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 criteriafor at least 3 months.

• Participant with DM diagnosed in the 3 years prior to screening must have undergonecancer screening (according to local standard of care or applicable guidelines)within 1 year prior to screening. Note: The evidence of cancer screening must bedocumented.

• Participant must present objective evidence of active disease as defined byfulfilling 1 of the criteria below (as confirmed by the sponsor):

• DM rash as defined by modified-Cutaneous Dermatomyositis Disease Area andSeverity 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 theproximal skeletal muscles within 3 months prior to screening, or

• electromyography showing acute changes, such as spontaneous activity andmyopathic changes not explained by other diseases within 3 months prior toscreening, or

• any other clinical evidence of active disease as confirmed by the steeringcommittee.

• 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 atscreening:

• Physician's Global Disease Activity score > 2/10 cm on the visual analog scale (VAS),

• Patient's Global Disease Activity score > 2/10 cm on VAS,

• extra-muscular disease activity > 2/10 cm on VAS,

• Health Assessment Questionnaire-Disability Index score > 0.25,

• elevated muscle enzymes (e.g. aldolase, CK, ALT, AST, and lactatedehydrogenase) with at least 1 muscle enzyme > 1.5x ULN.

• Participant previously demonstrated failure to or intolerance to first-linetreatment (defined as oral corticosteroid[s] and at least 1 other immunosuppressant/hydroxychloroquine) OR active disease despite treatment with first-line drugs.Currently, the participant is receiving maximum 3 treatments for DM (oralcorticosteroid[s] and/or allowed immunosuppressant[s]/hydroxychloroquine) for atleast 3 months and is on a stable dose (defined as no change in dose, type ofadministration, or dose regimen) for at least 4 weeks prior to screening and duringscreening within maximum allowed doses as specified in the study protocol. Note:Participants receiving 1 or no concomitant treatment for DM are also eligible.

• Open Label Extension : Participant must meet both of the following inclusioncriteria at Visit 8 to be eligible for participation in the OLE period of the study:participant who may benefit from open-label treatment with GLPG3667, according tothe investigator's judgment; participant who completed the 24-week double-blindtreatment period on investigational product.

Key Exclusion Criteria:

• Participant has cancer-associated myositis (defined as myositis diagnosed within 2years of cancer diagnosis with the exception of basal cell carcinoma, squamous cellcarcinoma of the skin, or in situ uterine cervical carcinoma that has been excisedand cured). Note: At least 1 year for basal cell carcinoma and squamous cellcarcinoma or 5 years for in situ uterine cervical carcinoma must have passed sincethe excision.

• Participant has other causes of myositis (e.g. connective tissue disease) associatedDM, polymyositis, juvenile DM, inclusion body myositis, or necrotizing idiopathicinflammatory myopathies (with or without rash) with the exception of overlap withsecondary Sjogren's syndrome.

• Participant has permanent muscle weakness due to muscle damage (e.g. participant iswheelchair bound or has significant muscle atrophy on magnetic resonance imaging [MRI]) or a non-DM cause (drug-induced myopathy, including glucocorticoid-inducedmyopathy as primary cause of muscle weakness), according to investigator'sjudgement.

• Participant has taken any prohibited therapies within the defined washout periodsbefore screening, and during screening as listed in the study protocol.

• Open Label Extension : Participant meeting one or more of the criteria at Visit 8 asdefined in the protocol, cannot be selected for the OLE period of this clinicalstudy. 1. Participant has total bilirubin >1.5x ULN; however, participant with anisolated increase in total bilirubin <3 x ULN due to Gilbert's Syndrome, with normaldirect bilirubin, can be enrolled in the OLE period. 2. Participant has AST or ALT >1.5 x ULN (hepatic injury), or AST or ALT >=5 x ULN if judged to be of muscularorigin (and confirmed by the steering committee) at Visit 7 and Visit 8.