Phenylbutazone is a synthetic pyrazolidinedione-class non-steroidal anti-inflammatory drug (NSAID) and analgesic active pharmaceutical ingredient (API). It has historically been used for its anti-inflammatory, analgesic, and antipyretic properties, particularly in the management of musculoskeletal and rheumatic conditions. Phenylbutazone is also recognized pharmacologically as a cyclooxygenase inhibitor and pyrazolone analgesic.
The API has CAS Number 50-33-9, molecular formula C₁₉H₂₀N₂O₂, and molecular weight 308.37 g/mol. PubChem identifies the compound as 4-butyl-1,2-diphenylpyrazolidine-3,5-dione, while the USP monograph lists the same substance under the chemical description 3,5-pyrazolidinedione, 4-butyl-1,2-diphenyl-.
Phenylbutazone is a neutral, achiral small molecule belonging to the pyrazolidine chemical class. It contains two phenyl groups and a butyl substituent attached to the pyrazolidinedione ring system. PubChem reports an exact mass of 308.152477885 Da, XLogP3 of 3.2, hydrogen-bond acceptor count of 2, hydrogen-bond donor count of 0, rotatable bond count of 5, and topological polar surface area of 40.6 Ų.
Phenylbutazone is described as an odorless white or off-white crystalline powder. Published data report a melting point around 105°C, although older sources contain varying values depending on measurement conditions and material history. For CMS purposes, the pharmacopoeial melting range is preferable where a specific monograph edition is being claimed.
The API has relatively low aqueous solubility. PubChem reports experimental values including 34 mg/L at 25°C and 47.5 mg/L at 30°C, while other literature values vary with temperature and experimental conditions. Phenylbutazone is substantially more soluble in certain organic solvents, including acetone, ether, and ethyl acetate.
Phenylbutazone has anti-inflammatory, analgesic, and antipyretic activity. Historically, it has been used in the treatment of inflammatory and painful musculoskeletal disorders, including certain rheumatic conditions. PubChem describes its historical pharmaceutical activity in conditions such as ankylosing spondylitis and rheumatoid arthritis.
Phenylbutazone inhibits cyclooxygenase-mediated prostaglandin synthesis, thereby reducing inflammatory signaling, pain, and fever. Its use in human medicine has become substantially restricted in many jurisdictions because of potentially serious adverse effects, particularly hematological and gastrointestinal toxicity. Consequently, its regulatory and pharmaceutical status should be considered carefully for the intended market and application.
Phenylbutazone has an established USP-NF monograph. The USP specifies that Phenylbutazone contains not less than 98.0% and not more than 102.0% of C₁₉H₂₀N₂O₂, calculated on the dried basis. This is the most useful verified numerical assay value for the CMS specification.
The accessible USP monograph information also specifies loss on drying NMT 0.5%, determined under vacuum at 80°C for four hours, and a melting range of 104–107°C.
Historical BP material provides additional specification information, including heavy metals NMT 20 ppm, loss on drying NMT 0.2%, and sulfated ash NMT 0.1%. However, because these values are from an older BP edition, they should not automatically be presented as current BP requirements without confirming the applicable pharmacopoeial edition.
Phenylbutazone API manufacturing requires controlled processing and analytical testing to ensure consistent identity, purity, potency, and physical quality. Typical quality-control programs evaluate identification, assay, moisture or loss on drying, related substances, residual solvents, elemental impurities, and other relevant physical characteristics.
For CMS publication, it is important to distinguish verified pharmacopoeial values from manufacturer-specific release specifications. The USP assay range of 98.0–102.0% and LOD limit of NMT 0.5% can be stated where USP compliance is claimed. For parameters without a current universally verified numerical value, specification-dependent wording is more appropriate than inserting generic limits.
Phenylbutazone therefore represents a chemically well-defined pyrazolidinedione API with established analytical standards, CAS 50-33-9, molecular weight 308.37 g/mol, and recognized pharmacological activity as an anti-inflammatory and analgesic compound.
| Parameter | Specification |
|---|---|
| Appearance | white to off-white crystalline powder |
| Identification | IR & HPLC compliant |
| Assay (HPLC) | 98.0% – 102.0% |
| Loss on Drying | NMT 0.5% |
| Residue on Ignition | NMT 0.1% |
| Heavy Metals | NMT 20 ppm |
| pH (1% Solution) | 2.0 – 4.0 |
| Individual Impurity | NMT 0.10% |
| Melting Point | 104–107°C |
| Solubility | Approximately 34 mg/L in water at 25°C; experimental values vary with temperature and method |