Fosphenytoin

CEREBYX

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Pronunciation: fos-FEN-i-toyn

Fosphenytoin ballicule: receptor binding, kinetics and half-life diagram — Mechanism of action: Fosphenytoin ballicule

Fosphenytoin CEREBYX mascot mnemonic cartoon — Visual mnemonic: Fosphenytoin mascot
**“Cerebral Fox phenytoin”**
Variation of phenytoin mascot
Water-soluble phenytoin prodrug for IV/IM loading

What it isWater-soluble IV or IM prodrug of phenytoin; status epilepticus loading and short-term parenteral substitution.

Why this oneCan be infused faster and causes less local tissue injury than IV phenytoin; IM administration is possible.

What limits itDose in phenytoin-equivalent units, as mix-ups are fatal; rapid infusion causes groin paresthesias and pruritus.

Pearls

Check phenytoin levels at least 2 hours after IV, 4 after IM.

Fosphenytoin molecular structure (skeletal formula) — Molecular structure: Fosphenytoin

FDA-approved for

seizures (IV/IM) (1996)

Classes and tags

Clinical profile

Therapeutic safety burden3 / 4

Overdose danger4 / 4

Weight-gain liability1 / 4

Sedation liability1 / 4

QT / Torsades0 / 4

Direct muscarinic antagonism0 / 4

Embryofetal risk2 / 4

Legacy pregnancy categoryD

Defining liabilityCardiovascular collapse with rapid administration and phenytoin-related nonlinear toxicity.

FDA label

Read the full label on DailyMed ↗

BOXED WARNING Cardiovascular risk with rapid infusion: do not exceed 150 mg PE/min in adults and 2 mg PE/kg/min (or 150 mg PE/min) in pediatric patients; monitor cardiac/respiratory status.

FDA dosing	Population	Start	Target	Max
Status epilepticus	Adult	15-20 mg PE/kg IV loading at 100-150 mg PE/min	maintenance 4-6 mg PE/kg/day divided	per level
Status epilepticus	Pediatric	15-20 mg PE/kg IV loading at 2 mg PE/kg/min (or 150 mg PE/min, whichever slower)	per level	per level
Non-emergent loading and maintenance	Adult	10-20 mg PE/kg IV or IM loading	4-6 mg PE/kg/day divided	per level
Non-emergent loading and maintenance	Pediatric	10-15 mg PE/kg loading at 1-2 mg PE/kg/min (or 150 mg PE/min, whichever slower)	Initial maintenance 2-4 mg PE/kg given 12 h after loading, then q12h individualized by phenytoin levels; infuse at 1-2 mg PE/kg/min or 100 mg PE/min, whichever slower	per level

Renal Monitor unbound phenytoin concentrations in renal disease or hypoalbuminemia

Hepatic Monitor unbound phenytoin concentrations in hepatic disease or hypoalbuminemia

Concentration is 50 mg PE/mL; a 10 mL vial contains 500 mg PE total.

Instructions

Always dose, concentrate, and infuse in phenytoin sodium equivalents (PE); no molecular-weight conversion needed
Do not confuse vial concentration (50 mg PE/mL) with total vial content
Should ordinarily not be given intramuscularly for status epilepticus
Discontinue gradually to avoid precipitating status epilepticus
IV: continuous ECG, blood pressure and respiratory monitoring; observe through peak phenytoin period (~10-20 min after end of infusion)

Available strengths

inj 50 mg PE/mL

Contraindications

Sinus bradycardia, sino-atrial block, second/third-degree AV block, or Adams-Stokes syndrome
History of prior acute hepatotoxicity attributable to fosphenytoin or phenytoin
Coadministration with delavirdine

Cautions

Dosing errors (PE vs concentration)
Withdrawal-precipitated seizure/status epilepticus
Serious dermatologic reactions (SJS/TEN)
DRESS/multiorgan hypersensitivity
Hypersensitivity (cross-sensitivity with hydantoins)
Angioedema
Hepatic injury
Hematopoietic complications
Cardiac effects with rapid infusion
Suicidal behavior/ideation

Adverse reactions

Nystagmus
Dizziness
Pruritus
Paresthesia
Headache
Somnolence
Ataxia

Pregnancy Prenatal phenytoin (active metabolite) exposure may increase risk of congenital malformations and adverse developmental outcomes.

Lactation Undefined

Cost

Fosphenytoin (CEREBYX)

Strength		#/mo	generic $/unit	CostPlus
50 mg PE/mL	inj	per unit		not offered

NADAC is a voluntary CMS survey of what retail community pharmacies pay for the drugs they stock. Fosphenytoin is administered under clinical supervision and dispensed outside retail pharmacies, so it rarely appears on surveyed invoices.

Generic entry2007

Active metabolites

Phenytoin

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Self-check

In what units should fosphenytoin be prescribed?
Phenytoin-equivalent units.
What must be monitored during IV fosphenytoin infusion?
ECG and blood pressure.