indication

For the treatment of overactive bladder.

pharmacology

Oxybutynin is an antispasmodic, anticholinergic agent indicated for the treatment of overactive bladder with symptoms of urge urinary incontinence, urgency, and frequency. Oxybutynin relaxes bladder smooth muscle. Oxybutynin exhibits only one-fifth of the anticholinergic activity of atropine on the rabbit detrusor muscle, but four to ten times the antispasmodic activity. Antimuscarinic activity resides predominantly in the R-isomer.

mechanism of action

Oxybutynin exerts a direct antispasmodic effect on smooth muscle and inhibits the muscarinic action of acetylcholine on smooth muscle. No blocking effects occur at skeletal neuromuscular junctions or autonomic ganglia (antinicotinic effects). By inhibiting particularily the M1 and M2 receptors of the bladder, detrusor activity is markedly decreased.

toxicity

LD₅₀=1220 mg/kg (Orally in rats, Goldenthal)

biotransformation

Hepatic, primarily by CYP3A4

absorption

Rapidly absorbed from gastrointestinal tract.

half life

12.4-13.2 hours

route of elimination

Oxybutynin is metabolized primarily by the cytochrome P450 enzyme systems, particularly CYP3A4, found mostly in the liver and gut wall. Oxybutynin is extensively metabolized by the liver, with less than 0.1% of the administered dose excreted unchanged in the urine. Also, less than 0.1% of the administered dose is excreted as the metabolite N-desethyloxybutynin.

drug interactions

Carbamazepine: Oxybutynin may cause carbamazepine toxicity

Donepezil: Possible antagonism of action

Galantamine: Possible antagonism of action

Rivastigmine: Possible antagonism of action

Tacrine: The therapeutic effects of the central acetylcholinesterase inhibitor, Tacrine, and/or the anticholinergic, Oxybutynin, may be reduced due to antagonism. The interaction may be beneficial when the anticholinergic action is a side effect. Monitor for decreased efficacy of both agents.

Trimethobenzamide: Trimethobenzamide and Oxybutynin, two anticholinergics, may cause additive anticholinergic effects and enhance their adverse/toxic effects. Monitor for enhanced anticholinergic effects.

Triprolidine: Triprolidine and Oxybutynin, two anticholinergics, may cause additive anticholinergic effects and enhance their adverse/toxic effects. Monitor for enhanced anticholinergic effects.

Trospium: Trospium and Oxybutynin, two anticholinergics, may cause additive anticholinergic effects and enhanced adverse/toxic effects. Monitor for enhanced anticholinergic effects.