• This is a single-center, open-label, single-arm study with a double-blind, placebo-controlled, randomized withdrawal extension. Patients are initiated on a DCCR dose of about 1.5 mg/kg (maximum starting dose of 145 mg) and are titrated every 14 days to about 2.4 mg/kg, 3.3 mg/kg, 4.2 mg/kg, and 5.1 mg/kg (maximum dose of 507.5 mg). These DCCR doses are equivalent to diazoxide doses of 1.03, 1.66, 2.28, 2.9, and 3.52 mg/kg. The administered dose will be as close to the mg/kg dosing as can be achieved by the available dose strengths of DCCR. Patients will be up-titrated at each visit at the discretion of the investigator. All patients will be continued in the double-blind, placebo-controlled, randomized withdrawal extension. Any patient who showed an increase in resting energy expenditure and/or a reduction in hyperphagia from Baseline through Day 55 or Day 69 will be designated a responder, whereas all others will be designated non-responders. Responders will be randomized in a 1:1 ratio either to continue on active treatment at the dose they were treated with on Day 69 or to the placebo equivalent of that dose for an additional 4 weeks. Non-responders will continue open label treatment during the extension.

Inclusion Criteria:

• Children. adolescents and young adults with genetically confirmed Prader-Willi syndrome
• Ages at ≥ 10 years and ≤ 22 years
• Generally healthy as documented by the medical history, physical examination, vital sign assessments, 12-lead electrocardiogram (ECG), and clinical laboratory assessments
• BMI exceeds the 95th percentile of the age specific BMI value on the CDC BMI charts
• Fasting glucose ≤ 126 mg/dL
• HbA1c ≤ 6.5 %

Exclusion Criteria:

• Administration of investigational drugs within 1 month prior to Screening Visit
• Anticipated requirement for use of prohibited medications
• History of allergic reaction or significant intolerance to: diazoxide, thiazides or sulfonamides
• Anticipate transitions in their care from family home to group home or other similar potentially disruptive changes
• Congestive heart failure or known compromised cardiac reserve
• Any other clinically significant endocrine, cardiovascular, pulmonary, neurological, psychiatric, hepatic, gastrointestinal, hematological, renal, or dermatological disease interfering with the assessments of the investigational drug, according to the Investigator