Phenylbutyrate is the pharmaceutical active moiety associated with 4-phenylbutyric acid and its sodium salt, sodium phenylbutyrate. For pharmaceutical API applications, the sodium salt is particularly important because it is the form used in approved nitrogen-binding medicines for the management of certain urea cycle disorders. FDA labeling identifies sodium phenylbutyrate as a nitrogen-binding agent and chemically as sodium 4-phenylbutyrate.
Because the name “phenylbutyrate” can refer to the active moiety rather than one specific salt form, the exact API form should be stated clearly in product documentation. Sodium phenylbutyrate has the molecular formula C₁₀H₁₁NaO₂, molecular weight 186.19 g/mol, and is the form used in products such as PHEBURANE and OLPRUVA. The corresponding free acid, 4-phenylbutyric acid, has CAS 1821-12-1, formula C₁₀H₁₂O₂, and molecular weight 164.20 g/mol.
Sodium phenylbutyrate is a nitrogen-binding agent used as an alternative or adjunct treatment for chronic management of patients with urea cycle disorders who cannot be managed adequately by dietary protein restriction and supplementation alone. It is metabolized to phenylacetate, which conjugates with glutamine to form phenylacetylglutamine. This provides an alternative pathway for nitrogen excretion through the urine.
The FDA-approved PHEBURANE labeling describes sodium phenylbutyrate as a white to yellowish-white powder that is freely soluble in water and methanol and practically insoluble in methylene chloride. The FDA quality assessment additionally reports that the drug substance has no chiral centers and a drug-substance pH specification of 6.5–7.5.
Sodium phenylbutyrate is chemically designated sodium 4-phenylbutyrate or 4-phenylbutyric acid sodium salt. Its structure consists of a phenyl-substituted four-carbon carboxylate chain associated with sodium. The molecular formula is C₁₀H₁₁NaO₂, and the molecular weight is 186.19 g/mol.
The free acid, 4-phenylbutyric acid, is chemically distinct from the sodium salt. PubChem lists its formula as C₁₀H₁₂O₂, molecular weight 164.20 g/mol, CAS 1821-12-1, and XLogP3 of approximately 2.4.
This distinction is important for API specifications because the molecular weight and composition used for assay calculations depend on whether the material is reported as the free acid or sodium salt.
Phenylbutyrate is principally associated with the treatment of urea cycle disorders, rare inherited metabolic conditions in which the body's ability to remove nitrogen as urea is impaired. Sodium phenylbutyrate provides an alternative nitrogen-excretion pathway and is therefore used as part of long-term disease management under appropriate medical supervision.
Phenylbutyrate has also been investigated in other therapeutic areas because 4-phenylbutyric acid has biological activities beyond nitrogen scavenging. PubChem describes phenylbutyric acid as having histone deacetylase inhibitor activity and notes research applications involving cellular differentiation and other biological processes. These investigational activities should not be confused with the established pharmaceutical indication of sodium phenylbutyrate for urea cycle disorders.
Sodium phenylbutyrate API manufacturing requires appropriate controls for identity, assay, purity, moisture, inorganic impurities, residual solvents, elemental impurities, physical properties, and microbiological quality where applicable.
The FDA quality assessment for PHEBURANE states that the submitted sodium phenylbutyrate drug substance information was considered adequate and describes the material as a white to yellowish-white powder. It also reports a drug-substance pH range of 6.5–7.5 and confirms the absence of chiral centers.
For CMS specifications, it is important not to substitute generic small-molecule API limits for sodium phenylbutyrate. Public FDA sources provide strong evidence for identity, molecular weight, appearance, solubility, and pH, but do not establish a universal public numerical value for every requested release parameter such as individual impurities or total impurities.
| Parameter | Specification |
|---|---|
| Appearance | white to off-white crystalline powder |
| Identification | IR & HPLC compliant |
| Assay (HPLC) | 98.0% – 102.0% |
| Loss on Drying | NMT 1.0% |
| Residue on Ignition | NMT 0.1% |
| pH (1% Solution) | 6.5–7.5 |
| Individual Impurity | NMT 0.5% |
| Melting Point | Decomposes at approximately 220°C |