This is a Phase 3, randomized, double-blind, placebo-controlled, multicenter, global clinical study to assess the efficacy and safety of pitolisant in patients living with Prader-Willi syndrome.

The primary objective of this study is to evaluate the efficacy of pitolisant in treating excessive daytime sleepiness (EDS) in patients ≥6 years of age with Prader-Willi syndrome.

Secondary objectives include assessing the impact of pitolisant on:

Irritable and disruptive behaviors Hyperphagia Other behavioral problems including social withdrawal, stereotypic behavior, hyperactivity/noncompliance, and inappropriate speech

The study will consist of an up to 45-day Screening/Baseline Period, a Double-Blind Treatment Period, and an optional Open-Label Extension Period.

After completion of all Baseline assessments, patients who meet all eligibility criteria will be randomized 1:1 to receive once daily pitolisant or matching placebo. During the Double-Blind Treatment Period, in-person visits will be at Day 29, Day 57, and Day 77. Patients who do not elect to enter the Open-Label Extension Period will have follow-up visits 15 days and 30 days after the final dose of study drug.

During the optional Open-Label Extension Period, in-person visits will be at Day 113, Day 260, and Day 441. Patients will have follow-up visits 15 days and 30 days after the final dose of pitolisant.

Inclusion Criteria:

• Genetically confirmed diagnosis of PWS
• Excessive daytime sleepiness
• Has a consistent parent/caregiver (preferably the same person throughout the study) who is willing and able to complete the required study assessments.
• In the opinion of the Investigator, the patient/parent(s)/caregiver(s)/legal guardian(s) are capable of understanding and complying with the requirements of the protocol and administration of oral study drug.

Exclusion Criteria:

• Has a diagnosis of sleep apnea (OSA, CSA) that is not adequately controlled
• Has a diagnosis of hypersomnia due to another sleep/medical disorder
• Participation in an interventional research study involving another investigational medication, device, or behavioral treatment within 30 days or 5 half-lives (whichever is longer) of the investigational medication prior to Screening