Oxybutynin Base is a synthetic antimuscarinic and antispasmodic pharmaceutical active ingredient used in medicines for the management of overactive bladder and related urinary conditions. Oxybutynin is chemically identified by CAS No. 5633-20-5, molecular formula C₂₂H₃₁NO₃, and molecular weight 357.49 g/mol. PubChem identifies oxybutynin as a racemic compound consisting of equimolar R- and S-isomers.
Oxybutynin Base is chemically distinct from Oxybutynin Hydrochloride (Oxybutynin Chloride), which has CAS No. 1508-65-2 and molecular weight 393.95 g/mol. The free-base form is particularly relevant to transdermal formulations because the non-ionized base can provide suitable skin penetration characteristics. EMA documentation for Kentera specifically describes the use of oxybutynin free base as the active substance in the transdermal system.
Oxybutynin has the IUPAC name 4-(diethylamino)but-2-ynyl 2-cyclohexyl-2-hydroxy-2-phenylacetate. Its molecular structure contains a tertiary amino group, ester functionality, hydroxyl group, cyclohexyl group and phenyl group. PubChem reports an exact mass of 357.23039385 Da, XLogP3-AA of 4.2, hydrogen-bond donor count of 1, hydrogen-bond acceptor count of 4, rotatable bond count of 8, and topological polar surface area of 49.8 Ų.
Oxybutynin is a racemic 50:50 mixture of R- and S-isomers. Pharmacological activity is predominantly associated with the R-isomer, while the marketed racemic drug contains both stereoisomers. The free base is pharmacologically equivalent to oxybutynin hydrochloride.
Oxybutynin acts as a competitive antagonist at muscarinic acetylcholine receptors. By reducing muscarinic-mediated stimulation of bladder smooth muscle, it promotes relaxation of the detrusor muscle and helps reduce urinary urgency, frequency and involuntary bladder contractions. FDA documentation describes oxybutynin as a racemic compound and identifies its mechanism as competitive antagonism of acetylcholine at postganglionic muscarinic receptors.
Oxybutynin is used in pharmaceutical products for the management of overactive bladder symptoms, including urinary urgency, frequency and urge urinary incontinence. It has been formulated in oral tablets, extended-release products, oral solutions and transdermal systems. The Indian Pharmacopoeia Commission has published specifications for Oxybutynin Hydrochloride Tablets, demonstrating its established pharmaceutical relevance in solid oral dosage forms.
One of the important characteristics of Oxybutynin Base is its application in transdermal drug-delivery systems. EMA's scientific discussion for Kentera states that the free base of oxybutynin was selected as the active substance because acid-based salts generally do not penetrate the stratum corneum satisfactorily. The free base therefore has specific importance for topical and transdermal pharmaceutical development.
FDA regulatory documentation for an oxybutynin gel describes the active pharmaceutical ingredient as oxybutynin free base, with the drug product containing 3.0% w/w oxybutynin free base. This provides an important regulatory example of the direct use of the free-base API in a pharmaceutical dosage form.
Oxybutynin Base is reported as a white to off-white solid/powder in commercial chemical references. PubChem establishes the fundamental molecular identity and physicochemical descriptors, while commercial analytical material specifications can differ according to grade and intended use.
A commercial analytical specification from MedKoo reports HPLC purity ≥98% for Oxybutynin free base. Another commercial certificate reports a sample melting point of 59–62°C. Because these are commercial/reference-material values rather than a universal pharmacopoeial API release requirement, they should not be represented as mandatory pharmaceutical specifications for every Oxybutynin Base batch.
Oxybutynin Base and Oxybutynin Hydrochloride should be clearly distinguished when preparing pharmaceutical API documentation. Oxybutynin Base has the molecular formula C₂₂H₃₁NO₃ and molecular weight 357.49 g/mol, whereas Oxybutynin Hydrochloride has the formula C₂₂H₃₁NO₃·HCl and molecular weight 393.95 g/mol. FDA's GSRS identifies Oxybutynin Chloride as a separate ingredient substance with CAS No. 1508-65-2.
The USP-NF monograph applies to Oxybutynin Chloride, not the free base. USP specifies 97.0–102.0% oxybutynin chloride on the dried basis. Consequently, this assay range should not be copied into a CMS specification for Oxybutynin Base.
For Oxybutynin Base, appropriate quality control can include identity confirmation, chromatographic purity, assay, residual solvents, moisture, elemental impurities and other process-related impurities. However, unlike Oxybutynin Chloride, a current universal USP numerical API specification for the free base was not verified in the authoritative sources reviewed.
For this reason, numerical values in a CMS specification should only be entered where they are supported by an applicable approved specification, DMF, customer specification or validated internal release standard. This is especially important for fields such as assay, individual impurities, total impurities, pH and loss on drying.
Pharmaceutical-grade Oxybutynin Base requires controlled manufacturing, purification, drying and packaging processes designed to maintain chemical identity and consistent quality. Analytical testing should be performed using validated methods suitable for the free-base substance.
API documentation may include a Certificate of Analysis, specification sheet, analytical methods, safety documentation, batch information and regulatory documentation appropriate to the intended pharmaceutical market. For transdermal applications, control of purity and physical characteristics is particularly important because the free-base form is intentionally selected for its formulation and permeation characteristics.
Oxybutynin Base is an important pharmaceutical raw material for formulations requiring the free-base form of oxybutynin. Its established antimuscarinic activity, racemic composition and suitability for transdermal drug delivery make it relevant to pharmaceutical formulation development. FDA and EMA regulatory documents confirm the pharmaceutical use of oxybutynin free base in topical/transdermal products.
For API procurement, formulation development and regulated manufacturing, the free base should be identified by its correct CAS 5633-20-5, molecular formula C₂₂H₃₁NO₃, and molecular weight 357.49 g/mol, while avoiding confusion with Oxybutynin Hydrochloride.
| 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% |
| Individual Impurity | NMT 0.5% |
| Total Impurities | NMT 1.0% |
| Water Content | NMT 0.5% |
| Melting Point | Reference value: 59–62°C |