The transdermal oxybutynin system is a matrix-type patch designed for controlled drug delivery to treat overactive bladder. It contains 36 mg of racemic oxybutynin in a 39 cm² patch, delivering 3.9 mg daily over 3-4 days. The system minimizes first-pass metabolism, reducing side effects like dry mouth, and offers flexible application sites (abdomen, buttock, or hip). Its design includes an acrylic adhesive with triacetin as a permeation enhancer, ensuring steady absorption through the skin. While effective, limitations include potential skin reactivity and dosage constraints due to patch size.
Key Points Explained:
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Patch Design and Composition
- The Oxybutynin Transdermal Patch is a 39 cm² matrix-type system with three layers:
- Peel-off protective layer
- Occlusive backing film
- Active drug layer containing 36 mg racemic oxybutynin (R- and S-isomers) and triacetin (permeation enhancer) in an acrylic adhesive matrix.
- Triacetin interacts with skin lipids to modulate drug diffusion, ensuring controlled release.
- The Oxybutynin Transdermal Patch is a 39 cm² matrix-type system with three layers:
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Dosage and Delivery Profile
- Delivers 3.9 mg/day of oxybutynin continuously for 3–4 days (total wear time: ~96 hours).
- Steady-state plasma concentrations are achieved within 24–48 hours.
- Bioequivalence studies confirm consistent delivery across application sites (abdomen, buttock, hip).
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Mechanism of Action
- Oxybutynin’s molecular properties (357 daltons, lipophilicity, weak base) enable transdermal permeation.
- Minimizes first-pass metabolism, reducing production of the metabolite DEO (linked to dry mouth).
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Application and Flexibility
- Approved for application on hairless, clean skin of abdomen, buttock, or hip.
- Twice-weekly dosing simplifies regimen adherence.
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Advantages Over Oral Formulations
- Fewer anticholinergic side effects (e.g., dry mouth incidence drops from ~70% oral to ~25% transdermal).
- Avoids gastrointestinal metabolism and erratic absorption.
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Limitations
- Skin reactivity: Potential for irritation/allergy, especially with larger/multiple patches.
- Dosage inflexibility: Higher doses require impractical patch sizes.
- Hair growth: May limit use in males with dense hair at application sites.
- Pediatric data gap: Not studied in children.
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Clinical Relevance
- Ideal for patients intolerant to oral oxybutynin’s side effects.
- Offers predictable pharmacokinetics but requires monitoring for local skin reactions.
The system exemplifies how transdermal technology balances efficacy with tolerability, though its fixed-dose design and cutaneous effects warrant careful patient selection.
Summary Table:
| Feature | Specification |
|---|---|
| Patch Size | 39 cm² matrix-type patch |
| Drug Content | 36 mg racemic oxybutynin |
| Daily Delivery | 3.9 mg/day over 3–4 days |
| Application Sites | Abdomen, buttock, or hip (hairless, clean skin) |
| Key Benefit | Minimizes first-pass metabolism, reducing side effects like dry mouth |
| Limitations | Potential skin reactivity, dosage inflexibility, not studied in children |
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