Clinical Research Directory

Inclusion Criteria: * Participants with a primary diagnosis of ADHD based on DSM-5 diagnostic criteria, differentiated from other mental disorders using the MINI-KID, excluding other specified ADHD or unspecified ADHD. * Participants with a symptom total raw score of\>=28 (if not receiving any pharmacological treatment for ADHD) or\>=22 (if receiving pharmacological treatment for ADHD) on the ADHD Rating Scale Version 5 (ADHD-RS-5) at screening. * Participants with a symptom total raw score of\>=28 on the ADHD-RS-5 at baseline. * Participants with a score of 4 or higher on the Clinical Global Impression Severity - ADHD (CGI-S-ADHD) at baseline. Exclusion Criteria: * Participants who have a positive pregnancy test result at baseline. * Participants determined to have the following diseases based on an interview using the MINI-KID. * Tourette's disorder * Panic disorder * Conduct disorder * Psychotic disorder * Post-traumatic stress disorder * Bipolar disorder * Participants with a generalized anxiety disorder requiring pharmacotherapy, based on the DSM-5 diagnostic criteria. * Participants with an autism spectrum disorder based on the DSM-5 diagnostic criteria. * Participants with a personality disorder, oppositional defiant disorder, or obsessive-compulsive disorder that is the primary focus of treatment, based on the DSM-5 diagnostic criteria. * Participants with a diagnosis of major depressive disorder (MDD), based on the DSM-5 diagnostic criteria who currently have a major depressive episode, or who have required treatment for MDD within the past 3 months prior to screening. Also, participants who, in the judgment of the investigator or subinvestigator, may have a worsening of MDD during the trial or may require treatment during the trial period. * Participants who have a diagnosis of intellectual disability with an intelligence quotient (IQ) score less than 70. * Participants who have a significant risk of committing suicide in the opinion of the investigator or subinvestigator, or based on the following evidence. * Active suicidal ideation as evidenced by an answer of "yes" on Questions 4 or 5 (over the last 6 months) on the section of suicidal ideation or a history of suicidal behavior (over the last 6 months) on the Baseline/Screening version of the Columbia-Suicide Severity Rating Scale (C-SSRS) at screening. * Participants with a diagnosis of substance use disorder. * Platelets \<= 130,000/mm3 * Hemoglobin \<= 11.2 g/dL * Neutrophils, absolute \<= 1000/mm3 * AST \> 2 x ULN * ALT \> 2 x ULN * eGFR \< 45 mL/min/1.73 m2, calculated by the CKiD U25 equation * CPK \>= 2 x ULN (except for the cases that the medical monitor determined that participant's inclusion is possible based on the discussion about the participant's condition with the investigator or subinvestigator) * Abnormal values for both free T4 and TSH * Participants who cannot agree to discontinuation of prohibited concomitant medication, such as ADHD medication or antidepressants.