This study is testing a new medicine called **GLPG3667** for adults with a condition called **dermatomyositis (DM)**, which causes muscle weakness and skin rashes. The study will last for up to 48 weeks. First, participants will take the medicine for 24 weeks and then can continue for another 24 weeks in an *open-label extension* phase, where everyone knows they are receiving the medicine.

To join, you must have had DM for at least 3 months and have tried other treatments that didn't work well. Also, you need to show signs of active disease, like a rash or high levels of muscle enzymes. You can't join if you have another type of myositis or if your muscle weakness is from other causes like drug side effects.

• **24-week treatment** with a follow-up period, and possible extension.
• **Regular checkups** to monitor progress and safety.
• No major compensation, but access to the study drug and assessments.