This study will examine the efficacy and safety of HLD200 in patients age 6-12 years with ADHD using a classroom study design.

The study consists of two distinct treatment phases. The first is the 6-week open-label, treatment optimization phase during which subjects are titrated to an optimal daily dose of HLD200. Subjects are then randomized to receive either optimal HLD200 or matched placebo treatment for an additional week prior to final testing during a laboratory school day.

Inclusion Criteria:

• Male and female children (6-12 years at study entry)
• Previous diagnosis of ADHD and confirmation using the Mini International Neuropsychiatric Interview for Children and Adolescents (MINI-KID)
• Able to swallow treatment capsules
• Available for entire study period
• Provision of informed consent (from the parent[s] and/or legal representative[s]) and assent (from the subject)
• Female subjects of childbearing potential (i.e., post-menarche) required to have a negative result on urine pregnancy test (and will be given specific instructions for avoiding pregnancy during trial).

Exclusion Criteria:

• Any known history or presence of significant cardiovascular, pulmonary, hepatic, renal, hematologic, gastrointestinal, endocrine, immunologic, dermatologic, neurologic, ophthalmologic disease
• Presence of any significant physical or organ abnormality
• Any illness during the 4 weeks before this study
• Comorbid psychiatric diagnosis that may affect subject safety or confound results (e.g., psychosis, bipolar disorder)
• Known history of severe asthma (in the opinion of the investigator) unless deemed currently controlled
• Known history of severe allergic reaction to MPH
• Known history of seizures (except febrile seizures prior to age 5), anorexia nervosa, bulimia or current diagnosis or family history of Tourette's disorder
• Subject who are severely underweight or overweight (in the opinion of the Investigator)
• Any clinical laboratory value outside of the acceptable ranges, unless deemed NCS significant per the Investigator
• Positive history for hepatitis B, hepatitis C or Human Immunodeficiency Virus (HIV)
• Positive screening for illicit drug use, and/or current health conditions or use of medications that might confound the results of the study or increase risk to the subject
• Use of prescription medications (except ADHD medications) within 7 days and over-the-counter medications (except birth control) within the 3 days preceding study enrollment, unless deemed acceptable by the Investigator and Clinical or Medical Monitor
• Participation in clinical trial with an investigational drug within the 30 days preceding study enrollment
• Current suicidal ideation or history of suicidality determined as a significant finding on the C-SSRS by the investigator (Baseline C-SSRS for adolescents; Pediatric Baseline C-SSRS for children).

1 Week (Double-blind Phase)