This study consists of a screening period (up to 4 weeks), a core treatment period (12 weeks double-blind, double-dummy), an optional open-label extension \[OLE\] period, and safety follow-up period.
Screening period: participants will have a screening period of a minimum of 7 days up to a maximum of 28 days to establish eligibility for the study. Participants meeting all the inclusion criteria and none of the exclusion criteria will be eligible to participate in the study.
Core treatment period (double-blind, double-dummy): approximately, 400 participants diagnosed with CSU inadequately controlled by sgH1-AH will be randomized in this study. Participants will be stratified based on prior exposure to anti-IgE biologics.
Eligible participants will be randomized in a 1:1 ratio to receive remibrutinib and placebo solution for injection or dupilumab and remibrutinib matching placebo, both as an add-on treatment to once daily standard label dose of sgH1-AH background therapy, until the Week 12 visit.
Additionally, all participants will be on a stable, standard label dose of a sgH1-AH ("background therapy") throughout the entire core treatment period (starting a minimum of 7 days prior to randomization until the end of Week 12). To treat unbearable symptoms of CSU, participants will be allowed to add more of the same sgH1-AH on an as-needed basis ("rescue therapy"). The total maximum daily dose of sgH1-AH (background plus rescue) should not exceed 4 tablets/day (4-fold the standard label dose).
Optional open-label extension \[OLE\] period: At the end of the 12-week double-blind treatment period, and in case remibrutinib is not commercially available, participants from both arms will be given the choice to roll over into an optional OLE safety period and receive remibrutinib (25 mg b.i.d. p.o.) for 12 weeks (up to Week 24).
Safety follow-up period:
* For participants who do not enter the OLE period: there will be a safety follow-up for 12 weeks with safety follow-up phone calls at Week 16 and Week 24 (Week 24 will be end of study).
* For participants who enter the OLE period: there will be 2 options at the end of Week 24:
* If remibrutinib is commercially available: participants will discontinue study treatment, perform their EOT visit and receive a safety follow-up phone call 4 weeks after the last dose of remibrutinib (this will be the EOS).
* If remibrutinib is not commercially available, participants may continue on remibrutinib 25 mg b.i.d. until its commercial availability and return for site visits for safety follow up and treatment dispensation every 3 months. When remibrutinib becomes commercially available, the participant will be contacted for a site visit to complete the EOT visit and will receive a safety follow-up phone call 4 weeks after the last dose of remibrutinib (this will be the EOS).
