Trastuzumab Deruxtecan (T-DXd) With or Without Pertuzumab Versus Taxane, Trastuzumab and Pertuzumab in HER2-positive Metastatic Breast Cancer (DESTINY-Breast09)

Eligible participants will be those diagnosed with HER2-positive (IHC 3+ or ISH+), metastatic breast cancer, who have received no prior chemotherapy or HER2-targeted therapy for advanced or metastatic breast cancer. The study aims to evaluate the efficacy, and safety of trastuzumab deruxtecan, alone or with pertuzumab, compared with the standard of care treatment (taxane \[docetaxel or paclitaxel\], trastuzumab and pertuzumab). This study aims to see if trastuzumab deruxtecan allows patients to live longer without the cancer getting worse, or simply to live longer, compared to patients receiving standard of care chemotherapy. This study is also looking to see how the treatment and the cancer affects patients' quality of life.