Medications> Neuropathy and bladder disorders>

INDICATIONS

• Urinary incontinence 
• Overactive bladder
• Uninhibited or reflex neurogenic bladder (i.e., urgency, frequency, urinary leakage, urge incontinence, dysuria).

MECHANISM

• Oxybutynin, a tertiary amine ester, exerts antimuscarinic (atropine-like) and antispasmodic (papaverine-like) actions on smooth muscle. In neurogenic bladder, oxybutynin inhibits detrusor muscle hyperreflexia.

USUAL ADULT DOSING

• Non-elderly adults: Immediate release oxybutynin 5 mg PO 4 times per day; dosage may be adjusted weekly by 5 mg increments based on efficacy and tolerability.
• Elderly: extended release oxybutynin 5 mg PO once daily is more appropriate for elderly patients based on the Beers criteria.
• Maximum dose 30 mg/day
• Transdermal patch: 3.9 mg/day system applied twice weekly to dry, intact skin on the abdomen, hip, or buttock
• Topical gel 10%: apply the content of one sachet to a dry, intact skin on the abdomen, upper arms/shoulders, or thighs.
• Patch and gel require patient to rotate sites.

FORMS

*Prices represent cost per unit specified and are representative of "Average Wholesale Price" (AWP). AWP Prices were obtained and gathered by Lakshmi Vasist Pharm D using the Red Book, manufacturer's information, and the McKesson database.

^Dosage is indicated in mg unless otherwise noted.

DOSING IN SPECIAL POPULATIONS

• Extended-release formulations of oxybutynin have not been evaluated in severe renal impairment and are not recommended.

• Lower dosages may be needed in hepatic disease because the drug is extensively metabolized in the liver.
• Extended-release formulations of oxybutynin have not been evaluated and are not recommended.

• Although oxybutynin is classified as FDA pregnancy risk category B, safe use during pregnancy has not been established.

• It is not known if oxybutynin is excreted in breast milk. Lactation suppression has been reported during post-marketing use of immediate-release oxybutynin.

ADVERSE DRUG REACTIONS

• Side effects are related to the anticholinergic effects and the antimuscarinic effects on GI smooth muscle.

• GI: Constipation, Gastroenteritis, Nausea, Xerostomia
• CNS: Dizziness, Headache, Somnolence

• Cardiac: Palpitations, sinus tachycardia, fluid retention, peripheral edema
• Endocrine: Hyperglycemia

• Severe allergic reactions, urticaria

DRUG INTERACTIONS

• Oxybutynin is metabolized primarily by the cytochrome P450 3A4 isoenzyme in the liver and gut wall.
• Potent inhibitors of CYP450 3A4 (e.g. HIV protease inhibitors, azole antifungals, macrolide antibiotics) should be avoided.
• Drugs that induce CYP450 3A4 (e.g. carbamazepine, phenytoin, phenobarbital, rifamycin antibiotics, nevirapine, efavirenz) may reduce the effects of oxybutynin.

PHARMACOKINETIC

COMMENTS

• Commonly used for outpatient treatment of diabetic neurogenic bladder; some evidence for reduction of daytime urinary frequency in the elderly.
• Contraindicated with diabetic gastroparesis, due to GI smooth muscle effects. 
• Closed angle glaucoma and urinary retention are absolute contraindications.
• May be useful in the treatment of gustatory sweating associated with autonomic dysfunction in diabetes

REFERENCES