Study of Trastuzumab Deruxtecan (T-DXd) vs Investigator's Choice Chemotherapy in HER2-low, Hormone Receptor Positive, Metastatic Breast Cancer

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* Patients must be ≥18 years of age* Pathologically documented breast cancer that:

1. is advanced or metastatic 2. has a history of HER2-low or negative expression by local test, defined as IHC 2+/ISH- or IHC 1+ (ISH- or untested) or HER2 IHC 0 (ISH- or untested) 3. has HER2-low or HER2 IHC \>0 \<1+ expression as determined by the central laboratory result established on a tissue sample taken in the metastatic setting 4. was never previously HER2-positive 5. is documented HR+ disease in the metastatic setting. * No prior chemotherapy for advanced or metastatic breast cancer. * Has adequate tumor samples for assessment of HER2 status * Must have either:

1. disease progression within 6 months of starting first line metastatic treatment with an endocrine therapy combined with a CDK4/6 inhibitor or 2. disease progression on at least 2 previous lines of endocrine therapy with or without a targeted therapy in the metastatic setting. Of note with regards to the ≥2 lines of previous ET requirement: disease recurrence while on the first 24 months of starting adjuvant ET, will be considered a line of therapy; these patients will only require 1 line of ET in the metastatic setting. * Has protocol-defined adequate organ and bone marrow function

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