Safety and Efficacy Study of LUM001 (Maralixibat) With a Drug Withdrawal Period in Participants With Alagille Syndrome (ALGS)

Inclusion Criteria: * Male or female between the ages of 12 months and 18 years inclusive. * Diagnosis of ALGS. * Evidence of cholestasis (one or more of the following): 1. Total serum bile acid \> 3x ULN for age. 2. Conjugated bilirubin \> 1 mg/dL. 3. Fat soluble vitamin deficiency otherwise unexplainable. 4. GGT \> 3x ULN for age. 5. Intractable pruritus explainable only by liver disease. * Females of childbearing potential must have a negative serum pregnancy test during Screening. * Males and females of child-bearing potential who are sexually active, or are not currently sexually active during the study, but become sexually active during the period of the study and 30 days following the last dose of study drug, must agree and use acceptable contraception during the trial. * Participant is expected to have a consistent caregiver(s) for the duration of the study. * Informed consent and assent (per IRB/IEC) as appropriate. * Access to phone for scheduled calls from study site. * Caregivers (and age-appropriate participants) must be willing and able to use an eDiary device during the study. * Caregivers (and age-appropriate participants) must digitally accept the licensing agreement in the eDiary software. * Caregivers (and age-appropriate participants) must complete at least 10 eDiary reports (morning or evening) during each of two consecutive weeks of the screening period (maximum possible reports = 14 per week). * Average daily score \>2 on the Itch Reported Outcome (ItchRO™) questionnaire (maximum possible daily score of 4) for two consecutive weeks in the screening period, prior to dosing. A daily score is the higher of the scores for the morning and evening ItchRO. The average daily score is the sum of all daily scores divided by the number of days the ItchRO was completed. Inclusion Criteria for participants to be eligible for the 52-week optional follow-up treatment period: * Completed the protocol through the Week 48 visit with no safety concerns. Participants who were discontinued due to safety reasons can be rechallenged if blood tests are back to relatively normal values for this patient population and participant does not meet any of the protocol's stopping rules. The decision will be made by the investigator in consultation with the sponsor medical monitor. * Participants who have undergone a surgical disruption of the enterohepatic circulation will not be eligible to enter into the follow up treatment period. * Participants who were discontinued for other reasons will be considered for the 52-week optional follow-up treatment period on an individual basis. The decision will be made by the investigator in consultation with the sponsor medical monitor. Inclusion Criteria for participants with LUM001dosing interruption \<7 days, or \>=7 days: * The Participant has either: completed the protocol through the Week 48 visit with no major safety concerns OR discontinued due to safety reasons judged unrelated to the study drug, and laboratory results have returned to levels acceptable for this patient population or individual and participant does not meet any of the protocol's stopping rules at the time of entry into the follow-up period. The decision will be made by the investigator in consultation with the sponsor medical monitor. \[Participants who were discontinued for other reasons will be considered on an individual basis.\] * Females of childbearing potential must have a negative urine or serum pregnancy test (beta- human chorionic gonadotropin \[β-hCG\]) at the time of entry into the long-term optional follow-up treatment period. * Males and females of child-bearing potential who are sexually active, or are not currently sexually active during the study, but become sexually active during the period of the study and 30 days following the last dose of study drug, must agree and use acceptable contraception during the trial. * Informed consent and assent (per IRB/EC) as appropriate. * Access to phone for scheduled calls from study site. * Caregivers (and age-appropriate participants) must be willing and able to use an eDiary device during the study. Exclusion Criteria: * Chronic diarrhea requiring ongoing intravenous fluid or nutritional intervention. * Surgical interruption of the enterohepatic circulation. * Previous liver transplant * Decompensated cirrhosis (ALT \>15 x ULN, INR \>1.5 \[unresponsive to vitamin K therapy\], albumin \<3.0 g/dL, history or presence of clinically significant ascites, variceal hemorrhage, and/or encephalopathy). * History or presence of other concomitant liver disease. * History or presence of any other disease or condition known to interfere with the absorption, distribution, metabolism or excretion of drugs, including bile salt metabolism in the intestine (eg, inflammatory bowel disease). * History or presence of gallstones or kidney stones. * Known diagnosis of human immunodeficiency virus (HIV) infection. * Cancers, except for in situ carcinoma, or cancers treated at least 5 years prior to Screening with no evidence of recurrence. * Recent medical history or current status that suggests that the participant may be unable to complete the study. * Any female who is pregnant or lactating or who is planning to become pregnant during the study period. * Known history of alcohol or substance abuse. * Administration of bile acid or lipid binding resins within 28 days prior to screening and throughout the trial. * Known hypersensitivity to LUM001 or any of its components. * Receipt of investigational drug, biologic, or medical device within 28 days prior to screening, or 5 half-lives of the study agent, whichever is longer. * History of non-adherence to medical regimens, unreliability, mental instability or incompetence that could compromise the validity of informed consent or lead to nonadherence with the study protocol based upon investigator judgment. * Any other conditions or abnormalities which, in the opinion of the investigator or sponsor medical monitor, may compromise the safety of the participant, or interfere with the participant participating in or completing the study. * Participants weighing over 50 kg at screening. Exclusion Criteria for participants with LUM001 dosing interruption \>=7 days: \- All exclusion criteria mentioned above apply upon entry into the long-term optional follow-up period, with the exception of participants weighing over 50 kg at screening.