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Prospective Pilot Study of Mirabegron in Pediatric Patients with Overactive Bladder
Anne-Sophie Blais, M.D, Geneviève Nadeau, M.D FRCSC, Lucie Genois, Nurse researcher, Stéphane Bolduc, M.D FRCSC.
Centre hospitalier universitaire de Québec, Québec, QC, Canada.

Background: Pediatric urologists frequently encounter children presenting symptoms of overactive bladder (OAB). Antimuscarinic agents are the current pharmacologic mainstay for management of OAB but bothersome side effects (S/E) limit their clinical use. Recently, significant innovation in the treatment of OAB has occurred as a new molecule with a distinct mechanism of action has entered the market. Mirabegron, a β3-adrenoreceptor agonist, is currently approved as monotherapy for idiopathic OAB in adults at dosages of 25 and 50 mg once daily. This is based on several large-scale Phase III studies that have shown significant improvement in clinical symptoms of OAB with a favorable tolerability profile. Without surprise, mirabegron has not been studied yet in the pediatric population and no recommendation with regards to its use has been issued by the manufacturer nor medical regulatory bodies. Therefore, the objective was to evaluate the efficacy and safety of mirabegron to treat urinary incontinence in children with idiopathic OAB that were refractory to antimuscarinics.
Methods: A prospective off-label study using adjusted-dose regimens of mirabegron (25 to 50 mg) was conducted on pediatric patients with refractory idiopathic OAB. Patients without symptom improvement under intensive behavioural and medical therapies and/or significant S/E with at least 2 different antimuscarinic agents were recruited. Efficacy and tolerability were assessed with: voiding diaries, post-void residuals, urine cultures, EKG, vital signs and urodynamic study if deemed necessary. Families were also questioned for continence, S/E, compliance, and patient perception of bladder condition (PPBC) questionnaire.
Results: Twenty-six patients (5 girls, 21 boys) with OAB were enrolled. Mean age at initiation was 10.5 years and patients were on mirabegron for a minimum of 3 months. Mean bladder capacity improved from 181mL to 253 mL. Continence improved in all patients but 1, with 4 being completely dry and 13 being moderately improved. No appearance of a significant PVR (> 20% of bladder capacity) occurred in any patient while on treatment. Mean PPBC score improved from 4.4 to 2.2. Four patients reported mild or moderate S/E, which included rhinitis, abdominal cramps, constipation and nausea. Three patients withdrew from the protocol because of S\E and\or lack of efficacy. No worrisome toxicity was found as EKG and vital signs remained normal throughout the study.
Conclusions: Mirabegron, a novel first-in-class therapy as a selective β3-agonist, appears as a safe and effective alternative for children with idiopathic overactive bladder refractory to antimuscarinics.

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