A Study of TAK-503 in Children and Teenagers With Attention Deficit Hyperactivity Disorder (ADHD)

Inclusion Criteria

Study Part A:

• Participant is a male or female aged 6 to 17 years inclusive at the time of consent/assent.
• Participant must meet Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria for a primary diagnosis of ADHD based on a detailed psychiatric evaluation using the Kiddie-Schedule for Affective Disorders-Present and Lifetime Version (K-SADS-PL) by a trained child and adolescent psychiatrist at screening (Visit 1A).
• Participant for whom prior stimulant therapy is not suitable, not tolerated, or shown to be ineffective as determined by investigator clinical assessment and review of the Prior Stimulant Medication Questionnaire (PSMQ) administered during screening (Visit 1A).
• Participant has an ADHD-RS-5 total score greater than or equal to (> =) 28 at baseline (Visit 2A).
• Participant has a baseline (Visit 2A) CGI-S score > = 4.
• Participant who is a female of childbearing potential (FOCP) and postmenarchal must have a negative serum beta-human chorionic gonadotropin (β-hCG) pregnancy test at screening (Visit 1A) and a negative urine pregnancy test at baseline (Visit 2A), be nonlactating, and agree to comply with any applicable contraceptive requirements described in the protocol. Female of child bearing potential is defined as any female participant who is at least aged 9 years or younger than 9 years and postmenarchal.
• Participants parent or legally authorized representative (LAR) must provide signature of informed consent. Documentation of assent (if applicable) must be provided by the participant indicating that the participant is aware of the investigational nature of the study and the required procedures and restrictions in accordance with the International Council for Harmonisation (ICH) Good Clinical Practice (GCP) Guideline E6[R2] and applicable regulations, before completing any study-related procedures.
• Participant and parent/LAR are willing and able to comply with all the testing and requirements defined in this protocol, including oversight of morning dosing. Specifically, the parent/LAR must be available for the duration of the study to administer the investigational medicinal product (IMP) dose each morning when the participant awakens.
• Participant has supine and standing blood pressure (BP) measurements less than the 95th percentile for age, sex, and height at both screening (Visit 1A) and baseline (Visit 2A).
• Participant is functioning at an age-appropriate level intellectually, as judged by the investigator.
• Participant is able to swallow intact tablets and capsules.

Study Part B:

• Female participants of child-bearing potential must have a negative serum β-hCG pregnancy test if a screening visit is conducted and/or a negative urine pregnancy test at baseline and agree to comply with any applicable contraceptive requirements of the protocol. An FOCP is defined as any female participant who is at least aged 9 years or younger than 9 years and postmenarchal.
• Participant has a supine and standing BP measurement less than the 95th percentile for age, sex, and height.

Exclusion Criteria

Study Part A:

• Participant has a current, controlled (requiring medication or therapy) or uncontrolled, comorbid psychiatric disorder (except oppositional defiant disorder), including but not limited to any of the following comorbid Axis I and Axis II disorders (the K-SADS-PL should be reviewed to confirm diagnosis, if necessary):
• Post-traumatic stress disorder (PTSD)
• Bipolar illness, psychosis, or family history in either biological parent
• Pervasive developmental disorder
• Obsessive-compulsive disorder (OCD)
• Psychosis/schizophrenia
• Serious tic disorder or a family history of Tourette's disorder
• Participant is currently considered to be a suicide risk by the investigator; has made a previous suicide attempt; has a history of, or currently demonstrating, active suicidal ideation.
• Participant has a substance abuse disorder as define