This is PART B of a 2-part, 6-week, double-blind, dose-optimization, parallel-group study in children and adolescents (ages 6-17 years) with ADHD with and without CNVs in specific genes implicated in glutamatergic signaling and neuronal activity. PART B will assess subjects who do not have CNVs in any of the specific gene mutation(s) implicated in glutamatergic signaling and neuronal connectivity.

Inclusion Criteria:

1. Subject and parent/legally authorized representative (LAR) can speak English fluently and have provided written informed consent, and assent (as applicable) for this study.
2. Subject is 6 to 17 years of age (inclusive) at the time of consent/assent. The date of signature of the informed consent/assent is defined as the beginning of the Screening Period. This inclusion criterion will only be assessed at the Screening Visit (Visit 1).
3. Subject is male or non-pregnant, non-lactating female, who if of childbearing potential agrees to comply with any applicable contraceptive requirements prior to administration of investigational product (IP).
4. Subject meets Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria for a primary diagnosis of ADHD based upon DSM 5 criteria.
5. Subject has a minimum score of ≥28 on the ADHD-RS-5 at the Baseline Visit (Visit 2).
6. Subject has been genotyped previously and has their identity confirmed (if required).

Exclusion Criteria:

1. Subject or parent/LAR is, in the opinion of the investigator, mentally or legally incapacitated, has significant emotional problems at the time of the Screening Visit (Visit 1) which could interfere with the conduct of study evaluations.
2. Subject has a current, controlled or uncontrolled, co-morbid major psychiatric diagnosis (aside from ADHD), including an anxiety disorder, major depression, bipolar disease, schizophrenia (or any psychotic disorder), and moderate or severe intellectual disability. Mild anxiety and/or depressive symptoms that do not meet diagnostic criteria for an anxiety disorder or major depression and/or do not require treatment are not exclusionary.
3. Subject has autism spectrum disorder to include a DSM-IV diagnosis of autistic disorder, Asperger's disorder, or pervasive developmental disorder.
4. Subject is currently taking any medication that might confound the results of safety assessments conducted in the study.
5. Subject has a known history of cardiovascular disease, advanced arteriosclerosis, structural cardiac abnormality, cardiomyopathy, serious heart rhythm abnormalities, coronary artery disease, cardiac conduction problems, exercise-related cardiac events including syncope and pre-syncope, or other serious cardiac problems.
6. Subject has any clinically significant abnormality on 12-lead ECG performed at the Screening Visit (Visit 1) and/or the Baseline Visit (Visit 2) such as serious arrhythmia, cardiac conduction problems, or other abnormalities deemed to be a potential safety issue.

ITT Analysis Set

3 months, 13 days

All AEs are collected from the time of the informed consent is signed until the follow-up call is completed. This includes events occurring during the screening phase of the study, regardless of whether investigational product (IP) is administered. An AE will be considered treatment emergent if it occurs after the first dose of IP and within 3 days of a subjects last dose of IP.