Efficacy and Safety of Extended-release Guanfacine Hydrochloride in Children and Adolescents Aged 6-17 Years With Attention-Deficit/Hyperactivity Disorder (ADHD)

Inclusion Criteria: 1. Male or female, aged 6 17 years at the time of consent/assent at Screening (Visit 1). 2. Subject's parent or legally authorised representative (LAR) must provide signature of informed consent, and there must be documentation of assent (if applicable) by the subject indicating that the subject is aware of the investigational nature of the study and the required procedures and restrictions in accordance with the International Conference on Harmonisation (ICH) Good Clinical Practice (GCP) Guidance E6, and applicable regulations before completing any study related procedures at Screening (Visit 1). 3. Subject meets Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision (DSM-IV-TR) criteria for a primary diagnosis of ADHD, combined sub-type, hyperactive/impulsive sub-type, or inattentive sub-type based on a detailed psychiatric evaluation using the Kiddie Schedule for Affective Disorders and Schizophrenia - Present and Lifetime version (K-SADS-PL). 4. Subject has a minimum ADHD-RS-IV total score of 32 at Baseline (Visit 2). 5. Subject has a minimum CGI-S score of 4 at Baseline (Visit 2). 6. Subject is functioning at an age-appropriate level intellectually, as judged by the Investigator. 7. Subject and parent/LAR understand, are willing, able, and likely to fully comply with the study procedures and restrictions defined in this protocol. 8. Subject is able to swallow intact tablets and capsules. 9. Subject who is a female of child-bearing potential (FOCP), defined as greater than or equal to 9 years of age or \<9 years of age and is menarchal, must have a negative serum beta Human Chorionic Gonadotropin (hCG) pregnancy test at Screening (Visit 1) and a negative urine pregnancy test at Baseline (Visit 2) and agree to comply with any applicable contraceptive requirements of the protocol. 10. Subject has supine and standing blood pressure (BP) measurement within the 95th percentile for age, sex, and height Exclusion Criteria: 1. Subject has a current, controlled (requiring a prohibited medication or behavioural modification program) or uncontrolled, co-morbid psychiatric diagnosis \[except oppositional defiant disorder (ODD)\], including any severe co-morbid Axis II disorders or severe Axis I disorders such as post traumatic stress disorder (PTSD), bipolar illness, psychosis, pervasive developmental disorder, obsessive-compulsive disorder (OCD), substance abuse disorder, or other symptomatic manifestations or lifetime history of bipolar illness, psychosis or conduct disorder that, in the opinion of the Investigator, contraindicate treatment with SPD503 or STRATTERA or confound efficacy or safety assessments. 2. Subject is well-controlled on their current medication, with acceptable tolerability, and the parent/caregiver does not object to the current medication. 3. Subject has any condition or illness including a clinically significant abnormal Screening (Visit 1) laboratory values which, in the opinion of the Investigator, represents an inappropriate risk to the subject and/or could confound the interpretation of the study. Mild stable asthma treated without the use of beta-2 agonist is not exclusionary. 4. Subject has a known history or presence of structural cardiac abnormalities, cardiovascular or cerebrovascular disease, serious heart rhythm abnormalities, syncope, tachycardia, cardiac conduction problems (eg, clinically significant heart block or QT interval prolongation), exercise-related cardiac events including syncope and pre syncope, or clinically significant bradycardia. 5. Subject has a known family history of sudden cardiac death, ventricular arrhythmia, or QT prolongation. 6. Subjects with orthostatic hypotension or a known history of hypertension. 7. Subject has glaucoma. 8. Subject has clinically significant ECG findings as judged by the Investigator with consideration of the central ECG laboratory's interpretation. 9. Subject has a history of a seizure disorder (other than a single childhood febrile seizure occurring before the age of 3 years) or the presence of a serious tic disorder including Tourette's Syndrome. 10. Current use of any prohibited medication or other medications, including monoamine oxidase inhibitors, herbal supplements, that affect BP or heart rate potent CYP2D6 inhibitors, medications known to prolong the QT/QTc interval, medications that lower seizure threshold, pressor agents, beta-2 agonists, medications that affect noradrenaline, medications that have central nervous system (CNS) effects or affect cognitive performance, such as sedating antihistamines and decongestant sympathomimetics (inhaled bronchodilators are permitted) or a history of chronic use of sedating medications \[ie, antihistamines\]) in violation of the protocol specified washout criteria at Baseline (Visit 2). 11. Subject has a history of alcohol or other substance abuse or dependence, as defined by DSM-IV (with the exception of nicotine) within the last 6 months. 12. Subject has taken another investigational product within 30 days prior to Baseline (Visit 2). 13. Subject is significantly overweight based on Center for Disease Control and Prevention Body Mass Index (BMI)-for-age gender specific charts at the Screening (Visit 1). Significantly overweight is defined as a BMI \>95th percentile. 14. Children aged 6 12 years with a body weight of less than 25kg or adolescents aged 13 17 years with a body weight of less than 34kg or greater than 91kg at Screening (Visit 1). 15. Subject has a known or suspected allergy, hypersensitivity, or clinically significant intolerance to guanfacine hydrochloride or atomoxetine hydrochloride, or any components found in SPD503 or STRATTERA. 16. Clinically important abnormality on drug and alcohol screen (excluding the subject's current ADHD stimulant if applicable) at Screening (Visit 1) 17. Subject is female and is pregnant or currently lactating. 18. Subject failed screening or was previously enrolled in this study. 19. Subject is currently considered a suicide risk in the opinion of the Investigator, has previously made a suicide attempt, or has a prior history of, or is currently demonstrating active suicide ideation. Subjects with intermittent passive suicidal ideation are not necessarily excluded based on the assessment of the Investigator. 20. History of failure to respond to an adequate trial of an α2-agonist or atomoxetine hydrochloride for the treatment of ADHD (consisting of an appropriate dose and adequate duration of therapy in the opinion of the investigator). 21. Subjects with renal or hepatic insufficiency.