What is Mirabegron?

Mirabegron: A Beta-3 Adrenergic Receptor Agonist for Overactive Bladder

Mirabegron is a beta-3 adrenergic receptor agonist that relaxes the detrusor smooth muscle during bladder filling, increasing bladder capacity and treating symptoms of overactive bladder (OAB) including urgency, frequency, and urgency incontinence. 1

Mechanism of Action

Mirabegron works differently from traditional antimuscarinic medications by:

• Selectively activating beta-3 adrenergic receptors in the bladder 1
• Relaxing detrusor smooth muscle during the storage phase of urination 1
• Increasing bladder capacity without significantly affecting voiding parameters 2

Clinical Indications

Mirabegron is indicated for:

• Treatment of overactive bladder symptoms including:
  • Urinary urgency
  • Frequency of urination
  • Urgency urinary incontinence 3
• Can be used in both men and women with OAB symptoms 2
• May be particularly valuable for patients who cannot tolerate antimuscarinic side effects, especially dry mouth 3

Dosing and Administration

• Starting dose: 25 mg orally once daily 3
• Can be increased to 50 mg once daily after 4-8 weeks if needed for symptom control 3
• Dose adjustments required for:
  • Severe renal impairment (eGFR 15-29 mL/min/1.73 m²): maximum 25 mg daily 1
  • Moderate hepatic impairment (Child-Pugh Class B): maximum 25 mg daily 1
  • Not recommended in severe hepatic impairment or end-stage renal disease 1

Efficacy

Mirabegron demonstrates significant improvements in:

• Micturition frequency (reduced number of urinations per day) 3
• Urgency incontinence episodes 3
• Mean voided volume per micturition 3

Benefits are typically seen as early as 4 weeks after starting treatment and are maintained throughout continued therapy 4.

Safety Profile and Adverse Events

The most common adverse events include:

• Hypertension (monitor blood pressure before and during treatment) 3, 1
• Nasopharyngitis 1, 4
• Urinary tract infection 1, 4

Notably, mirabegron has a significantly lower incidence of dry mouth compared to antimuscarinic medications (similar to placebo rates), which may improve medication adherence 3, 4.

Special Considerations

Blood Pressure Effects

• Mirabegron can increase blood pressure in a dose-dependent manner 1
• Regular blood pressure monitoring is recommended, especially in patients with pre-existing hypertension 3
• Contraindicated in patients with severe uncontrolled hypertension 3

Use in Specific Populations

• Effective and well-tolerated in elderly patients (≥65 years) 3, 4
• Can be used in men with lower urinary tract symptoms (LUTS) without significantly affecting voiding function 2, 1
• May be combined with alpha-blockers in men with LUTS 3
• Can be combined with antimuscarinic agents for enhanced efficacy in refractory cases 3

Advantages Over Antimuscarinic Agents

• Different mechanism of action (beta-3 agonist vs. muscarinic antagonist) 1
• Significantly lower incidence of dry mouth (3-5 times less than antimuscarinics) 4
• Lower rates of constipation 4
• Fewer CNS effects 5
• Effective in patients who previously discontinued antimuscarinic therapy due to lack of efficacy or poor tolerability 4

Contraindications

• Severe uncontrolled hypertension 3
• Severe hepatic impairment (Child-Pugh Class C) 1
• End-stage renal disease (eGFR <15 mL/min/1.73 m²) 1

Mirabegron represents an important alternative to antimuscarinic medications for OAB treatment, particularly for patients who experience intolerable side effects from antimuscarinics or have inadequate response to them.