Overactive bladder is a condition where the bladder muscle contracts involuntarily. Overactive bladder can lead to urine leakage, urgency to urinate,
and frequent urination. Overactive bladder is typically treated with a class of drugs called anticholinergics. Examples include Detrol®, Ditropan®, Vesicare®,
and Enablex®. Mirabegron is a new drug approved for overactive bladder that has a different mechanism of action than
Mirabegron (Myrbetriq®) stimulates Beta-3 adrenergic receptors. Stimulation of Beta-3 adrenergic receptors in the bladder muscle causes it to relax.
DOSAGE: Mirabegron (Myrbetriq®) is available as a 25 and 50 mg extended-release tablet.
- Starting: 25 mg once daily
- Maximum: 50 mg once daily
- Mirabegron may be taken without regard to food
- It may take up to 8 weeks for a noticeable effect
STUDIES: The Drug's package insert cites three 12-week studies.
- In those studies, the following effects were seen:
- Mirabegron decreased the number of incontinence episodes per 24 hours by 0.34 - 0.42 when compared to placebo
- Mirabegron decreased the number of urinations per 24 hours by 0.42 - 0.61 when compared to placebo
- Mirabegron increased the volume of urine voided per urination by 4.6 - 12.4 ml when compared to placebo
CONTRAINDICATIONS / PRECAUTIONS:
- In trials, the following side effects were more common in patients taking mirabegron than in patients taking placebo:
- Nasopharyngitis - runny nose, sore throat
- Urinary tract infection
- Increase in blood pressure - mirabegron may increase blood pressure in some individuals. In studies, the mean maximum increase in systolic/diastolic blood pressure
was approximately 3.5/1.5 mmHg greater than placebo in patients taking 50 mg.
- Urinary retention - may occur in patients with bladder obstruction and/or in patients taking anticholinergics
- Angioedema - cases of angioedema have been reported in patients taking mirabegron
- Kidney disease - dose should not exceed 25 mg/day in patients with a CrCl of 15 - 29 ml/min. Do not use in patients with CrCl < 15 ml/min.
- Liver disease - dose should not exceed 25 mg/day in patients with Child-Pugh Class B liver disease. Do not use in patients with Child-Pugh Class C.
- Mirabegron is a moderate CYP2D6 inhibitor. Dosage adjustments may be necessary when it is taken with CYP2D6 substrates.
- Mirabegron may increase digoxin levels
Mirabegron has a unique mechanism that differs from the widely used anticholinergic drugs. Based on the studies
presented in the package insert, its effects appear to be modest. Increases in blood pressure is a worrisome side effect. Mirabegron is a newer drug, and it will be much more expensive than
the anticholinergic agents that have generics (e.g. Ditropan®, Detrol®).