Study to Investigate the Pharmacokinetics and Safety and to Provide Proof of Mechanism of Alogabat in Children and Adolescents Aged 5-17 Years With Angelman Syndrome (AS) With Deletion Genotype

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

Inclusion Criteria:Clinical diagnosis of AS and a genetic subtype of deletion on the maternally inherited chromosome 15q11q13 confirmed by a historical molecular diagnosis. The deletion must include UBE3A, GABRB3, GABRA5, and GABRG3 genes, and be less than 7 Mb in size.Body mass index (BMI) below the 97th percentile and above the 3rd percentile for the same age and sexThe 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 intercourseExclusion Criteria:A molecular diagnosis of AS with genotypic classification of any type besides the molecular diagnosis as specified in Inclusion CriterionConcurrent cardiovascular disease considered not well controlled by drug treatment, including participants with clinically significant hypertension, bradycardia and arrhythmias, myocardial infarction within 12 months of screening or uncompensated heart failureConfirmed clinically significant abnormality on 12-lead ECG, including:a QTcF of >/= 450 ms (based on the average of 3 consecutive measurements) for participants older than 10 years olda QTcB of >/= 450 ms (based on the average of 3 consecutive measurements) for participants up to, and including, the age of 10 years oldCongenital heart diseases not treated and congenital QTc prolongation or family history of Long QT SyndromeMedical 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 treatedConcomitant 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 ASKnown 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)Participation in an investigational drug study within 6 weeks or 5 times the t1/2 of the investigational molecule (whichever is longer) prior to randomization or participation in a study testing an investigational medical device within 6 weeks prior to randomization 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 studyClinically significant vital sign or ECG abnormalities at ScreeningConfirmed clinically significant abnormality in hematological, chemistry or coagulation laboratory parametersUncorrected hypokalemia or hypomagnesaemiaPositive 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 RNA may be considered eligible for entry into the study.