This is a two-part, Phase IIa, multicenter, 12-week, open-label study. Up to 56 participants with deletion AS aged 5-17 years (inclusive) will be enrolled in the study.

The study will have Part 1-dose confirmations and Part 2 with dose levels to be decided based on the cumulative PK, electroencephalography (EEG), and safety data emerging from Part 1.

The dose levels for the first cohort of Part 2 will be decided based on the cumulative PK, EEG, and safety data emerging from Part 1.

Part 2 will explore the change in EEG beta-band power relative to baseline at Week 2, Week 4 (i.e., approximately 2 weeks after the start of the Dose B), and at the end of the 12-week treatment period after daily administration of alogabat.

Inclusion Criteria:

• Clinical diagnosis of AS and a genetic subtype of deletion on chromosome 15q11q13 confirmed by a historical molecular diagnosis
• The participant's general health status, in the context of the disease under study, allows them to participate in a clinical trial in the opinion of the investigator
• The reliability of sexual abstinence for male and/or female enrollment eligibility needs to be evaluated in relation to the duration of the clinical study and the preferred and usual lifestyle of the participant. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or post-ovulation methods) and withdrawal are not acceptable methods of preventing drug exposure
• Female participants:

A female participant is eligible to participate if she is not pregnant, not breastfeeding, and non-childbearing or remain abstinent and/or Hormonal contraceptive methods must be supplemented

-Male participants: Male contraception is not required in this study because of the minimal seminal dose transmitted through sexual intercourse

Exclusion Criteria:

• A molecular diagnosis of AS with genotypic classification of any type besides the molecular diagnosis as specified in Inclusion Criterion
• Concurrent cardiovascular disease considered not well controlled by drug treatment, including participants with clinically significant hypertension, bradycardia and arrhythmias, myocardial infarction (MI) within 12 months of screening or uncompensated heart failure
• Confirmed clinically significant abnormality on 12-lead electrocardiogram (ECG), including:
• a QT corrected for heart rate using the Fridericia's correction factor (QTcF) of >/= 450 ms (based on the average of 3 consecutive measurements) for participants older than 10 years old
• a QT corrected for heart rate using Bazett's formula (QTcB) of >/= 450 ms (based on the average of 3 consecutive measurements) for participants up to, and including, the age of 10 years old
• Congenital heart diseases not treated and congenital QT corrected for heart rate (QTc) prolongation or family history of Long QT Syndrome
• Medical history of malignancy if not considered cured or if occurred within the last 5 years with the exception of fully excised non-melanoma skin cancers or in-situ carcinoma of the cervix that has been successfully treated
• Concomitant disease, condition, or treatment that would either interfere with the conduct of the study or pose an unacceptable risk to the participant in the opinion of the investigator
• Known active or uncontrolled bacterial, viral, or other infection (excluding fungal infections of nail beds) or any major episode of infection or hospitalization (relating to the completion of the course of antibiotics) within 6 weeks prior to the start of drug administration. Rescreening is allowed once the infection is cured and if the rescreening criteria are met
• Any concomitant condition that might interfere with the clinical evaluation of AS and that is not related to AS
• Known history of human immunodeficiency virus (HIV) or hepatitis B virus (HBV) or hepatitis C virus (HCV)
• Hospitalization for any major medical or surgical procedure involving general anesthesia within 12 weeks of Screening or planned during the study. Rescreening is allowed not earlier than 12 weeks after the surgery and if the rescreening criteria are met.
• Use of prohibited medications within 6 weeks or 5 half-lives (t1/2) prior to start of study medication on Day 1 (whichever is longer)
• Clinically significant loss of blood within 3 months prior to screening defined by participant age and weight per recommendations from Duke University (2012)
• Any prior or current treatment with an investigational study drug within 6 weeks or 5 times the t1/2 of the investigational molecule (whichever is longer) prior to baseline or prior or current use of an investigational medical device within 6 weeks prior to baseline or if the device is still active. Concurrent or planned concurrent participation in any clinical study (including observational and non-interventional studies) without approval of the Investigator.
• Previous participation in a cellular therapy, gene therapy, or gene editing clinical study
• Clinically significant vital sign or ECG abnormalities at Screening
• Confirmed clinically significant abnormality in hematological, chemistry or coagulation laboratory parameters
• Uncorrected hypokalemia or hypomagnesaemia
• Positive test result at screening for hepatitis B surface antigen (HBsAg), HCV (untreated), or HIV-1/2. Participants with HCV who have been successfully treated and who test negative for HCV ribonucleic acid (HCV RNA) may be considered eligible for entry into the study

For eligible studies, qualified researchers may request access to individual patient level clinical data. See Roche's commitment to transparency of clinical study information here: https://go.roche.com/data\_sharing