This study is open to people with a type of cancer called dedifferentiated liposarcoma. People with advanced liposarcoma aged 18 or older who are not receiving any other cancer treatment can participate.

The purpose of this study is to compare a medicine called brigimadlin (BI 907828) with doxorubicin in people with liposarcoma. Brigimadlin (BI 907828) is a so-called MDM2 inhibitor that is being developed to treat cancer. Doxorubicin is a medicine already used to treat cancer including liposarcoma.

During the study, participants get either brigimadlin (BI 907828) or doxorubicin. Every 3 weeks, participants take brigimadlin (BI 907828) as tablets or doxorubicin as an infusion into a vein. Participants can switch to brigimadlin (BI 907828) treatment if they did not benefit from doxorubicin treatment.

Participants can continue treatment in the study as long as they benefit from it and can tolerate it.

Doctors regularly check the size of the tumour and check whether it has spread to other parts of the body. The doctors also regularly check participants' health and take note of any unwanted effects.

Inclusion Criteria:

• Provision of signed and dated, written informed consent form (ICF) in accordance with ICH-GCP and local legislation prior to any trial-specific procedures, sampling, or analyses.
• Male or female patients ≥18 years old at the time of signature of the informed consent form (ICF). Women of childbearing potential (WOCBP) and men able to father a child must be ready and able to use 2 medically acceptable 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 beginning at screening, during trial 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.
• Histologically proven locally advanced or metastatic, unresectable (surgery morbidity would outweigh potential benefits), progressive or recurrent dedifferentiated liposarcoma (DDLPS). Locally performed histopathological diagnosis will be accepted for entry into this trial but will be confirmed by independent pathological review while the patients receive treatment in this trial.
• Written pathology report indicating the diagnosis of DDLPS with positive mouse double minute 2 homolog (MDM2) immunohistochemistry or MDM2 amplification as demonstrated by fluorescence in situ hybridization or next generation sequencing (NGS) must be available.
• Formalin fixed paraffin embedded tumor blocks or slides must be available for retrospective histopathological central review.
• Presence of at least one measurable target lesion according to Response Evaluation Criteria In Solid Tumors (RECIST) version 1.1. In patients who only have one target lesion, the baseline imaging must be performed at least 2 weeks after any biopsy of the target lesion.
• Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1.
• Patient must be willing to donate blood samples for the pharmacokinetics, pharmacodynamics, and tumor mutation analysis.
• Patient willing to undergo a mandatory tumor biopsy at the time point specified in the flowchart unless exempt.
• Adequate organ function.

Exclusion Criteria:

• Known mutation in the TP53 gene (screening for TP53 status is not required).
• Major surgery (major according to the investigator's assessment) performed within 4 weeks prior to randomization or planned within 6 months after screening.
• Prior systemic therapy for liposarcoma in any setting (including adjuvant, neoadjuvant, maintenance, palliative).
• Previous or concomitant malignancies other than DDLPS or WDLPS, treated within the previous 5 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.
• Previous treatment with anthracyclines in any setting (systemic treatment with other anticancer agents is allowed if completed at least 5 years prior to study entry with the exception of hormone therapy).
• Patients who must or intend to continue the intake of restricted medications or any drug considered likely to interfere with the safe conduct of the trial.
• Currently enrolled in another investigational device or drug trial, or less than 30 days since ending another investigational device or drug trial(s) or receiving other investigational treatment(s).
• Patients not expected to comply with the protocol requirements or not expected to complete the trial as scheduled (e.g. chronic alcohol or drug abuse or any other condition that, in the investigator's opinion, makes the patient an unreliable trial participant).
• Further exclusion criteria apply

Once the time frame criteria given under number 4 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.