The primary objective of the study is to evaluate the long-term safety and tolerability of maralixibat. Secondary objectives: To evaluate the long-term efficacy of maralixibat, including the maintenance of severity and frequency of pruritus as well as serum bile acids (sBA) over time and growth in the primary cohort Exploratory objectives: The long-term treatment effects of maralixibat will be evaluated, including liver biochemistry, other pruritus measures, time to liver-associated events, health-related quality of life and biomarkers of interest in the primary cohort and all of the above secondary objectives in the supplementary cohort. Maralixibat’s impact on healthcare utilization and caregiver burden will be explored in both cohorts. The pharmacokinetics (PK) of maralixibat will also be explored.