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Oxybutynin: Mechanism of Action, Side Effects, Contraindications, and Dosage

• Anticholinergics: general pharmacology
• trospium chloride
• oxybutynin
• tolterodine
• fesoterodine
• propiverine
• darifenacin
• solifenacin
• butylscopolamine

Mechanism of Action

Oxybutynin is a tertiary amine with anticholinergic properties at the M₁, M₂, and M₃ muscarinic receptors. Oxybutynin also has a relaxing effect on smooth muscle by inhibiting calcium channels.

The inhibition of M₃ receptors with oxybutynin reduces smooth muscle detrusor contraction, increases functional bladder capacity, and alleviates urgency symptoms. Oxybutynin is used to treat enuresis, overactive bladder, urge incontinence, and neurogenic lower urinary tract dysfunction.

Pharmacokinetics of Oxybutynin:

Poor bioavailability of only 10% due to high first-pass metabolism in the liver (CYP3A4). High plasma protein binding of 99%. Immediate-release oxybutynin has a half-life of 2–3 hours, and up to 5 hours in elderly patients. Renal excretion of metabolites.

Side Effects of Oxybutynin

Muscarinic receptors are present in numerous organs. M₃ receptors are mainly found in smooth muscle and glands. Oxybutynin lacks organ specificity for the bladder, causing a high rate of unwanted side effects.

Eye:

• Mydriases
• Disturbed accommodation
• Dry eyes due to inhibited lacrimal gland
• Risk of increased intraocular pressure in patients with glaucoma

Gastrointestinal tract:

• Dry mouth due to inhibited salivary glands
• Inhibition of intestinal motility (constipation)

Heart:

• Tachycardia

CNS:

• Dizziness, loss of concentration, drowsiness
• Aggravation of dementia
• Headache
• Fear, anxiety, and confusion are possible psychiatric side effects.

Skin:

• Dry skin due to inhibited sweat glands.
• Impaired temperature regulation in fever.

Drug Interactions

Increased anticholinergic effect with antiparkinson drugs, antihistamines, antipsychotics, and tricyclic antidepressants. Amplification of the tachycardic effect of sympathomimetic medications. Oxybutynin mitigates the effects of gastrointestinal prokinetics.

Contraindications of Oxybutynin

• Risk of urinary retention
• Narrow-angle glaucoma
• Ileus, gastrointestinal stenosis
• Myasthenia gravis
• Tachyarrhythmias
• Pregnancy, children under 5 years of age
• Severe liver failure

Dosage of Oxybutynin:

• Adults: gradual dose titration starting with 2.5 mg 1-1-1, gradually increasing until the desired effect is achieved. The maximum daily dose is 20 mg. There are sustained-release preparations available for once-daily administration.
• Children over 5 years of age: initially 2.5 mg 1-0-1, then gradually increase. The maximum daily dose is 0.4 mg/kg body weight (maximum 15 mg).
• Transdermal application: with better tolerability. As a patch (3.9 mg/24 h, changed every 3–4 days) or gel (4 mg as a daily application, only available in North America).
• Intravesical use: to reduce episodes of incontinence in patients with neurogenic bladder dysfunction and self-catheterization. The daily dose of 10–40 mg is divided into 2–4 injections; the active ingredient remains in the bladder until the subsequent catheterization. Approved only in the EU and UK.

References

Ouslander 2004 OUSLANDER, J. G.: Management of overactive bladder.
In: N Engl J Med
350 (2004), Nr. 8, S. 786–99

Yoshimura und Chancellor 2002 YOSHIMURA, N. ; CHANCELLOR, M. B.: Current and future pharmacological treatment for overactive bladder.
In: J Urol
168 (2002), Nr. 5, S. 1897–913

  Deutsche Version: Trospium Chlorid

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