Inclusion Criteria:

• Subject must be 18-55 years of age, inclusive, at the time of consent of the NRP104.303 study.
• Subject must have been randomized and must have met all inclusion/exclusion criteria in the NRP104.303 study.
• Subject must be male or non-pregnant female. Females of childbearing potential (FOCP) must comply with contraceptive restrictions noted in the protocol.
• Subject must have a satisfactory medical assessment with no clinically significant or relevant abnormalities as determined by medical history, PE, clinical and laboratory evaluation.
• In the opinion of the investigator, the subject understands and is able, willing, and likely to fully comply with the study procedures and restrictions.
• Subject must have given written, personally signed and dated 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 specific procedures.
• Subject experienced no adverse events in a previous study of NRP104 or elsewhere that would preclude continued exposure to NRP104.

Exclusion Criteria:

• Subject 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 who have a history of mental retardation or a severe learning disability are excluded.
• Subject has a known cardiac structural abnormality or any other condition that may affect cardiac performance.
• Subject has any clinically significant ECG or laboratory abnormality known to the investigator prior to dispensation of study medication.
• Subject has a resting sitting systolic blood pressure or diastolic blood pressure deemed clinically significant by the investigator.
• Subject has used any prohibited prescription medication except for medications used to treat ADHD within 30 days of screening visit. Hormonal contraceptives are acceptable.
• Subject has a positive urine drug result at Screening (with the exception of subject's current stimulant therapy, if any).
• Subject has taken an investigational drug or taken part in a clinical trial within 30 days prior to Screening (except for participating in an NRP104 study).
• The female subject is pregnant or lactating.