A Study to Investigate the Pharmacokinetics (PK) 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.

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

What this study is about