Dose-Expansion Modular Study To Explore the Safety, Tolerability, and Anti-tumor Activity of HER3- DXd Monotherapy and Combinations in Patients With Inoperable Advanced Breast Cancer (ABC) After Progression on T-DXd

This study tests HER3-DXd, a new antibody-drug treatment that targets a protein called HER3 on breast cancer cells, in patients with advanced breast cancer that has progressed while on or after treatment with a similar drug called T-DXd (trastuzumab deruxtecan). **You may be eligible if...** - You are 18 or older with confirmed advanced (metastatic or locally unresectable) breast cancer - You have been previously treated with T-DXd and your cancer progressed during or within 2 months of stopping that treatment - You have HER2-positive breast cancer and have also received trastuzumab and a taxane chemotherapy - OR you have HER2-low breast cancer and have received a taxane; if HR-positive, you also need prior endocrine therapy and CDK4/6 inhibitor treatment **You may NOT be eligible if...** - Your cancer has never been treated with T-DXd - You have not met the required prior therapy criteria specific to your cancer subtype - You are pregnant Talk to your doctor to see if this trial is right for you.