fosphenytoin (Cerebyx)
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Introduction
Tradename: Cerebyx.
Indications
- generalized tonic clonic seizures
- partial seizures
- status epilepticus[6]
- 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:
- 150 mg (equivalent to 100 mg phenytoin) in 2 mL vials
- 750 mg (equivalent to 500 mg phenytoin) in 10 mL vials
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
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 pruritus
- increased risk of Stevens Johnson syndrome & toxic epidermal necrolysis in Asian patients positive for HLA allele, HLA-B*1502[5]
Test interactions
- increases serum glucose & alkaline phosphatase
- decreases serum thyroxine, Ca+2, testosterone
- serum Na+ increases with overdose
Laboratory
More general terms
Additional terms
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
- ↑ 5.0 5.1 FDA MedWatch http://www.fda.gov/medwatch/safety/2008/safety08.htm#Phenytoin
- ↑ 6.0 6.1 Deprecated Reference