A Clinical Study of the Anti-cancer Effects of an Investigational Therapy or Chemotherapy in Patients With Recurring Uterine Cancer

The study is divided into two cohorts (Cohort 1 and Cohort 2), to which participants will be enrolled based on the amount of human epidermal growth factor receptor 2 (HER2) in their tumor sample. In Cohort 1, the main goal is to assess how well BNT323 (also known as DB-1303) or chemotherapy (doxorubicin or paclitaxel \[or docetaxel, if participants cannot take paclitaxel\]) works by determining the progression-free survival (PFS) of participants who have been previously treated with immune checkpoint inhibitors (ICIs). In Cohort 2, the main goal is to assess how well BNT323 works by determining the objective response rate (ORR), that is, the percentage of participants whose tumor shrinks (partial response) or disappears (complete response) after treatment. The safety of BNT323 will also be assessed by following the occurrence of unfavorable/adverse effects that are seen after treatment. Other measures include the pharmacokinetics of BNT323 (or how BNT323 moves through and out of the body), the body's immune response, and the impact on quality of life.

Key Inclusion Criteria: * Are female adults (defined as ≥18 years of age or acceptable age according to local regulations at the time of voluntarily giving informed consent). * Have histologically confirmed endometrial cancer that: * Is recurrent, * Has a HER2 IHC score of 1+, 2+ (Cohort 1), or 3+ (Cohort 2) as determined by central laboratory testing for HER2 expression, and * Is not defined as a true sarcoma (i.e., leiomyosarcoma or endometrial stromal sarcoma). Note: Uterine carcinosarcoma is allowed. * Have measurable disease defined by RECIST v1.1. * Have Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1 or 2. * Have recurrent endometrial cancer and meet any of the following: * developed recurrence \