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

People with neurogenic detrusor overactivity (NDO) have poor bladder control because of how their nerves to the bladder are wired. This can cause high pressure in the bladder, causing it to leak urine by accident (incontinence). Mirabegron has already been approved for adults with bladder problems and for children 3 years and older. This study will learn if mirabegron can help young children with NDO. The children will be from 6 months to up to 3 years old. The main aim of this study is to learn if mirabegron increases how much urine the bladder holds (maximum cystometric capacity, or Maximum Cystometric Capacity \[MCC\]) in young children with NDO. An increase in MCC will prevent high pressure in the bladder. Children from 6 months to up to 3 years old who have NDO can take part. They must weigh 9 kilograms (kg) or more. They will already be fitted with a tube (catheter) in their bladder. They will use this to drain urine from their bladder regularly during the day. This is called clean intermittent catheterization (CIC). There will be 2 groups in the study. Young children who aren't taking certain medicines for NDO will be in group A. Young children who are taking certain medicines for NDO will be in group B. Children in group B will stop taking these medicines before they start taking mirabegron. Treatment in group B will be delayed to allow the medicines to be cleared from the body before they start taking mirabegron. Both groups (A and B) will have the same treatment and dose of mirabegron and will have the same checks throughout the study. Mirabegron will be squirted from a syringe into the children's mouths, followed by a sip of water. This will happen once a day for up to 52 weeks (1 year). They will start on a low dose, adjusted for their weight. The dose may be increased to a higher dose if the study doctor thinks the child will benefit from the higher dose. Children will have safety checks throughout the study. Other tests will include checking how the bladder fills and empties plus an ultrasound of the bladder area. There will be several clinic visits during the study. There will be fewer clinic visits if a child stays on the low dose of mirabegron. Then, the clinic will phone the caregiver about 1 month after the last dose of mirabegron to check if there are any further medical problems.