Oxybuprocaine is a synthetic ester-type local anaesthetic used primarily for topical anaesthesia, particularly in ophthalmic and otolaryngological applications. For pharmaceutical manufacturing, the API is commonly supplied as Oxybuprocaine Hydrochloride, also known as Benoxinate Hydrochloride. The current European Pharmacopoeia monograph identifies Oxybuprocaine Hydrochloride with CAS No. 5987-82-6, molecular formula C₁₇H₂₉ClN₂O₃, and molecular weight 344.9 g/mol.
Oxybuprocaine, also known as Benoxinate or Oxybucaine, is chemically a benzoate ester formed from 4-amino-3-butoxybenzoic acid and 2-(diethylamino)ethanol. The free base has CAS No. 99-43-4, molecular formula C₁₇H₂₈N₂O₃, and molecular weight 308.4 g/mol. The hydrochloride salt is the important pharmacopoeial pharmaceutical form and has a molecular weight of 344.9 g/mol.
Oxybuprocaine works as a local anaesthetic by reversibly interacting with voltage-gated sodium channels in neuronal membranes. This reduces sodium-ion permeability and prevents the generation and propagation of nerve impulses, producing localized loss of sensation. PubChem classifies oxybuprocaine as an ester-based local anaesthetic and topical anaesthetic, with particular relevance to ophthalmology and otolaryngology.
The molecular structure contains an aromatic amino group, a butoxy substituent, an ester linkage and a diethylaminoethyl side chain. These structural features contribute to the compound's pharmacological activity and physicochemical behaviour. Oxybuprocaine Hydrochloride is considerably more water-compatible than the free base, which is important when developing suitable pharmaceutical formulations.
The European Pharmacopoeia monograph 1251 provides important quality requirements for Oxybuprocaine Hydrochloride. The material is described as a white or almost white crystalline powder or colourless crystals and is very soluble in water and freely soluble in ethanol 96%. The monograph also notes that Oxybuprocaine Hydrochloride exhibits polymorphism.
The pharmacopoeial content requirement is 98.5% to 101.5% calculated on the dried substance. This is a critical specification for pharmaceutical-grade material. The official assay is performed by potentiometric titration with 0.1 M perchloric acid, rather than by HPLC. Therefore, when a CMS has a field called “Assay (HPLC),” the value should be accompanied by a note that the pharmacopoeial assay method is potentiometric titration.
Identification of Oxybuprocaine Hydrochloride can be established using pharmacopoeial identification procedures, including infrared spectroscopy, melting-point characteristics and confirmation of the hydrochloride/chloride component. The Ph. Eur. monograph identifies 4-aminobenzoic acid, 4-amino-3-butoxybenzoic acid and 4-amino-3-hydroxybenzoic acid as relevant impurity structures.
For related substances, the Ph. Eur. monograph uses liquid chromatography. Each individual impurity is limited to 0.1%, total impurities are limited to 0.25%, and the disregard limit is 0.0125%. The chromatographic system also includes a resolution requirement between Oxybuprocaine and impurity B, with a minimum resolution of 12. These controls are useful for monitoring degradation and process-related impurities during API manufacture.
Moisture control is another important quality attribute. The Ph. Eur. requirement for loss on drying is NMT 0.5%, determined on 1.000 g by drying at 105°C. Sulfated ash is controlled at NMT 0.1%. These parameters help control residual moisture and inorganic residue in the API.
The pharmacopoeial solution has a specified pH of 4.5–6.0. Solution S is prepared by dissolving 5.0 g in carbon-dioxide-free water and diluting to 50 mL. The solution must also meet the specified appearance requirement, being clear and no more intensely coloured than reference solution Y5.
The Japanese Pharmacopoeia provides additional characterization for Oxybuprocaine Hydrochloride. It describes the substance as white crystals or crystalline powder and specifies a melting point of approximately 158–162°C. JP XVIII specifies that the dried substance contains NLT 99.0% of Oxybuprocaine Hydrochloride.
The Japanese Pharmacopoeia also specifies that a solution containing 1.0 g in 10 mL of water has a pH between 5.0 and 6.0. Its heavy-metals test uses a control corresponding to NMT 20 ppm. The JP monograph additionally specifies loss on drying of NMT 0.5% and residue on ignition of NMT 0.1%.
Pharmaceutical-grade Oxybuprocaine Hydrochloride manufacturing requires controlled synthesis, purification, salt formation, crystallization and drying, followed by analytical testing against the applicable specification. Critical quality attributes include identity, assay, related substances, pH, moisture, sulfated ash, appearance and solubility.
A robust quality-control program can include FTIR identification, chromatographic impurity profiling, assay testing, loss-on-drying determination, residue testing and pH measurement. For regulated pharmaceutical applications, the batch documentation should correspond to the applicable pharmacopoeial or customer-approved specification.
Oxybuprocaine is particularly associated with topical local anaesthesia in ophthalmology and otolaryngology. It is used in appropriate pharmaceutical formulations where temporary surface anaesthesia is required. PubChem describes it as an ester local anaesthetic and topical anaesthetic.
The hydrochloride form is especially relevant to pharmaceutical manufacturing because of its favourable aqueous solubility. The Ph. Eur. describes Oxybuprocaine Hydrochloride as very soluble in water and freely soluble in ethanol 96%, making its physicochemical profile suitable for formulation development involving the salt form.
High-quality Oxybuprocaine Hydrochloride should demonstrate consistent identity, potency, purity and physical characteristics from batch to batch. Pharmacopoeial controls provide measurable benchmarks, including 98.5–101.5% assay on dried substance, NMT 0.5% loss on drying, NMT 0.1% sulfated ash, pH 4.5–6.0, individual impurity NMT 0.1% and total impurities NMT 0.25% under the cited Ph. Eur. specification.
For pharmaceutical and ophthalmic formulation manufacturers, selecting an API supported by appropriate analytical documentation and a clearly defined quality specification is essential. Oxybuprocaine Hydrochloride should be evaluated against the applicable pharmacopoeial standard and regulatory requirements of the intended market.
| Parameter | Specification |
|---|---|
| Appearance | white to off-white crystalline powder |
| Identification | IR & HPLC compliant |
| Assay (HPLC) | 98.5% – 101.5% on dried substance |
| Loss on Drying | NMT 0.5% |
| Residue on Ignition | NMT 0.1% |
| Heavy Metals | NMT 20 ppm |
| pH (1% Solution) | 4.0 – 6.0 |
| Individual Impurity | NMT 0.1% |
| Total Impurities | NMT 0.25% |
| Melting Point | 158 – 162°C |