The study will enroll 96 participants with 2 idiopathic inflammatory myositis populations:~Population 1 or dermatomyositis (DM): participants with DM with definite or probable myositis according to the American College of Rheumatology/European League Against Rheumatism 2017 (ACR/EULAR 2017) criteria and a DM rash.~Population 2 or anti-synthetase inflammatory myositis (ASIM): participants with ASIM with definite or probable myositis according to ACR/EULAR 2017 criteria and a positive ASIM associated antibody.~Participants will be randomized by population in a 1:1 ratio and receive investigational product (IP) daxdilimab or placebo by subcutaneous injection.~The estimated total study duration will be up to 60 weeks.
The primary efficacy objective:~To evaluate the effect of daxdilimab compared with placebo in reducing disease activity at Week 24.~The secondary efficacy objectives include:~To evaluate the effect of daxdilimab compared with placebo in reducing disease activity at Week 24.~To evaluate the effect of daxdilimab compared with placebo on skin symptoms at Week 24.~To evaluate the effect of daxdilimab on decreasing the use of corticosteroid at Week 24.~Other secondary objectives include:~To characterize the pharmacokinetics (PK) and immunogenicity of daxdilimab in participants.~To evaluate the safety and tolerability of daxdilimab in participants.