This is a Phase 2, open-label study designed to assess the safety and efficacy of setmelanotide in patients with obesity due to Prader-Willi Syndrome (PWS) who are 6 to 65 years of age. Up to 20 patients are planned to be enrolled. Patients will take a daily subcutaneous dose of open-label setmelanotide for up to 52 weeks.

Key Inclusion Criteria:

• Confirmed diagnosis of Prader-Willi Syndrome (PWS)
• Age 6 to 65
• BMI ≥30 kg/m2 for patients ≥18 years of age or BMI ≥95th Percentile for age and sex
• Agree to use a highly effective form of contraception and follow the study contraception requirements throughout study duration and for 90 days after.
• Patient and/or guardian is able to communicate well with the Investigator, understand and comply with the requirements of the study, and understand English and sign the written informed consent.

Exclusion Criteria:

• Use of weight modulating medications
• Abnormal eGFR, ALT, AST, and bilirubin values
• Current, clinically significant pulmonary, cardiac, metabolic, or oncologic disease considered severe enough to interfere with the trial and/or confound the results.
• Participation in any clinical trial with an investigational drug/device within 3 months or 5 half-lives, whichever is longer, prior to the first trial dose
• Hypersensitivity to setmelanotide
• Diagnosis of severe psychiatric disorders
• Pregnant and/or breastfeeding

Other protocol defined Inclusion/Exclusion criteria may apply.