To avoid the inclusion of subjects with a mild or intermittent from of cholestasis, the Inclusion Criteria 6 and Exclusion Criteria 1 were amended with further clarification on the expectation of Chronic nature of cholestasis for subjects to be allowed in the study, as well as further clarification for the supplemental cohort. On top of that, changes to clarify and align the timing of pregnancy testing in different sections of the protocol were made .

In order to improve the safety of study participants and to reflect other changes in the protocol, an urinary pregnancy test was added to visit 4,6 and 8, as well as a PIC and CIC to EOT/ET, a clinical scratch scale to the screening and a Lipid Panel. Furthermore, Exclusion Criteria# 2 (to exclude subjects with recurrent intrahepatic cholestasis to avoid enrolling subjects who have intermittent spontaneous near-normalization of pruritus and sBA or remission of peritus), Exclusion Criteria #3 (Exclusion of subjects with pruritus of non-cholestatic origins, as this would directly confound the primary endpoint of the study.), and a questionnaire as an anchor for further ItchRo validation were added to the Protocol. Further Clarifications were also provided for, Exclusion Criteria #6 (Patients should not be excluded unless they have an imminent need for liver transplant.), Exclusion criteria # 11 (Excusing patients with possible malignant liver mass.), for when subjects should be discontinued (Subjects should only be discontinued if there is an imminent need for a liver transplant.), and of the permitted dose adaptations for permitted treatments (Dose adaptations of permitted treatments are allowed if the body weight changes during the study.), as well as updates on the volume of blood that is to be drawn. Finally, the option of long-term follow-up on disease progression for subjects who discontinue prematurely, the clarification that subjects should be informed about the extension study MRX-503 prior to Visit 9, and the correction of inconsistency in the duration for which female subjects should use contraception after their last dose of study medication were added.

Various texts were added, revised, and deleted in order to provide reference to the appendix for management of study procedures during the global pandemic, allow subjects to meet inclusion age criterion by baseline to allow for earlier screening of subjects, clarify that criterion only applies to the primary cohort, clarify that subjects must have pruritus to be eligible, include subjects with intermittent cholestasis in the supplemental cohort, exclude any phenylbuterates, deleted criterion for exclusion of administration of growth hormones, instruct sites to follow country-specific guidelines for acceptable methods of contraception, update the description of study medication packaging to current form, revise limit of propylene glycol exposure(the maximum aily exposure stated in the study protocol is 25mg/kg/day; after recalculation, the maximum may be ≤26 mg/kg/day for BID dosing), and to add expedited safety reporting as a reason for unblinding of treatment assignment.

Changes made to the Protocol include, clarifying the rational of the Study, updating the secondary objectives and endpoints, as well as the exploratory objectives and endpoints to better capture clinically meaningful measures in the primary cohort and the broader PFIC population, specifying that the SAP will detail how the assignment of siblings occurs, including a blinded validation analysis of the ItchRO to confirm the responder definition, clarifying sample size calculation and that the ITT population will include all randomized subjects, defining of analysis groups, clarifying the scoring of ItchRO(Obs) and ItchRO(Pt) severity scores, and how to construct monthly scores, Updating the hypothesis tests to be performed due to the adjustment of the secondary endpoint, and finally providing guidance regarding COVID-19 vaccinations.