A Trial of Centanafadine Long-term Safety in Children and Adolescents With Attention-deficit/Hyperactivity Disorder.

Rollover subjects from double-blind parent trials inclusion criteria: * Subjects who completed the 6-week double-blind treatment period and the 7 (+2) day follow-up in a double-blind parent trial and who, in the opinion of the investigator, could potentially benefit from centanafadine QD XR for ADHD De novo subjects inclusion criteria: * Males and females aged 4 to 17 years (inclusive) at the time of informed consent/assent. * A primary diagnosis of ADHD based on DSM-5 diagnosis criteria as confirmed with the MINI KID. * A minimum symptoms total raw score of ≥ 28 on the ADHD-RS-5 at baseline. * A score of 4 or higher on the CGI-S-ADHD at baseline. * Subjects aged 4 or 5 years only; has failed adequate psychotherapy or in the investigator's opinion, is severe enough to require pharmacotherapy in the absence of prior psychotherapy. Rollover subjects from double-blind parent trials exclusion criteria: * Subjects who, during the double-blind parent trials, experienced, in the opinion of the investigator, poor tolerability to IMP or whose safety assessments resulted in new concerns that would suggest the subject may not be appropriate for a 52-week treatment with IMP. De novo subjects exclusion criteria: * A comorbid diagnosis of: Tourette's Disorder, Generalized Anxiety Disorder that is severe enough to interfere with trial procedures, Panic Disorder, Conduct Disorder, Psychosis, Post-traumatic Stress Disorder, Bipolar Disorder, Autism Spectrum Disorder, Binge Eating Disorder (adolescents only), Anorexia (adolescents only), Bulimia (adolescents only), Oppositional Defiant Disorder that is severe enough to interfere with trial conduct (allowed if it is not the primary focus of treatment), Obsessive-Compulsive Disorder that is severe enough to interfere with trial conduct (allowed if it is not the primary focus of treatment), MDD with current major depressive episode, or has required treatment within the 3 months prior to screening, or in investigator's opinion, MDD may worsen or could be expected to require treatment during the course of this trial. * Subjects who are breast-feeding and/or have a positive pregnancy test result prior to receiving IMP. * A significant risk of committing suicide based on history and the investigator's clinical judgment, or routine psychiatric status examination, current suicidal behavior, Imminent risk of injury to self, active suicidal ideation, history of suicidal behavior (over the last 6 months). * Body weight \< 13 kg * BMI ≥ 40 kg/m2 * Has initiated, changed, or discontinued receiving psychological interventions for ADHD within the 30 days before the screening visit, or are anticipated to start new treatment during the trial.