Fosphenytoin
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Contents |
Introduction
- Tradename: Cerebyx.
Indications
- generalized & partial seizures
- one of the advantages of fosphenytoin is faster loading than with phenytoin
Contraindications
- Caution:
- use with caution in patients with severe cardiovascular, hepatic or renal disease, diabetes mellitus, porphyria, hypothyroidism or fever
- increased toxicity in patients with nephrotic syndrome
- may increase frequency of petit mal seizures
Dosage
- load 15-30 mg/kg up to 1000 mg IV/ IM
- infusion rate at 100-150 mg/min
- prescribe & dispense in phenytoin equivalents
- Injection:
Pharmacokinetics
- following parenteral administration, fosphenytoin is converted to phenytoin
- for every mmol of fosphenytoin, 1 mmol of phenytoin is produced
- the pharmacologic effects of fosphenytoin include those of phenytoin
- fosphenytoin hydrolysis also results in formaldehyde & phosphate
- formaldehyde is subsequently converted to formate which is metabolized via folate-dependent mechanisms
- although formaldehyde has toxic effects, these occur at concentrations in excess of those produced by therapeutic fosphenytoin administration
- 100% bioavailability via IM or IV route
- elimination via liver
Monitor
- blood pressure
- vital signs
- complete blood count
- liver function tests
- free phenytoin levels inpatients with hepatic or renal insufficiency
- monitor with anticonvulsants
Adverse-effects
- not common (1-10%)
- facial edema, slurred speech, dizziness, drowsiness, choreoathetosis, fever, visual hallucinations, rash, exfoliative dermatitis, erythema multiforme, acne, folic acid depletion, osteomalacia, gynecomastia, nausea/vomiting, gingival hyperplasia, priapism, lymphadenopathy, neutropenia, thrombocytopenia, megaloblastic anemia, nystagmus, blurred vision, diplopia, nephrotic syndrome
- other
- irritation & necrosis at site of injection is reduced relative to phenytoin
- perineal pruritis
- increased risk of Stevens Johnson syndrome & toxic epidermal necrolysis in Asian patients positive for HLA allele, HLA-B*1502 [5]
- drug adverse effects of anticonvulsants
Test-interactions
- increases serum glucose & alkaline phosphatase
- decreases serum thyroxine, Ca+2, testosterone
- serum Na+ increases with overdose
More General Terms
Additional Terms
Internet Database
References
- PDR 1997
- Alan Gelb, UCSF, Department of Emergency Services, San Francisco General Hospital, 1998
- Drug Information & Medication Formulary, Veterans Affairs, Central California Health Care System, 1st ed., Ravnan et al eds, 1998
- Kaiser Permanente Northern California Regional Drug Formulary, 1998
- FDA MedWatch [1]
