Fern-EC-01: This study is recruiting people with previously treated endometrial cancer to investigate how well a drug called BNT323 works compared to chemotherapyA Phase III, Randomized, Multi-site, Open-label Trial of BNT323/DB-1303 Versus Investigator's Choice of Chemotherapy in Previously Treated Patients With HER2- Expressing Recurrent Endometrial Cancer

Exclusion

• Are ineligible for all options in the investigator's choice of chemotherapy arm, per local prescribing information and institutional guidelines (applicable to Cohort 1 only).
• Have a history of small bowel obstruction requiring hospitalization within the past 3 months prior the first dose of study treatment.
• Have an uncontrolled intercurrent illness that would limit compliance with study requirement or substantially increase risk of incurring adverse events.
• Have clinically uncontrolled pleural effusion, ascites or pericardial effusion requiring drainage, or peritoneal shunt within 2 weeks prior to the first dose of study treatment.
• Have a history of (non-infectious) interstitial lung disease (ILD)/pneumonitis that required steroids, have current ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at screening.
• Participants with prior use of immunosuppressive medication within 14 days prior to the first dose of study treatment, except for intranasal and inhaled corticosteroids or systemic corticosteroids at doses of less than 10 mg/day of prednisone or equivalent, and topical corticosteroids. Participants receiving corticosteroids may continue if the dose is stable upon giving main informed consent.
• Have a lung-specific intercurrent clinically significant illness including, but not limited to, any underlying pulmonary disorder (e.g., pulmonary emboli within 3 months prior to the first dose of study treatment, severe asthma, chronic obstructive pulmonary disorder with moderate acute exacerbations, restrictive lung disease, pulmonary fibrosis, radiation pneumonitis, significant pleural effusion etc.), or any autoimmune, connective tissue or inflammatory disorder with pulmonary involvement (i.e., rheumatoid arthritis, Sjogren's syndrome, sarcoidosis etc.), and/or prior pneumonectomy (complete).
• Have uncontrolled infection requiring systemic antibiotics, antivirals, or antifungals within 2 weeks prior to the first dose of study treatment.
• Have unresolved toxicities from previous anti-cancer therapy, defined as toxicities (other than alopecia, fatigue, or endocrinopathies that are well controlled) not yet resolved to Grade ≤1 or baseline.
• Are pregnant or breastfeeding or are planning pregnancy during the study or within 7 months after the last dose of study treatment.
• Have a history of allergies, hypersensitivities, or intolerance to study treatments (investigational medicinal products and auxiliary medicinal product) including any excipients thereof or to other monoclonal antibodies. Participants who have successfully undergone a desensitization process and are able to tolerate the drug are eligible.
• Had prior treatment with topoisomerase I inhibitors, including ADCs.
• Have left ventricular ejection fraction <55% by either echocardiography or multiple-gated acquisition within 28 days prior to the first dose of study treatment. This includes participants with tissue doppler E/e' ratio >15.

NOTE: Other protocol defined Inclusion/Exclusion criteria apply.