Oxybutynin

Oxybutynin is an antimuscarinic agent with pronounced antispasmodic effects and local anesthetic activity, and it has served as the gold standard for treatment of OAB for 3 decades. Several double-blind, controlled studies have demonstrated its efficacy with reports of 50% symptomatic improvement in 61% to 86% of subjects at 15mg daily.4

The original formulation (OXY-IR) is an immediate-release oral administration dosed at 5 mg three to four

Table 7-1.1. Available anticholinergic agents for treatment of OAB

Name of Drug

Class

Dosage

Route

Side effects

Contraindications

Oxybutynin

Antimuscarinic

IR = 2.5mgBID-5mgQID

PO

Xerostomia

Closed-angle glaucoma

Local anesthetic

ER = 5mg-30mgQD

TD

Blurred vision

Gastric retention

Smooth muscle relaxant

TD = 3.9mg 2x/week

Constipation

Arrhythmia

tachycardia

Myasthenia gravis

Tolterodine

Antimuscarinic

IR = 1-2mgBID

PO

antincholinergic

Closed-angle glaucoma

Smooth muscle relaxant

ER = 2-4mgQD

Gastric retention

Arrhythmia

Myasthenia gravis

Hyoscyamine

Anticholinergic

0.125-0.25mgQ4h

PO

Anticholinergic

Closed-angle glaucoma

Antispasmodic

or

SL

Gastric retention

0.375-0.75mgBID

Arrhythmia

Myasthenia gravis

Imipramine

Tricyclic antidepressant

25-100mgQHS

PO

Orthostatic hypotension

Recent MI

Anticholinergic

Urinary retention

MAO inhibitor usage

Alpha-adrenergic

anticholinergic

Antihistaminic

Propantheline

Anticholinergic

15-30 mgTID/QID

PO

anticholinergic

Closed-angle glaucoma

Gastric retention

Arrhythmia

Myasthenia gravis

Solifenacin

Antimuscarinic

5-10mg QD

PO

Anticholinergic

Closed angle glaucoma

Urinary retention

Gastric retention

Darifenacin

Antimuscarinic

7.5-15mg QD

PO

Anticholinergic

Closed angle glaucoma

Urinary retention

Gastric retention

Hepatic impairment

Trospium

Antimuscarinic

20 mg BID

PO

Anticholinergic

Closed angle glaucoma

Antispasmodic

Urinary retention

Gastric retention

IR=immediate release, ER=extended release,TD=transdermal, SL=sublingual

times a day with the maximum dosage of 20 mg per day. Thuroff et al.5 demonstrated a mean decrease in incontinence episodes of 52%, and in urinary frequency of 33%. Significant levels of discontinuation were observed despite the high subjective improvement rates (74%) secondary to the incidence of reported side effects (70%), most notably dry mouth. Other placebo-controlled studies demonstrated reduced incontinence frequency by 19% to 58%.

Approximately 80% of patients taking OXY-IR 5 mg four times a day report intolerable adverse side effects. Oral administration of oxybutynin results in the metabolism of oxybutynin to N-desethyloxybutynin (DEO), its primary metabolite. A strong correlation has been identified between DEO levels and the severity of dry mouth. Thus, reducing small bowel and hepatic production of DEO results in lessened anticholinergic side effects. This led to the release of an extended-release oral oxybutynin and a transdermal delivery system. In 1999, oxybutynin extended release - OXY-ER (Ditropan XL) was approved. It is a once daily, controlled-release formulation using a push-pull osmotic system (OROS) allowing for consistent release of oxybutynin, affording a lower mean degree of fluctuation in plasma levels, and avoiding enterohepatic circulation by increased delivery in the large intestine, thus reducing the side effect profile whereas maintaining clinical efficacy. As a result, the incidence of moderate to severe xerostomia is reduced to 23%.6

Various clinical studies have demonstrated the effectiveness of oxybutynin in controlling OAB. In a randomized study that compared the immediate- and extended-release formulations of oral oxybutynin, the mean number of weekly urinary incontinence episodes was reduced by 88% and 84%, respectively. Although effectiveness was high, so was the incidence of dry mouth - 68% for extended-release oxybutynin and 87% for immediate-release oxybutynin.7

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