Mirabegron: Pharmacology, Indications, and Dosing
Pharmacology
Mirabegron is a selective β3-adrenergic receptor agonist that relaxes the detrusor muscle during bladder filling, thereby increasing bladder capacity and reducing urgency. 1 Unlike antimuscarinics that block bladder contraction, mirabegron works by activating β3-adrenoceptors in the bladder wall, causing smooth muscle relaxation through nitric oxide-dependent pathways 2. This mechanism may also reduce sensory nerve activity in the bladder 2.
Key Pharmacologic Properties:
- Mechanism: β3-adrenoceptor activation → detrusor relaxation → increased bladder storage capacity
- Drug interactions: Mirabegron is a CYP2D6 inhibitor, requiring dose adjustment of narrow therapeutic index drugs metabolized by this pathway (e.g., digoxin) 1
- Onset of action: Significant symptom improvement occurs as early as 1 week, with consistent benefits by 4 weeks 3
Indications
Mirabegron is FDA-approved for two distinct populations: 1
- Adult Overactive Bladder (OAB): Treatment of symptoms including urge urinary incontinence, urgency, and urinary frequency
- Pediatric Neurogenic Detrusor Overactivity (NDO): For patients aged ≥3 years and weighing ≥35 kg
The 2019 AUA/SUFU guideline supports mirabegron as monotherapy for OAB and endorses combination therapy with solifenacin (5 mg mirabegron + 25-50 mg solifenacin) based on the SYNERGY I/II and BESIDE trials 4.
Dosing Regimen
Adult OAB (Monotherapy)
Start with 25 mg orally once daily; increase to 50 mg once daily after 4-8 weeks if needed. 1
- Administration: Swallow whole with water; do not crush, chew, or divide
- Food: May take with or without food
- Maximum dose: 50 mg daily
Pediatric NDO (≥35 kg)
Same dosing as adults: 25 mg once daily, titrate to 50 mg after 4-8 weeks if needed. 1
- Critical difference: Must take WITH food in pediatric patients 1
Dose Adjustments for Renal Impairment
| eGFR | Starting Dose | Maximum Dose |
|---|---|---|
| 30-89 mL/min/1.73m² | 25 mg daily | 50 mg daily |
| 15-29 mL/min/1.73m² | 25 mg daily | 25 mg daily (do not increase) |
| <15 mL/min/1.73m² or dialysis | Not recommended | — |
Dose Adjustments for Hepatic Impairment
| Child-Pugh Class | Starting Dose | Maximum Dose |
|---|---|---|
| A (mild) | 25 mg daily | 50 mg daily |
| B (moderate) | 25 mg daily | 25 mg daily (do not increase) |
| C (severe) | Not recommended | — |
Missed Dose Management
Take the missed dose immediately unless >12 hours have passed; if >12 hours, skip and resume regular schedule. 1
Critical Safety Considerations
Blood Pressure Monitoring
Mirabegron can increase blood pressure—monitor periodically, especially in hypertensive patients. 1 The drug is not recommended in severe uncontrolled hypertension 1.
Urinary Retention Risk
Use with caution in patients with bladder outlet obstruction or those taking antimuscarinics concurrently, as urinary retention risk increases. 1 The BESIDE trial showed increased urinary retention events with combination therapy compared to monotherapy 4.
Common Adverse Effects
Most frequent (>2% in adults): hypertension, nasopharyngitis, urinary tract infection, headache 1. Notably, dry mouth incidence is substantially lower than with antimuscarinics 5, 2.
Clinical Efficacy Highlights
Phase III trials (SCORPIO, ARIES, CAPRICORN) demonstrated statistically significant reductions in incontinence episodes and micturition frequency versus placebo 6, 7. The 12-month TAURUS study confirmed sustained efficacy and tolerability 2. Real-world data suggest better persistence and adherence compared to antimuscarinics 5.
Important caveat: In the head-to-head comparison with solifenacin in patients previously dissatisfied with antimuscarinics, mirabegron did not demonstrate noninferiority 5, though combination therapy (mirabegron + solifenacin) showed superior efficacy to either monotherapy 4.