A Clinical Study to Find the Optimal Dose of an Investigational Treatment Called BNT323 When Used in Combination With Another Investigational Treatment, BNT327, and to Test if That Combination Treatment is Safe and Beneficial for Patients With Advanced Breast Cancer

The study consists of two parts: * Part 1 - Dose escalation: In this part of the study, participants with histologically confirmed, chemotherapy-pretreated advanced HR+, HER2-low or HER2-ultralow BC will receive BNT323 in combination with BNT327 (BNT323 + BNT327) in a dose escalation design. This will define the recommended Phase 2 dose (RP2D) for the BNT323 + BNT327 combination therapy. * Part 2 - Dose optimization and exploratory cohorts: This part of the study will be an expansion phase, aiming to evaluate the efficacy and safety of the optimal dose combination and providing a more robust comparison against the other treatments. It will start once the enrollment in Part 1 is completed and the sponsor in conjunction with the Safety Review Committee has assessed available Part 1 efficacy and safety data. Part 2 of the study will have four cohorts, i.e., Cohorts 1 (dose optimization cohort), and Cohorts 2, 3, and 4 (exploratory cohorts). Recruitment to Cohorts 2, 3, and 4 will begin with RP2D from Part 1 and in parallel to randomization in Cohort 1. Randomization is planned for Cohort 1 in Part 2, i.e., participants will be randomized in 2:2:1:1 ratio into one of the four arms (Arms 1-4). No randomization is planned for any other cohort in Part 2.