Oxybutynin

Oxybutynin is an anticholinergic medication used to relieve urinary and bladder difficulties, including frequent urination and inability to control urination (urge incontinence), by decreasing muscle spasms of the bladder.^[1] It competitively antagonizes the M1, M2, and M3 subtypes of the muscarinic acetylcholine receptor. It also has direct spasmolytic effects on bladder smooth muscle as a calcium antagonist and local anesthetic, but at concentrations far above those used clinically. It is available orally in generic formulation and as the brand-names Ditropan and Lyrinel XL, and as a transdermal patch under the brand-name Oxytrol.

Oxybutynin is also a very effective drug for treatment of hyperhidrosis, or hyper-active sweating.^[2][3][4]

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Chemistry

Oxybutynin contains one stereocenter. Commercial formulations are sold as the racemate. The (R)-enantiomer is a more potent anticholinergic than either the racemate or the (S)-enantiomer, which is essentially without anticholinergic activity at the doses used in clinical practice.^[5][6] However, (R)-oxybutynin administered alone offers little or no clinical benefit above and beyond the racemic mixture. The other actions (calcium antagonism, local anesthesia) of oxybutynin are not stereospecific. (S)-Oxybutynin has not been clinically tested for its spasmolytic effects, but may be clinically useful for the same indications as the racemate, without the unpleasant anticholinergic side effects.

Adverse effects

Adverse effects can limit the use of oxybutynin, but can be reduced through the use of modified release preparations (e.g. Lyrinel XL) or by slowly increasing dosage.

Common adverse effects associated with oxybutynin and other anticholinergics include: dry mouth, difficulty in micturition, constipation, blurred vision, drowsiness and dizziness.^[7] Anticholinergics have also been known to induce delirium.^[8] These are dose-related and sometimes severe; in one population studied, after six months more than half of the patients had stopped taking the medication due to side effects. Dry mouth may be particularly severe; one estimate is that over a quarter of patients who begin oxybutynin treatment may have to stop because of dry mouth.

N-Desethyloxybutynin is an active metabolite of oxybutynin that is thought to be responsible for much of the adverse effects associated with the use of oxybutynin.^[9] N-Desethyloxybutynin plasma levels may reach as much as six times that of the parent drug after administration of the immediate-release oral formulation.^[10] Alternative dosage forms have been developed in an effort to reduce blood levels of N-desethyloxybutynin and allow for a more steady concentration of oxybutynin to be achieved than is possible with the immediate release form. The long-acting formulations also allow once-daily administration instead of the twice-daily doseage required with the immediate-release form. The transdermal patch, in addition to the benefits of the extended-release oral formulations, bypasses the first-pass hepatic effect that the oral formulations are subject to.^[11]

Clinical pharmacology

Oxybutynin chloride exerts direct antispasmodic effect on smooth muscle and inhibits the muscarinic action of acetylcholine on smooth muscle. It exhibits on one-fifth of the anticholinergic activity of atropine on the rabbit detrusor muscle, but four to ten times the antispasmodic activity. No blocking effects occur at skeletal neuromuscular junctions or autonomic ganglia (antinicotinic effects).

Contraindications

Oxybutynin chloride is contraindicated in patients with untreated angle closure glaucoma and in patients with untreated narrow anterior chamber angles since anticholinergic drugs may aggravate these conditions. It is also contraindicated in partial or complete obstruction of the gastrointestinal tract, paralytic ileus, intestinal atony of the elderly or debilitated patient, megacolon, toxic megacolon complicating ulcerative colitis, severe colitis and myasthenia gravis. It is contraindicated in patients with obstructive uropathy and in patients with unstable cardiovascular status in acute hemorrhage. Oxybutynin chloride is contraindicated in patients who have demonstrated hypersensitivity to the product.

References