This study is open to adults with Progressive Fibrosing Interstitial Lung Disease (PF-ILD). People who have a form of PF-ILD other than Idiopathic Pulmonary Fibrosis (IPF) can join the study. If they already take nintedanib or pirfenidone, they can continue treatment throughout the study.

The purpose of this study is to find out whether a medicine called BI 1015550 helps people with PF-ILD.

Participants are put into 2 groups randomly, which means by chance. One group takes BI 1015550 tablets twice a day. The other group takes placebo tablets twice a day. Placebo tablets look like BI 1015550 tablets but do not contain any medicine.

Participants are in the study for up to two and a half years. During the first year, they visit the study site 10 times. Afterwards, they visit the study site every 3 months. The doctors regularly test participants' lung function. The results of the lung function tests are compared between the groups.

The doctors also regularly check participants' health and take note of any unwanted effects.

The L-PF is a 49-item questionnaire with two modules: Symptoms (28 items) and Impact (21 items).

The Symptom score has three domain scores: dyspnea, cough, and fatigue, as well as a total symptom score.

Items have response options on a five-option numeric rating score with an anchor of 0 "Not at all" to 4 "Extremely", with higher numbers indicating a greater impairment.

The L-PF is a 44-item questionnaire divided into two modules: Symptoms (23 items) and Impacts (21 items).

The Symptoms module assesses shortness of breath, cough and fatigue in the past 24 hours.

Items have response options on a five-option numeric rating score with an anchor of 0 "Not at all" to 4 "Extremely", with higher numbers indicating a greater impairment.

Inclusion criteria:

1. Written Informed Consent consistent with ICH-GCP and local laws signed prior to entry into the study (and prior to any study procedure including shipment of high-resolution computed tomography (HRCT) to reviewer).
2. Patients (male/female), aged ≥ 18 years when signing the informed consent, with a physician diagnosed progressive fibrosing Interstitial Lung Disease (PF-ILD) other than Idiopathic Pulmonary Fibrosis (IPF) (presence of reticular abnormality with traction bronchiectasis with or without honeycombing on HRCT, performed within 12 months of Visit 1) AND who fulfil at least one of the following criteria for PF-ILD within 24 months of Visit 1 despite management according to current clinical practice to treat ILD, as assessed by the investigator (refer to Exclusion Criteria):
   1. Clinically significant decline in Forced Vital Capacity (FVC) % predicted (pred) based on a relative decline of ≥10%
   2. Marginal decline in FVC % pred based on a relative decline of ≥5-<10% combined with worsening of respiratory symptoms
   3. Marginal decline in FVC % pred based on a relative decline of ≥5-<10% combined with increasing extent of fibrotic changes on chest imaging
   4. Worsening of respiratory symptoms as well as increasing extent of fibrotic changes on chest imaging
3. Patients need to be either

   -- on a stable therapy* with nintedanib or pirfenidone for at least 12 weeks prior to Visit 1 and during screening and planning to stay on this background treatment after randomization. Combination of nintedanib plus pirfenidone is not allowed.

   [*stable therapy is defined as the individually and general tolerated regimen of either nintedanib or pirfenidone (no dose changes) for at least 12 weeks] OR

   -- not on a therapy with nintedanib or pirfenidone for at least 8 weeks prior to Visit 1 or during the screening period. (e.g. either antifibrotic (AF)-treatment naïve or previously discontinued) and start or re-start of an antifibrotic is not planned.

4. Patients treated with permitted immunosuppressive agents for the underlying disease (e.g. Mycophenolat-Mofetil (MMF), methotrexat (MTX), azathioprine (AZA) …) need to be on a stable treatment for at least 12 weeks prior to visit 1 and during screening period:

   For oral corticosteroids, the dose should not have been > 20 mg prednisolone/day (or equivalent) within 4 weeks prior to visit 1 For patients with underlying Connective Tissue Disease (CTD), the CTD should be stable, defined as no initiation of new therapy or withdrawal of therapy for CTD within 12 weeks prior to Visit 1 and during the screening period.

5. Forced Vital Capacity (FVC) ≥ 45% of predicted normal at Visit 1
6. Diffusing capacity for carbon monoxide (DLCO) corrected for Haemoglobin (Hb) [visit 1] ≥ 25% and <90% predicted of normal at Visit 1
7. Female and male patients: Women of childbearing potential (WOCBP) and men able to father a child must be ready and able to use highly effective methods of birth control per ICH M3 (R2) that result in a low failure rate of less than 1% per year when used consistently and correctly. A list of contraception methods meeting these criteria and instructions on the duration of their use is provided in the patient information

Key exclusion criteria

1. Relevant airways obstruction (pre-bronchodilator Forced Expiratory Volume (FEV)1/FVC < 0.7) at Visit 1.
2. In the opinion of the Investigator, other clinically significant pulmonary abnormalities.
3. Acute IPF or Interstitial Lung Disease (ILD) exacerbation within 3 months prior to screening and/or during the screening period (investigator-determined).
4. Lower respiratory tract infection requiring antibiotics within 4 weeks prior to Visit 1 and/or during the screening period.
5. Relevant chronic or acute infections including human immunodeficiency virus (HIV) and viral hepatitis and a confirmed infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within the 4 weeks prior to Visit 1 or during the screening period.

   A patient can be re-screened if the patient was treated and/or cured from the acute infection.,

6. Any documented active or suspected malignancy or history of malignancy within 5 years prior to Visit 1, except appropriately treated basal cell carcinoma of the skin, "under surveillance" prostate cancer or in situ carcinoma of uterine cervix.
7. Major surgery (major according to the investigator's assessment) performed within 3 months prior to Visit 1 or planned during the course of the trial. Being on a transplant list is allowed.
8. Aspartate amino transferase (AST) or Alanine amino transferase (ALT) > 2.5 x upper limit of normal (ULN) or total Bilirubin > 1.5 x ULN at Visit 1.

Further exclusion criteria apply.

Once the criteria in section "Time Frame" are fulfilled, researchers can use the following link https://www.mystudywindow.com/msw/datasharing to request access to the clinical study documents regarding this study, and upon a signed "Document Sharing Agreement".

Furthermore, researchers can request access to the clinical study data, for this and other listed studies, after the submission of a research proposal and according to the terms outlined in the website.