Sponsor decision.

This study is open to adults with specific types of advanced soft tissue sarcoma. People with undifferentiated pleomorphic sarcoma (UPS) or myxofibrosarcoma (MFS) can join the study if they have a normal version of the TP53 gene. This is a study for people whose earlier treatment isn't working anymore, and their doctors suggest a new treatment to stop the sarcoma from getting worse.

The purpose of this study is to compare a medicine called brigimadlin in combination with another medicine called ezabenlimab with chemotherapy. Brigimadlin is a so-called MDM2-p53 antagonist that is being developed to treat cancer. Ezabenlimab is an antibody that may help the immune system fight cancer.

Participants are put into 3 groups by chance:

• Ezabenlimab group: Participants receive ezabenlimab as an infusion into a vein every 3 weeks
• Brigimadlin + ezabenlimab group: Participants take brigimadlin as tablets and receive ezabenlimab as an infusion into a vein every 3 weeks
• Chemotherapy group: Participants get chemotherapy as an infusion into a vein on 2 days every 3 weeks. Chemotherapy is a combination of gemcitabine and docetaxel which is often used in the treatment of sarcoma.

There are twice as many participants in the brigimadlin + ezabenlimab group and in the chemotherapy group, compared to those in the ezabenlimab group.

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 tumor 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. Participants in this study use an app on a mobile phone to regularly answer questions about their health and well-being. This is to find out if their quality of life is changing.

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 - Good clinical practice (ICH-GCP) and local legislation prior to any trial-specific procedures, sampling, or analyses

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

3. Histologically proven diagnosis of one of the following:

   • Undifferentiated pleomorphic sarcoma (UPS) with TP53 wild-type status
   • Myxofibrosarcoma (MFS) with TP53 wild-type status

4. Written pathology report indicating the diagnosis of UPS or MFS must be available

5. Written report from the trial central laboratory indicating TP53 wild-type status

6. One prior systemic regimen for advanced disease with documented progressive disease as per serial radiologic imaging. If prior systemic cytotoxic treatment for soft tissue sarcoma was given in a neoadjuvant, adjuvant or perioperative setting that treatment would count as one line of therapy in case of radiological progression during that treatment or within 3 months after completing that systemic therapy. If progression or relapse occurred after >3 months after completing that systemic therapy, the pre-, post or perioperative treatment would not count as a line of treatment for advanced disease

7. Patient must be willing to donate mandatory blood and tissue samples for the pharmacokinetics, pharmacodynamics, and biomarker analyses

8. Presence of at least one target lesion according to RECIST Version 1.1 Further inclusion criteria apply

Exclusion Criteria:

1. Prior treatment with immunotherapy in the context of anticancer therapy
2. Previous administration of gemcitabine, taxanes, brigimadlin or any other mouse double minute 2 homolog-p53 (MDM2-p53) or MDMX (MDM4)-p53 antagonist
3. Previous or concomitant malignancies other than the one treated in this trial 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
4. Patient unable to swallow the trial medication
5. Any history or presence of uncontrolled gastrointestinal disorders that could affect the intake and/or absorption of the trial drug (e.g., nausea, vomiting, Crohn's disease, ulcerative colitis, chronic diarrhea, malabsorption) in the opinion of the investigator
6. Active bleeding, significant risk of hemorrhage (e.g., previous severe gastrointestinal bleeding, previous hemorrhagic stroke at any time), or current bleeding disorder (e.g., hemophilia, von Willebrand disease)
7. Known history of severe hypersensitivity reactions to other monoclonal antibodies or known hypersensitivity or known contraindication to the trial medications or their excipients
8. Active autoimmune disease or a documented history of autoimmune disease, that requires systemic treatment, i.e., corticosteroids or immunosuppressive drugs, except for vitiligo or resolved childhood asthma/atopy, alopecia, or any chronic skin condition that does not require systemic therapy; patients with autoimmune-related hypothyroidism on a stable dose of thyroid hormone replacement and/or controlled Type 1 diabetes mellitus on a stable insulin regimen are eligible 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.

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.