Long-term Safety and Efficacy of Odevixibat in Patients With Alagille Syndrome

This Phase 3, open-label, multi-center extension study will have two groups of participants: Cohort 1 (participants who participated in Study A4250-012 \[NCT04674761; ASSERT\] and meet the entry criteria for this study) and Cohort 2 (infants under 12 months of age) with ALGS. The study will consist of 2 or 3 periods: 1. A 'Treatment period' of 72 weeks (cohort 1) or 12 weeks (cohort 2). Participants will visit the clinic every 4 to 12 weeks and will receive a dose of 120 μg/kg odevixibat daily. 2. An 'Optional extension period' where participants who wish to continue receiving odevixibat after the 'treatment period' will have the opportunity to remain on treatment with visits every 16 weeks until the drug is commercially available. The optional extension is available provided continued use is supported by the risk-benefit profile, the participant has not been previously withdrawn or discontinued from the study, and the study is not terminated by the Sponsor. 3. A 'Safety follow-up period' of 4 weeks (cohort 1) or 2 weeks (cohort 2). The Safety Follow-up Period will not occur for those who remain on treatment in the optional extension period. Participants will need to complete an e-diary and questionnaires throughout the study (cohort 1 only). Participants will undergo blood samplings, urine collections (cohort 1 only), physical examinations, and clinical evaluations. They may continue some other medications, but the details need to be recorded.