Safety, Pharmacokinetics, Tolerability and Efficacy of Tolvaptan in Children and Adolescents With ADPKD (Autosomal Dominant Polycystic Kidney Disease)

Tolvaptan has been demonstrated to delay the decline of kidney function in adults with rapidly progressing ADPKD (chronic kidney disease \[CKD\] stages 1 to 3) as measured by estimated glomerular filtration rate (eGFR) and Total Kidney Volume (TKV). This trial will be the first trial of tolvaptan in children and adolescents with ADPKD. Participants in this study will be randomly assigned to one of two groups in Phase A; tolvaptan or placebo. Participants will have an equal chance of being assigned to either treatment group and will be stratified by age and gender into the following cohorts: * Female participant ages 12 to 14 years, inclusive * Female participant ages 15 to 17 years, inclusive * Male participant ages 12 to 14 years, inclusive * Male participant ages 15 to 17 years, inclusive Phase (A) of this study will last 12 months. After that time, all participants who qualify will be assigned tolvaptan and will be treated with tolvaptan for 24 months (Phase B). A qualified participant is defined as one who has completed Phase A on investigational medicinal product (IMP), is willing to continue in the trial, and who does not have any adverse events (AEs), which would require IMP discontinuation. Participants in this study will be required to make monthly visits to the study clinic and will be closely monitored over the course of the study.