A Study Evaluating the Effects of GLPG3667 Given as Oral Treatment for up to 24 Weeks in Adults With Dermatomyositis

Key Inclusion Criteria: * Participant has probable or definite DM in accordance with the ACR/EULAR criteria for at least 3 months. * Participant with DM diagnosed in the 3 years prior to screening must have undergone cancer screening (according to local standard of care or applicable guidelines) within 1 year prior to screening. Note: The evidence of cancer screening must be documented. * Participant must present objective evidence of active disease as defined by fulfilling 1 of the criteria below (as confirmed by the sponsor): * DM rash as defined by modified-Cutaneous Dermatomyositis Disease Area and Severity Index Activity Score (m-CDASI-A) ≥ 6 at screening, or * Creatine kinase (CK) \> 4x upper limit of normal (ULN) at screening, or * muscle biopsy evidence of active disease within 3 months prior to screening (defined as presence of active inflammation in muscle biopsy), or * muscle magnetic resonance imaging showing active inflammation (edema) of the proximal skeletal muscles within 3 months prior to screening, or * electromyography showing acute changes, such as spontaneous activity and myopathic changes not explained by other diseases within 3 months prior to screening, or * any other clinical evidence of active disease as confirmed by the steering committee. * Participant has reduced muscle strength (defined as Manual Muscle Test-8 \< 142/150) and at least 2 additional abnormal core set measurements out of the following 5 at screening: * Physician's G…