24 Weeks Double-blind Randomized Placebo-controlled Trial to Evaluate Efficacy, PK, Safety of LOU064 in Adolescents (12 - <18) With CSU and Inadequate Response to H1-antihistamine Followed by Optional 3 Years Open-label Extension and an Optional 3 Years Safety Long-term Treatment-free Follow-up

This trial consists of 3 different periods: 1. the "core period", which is randomized and double-blind, during which 2/3 participants will receive remibrutinib and 1/3 will receive placebo for 24 weeks. Total duration: approximately 32 weeks (10 site visits). 2. an optional "open-label extension (OLE) period" proposed to all participants who completed 24 weeks of treatment of the "core period" and all scheduled assessments planned at week 24 visit . Depending on their CSU symptoms (as assessed by the doctor), participants will either receive remibrutinib for 24 weeks, or enter an observational treatment-free period for 1 year. If the CSU symptoms return during the observational period, the participants can switch to the treatment period at any time (decided by the doctor). At the end of the 24-week treatment period, if CSU is controlled, participants will enter the 1-year observational period, otherwise, they can continue with another cycle of 24-week remibrutinib treatment. The number of remibrutinib treatment or observational cycles will be limited to 6 times each. Total duration: from 1 year to approximately 3 years, and number of visits: from 3 to 15 (depending on the CSU symptoms). 3. an optional "long-term treatment-free follow-up period" proposed to all participants who completed at least 4 months treatment in the "OLE period". No treatment will be given. Duration: 3 years with 1 site visit and up to 4 phone call follow-up visits. The primary clinical question of interest is what is the effect of remibrutinib treatment versus placebo on the change from baseline in UAS7, ISS7 and HSS7 scores after 12 weeks of treatment