Alpelisib in Pediatric and Adult Patients With Lymphatic Malformations Associated With a PIK3CA Mutation.

This is a phase II/III multi-center study with two stages: * Stage 1 is designed to select the dose(s) for the confirmatory phase (DSCP) for alpelisib in Stage 2 and will comprise a 24-week open-label core phase in adult (≥18 years of age) and pediatric participants (6-17 years of age) with PIK3CA-mutated LyM, followed by an extension. After eligibility has been confirmed at screening, participants will be randomized in a 1:1 ratio to the different alpelisib doses according to their age. Depending on the results at the end of Stage 1 core phase, the Stage 2 will be opened to adult and/or pediatric participants or the study may be stopped. * Stage 2 is designed to confirm the efficacy and assess safety of alpelisib at the DSCP in participants with PIK3CA-mutated LyM and will comprise a 24-week randomized, double blind, placebo-controlled confirmatory phase in adult (≥18 years of age) and pediatric participants 6-17 years of age followed by an open-label extension. After eligibility has been confirmed at screening participants will be randomized in a 2:1 ratio to alpelisib or placebo. Additionally, in parallel, Stage 2 will include a 24-week open-label core phase in pediatric participants 0-5 years of age followed by an extension, if pediatric participants will be enrolling in Stage 2. Based on the results of the 24-week open-label core phase of Stage 1, the dose(s) for Stage 2 will be selected by Novartis in consultation with the Steering Committee (SC). During the 24-week randomized, double blind, placebo-controlled core phase of Stage 2, an Independent Data Monitoring Committee (DMC) will conduct periodic safety and efficacy reviews to assess the risk benefit profile of the treatment.