An Extension Study to Evaluate the Long-Term Safety and Durability of Effect of LUM001 in the Treatment of Cholestatic Liver Disease in Pediatric Subjects With Alagille Syndrome

Inclusion Criteria: 1. Male or female, 12 months to 18 years of age. 2. Competent to provide informed consent and assent (per institutional review board/Ethics Committee \[IRB/EC\]), as appropriate. 3. Completed participation in the LUM001-301 protocol. 4. Females of childbearing potential must have a negative urine pregnancy test \[beta human chorionic gonadotropin (beta-hCG)\] at the Baseline Visit. 5. Sexually active females must be prepared to use an effective method of contraception during the trial. Effective methods of contraception are considered to be: 1. Hormonal (for example, contraceptive pill, patch, intramuscular implant or injection); or 2. Barrier method, for example, (a) condom with spermicide, or (b) diaphragm, with spermicide; or 3. Intrauterine device (IUD). 6. Participants above the age of assent and caregivers and children must be able to read and understand English or Spanish. 7. Caregivers (and age appropriate participants) must have access to phone for scheduled calls from study site. 8. Caregivers (and age appropriate participants) must be willing and able to complete a daily electronic diary (ItchRO) during the first consecutive 12 weeks of the study and then for 4 consecutive weeks following the Week 24 and Week 44 visits. 9. Caregivers (and age appropriate participants) must digitally accept the licensing agreement in the ItchRO electronic diary software at the outset of the study. 10. Eligible participants must be able to adhere to local Ethics Committee or Institutional Review Board (IRB) blood volume limits for laboratory testing. 11. The participant has completed the protocol either through Week 144, or the End of Trial visit, or has received permission from the sponsor and the Premier Medical monitor to re-enter the study in the long-term, optional follow-up treatment period 2. 12. Females of child-bearing potential must have a negative urine or serum pregnancy test (beta-HCG\]) at the time of entry into the long-term optional follow-up treatment period 2. 13. Male and female participants of child-bearing potential who are sexually active, or are not currently sexually active, but become sexually active during the study or for 30 days following the last dose of study drug, must agree to use acceptable contraception during the study. 14. Informed consent and assent (per IRB/EC) as appropriate. 15. Caregivers (and age appropriate participants) must have access to phone for scheduled calls from study site. 16. Caregivers (and age appropriate participants) must be willing to follow the rules of eDiary completion. Exclusion Criteria: 1. Experienced an adverse event or serious adverse event (SAE) related to the study drug during the LUM001-301 protocol that led to the discontinuation of the participant from the core study. 2. Any conditions or abnormalities (including laboratory abnormalities) which in the opinion of the Investigator, Medical Monitor or ChiLDReN Protocol Chair, may compromise the safety of the participant, or interfere with the participant participating in or completing the study. 3. History or known presence of gallstones or kidney stones. 4. History of non-adherence during the participant's participation in the LUM001-301 protocol. Non-adherence is defined by dosing compliance (dosing compliance is calculated by \[the total number of doses that were actually taken by the participant\] divided by \[the total number of doses that should have been taken by the participant\] multiplied by 100) of less than 80% in the LUM001-301 protocol. 5. Unlikely to comply with the study protocol, or unsuitable for any other reason, as judged by the investigator. 6. All above exclusion criteria will apply upon re-entry into the long-term, optional follow-up treatment period 2.