SODIUM PHENYLBUTYRATE
Clinical safety rating: safe
Animal studies have demonstrated safety
Sodium phenylbutyrate is a prodrug that is metabolized to phenylacetate, which conjugates with glutamine via acetylation to form phenylacetylglutamine. This provides an alternative pathway for nitrogen excretion, reducing ammonia levels in patients with urea cycle disorders.
| Metabolism | Sodium phenylbutyrate is metabolized via beta-oxidation to phenylacetate. Phenylacetate then conjugates with glutamine to form phenylacetylglutamine, which is excreted by the kidneys. |
| Excretion | Renal: 80-100% as phenylacetylglutamine; fecal: minimal (<5%). |
| Half-life | Sodium phenylbutyrate: 0.76-1.7 hours; phenylacetate: 1.1-1.9 hours; clinical context: short half-life, requires multiple daily dosing. |
| Protein binding | 80-95% to albumin; phenylacetate is primarily bound. |
| Volume of Distribution | 0.14-0.36 L/kg; clinical meaning: small Vd, primarily in extracellular fluid. |
| Bioavailability | Oral: approximately 100% (prodrug rapidly converted to phenylacetate). |
| Onset of Action | Oral: 1-2 hours; intravenous: N/A (not available IV). |
| Duration of Action | 6-8 hours after oral dose; clinical notes: requires thrice-daily dosing. |
450-600 mg/kg/day orally in 3-6 divided doses; maximum 20 g/day.
| Dosage form | TABLET |
| Renal impairment | GFR 30-49 mL/min: reduce dose by 25%; GFR 15-29 mL/min: reduce dose by 50%; GFR <15 mL/min: contraindicated. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 50%; Child-Pugh C: avoid use. |
| Pediatric use | 300-600 mg/kg/day orally in 3-6 divided doses; maximum 20 g/day for children >20 kg. |
| Geriatric use | Start at lower end of dosing range; monitor for fluid retention and electrolyte imbalances due to high sodium content. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
No significant drug interactions Can cause body odor and menstrual irregularities.
| Breastfeeding | It is unknown if sodium phenylbutyrate or its metabolites (phenylacetate) are excreted in human milk. M/P ratio not established. Due to potential for serious adverse reactions in nursing infants (e.g., neurotoxicity), breastfeeding is not recommended during therapy and for 2 weeks after last dose. |
| Teratogenic Risk | Sodium phenylbutyrate is classified as Pregnancy Category C. First trimester: In animal studies, administration produced adverse effects (reduced fetal weight, skeletal variations) at doses below the maximum human dose. No adequate human studies exist; potential risk to fetus cannot be ruled out. Second and third trimesters: No specific data; use only if maternal benefit outweighs fetal risk. Avoid in pregnancy unless clearly needed. |
■ FDA Black Box Warning
None.
| Common Effects | Amenorrhea |
| Serious Effects |
Hypersensitivity to sodium phenylbutyrate or any component of the formulation.
| Precautions | Neurotoxicity (somnolence, confusion, headache) due to phenylacetate accumulation; monitor ammonia levels and neurological status; sodium overload in patients with congestive heart failure or hypertension; contain 11% sodium by weight; use with caution in renal impairment; monitor serum electrolytes, liver function, and blood counts. |
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| Fetal Monitoring | Monitor serum ammonia levels, liver function tests, electrolytes (especially sodium, potassium, bicarbonate), and blood urea nitrogen (BUN). Monitor for signs of hyperammonemic encephalopathy (lethargy, vomiting, change in mental status). In pregnant patients, monitor fetal growth and well-being with serial ultrasound and antepartum testing if indicated. |
| Fertility Effects | No specific human studies on fertility. In animal studies, no adverse effects on male or female fertility were observed at relevant doses. However, underlying condition (urea cycle disorders) may affect fertility. Use with caution in patients of reproductive potential; contraceptive counseling may be warranted. |