This study tests if a medicine called nerandomilast helps adults with lung fibrosis due to rheumatic diseases. Lung fibrosis is when lungs get scarred and can't work well. Participants must be 18 or older and have not improved with standard treatment using immunosuppressants (medicines that lower the body's defense system). They will be split into two groups by chance: one group takes nerandomilast and the other takes a placebo (a pill that looks the same but has no medicine). Participants take tablets twice a day for 26 weeks to 1 year while continuing their regular treatment. They visit the study site 9-10 times over 7.5 to 13 months. During visits, their lung function is tested, and sometimes chest images are taken. They also fill in forms about symptoms and life quality.

• The study lasts 7.5 to 13 months with 9-10 visits.
• Participants continue their usual treatment during the study.
• Regular check-ups ensure safety and track any side effects.