Neonatal Drug Therapy Manual

Fosphenytoin

Disclaimer: Official controlled document is the CHEO and Ottawa Hospital online copy. It is the responsibility of user to ensure that any paper copy version is the same as the online version before use.

Updated January 26, 2022 - 6:00pm

Alternate Name(s): 
Cerebyx
Classification: 
Anticonvulsivant
Original Date: 
June 2011
Revised Date: 
January 2016
Indications: 
  • Treatment of Phenobarbital resistant seizures
Administration: 
  • IV intermittent infusion:over 10 - 30 minutes
    • (maximum rate: 3 mg of  Phenytoin Equivalents (PE)/kg/min)
  • IM
Dosage: 

***  Fosphenytoin should always be prescribed in mg of Phenytoin Equivalents (PE)

  • Loading dose: 20 mg PE/kg IV/IM
  • Maintenance dose: 5 - 8 mg/kg/day PE/kg/day IV/IM divided BID

* Start at lower end of dosing range and titrate upwards based on efficacy/side effects

 

Side Effects: 
  • CVS: hypotension (with rapid IV administration), vasodilation, tachycardia, bradycardia
  • Dermatologic: rash,exfoliative dermatitis
  • Endocrine and metabolic: hyperglycemia
  • Hematologic: neutropenia, thrombocytopenia, anemia
  • Local: pain on injection (less irritation at injection site and phlebitis than with phenytoin), paresthesia, pruritis

 

Parameters to Monitor: 
  • BP, HR
  • CBC with differential, platelets
  • Serum glucose
  • Liver enzymes
  • Therapeutic Phenytoin levels: 40 - 80 micromol/L
    • Usual sampling time:
      • After load: at least 2 hours after end of IV infusion or 4 hours after IM load injection
      • Maintenance: trough level for IV or IM administration (0 - 60 minutes before next dose) on day 3
Reconstitution and Stability: 

*** Dose, concentration and infusion rates are expressed as Phenytoin Equivalents (PE)

  • Fosphenytoin PE 50 mg/mL
    • Take 1 mL (PE 50 mg) and add to 9 mL of 0.9% NaCl
    • Final concentration: PE 5 mg/mL
  • Store vials in refrigerator.  Vials stable 48 hours at room temperature
Compatibility: 

- Solutions Compatible: D5W, D10W, 0.9 % NaCl, dextrose-saline combinations

- Y-site Compatible: lorazepam, phenobarbital, potassium chloride

- Incompatible: calcium gluconate, midazolam, TPN

Notes: 

*Dose, concentration and infusion rates are expressed as Phenytoin Equivalents (PE)

Fosphenytoin 1 mg PE = Phenytoin 1 mg

 

IM or IV substitution for oral Phenytoin:

Initial: Use the same total daily dose in PE of Fosphenytoin.  Monitor clinical response and Phenytoin serum levels to further guide dose adjustment after 3 - 4 days.  Plasma concentrations may increase slightly with this method since oral phenytoin is 90% bioavailable and phenytoin derived from IM or IV Fosphenytoin is 100% bioavailable.

 

  • Each 1 mg of PE contains 0.0037 mmol of phosphate
References: 

-Taketomo CK, Hodding JH, Kraus DM.  Pediatric Dosage Handbook 17th Edition. Hudson: Lexi-Comp Inc; 2010

-Lau E. (Editor).  Drug Handbook and Formulary- The Hospital for Sick Children. Toronto: 2010 - 2011

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