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

Eligibility Criteria

• * Have history of small bowel obstruction requiring hospitalization within the past 3 months prior to the first dose of IMP.
• * 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, peritoneal shunt, or cell-free concentrated ascites reinfusion therapy within 2 weeks prior to randomization/enrollment.
• * 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.
• * Had prior treatment with topoisomerase I inhibitors, including ADCs with exatecans.
• * Have received any of the following therapies or drugs prior to the initiation of the study:
• * Participants who have previously been randomized to or received treatment in a previous study with BNT323, regardless of treatment assignment.
• * Participants who received prior treatment with a PD-L1/VEGF bispecific antibody.
• * Have received other systemic immunostimulatory agents or immunosuppressive therapies (such as interferon-α, interleukin-2, or methotrexate) within 4 weeks prior to the initiation of study treatment or are within five half-lives of the treatment drug (whichever is longer). Exception: excluding local, intranasal, intraocular, intra-articular or inhaled corticosteroids, short term use (≤7 days) of corticosteroids for prophylaxis (e.g., prevention of contrast agent allergy) or treatment of non-autoimmune conditions (e.g., delayed hypersensitivity reactions caused by exposure to allergens).
• * Have received systemic corticosteroids (at a dosage greater than 10 mg/day of prednisone or an equivalent dose of other corticosteroids) within 3 weeks prior to the initiation of study treatment.