Pharm

Fosphenytoin

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Fosphenytoin, Cerebyx

  • See Also
  • Indications
  1. Status Epilepticus
  2. Seizure Disorder and unable to take oral medications
  • Contraindications
  1. Pregnancy
  2. Cardiac conduction delays
  • Mechanism
  1. Hydantoin anticonvulsant
  2. Water-soluble phosphate ester prodrug of Phenytoin
    1. Fosphenytoin is hydrolyzed to Phenytoin by phosphatases
  3. Increases Neuronal Sodium efflux and stabilizes Neuronal membranes in the motor cortex
  4. Suppresses the propagation of Seizure activity in the motor cortex
  • Dosing
  1. Dosed in Phenytoin equivalents (PE)
  2. Transition to oral Phenytoin as soon as possible
  3. Therapeutic serum level range: 10 to 20 mcg/ml
  4. Status Epilepticus
    1. Dose: 15 to 20 PE mg/kg IV or IM (at 3 mg/kg/min up to 100 to 150 mg/min) up to 1000 mg maximum
    2. Avoid IM dosing if possible
  5. Non-Emergent Loading Infusion
    1. Dose: 10 to 15 PE mg/kg IV (at 2 mg/kg/min up to 100 to 150 mg/min) up to 1000 mg maximum
  6. Maintenance
    1. Follows loading dose by 12 hours (Status Epilepticus or non-emergent loading)
    2. For initial dosing only (later dosing should be based on serum levels)
      1. Start: 2 to 4 PE mg/kg infused slowly (1-2 mg/kg/min up to 100 mg/min) every 12 hours
    3. Subsequent dosing (based on serum levels)
      1. Dose 4 to 8 mg PE/kg/day divided every 12 hours based on serum levels
  7. Preferred over Phenytoin for Status Epilepticus
    1. Fosphenytoin can be infused with dextrose
    2. Fosphenytoin has lower risk of Arrhythmia (due to no Ethylene Glycol in base)
    3. Fosphenytoin may be given IM or delivered a faster IV rate (not tissue toxic)
      1. However onset of activity is similar to that with Phenytoin (as Fosphenytoin is converted to active Phenytoin form)
  • Safety
  1. Pregnancy Category X (Known Teratogen)
    1. See Fetal Hydantoin Syndrome
  2. Unknown Safety in Lactation
  • Adverse Effects
  1. See Phenytoin
  2. Severe Hypotension and Cardiac Dysrhythmias
    1. Infuse slowly
    2. Cardiac and hemodynamic monitoring during infusion
  • Drug Interactions
  1. See Phenytoin
  • References
  1. (2022) Presc Lett, Resource #361206, Antiseizure Medications
  2. Olson (2020) Clinical Pharmacology, Medmaster Miami, p. 56-7
  3. Hamilton (2020) Tarascon Pocket Pharmacopoeia