A Study to Learn How Effective and Safe the Drug 'Mirabegron' is and How Long it Stays in the Body of Children Aged 6 Months to Less Than 3 Years of Age With Neurogenic Detrusor Overactivity

Inclusion Criteria:

• Participant's weight is a minimum of 9 kg.

• Participant has a previous myelomeningocele (documented at the screening visit).

• Participant has a diagnosis of neurogenic detrusor overactivity (NDO) confirmed byurodynamic investigation at baseline (day 1). The diagnosis of NDO should beconfirmed by the presence of ≥ 1 involuntary detrusor contraction > 15 cm H2O frombaseline detrusor pressure, and/or a decrease in bladder compliance leading to anincrease in baseline detrusor pressure of > 20 cm H2O.

• Participant has a diagnosis of detrusor sphincter dyssynergia (DSD).

• Participant is using clean intermittent catheterization (CIC).

• Participant is suitable for a regimen of 4 to 6 CICs per day, fixed for the durationof the study using the 7-day baseline e-diary.

• Participant is able to swallow the study drug.

• Participant's legally authorized representative (LAR) is willing and able to complywith the study requirements (including compliant use of the e-diary) and with theconcomitant medication restrictions.

• Participant's LAR agree not to allow participant to participate in anotherinterventional study while receiving study intervention and throughout thepretreatment period.

Exclusion Criteria:

• Participant has a bladder capacity less than 25% of expected age-related capacity,confirmed by urodynamic investigation at baseline (day 1).

• Participant has vesicoureteral reflux grade 3 to 5.

• Participant has a known genitourinary condition, other than NDO, that may causeoveractive contractions and/or incontinence (e.g., bladder exstrophy, urinary tractobstruction, urethral diverticulum or fistula) or kidney/bladder stones or anotherpersistent local pathology that may cause urinary symptoms.

• Participant has had an indwelling urinary catheter within 4 weeks prior to thebaseline visit.

• Participant has undergone bladder augmentation surgery.

• Participant with surgically corrected underactive sphincter.

• Participant receives electrostimulation therapy, if started within 30 days beforevisit 1 screening or is expected to start during the study period. Participants whoare on an established regimen (defined as starting more than 30 days before visit 1screening) may remain on this for the duration of the study.

• Participant has been administered intravesical botulinum toxin; except if given > 4months prior to visit 1 screening and the participant experiences symptomscomparable to those existing prior to the botulinum toxin injections.

• Participant has a current symptomatic urinary tract infection (UTI) confirmed byurinalysis (urine culture containing > 100,000 cfu/mL) at baseline. If at screeningand start of washout a UTI is present, the participant will be eligible forenrollment if the UTI has been treated successfully prior to baseline. If asymptomatic UTI is present at baseline, all baseline assessments should be postponedfor a maximum of 7 days until the UTI is successfully treated. Successful treatmentis defined as a symptom free patient with a white blood cell count in the urine < 100/microliter and urine culture below 100,000 cfu/mL.

• Participant is using prohibited medications.

• Participant has a diagnosis of central or congenital nephrogenic diabetes insipidus.

• Participant with severe gastrointestinal (GI) condition (including toxic megacolon)or any of the following GI conditions: partial or complete obstruction, decreasedmotility like paralytic ileus or at risk for gastric retention.

• Participant suffers from malnutrition or is severely overweight.

• Participant has an average QT interval corrected by Bazett's formula (QTcB) > 440 ms (based on the QTcB mean from the screening and baseline ECG triplicates), history ofQTc prolongation or risk of QT prolongation (e.g., hypokalemia, Long QT Syndrome (LQTS), or family history of LQTS, exercise induced syncope).

• Participant has severe renal impairment (estimated glomerular filtration rate (eGFR) < 30 mL/min per 1.73 m^2 for participants 1 year of age and older; serum creatinine ≥ 2 × ULN, with ULN defined as 97.5th percentile for participants 6 to < 12 monthsof age.).

• Participant's aspartate aminotransferase (AST) or alanine aminotransferase (ALT) is ≥ 2 × upper limit of normal (ULN) or total bilirubin (TBL) greater than or equal to 1.5 × ULN.

• Participant has a current or previous history of epilepsy.

• Participant has a history or presence of any malignancy prior to visit 1 screening.

• Participant has any other clinically significant out of range results of urinalysis,biochemistry, hematology or coagulation.

• Participant has an established hypertension and systolic or diastolic blood pressuregreater than the 99th percentile of their normal range determined by gender, bodysize and age, plus 5 mmHg.

• Participant has a (median) resting heart rate > 99th percentile.

• Participant has any clinically significant or unstable medical condition or disorderwhich precludes the participant from participating in the study.

• Participant has known or suspected hypersensitivity to mirabegron, any of theexcipients used in the current formulation or previous severe hypersensitivity toany drug.

• Participant has participated in another clinical trial and/or has taken aninvestigational drug within 30 days (or 5 half-lives of the drug, or the limit setby national law, whichever is longer) prior to visit 1 screening.

• Participant is being breast-fed by a woman taking any prohibited medication or fedwith a milk product in which the presence of prohibited medication ingredientscannot be excluded.