

Metabolism

Fosphenytoin converts to phenytoin by hydrolysis. Phenytoin is extensively metabolized in the liver.

Elimination

The t 1/ 2 is about 15 min; the mean t 1/ 2 is 12 to 28.9 (phenytoin). Fosphenytoin is primarily excreted in the urine as metabolites.

Special Populations

Renal Function Impairment

Increased fraction of unbound phenytoin may occur.

Hepatic Function Impairment

Increased fraction of unbound phenytoin may occur.

Elderly

Cl decreases about 20% in patients over 70 yr.

Hypoalbuminemia

Increased fraction of unbound phenytoin may occur.

Indications and Usage

Short-term parenteral administration when other means of phenytoin administration are unavailable, inappropriate, or less advantageous; treatment of generalized convulsive status epilepticus; prevention and treatment of seizures occurring during neurosurgery; short-term substitution for oral phenytoin.

Contraindications

Hypersensitivity to phenytoin or other hydantoins; patients with sinus bradycardia, sino-atrial block, second- and third-degree AV block, and Adams-Stokes syndrome.

Dosage and Administration

To avoid the need to perform molecular weight-based adjustments when converting between fosphenytoin and phenytoin sodium, the fosphenytoin dose is expressed as phenytoin sodium equivalents (PE).

Status Epilepticus
Adults

IV

Initial/Loading dose

15 to 20 mg PE/kg.

Maintenance and Non-Emergent Dose
Adults

IV/IM

Loading dose

10 to 20 mg PE/kg

Maintenance dose

4 to 6 PE/kg/day.

General Advice

  • Do not administer solution if particulate matter or discoloration is noted.
  • May use D5W or NS for dilution prior to administration.
  • Administer IV no faster than 150 mg/min to prevent hypotension.

Storage/Stability

Store in refrigerator at 36° to 46° F. Do not keep at room temperature for more than 48 hours.

Drug Interactions

Amiodarone, benzodiazepines, chloramphenicol, cimetidine, disulfiram, estrogens, felbamate, fluconazole, fluoxetine, isoniazid, oxyphenbutazone, phenacemide, phenylbutazone, succinimides, sulfonamides

May increase phenytoin serum concentrations and effects.

Antineoplastic drugs, carbamazepine, diazoxide, enteral nutritional therapy, rifabutin, rifampin, sucralfate

May decrease serum phenytoin concentrations and effects.

Corticosteroids, coumarin anticoagulants, doxycycline, estrogens, felodipine, levodopa, loop diuretics, methadone, oral contraceptives, mexiletine, quinidine, rifabutin, rifampin

The effects of these agents may be impaired.

Cyclosporine

Cyclosporine concentrations may be decreased.

Disopyramide

Disopyramide concentrations and bioavailability may be decreased, while anticholinergic actions may be enhanced.

Divalproex sodium, phenobarbital, sodium valproate, valproic acid

May increase or decrease phenytoin concentrations and effects.

Folic acid

May cause folic acid deficiency.

Itraconazole

Effects of itraconazole may be decreased, while those of phenytoin may be increased.

Metyrapone

Phenytoin may cause subnormal response to metyrapone.

Non-depolarizing muscle relaxants

May cause these agents to have shorter duration or decreased effects.

Primidone

May increase concentrations of primidone and metabolites, increasing the effects.

Sympathomimetics (eg, dopamine)

May cause profound hypotension and possibly cardiac arrest.

Theophyllines

Effects of either agents may be decreased.

Incompatibility

Do not mix with other drugs.

Laboratory Test Interactions

Fosphenytoin may interfere with metapyrone and dexamethasone tests, causing inaccurate results because of increased metabolism of these agents. Drug may cause decrease in serum levels of protein-bound iodine. It may cause increased levels of glucose, alkaline phosphatase, and gamma glutamyl-transpeptidase.

Adverse Reactions

Cardiovascular

CV collapse; hypotension; vasodilation; tachycardia; atrial and ventricular conduction depression; ventricular fibrillation; hypertension.

CNS

Nystagmus; headache; dizziness; somnolence; ataxia; stupor; incoordination; paresthesia; extrapyramidal syndrome; tremor; agitation; hypesthesia; dysarthria; vertigo; brain edema.

Dermatologic

Pruritus; rash; ecchymosis (IM).

EENT

Diplopia; amblyopia; tinnitus; deafness.

GI

Nausea; vomiting; constipation; tongue disorder; taste perversion; dry mouth.

Metabolic

Hypokalemia.

Respiratory

Pneumonia.

Miscellaneous

Pelvic and back pain; weakness; asthenia; myasthenia; fever; chills; face edema; injection site inflammation.

Precautions

Pregnancy

Category D .

Lactation

Undetermined.

Children

Safety and efficacy not established.

Special Risk Patients

Use drug with caution with hepatic or renal impairment, hypotension, severe myocardial insufficiency, alcohol abuse, and porphyria.

Age

Age does not affect fosphenytoin pharmacokinetics. Phenytoin dosing requirements are variable and should be individualized.

Withdrawal

Abrupt withdrawal may precipitate status epilepticus. Dosage must be reduced or other anticonvulsant medicine substituted gradually.

Overdosage

Symptoms

Nystagmus, ataxia, dysarthria, hypotension, diminished mental capacity, coma, unresponsive pupils, respiratory and cardiovascular depression.

Patient Information

  • Explain to family and patient that the medication is a short-term substitute for the regular use of phenytoin.
  • Explain to family that sedation or drowsiness might occur as a result of the medication.
  • Avoid alcohol or other CNS drugs while taking this medication.
  • Never suddenly discontinue the medication; may lead to status epilepticus.
  • Instruct patient what to do in case of a missed dose.

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