Inclusion Criteria:

• Must be 18-55 years of age, inclusive.
• Must be male or non-pregnant female. Females of childbearing potential (FOCP) must use contraception.
• Must have a medical assessment with no clinically significant or relevant abnormalities as determined by medical history, PE, clinical and lab evaluation.

• Must have 12-lead ECGs defined by the following parameters:

  1. QT/QTcF interval < 450 msec for males and < 470 msec for females
  2. Resting heart rate is between 40 and 100 beats per minute
  3. P-R interval < 200 msec
  4. QRS interval <110 msec.
• Meets Diagnostic and Statistical Manual of Mental Disorders Fourth Edition; Text Revision (DSM-IV-TR™) criteria for a primary diagnosis of ADHD (diagnostic code 314.00 and 314.01) established by a psychiatric evaluation that reviews DSM-IV-TR™ criteria with at least 6 of the 9 subtype criteria met. The Adult ADHD Clinical Diagnostic Scale (ACDS v1.2) will be utilized as the diagnostic tool.
• Has a baseline ADHD-RS score greater than or equal to 28 assessed using adult DSM-IV prompts.
• Understands and is able, willing, and likely to fully comply with the study procedures and restrictions.
• Has given written informed consent to participate in the study in accordance with the International Conference on Harmonization (ICH) Good Clinical Practice (GCP) Guidelines and applicable regulations before completing any study procedures.

Exclusion Criteria:

• In the opinion of the investigator, the subject is significantly underweight [e.g., Body Mass Index (BMI) < 18.5] or morbidly obese.
• Has any comorbid psychiatric diagnosis with significant symptoms such as any severe comorbid Axis II disorders or severe Axis I disorders including Post Traumatic Stress Disorder (PTSD), psychosis, bipolar illness, severe obsessive compulsive disorder, severe depressive or severe anxiety disorder or other symptomatic manifestations that will contraindicate NRP104 treatment or confound efficacy or safety assessments. Specifically, subjects with mild to moderate forms of Axis I disorders including social phobia and dysthymia may be included while subjects with a lifetime history of psychosis or bipolar disorder will be excluded from participation. Comorbid psychiatric diagnoses will be established by a psychiatric evaluation that includes the Structured Clinical Interview for DSM-IV-TR™ disorders (SCID-I) interview at the screening visit.
• Has any concurrent chronic or acute illness or unstable medical condition that could confound the results of safety assessments, increase risk to the subject or lead to difficulty complying with the protocol. Subjects with mental retardation or a severe learning disability are excluded.
• Has a history of seizure (other than infantile febrile seizures), any tic disorder, or a current diagnosis and/or family history of Tourette's Disorder.
• Has a known cardiac structural abnormality or any other condition that may affect cardiac performance.
• Has any clinically significant ECG or laboratory abnormality at Screening or Baseline.
• Subject has a history of hypertension or has a resting sitting systolic blood pressure > 139mmHg or diastolic blood pressure > 89mmHg.
• Has used any prohibited medication except for ADHD medications within 30 days of screening visit. Hormonal contraceptives are acceptable.
• Has a documented allergy, intolerance, or documented history of non-responsivity to methylphenidate or amphetamine.
• Currently has (or had a history within the last 6 months of) a drug dependence or substance abuse disorder according to DSM-IV-TR™ criteria (excluding nicotine) as established by a SCID-I at the screening visit.
• Has a positive urine drug result at Screening (with the exception of subject's current stimulant therapy, if any) or at Baseline.
• Has taken an investigational drug or taken part in a clinical trial within 30 days prior to Screening.
• The female subject is pregnant or lactating.