A Trial Evaluating the Efficacy, Safety, & Tolerability of Centanafadine Sustained-release Tablets in Adults With Attention-deficit/Hyperactivity Disorder

Inclusion Criteria: * Participants must meet the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5) criteria for ADHD (including predominantly inattentive presentation, hyperactive presentation, or combined presentation) as confirmed by the Adult ADHD Clinical Diagnostic Scale (ACDS) Version 1.2. To confirm that ADHD is the primary diagnosis, the Mini International Neuropsychiatric Interview (MINI) will be used to identify and exclude other psychiatric conditions which would preclude enrollment. * Participants who were not receiving any pharmacological treatment for ADHD must have an Adult ADHD Investigator Symptom Rating Scale (AISRS) score of ≥ 28 at screening and baseline. Participants who were receiving pharmacological treatment for ADHD at screening must have a minimum AISRS score of ≥ 22 at screening, and a score of ≥ 28 at baseline. * All participants must be willing to discontinue all prohibited psychotropic medications starting from the time of signing the informed consent through the 7-day follow-up period. Participants that do not rollover into Trial 405-201-00015 (NCT03605849) must be willing to discontinue all prohibited psychotropic medications starting from the time of signing the informed consent until after the follow-up telephone call 10 days after the last dose of investigational medicinal product (IMP). * Participants must have a Clinical Global Impression-Severity of Illness Scale (CGI-S) score of ≥ 4 (≥ moderate impairment) at b…