Oxybutynin Hydrochloride, commonly abbreviated as Oxybutynin HCl, is an established pharmaceutical active ingredient belonging to the antimuscarinic and antispasmodic class. It is primarily used in pharmaceutical formulations for the management of overactive bladder, urinary urgency, urinary frequency and urge urinary incontinence. Chemically, Oxybutynin HCl is the hydrochloride salt of oxybutynin and is also officially referred to as Oxybutynin Chloride. PubChem identifies the substance under CAS No. 1508-65-2, molecular formula C₂₂H₃₂ClNO₃, and molecular weight 393.9 g/mol.
Oxybutynin acts as a competitive antagonist at muscarinic acetylcholine receptors. By reducing acetylcholine-mediated stimulation of bladder smooth muscle, it decreases involuntary detrusor contractions and helps increase bladder capacity. This pharmacological activity makes Oxybutynin HCl an important API for pharmaceutical products designed to control symptoms associated with overactive bladder.
Oxybutynin HCl has the IUPAC name 4-(diethylamino)but-2-ynyl 2-cyclohexyl-2-hydroxy-2-phenylacetate hydrochloride. Its molecular structure contains a tertiary amine, ester group, hydroxyl functionality, cyclohexyl group and phenyl group. Protonation of the tertiary amine with hydrochloric acid produces the hydrochloride salt.
The API has a molecular weight of approximately 393.95 g/mol and an exact mass of 393.2070716 Da. PubChem reports a topological polar surface area of 49.8 Ų, hydrogen-bond donor count of 2, hydrogen-bond acceptor count of 4, and 8 rotatable bonds.
Oxybutynin itself is a racemic compound containing approximately equimolar amounts of R-oxybutynin and S-oxybutynin. Therefore, Oxybutynin HCl should not be confused with the free-base API, which has CAS No. 5633-20-5 and molecular weight 357.49 g/mol.
Oxybutynin HCl is primarily associated with pharmaceutical treatment of overactive bladder. Its antimuscarinic activity helps reduce involuntary bladder contractions and associated urinary symptoms. Oxybutynin products have been developed in multiple dosage forms, including immediate-release tablets, extended-release tablets and oral formulations.
The API is therefore relevant to pharmaceutical manufacturers developing urinary antispasmodic and anticholinergic products. Consistent assay, impurity control, moisture control and salt composition are important to maintain reproducible pharmaceutical quality.
Oxybutynin HCl is covered by the USP-NF Oxybutynin Chloride monograph. The USP specification states that Oxybutynin Chloride contains 97.0% to 102.0% of C₂₂H₃₁NO₃·HCl, calculated on the dried basis.
Importantly, the USP assay uses liquid chromatography, making the value appropriate for a CMS field labelled Assay (HPLC). The pharmacopoeial chromatographic method uses UV detection and controls system suitability before calculation of the assay.
USP provides defined limits for several related compounds. Oxybutynin Related Compound A is NMT 0.5%, the diphenyl analog is NMT 0.1%, Related Compound B is NMT 1.0%, Related Compound C is NMT 1.0%, the cyclohexenyl analog is NMT 1.0%, and the ethylpropyl analog is NMT 0.1%.
Any other individual impurity is limited to NMT 0.1%, while the total impurities limit is NMT 1.0%. These limits provide a useful impurity-control framework for pharmaceutical-grade Oxybutynin HCl.
The USP chromatographic system also includes a resolution requirement of NLT 1.1 between Oxybutynin Related Compound B and Related Compound C. This helps demonstrate adequate chromatographic separation during impurity analysis.
Because Oxybutynin HCl is a hydrochloride salt, chloride content is another important quality characteristic. The USP specification establishes chloride content at 8.0% to 10.0%, determined by titration with silver nitrate.
USP also specifies residue on ignition NMT 0.1% and loss on drying NMT 3.0%. The loss-on-drying test is performed at 105°C for 2 hours. These numerical controls help ensure appropriate moisture and inorganic-residue characteristics of the API.
Oxybutynin HCl is a crystalline pharmaceutical substance with a USP melting range of approximately 124–129°C. The API is preserved in well-closed containers according to the USP monograph.
The physical characteristics should be monitored during manufacturing and release because particle characteristics, moisture and crystallinity can influence downstream processing and pharmaceutical formulation performance.
Manufacturing pharmaceutical-grade Oxybutynin HCl involves controlled synthesis, purification, hydrochloride salt formation, crystallization and drying. Appropriate process controls should be implemented to maintain chemical identity, assay, impurity profile and physical consistency.
A comprehensive analytical program can include FTIR identification, HPLC assay, HPLC related-substance testing, loss-on-drying analysis, residue-on-ignition testing and chloride determination. Reference standards should be appropriately qualified for identity and impurity testing.
For regulated pharmaceutical applications, each batch should be released against the applicable pharmacopoeial or approved specification. Typical documentation can include a Certificate of Analysis, specification sheet, analytical documentation, safety information and regulatory documentation appropriate to the destination market.
High-quality Oxybutynin HCl should demonstrate consistent chemical identity and pharmaceutical purity. The USP specification provides clearly defined numerical benchmarks including 97.0–102.0% assay on dried basis, NMT 3.0% loss on drying, NMT 0.1% residue on ignition, 8.0–10.0% chloride content and NMT 1.0% total impurities.
For pharmaceutical manufacturers, it is important to distinguish Oxybutynin HCl from Oxybutynin Base. Oxybutynin HCl has CAS 1508-65-2 and molecular weight approximately 393.95 g/mol, whereas Oxybutynin Base has CAS 5633-20-5 and molecular weight 357.49 g/mol.
| Parameter | Specification |
|---|---|
| Appearance | white to off-white crystalline powder |
| Identification | IR & HPLC compliant |
| Assay (HPLC) | 97.0% – 103.0% |
| Loss on Drying | NMT 3.0% |
| Residue on Ignition | NMT 0.1% |
| Heavy Metals | NMT 20 ppm |
| Individual Impurity | ≤ 0.5% |
| Total Impurities | NMT 1.0% |
| Melting Point | 124–129°C |