This study is open to adults with idiopathic pulmonary fibrosis (IPF) who are at least 40 years old. People taking standard medicines for IPF, including antifibrotic medicines, can continue taking them throughout the study.

The purpose of the study is to find out whether a medicine called BI 1015550 can slow down the worsening of lung function. Participants are in the study for about 4 months. During this time, they visit the study site about 7 times. At the beginning, they visit the study site every 2 weeks.

After 1 month of treatment, they visit the study site every 4 weeks.

The participants are put into 2 groups by chance. 1 group gets BI 1015550. The other group gets placebo. Placebo tablets look like BI 1015550 tablets but contain no medicine. The participants take BI 1015550 or placebo tablets twice a day.

The participants have lung function tests at study visits. The results of the lung function tests are compared between the BI 1015550 group and the placebo group. The doctors also regularly check the general health of the participants.

Inclusion Criteria:

1. Patients aged ≥40 years when signing the informed consent.

2. Diagnosis:

   1. IPF based on 2018 ATS/ERS/JRS/ALAT Guideline as confirmed by the investigator based on chest High Resolution Computed Tomography Scan (HRCT) scan taken within 12 months of Visit 1 and if available surgical lung biopsy.

      and

   2. Usual interstitial pneumonia (UIP) or probable UIP HRCT pattern consistent with the clinical diagnosis of IPF, as confirmed by central review prior to Visit 2*

      • if indeterminate HRCT finding IPF may be confirmed locally by (historical) biopsy

3. Stable for at least 8 weeks prior to Visit 1. Patients have to be either :

   • not on therapy with nintedanib or pirfenidone for at least 8 weeks prior to Visit 1 (combination of nintedanib plus pirfenidone not allowed), or
   • on stable* therapy with nintedanib or pirfenidone for at least 8 weeks prior to Visit 1 and planning to stay stable on this background therapy after randomisation.

   [*stable therapy is defined as the individually and general tolerated regimen of either pirfenidone or nintedanib]

4. Forced Vital Capacity (FVC) ≥45% of predicted normal at Visit 1

5. Diffusion capacity of the lung for carbon monoxide (DLCO) (corrected for haemoglobin [Hb] [Visit 1]) ≥ 25% to < 80% of predicted normal at Visit 1.

6. Signed and dated written informed consent in accordance with ICH-GCP and local legislation prior to admission to the trial.

Exclusion Criteria:

1. Relevant airways obstruction (pre-bronchodilator Forced Expiratory Volume in one second (FEV1)/Forced Vital Capacity (FVC) < 0.7) at Visit 1.
2. In the opinion of the Investigator, other clinically significant pulmonary abnormalities.
3. Acute IPF exacerbation within 4 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. 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).
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. Evidence of active infection (chronic or acute) based on clinical exam or laboratory findings at Visit 1 or at Visit 2.
8. Any suicidal behaviour in the past 2 years (i.e. actual attempt, interrupted attempt, aborted attempt, or preparatory acts or behavior).
9. The patient has a confirmed infection with SARS-CoV-2 within the 4 weeks prior to Visit 1 and/or during the screening period.

Further exclusion criteria apply.

After the study is completed and the primary manuscript is accepted for publishing, researchers can use this 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".

Also, Researchers can use the following link https://www.mystudywindow.com/msw/datasharing to find information in order to 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.

The data shared are the raw clinical study data sets.

After all regulatory activities are completed in the US and EU for the product and indication, and after the primary manuscript has been accepted for publication.

For study documents - upon signing of a 'Document Sharing Agreement'. For study data - 1. after the submission and approval of the research proposal (checks will be performed by both the independent review panel and the sponsor, including checking that the planned analysis does not compete with sponsor's publication plan); 2. and upon signing of a 'Data Sharing Agreement'.