FOSPHENYTOIN SODIUM
Clinical safety rating: avoid
Positive evidence of fetus risks but benefits may outweigh risks in some cases
Fosphenytoin is a water-soluble prodrug of phenytoin. It is converted to phenytoin, which stabilizes neuronal membranes by blocking voltage-gated sodium channels, thereby inhibiting repetitive firing of action potentials and reducing seizure propagation.
| Metabolism | Fosphenytoin is rapidly and completely converted to phenytoin by phosphatases (primarily alkaline phosphatase) in the liver and other tissues. Phenytoin is extensively metabolized by hepatic CYP450 enzymes, mainly CYP2C9 and CYP2C19. |
| Excretion | Renal excretion of inactive metabolites (primarily fosphenytoin metabolites including phenytoin metabolites) accounts for approximately 80-90% of elimination; less than 5% excreted unchanged in urine; biliary/fecal excretion minimal. |
| Half-life | The terminal elimination half-life of fosphenytoin is approximately 15 minutes (range 8-30 minutes) following IV administration; however, the half-life of the active metabolite phenytoin is 20-30 hours (dose-dependent) in adults, requiring careful monitoring for accumulation. |
| Protein binding | Fosphenytoin is approximately 90-95% bound to plasma proteins, primarily albumin; its active metabolite phenytoin is also highly bound (88-95%) to albumin. Binding is saturable and decreases in renal failure and hypoalbuminemia. |
| Volume of Distribution | Volume of distribution for fosphenytoin is approximately 6-10 L/kg, reflecting extensive distribution into tissues; for the metabolite phenytoin, Vd is 0.5-1.0 L/kg, indicating distribution primarily into total body water with accumulation in fat over time. |
| Bioavailability | Intramuscular: 100% bioavailable (complete conversion to phenytoin); IV: 100% bioavailable. Oral fosphenytoin is not available; bioavailability of oral phenytoin is variable (70-90%) but fosphenytoin is only administered parenterally. |
| Onset of Action | IV: Therapeutic phenytoin levels are achieved within 5-10 minutes after IV administration (time to peak phenytoin concentration ~5-30 minutes post-infusion). IM: Therapeutic phenytoin levels are achieved within 30-60 minutes; peak phenytoin concentrations occur approximately 3 hours after IM injection. |
| Duration of Action | Duration of anticonvulsant effect corresponds to phenytoin's half-life, approximately 24-48 hours for a single dose; steady-state is achieved in 7-10 days with regular dosing. IM administration may have slower absorption and prolonged time to steady-state. |
Loading dose: 15-20 mg PE/kg IV at 100-150 mg PE/min; maintenance: 4-6 mg PE/kg/day IV divided every 8-12 hours.
| Dosage form | INJECTABLE |
| Renal impairment | No specific adjustment for GFR; monitor phenytoin levels closely due to altered protein binding in renal impairment. |
| Liver impairment | Child-Pugh A: reduce maintenance dose by 25%; Child-Pugh B: reduce by 50%; Child-Pugh C: avoid or reduce by 75% and monitor levels. |
| Pediatric use | Loading dose: 15-20 mg PE/kg IV; maintenance: 5-7 mg PE/kg/day IV divided every 8-12 hours (max 300 mg PE/day). |
| Geriatric use | Start at lower maintenance doses (e.g., 3-4 mg PE/kg/day) due to decreased clearance and increased sensitivity; monitor levels and adjust. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Many drugs can increase or decrease phenytoin levels Can cause life-threatening dermatologic reactions like Stevens-Johnson syndrome.
| Breastfeeding | Fosphenytoin is metabolized to phenytoin; phenytoin is excreted into breast milk with milk-to-plasma ratio 0.18–0.45. Infant serum levels are typically low (<1–2 mcg/mL). Benefit of breastfeeding likely outweighs risk, but monitor infant for sedation, poor sucking, and rash. |
| Teratogenic Risk | First trimester: Increased risk of major congenital malformations including cleft lip/palate, cardiac defects, and neural tube defects, associated with fetal hydantoin syndrome. Second and third trimesters: Risk of hemorrhagic disease of the newborn due to vitamin K deficiency, neonatal coagulopathy, and potential for poor neurodevelopmental outcomes. |
■ FDA Black Box Warning
Intravenous administration: Risk of hypotension and cardiac arrhythmias, including bradycardia and cardiac arrest, particularly with rapid infusion. Continuous monitoring of ECG, blood pressure, and respiratory function is required during IV infusion.
| Common Effects | Gingival hyperplasia |
| Serious Effects |
Hypersensitivity to fosphenytoin, phenytoin, or any component. Sinus bradycardia, sinoatrial block, second- and third-degree AV block, or Adams-Stokes syndrome. History of prior acute hepatotoxicity due to phenytoin. Concomitant use with delavirdine (reduced antiretroviral efficacy).
| Precautions | Hypotension and cardiac arrhythmias with rapid IV administration; monitor ECG and BP. Purple glove syndrome (edema, discoloration, and pain) with IV extravasation. Hepatic enzyme induction leading to reduced efficacy of oral contraceptives and other drugs. Exacerbation of porphyria. Risk of Stevens-Johnson syndrome and toxic epidermal necrolysis. Abrupt withdrawal may precipitate status epilepticus. May cause fetal harm (phenytoin exposure); use effective contraception. |
Loading safety data…
| Fetal Monitoring | Monitor maternal serum free phenytoin levels (due to pregnancy-induced hypoalbuminemia and altered protein binding), complete blood count, liver function. Fetal monitoring: targeted ultrasound for anomalies at 18–20 weeks, fetal echocardiography, and serial growth scans. Monitor for neonatal bleeding with vitamin K prophylaxis at birth. |
| Fertility Effects | Phenytoin may cause decreased serum folate levels, which can contribute to infertility or adverse pregnancy outcomes. No direct effect on ovulation or spermatogenesis documented, but enzyme induction may affect hormonal contraception efficacy. |