Study is to assess if TC-5619 improves symptoms for adults diagnosed with the inattentive-predominant subtype of ADHD over a 4-week treatment period.

A 3-arm, double-blind, randomized, placebo-controlled, multicenter, fixed dose study to assess efficacy and safety of TC-5619 in adults with inattentive-predominant attention deficit/hyperactivity disorder (ADHD) utilizing the Connor's Adult ADHD Rating Scale-Investigator Completed Version (CAARS-INV) inattentive subscale scores through 4 weeks of treatment.

Inclusion Criteria:

1. Diagnosis of Inattentive predominant ADHD per DSM-IV TR) criteria at Screening
2. Score ≥ 2 on at least 6 of 9 items in the inattentive subscale of the CAARS-INV at Baseline (Day 1)
3. Score of < 2 on at least 4 of 9 items in the hyperactive/impulsivity subscale of the CAARS-INV at Baseline (Day 1)
4. Score ≥ 4 (at least moderate) on the CGI-S
5. Tobacco non-users as indicated by lack of tobacco use within the last 6 months prior to Screening, and by negative urinary cotinine level of < 50 ng/mL after quantification

Exclusion Criteria:

1. Current DSM-IV Axis I psychiatric disorder other than ADHD;
2. Use of tobacco cessation agents within 4 weeks prior to Screening
3. Known or suspected drug abuse within the last 6 months prior to Screening
4. Urine drug screen positive for illegal or non-prescribed drugs at Screening
5. Patients at imminent risk of suicide or of danger to themselves or others as judged by the investigator
6. Use of drugs affecting cognitive function within 4 weeks prior to Day 1. This includes use of any concomitant medications for treatment of ADHD.
7. History of significant other major or unstable neurological, metabolic, hepatic, renal, hematological, pulmonary, cardiovascular (CV), gastrointestinal (GI), or urological disorder; or diagnosis of major depressive disorder
8. Myocardial infarction within past year
9. Seizure disorder within past year
10. Type 1 diabetes mellitus (DM); type 2 DM that requires medication (diet-controlled allowed)
11. HbA1C > 7.4 at Screening
12. BMI < 15 or > 35; male weight < 100 lbs; female weight < 80 lbs.
13. Current tuberculosis (TB) or known systemic infection [Hepatitis B Virus (HBV), Hepatitis C Virus (HCV), Human Immunodeficiency Virus (HIV)]
14. Women of child-bearing potential and male participants unwilling or unable to use accepted methods of birth control
15. Participation in another Central Nervous System (CNS)-related clinical trial in the last 3 months and any other clinical trial in last 30 days prior to Screening, or participation in a previous TC-5619 clinical trial