A Study to See Iftolvaptan is Safe in Infants and Children Who at Enrollment Are 28 Days to Less Than 18 Years Old withAutosomal Recessive Polycystic Kidney Disease (ARPKD)

Inclusion Criteria: 1. Male or female subjects between 28 days and less than 18 years of age, with clinical features that are consistent with a diagnosis of ARPKD. 2. Ability for parent/legal guardian to provide written, informed consent prior to initiation of any trial-related procedures, and ability, in the opinion of the principal investigator, to comply with all the requirements of the trial. Ability to provide written informed assent from all subjects old enough per local laws to provide assent. Exclusion Criteria: 1. Premature birth (≤ 32 weeks gestational age) for infants 28 days to \< 12 weeks of age. 2. Anuria or RRT defined as intermittent or continuous hemodialysis, peritoneal dialysis, hemofiltration, hemodiafiltration or history of kidney transplantation. 3. Evidence of syndromic conditions associated with renal cysts (other than ARPKD). 4. Abnormal liver function tests including ALT and AST, \> 1.2 × ULN (upper limit of normal). 5. Has splenomegaly or portal hypertension (HTN). 6. Parents with renal cystic disease. 7. Receiving chronic diuretic that could not be adjusted after tolvaptan initiation. 8. Cannot be monitored for fluid balance. 9. Has or at risk of having sodium and potassium electrolyte imbalances, as determined by the investigator. 10. Has or at risk of having significant hypovolemia as determined by investigator. 11. Clinically significant anemia, as determined by investigator. 12. Platelets \< 50000 µL. 13. Severe systolic dysfunction defined as ejection fraction \< 14%. 14. Serum sodium levels \< 130 mmol/L or \>145 mmol/L. 15. Taking any other experimental medications. 16. Require ventilator support. 17. Taking medications known to induce CYP3A4 (CYP = Cytochrome P). 18. Having an infection including viral that would require therapy disruptive to IMP dosing. 19. Females who are breast-feeding or who have a positive pregnancy test result prior to receiving IMP. 20. Subjects with a history of substance abuse (within the last 6 months). 21. Subjects who have bladder dysfunction and/or difficulty voiding. 22. Subjects taking a vasopressin agonist (eg, desmopressin). 23. Subjects with a history of persistent noncompliance with antihypertensive or other important medical therapy. 24. Subjects taking medications or having concomitant illnesses likely to confound endpoint assessments, including taking approved (ie, marketed) therapies for the purpose of affecting PKD cysts such as tolvaptan, vasopressin antagonists, anti-sense ribonucleic acid (RNA) therapies, rapamycin, sirolimus, everolimus, or somatostatin analogs (ie, octreotide, sandostatin). 25. Received or are scheduled to receive a liver transplant. 26. History of cholangitis within the last 6 months. 27. Has findings consistent with clinically significant portal hypertension (eg, varices, variceal bleeding, hypersplenism indicated by thrombocytopenia).