This study is open to adults and adolescents aged 12 to under 18 with bronchiectasis. People can participate in this study if they produce sputum and have had flare-ups (also called exacerbations).

The purpose of this study is to find out whether a medicine called BI 1291583 helps people with bronchiectasis. Participants are put into 2 groups randomly, which means by chance. One group takes BI 1291583 tablets and the other group takes placebo tablets. A placebo tablet looks like the BI 1291583 tablet but does not contain any medicine. Participants take 1 tablet once a day for up to 1 year and 6 months.

Participants are in the study for up to 1 year and 8 months. During this time, participants visit the study site up to 10 times and get about 13 phone calls from the site staff. Participants regularly complete a diary on a smartphone about their bronchiectasis symptoms and study doctors regularly check for any changes. The study doctors document when participants experience flare-ups. The number of flare-ups is compared between the participants who receive BI 1291583 and those who receive the placebo. The study doctors also regularly check participants' health and take note of any unwanted effects.

Inclusion criteria:

• Male or female participants. Woman of childbearing potential (WOCBP) must be ready and able to use highly effective methods of birth control per International Council of Harmonisation (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 is provided in the participant information.

• Signed and dated written informed consent and assent, if applicable, prior to admission to the study, in accordance with GCP and local legislation.

• Age of participants when signing the informed consent/assent ≥12 years.

  -- Adolescents need to weigh at least 35 kg at Visit 1.

• Clinical history consistent with bronchiectasis (e.g. cough, chronic sputum production, recurrent respiratory infections) and investigator confirmed diagnosis of bronchiectasis by CT scan where bronchiectasis has been documented by a radiologist.

Participants whose past CT scan image records are not available will undergo a chest CT scan during Screening. Historical scans must not be older than five years.

• Adult participants should be able to produce sputum for Pseudomonas aeruginosa assessment during the screening period.

• History of documented pulmonary exacerbations (assessed and recorded by the investigator) requiring antibiotic treatment. In the 12 months before Visit 1, participants must have had either:

  • at least 2 exacerbations, or
  • at least 1 exacerbation and an St. George's Respiratory Questionnaire (SGRQ) Symptoms score of >40 at screening Visit 1 (adults only)
  • at least 1 exacerbation and high symptom burden according to the investigator's judgement (adolescents only) For participants on oral or inhaled antibiotics as chronic treatment for bronchiectasis and participants on Cystic Fibrosis Transmembrane Conductance Regulator Modulator Therapy (CFTR-MT), at least one exacerbation must have occurred since initiation of antibiotics or CFTR-MT.

Exclusion criteria:

• Any new or newly diagnosed condition of primary or secondary immunodeficiency within 1 year before randomisation.
• Allergic bronchopulmonary aspergillosis being treated or requiring treatment.
• Tuberculosis or non-tuberculosis mycobacterial infection being treated or requiring treatment
• Any findings in the medical examination and/or laboratory value assessed at Screening Visit 1 or during screening period, that in the opinion of the investigator may put the participant at risk by participating in the trial.
• Any clinically relevant (at the discretion of the investigator) acute respiratory infection or ongoing pulmonary exacerbation at screening visit or during the screening unless recovered in the opinion of the investigator prior to Visit 2.
• Any relevant pulmonary, gastrointestinal, hepatic, renal, cardiovascular, metabolic, immunological, hormonal, or other disorder that, in the opinion of the investigator, may put the participant at risk by participating in the study.
• Major surgery (major according to the investigator's assessment) performed within 6 weeks prior to randomisation or scheduled during trial period.
• Any documented active or suspected malignancy or history of malignancy within 5 years prior to screening, except appropriately treated in situ non-melanoma skin cancers or in situ carcinoma of uterine cervix.
• Evidence or medical history of moderate or severe liver disease (Child-Pugh score B or C hepatic impairment).
• estimated Glomerular Filtration Rate (eGFR) according to Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula (adults) or Chronic Kidney Disease Under 25 (CKiD-U25) (adolescents) <30 mL/min at Visit 1.
• Previous treatment with a dipeptidyl peptidase-1 (DPP1) (Cathepsin C (CatC)) inhibitor. (Note: Participants that were randomised and only received placebo in studies with DPP1 (CatC) inhibitor are allowed.) Further exclusion criteria apply.

Once the criteria in section "Time Frame" are fulfilled, researchers can use the following link https://www.clinicalstudies.boehringer-ingelheim.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.

One year after the approval has been granted by major Regulatory Authorities and after the primary manuscript has been accepted for publication, or after termination of the development program.

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 the sponsor and/or the independent review panel, including checking that the planned analysis does not compete with sponsor's publication plan); 2. and upon signing of a legal agreement.