Inclusion Criteria:  Aged 18-70 years Female subjects of child-bearing potential must test negative to pregnancy and use appropriate birth control method from the beginning of study to the 15 days later after ending of study Subjects must be able to understand and willing to sign informed consent Able to discontinue the use of any psychotropic medications for the treatment of ADHD symptoms at screening Meet strict operational criteria for adult ADHD according to the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) A total score of 28 or higher of ADHD Rating Scale-Investigator Rated (ADHD-RS-IV) at screening Have a moderate or severe symptom of ADHD with score of 4 or higher in Clinical Global Impression- Severity (CGI-S) at screening  Exclusion Criteria:  Have any clinically significant concurrent medical condition (endocrine, renal, respiratory, cardiovascular, hematological, immunological, cerebrovascular, neurological, anorexia, obesity or malignancy) that has become unstable and may interfere with the interpretation of safety and efficacy evaluations Have any clinically significant abnormal laboratory, vital sign, physical examination, or electrocardiogram (ECG) findings at screening that, in the opinion of the investigator, may interfere with the interpretation of safety or efficacy evaluations Have known serological evidence of human immunodeficiency virus (HIV) antibody Are pregnant as confirmed by a positive pregnancy test at screening Have QTc values >450 msec at screening using Fridericia's QTc formula Have current of bipolar and psychotic disorders Have a current major depression disorder, obsessive-compulsive disorder, post-traumatic stress disorder, generalized anxiety disorder, panic disorder and eating disorder (also if treated but not currently symptomatic) NOTE: Comorbid diagnoses identified during screening and baseline are acceptable provided that ADHD is the primary diagnosis and the comorbid diagnoses will not confound study data or impair subject's ability to participate (per the Investigator's judgement and documented in source note). Have any history of a significant suicide attempt, or possess a current risk of attempting suicide, in the investigator's opinion, based on clinical interview and responses provided on the Columbia-Suicide Severity Rating Scale (C-SSRS). Have a history of jailing or imprisonment in the past 6 months due to worsening of symptoms of ADHD