What is the comparison between Myrbetriq (mirabegron) and oxybutynin for treating overactive bladder?

Mirabegron vs. Oxybutynin for Overactive Bladder

Mirabegron (Myrbetriq) is superior to oxybutynin for treating overactive bladder due to its comparable efficacy with significantly fewer anticholinergic side effects, particularly in elderly patients. 1, 2

Mechanism of Action and Efficacy

• Mirabegron is a β3-adrenergic receptor agonist that relaxes the detrusor muscle during the storage phase, improving bladder capacity without affecting voiding 3, 4
• Oxybutynin is an antimuscarinic agent that blocks acetylcholine receptors, reducing involuntary detrusor contractions 1
• Both medications demonstrate significant improvements in key overactive bladder symptoms compared to placebo, including reduction in:
  • Incontinence episodes
  • Urinary frequency
  • Urgency episodes 5, 6

Side Effect Profile Comparison

• Mirabegron has a significantly better tolerability profile than oxybutynin, particularly regarding:
  • Dry mouth (2.8% with mirabegron vs. significantly higher rates with antimuscarinics) 3, 6
  • Constipation (less common with mirabegron) 6
  • Blurred vision (less common with mirabegron) 1, 6
• Oxybutynin carries a substantially higher risk of anticholinergic side effects, which can significantly impact quality of life and medication adherence 1
• Mirabegron has demonstrated favorable cardiovascular safety in clinical analyses 7, 5

Special Populations

• Elderly patients:
  • Mirabegron 25mg has demonstrated safety and efficacy in older patients with multiple comorbidities 2, 7
  • Oxybutynin should be used with extreme caution in elderly patients due to increased risk of anticholinergic side effects and cognitive impairment 1
• Men with lower urinary tract symptoms (LUTS):
  • Mirabegron can be used safely in men with BPH-related symptoms with minimal risk of urinary retention 8
  • Oxybutynin carries a theoretical higher risk of urinary retention in men with BPH 1

Treatment Algorithm

1. First-line therapy should include behavioral interventions:

   • Bladder training
   • Pelvic floor exercises
   • Fluid management 1, 7

2. For pharmacological treatment:

   • For most patients, especially elderly: Start with mirabegron 25-50mg due to better tolerability profile 2, 7
   • For younger patients without comorbidities: Either medication may be appropriate, with patient preference considering side effect profiles 5
   • For patients with narrow-angle glaucoma, urinary retention, or impaired gastric emptying: Avoid oxybutynin and use mirabegron 1

3. For inadequate response to monotherapy:

   • Consider combination therapy with mirabegron plus an antimuscarinic agent (preferably solifenacin) 1, 7
   • Combination therapy has demonstrated superior efficacy compared to either monotherapy alone 7, 6

Common Pitfalls and Caveats

• Avoid oxybutynin in patients with:
  • Narrow-angle glaucoma
  • History of urinary retention
  • Impaired gastric emptying
  • Cognitive impairment or dementia 1
• Mirabegron starting dose should be 25mg in patients with:
  • Severe renal impairment (eGFR 15-29 mL/min)
  • Moderate hepatic impairment (Child-Pugh B) 2, 5
• Monitor blood pressure with mirabegron, although cardiovascular safety analyses have shown no significant concerns 7, 5
• Consider that dry mouth from oxybutynin often leads to treatment discontinuation, negatively impacting long-term outcomes 3, 6