A Trial Evaluating the Long-term Safety and Tolerability of Centanafadine Sustained-release Tablets in Adults With Attention-Deficit/Hyperactivity Disorder

De Novo Participants \[De Novo Enrollment has ended 20Sep2019\]. Inclusion Criteria: * De novo 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 are 18 to 55 years of age, inclusive, at the time of consent. * Participants have BMI of 18 to 40, inclusive * Participants are willing to discontinue all prohibited psychotropic medications starting from the time of signing the informed consent and up to the 10-day safety follow-up period. Exclusion Criteria: * Participants has a DSM-5 diagnosis of Other Specified or Unspecified Attention-Deficit/Hyperactivity Disorder as confirmed by ACDS Version 1.2. * Participant has a current comorbid psychiatric disorder that is either controlled with medications prohibited in this trial or is uncontrolled and associated with significant symptoms, including but not limited to: a current major depressive episode (per DSM-5 criteria), current symptoms (past 90 days) meeting the DSM-5 criteria for a diagnosis of generalized anxiety disorder, obsessive compulsive disorder, panic diso…