This study is open to adults with a type of cancer called dedifferentiated liposarcoma (DDLPS). They can join the study if their tumours are positive for MDM2. The purpose of this study is to find out whether a medicine called brigimadlin (BI 907828) is tolerated by and helps people with DDLPS. Brigimadlin is a so-called MDM2 inhibitor that is being developed to treat cancer.

Participants take brigimadlin as a tablet once every 3 weeks. Participants may continue to take brigimadlin as long as they benefit from treatment and can tolerate it. They visit the study site regularly. At the study site, doctors regularly check participants' health and take note of any unwanted effects. The doctors also regularly check tumour size.

Inclusion Criteria:

1. Provision of signed and dated, written informed consent form (ICF) in accordance with International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use (ICH) - Good Clinical Practice (GCP) and local legislation prior to any study-specific procedures, sampling, or analyses

2. Male or female patients ≥18 years old at the time of signature of the ICF

3. Women of childbearing potential (WOCBP) and men able to father a child must be ready and able to use two medically acceptable methods of birth control per ICH M3 (R2) that result in a low failure rate of <1% per year when used consistently and correctly beginning at screening, during study participation, and until 6 months and 12 days after last dose for women and 102 days after last dose for men. A list of contraception methods meeting these criteria is provided in the patient information

4. Histologically documented locally advanced or metastatic, unresectable (i.e. surgery morbidity would outweigh potential benefits), progressive or recurrent Dedifferentiated liposarcoma (DDLPS), meeting the criteria for an open study cohort:

   • Cohort A: patient has not received prior systemic therapy for DDLPS in any setting (including adjuvant, neoadjuvant, maintenance, palliative)
   • Cohort B: patient has received any prior systemic therapy for DDLPS in any setting (including adjuvant, neoadjuvant, maintenance, palliative)

5. Written pathology report indicating the diagnosis of DDLPS with positive MDM2 immunohistochemistry or MDM2 amplification as demonstrated by fluorescence in situ hybridisation (FISH) or next-generation sequencing (NGS)

6. Presence of at least 1 measurable target lesion according to RECIST version 1.1. In patients who only have 1 target lesion, the baseline imaging must be performed at least 2 weeks after any biopsy of the target lesion

7. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

8. Life expectancy ≥3 months at the start of treatment in the opinion of the investigator Further inclusion criteria apply.

Exclusion Criteria:

1. Known mutation in the TP53 gene (screening for TP53 status is not required)
2. Major surgery (major according to the investigator's assessment) performed within 4 weeks prior to start of study treatment or planned within 6 months after screening
3. Previous administration of brigimadlin or any other MDM2-p53 or MDM4 regulator of p53 (MDM4/MDMX)-p53 antagonist
4. Previous treatment in study 1403-0008 (Brightline-1)
5. Having to receive, or intending to receive, restricted medications or any drug considered likely to interfere with the safe conduct of the study
6. Receiving treatment for brain metastases or leptomeningeal disease (LMD) which may interfere with safety and/or endpoint assessment
7. Unable to swallow the study treatment
8. Previous or concomitant malignancies other than the one treated in this study within the previous 2 years, except effectively treated non-melanoma skin cancers, carcinoma in situ of the cervix, ductal carcinoma in situ, or other malignancy that is considered cured by local treatment 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.

After structured results have been posted, 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 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.