Key Inclusion Criteria:

          -  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 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

               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

          -  Either disease progression on at least 2 previous lines of endocrine therapy with or
             without a targeted therapy in the metastatic setting or disease progression within 24
             months of starting adjuvant endocrine therapy and on at least 1 previous line of
             endocrine therapy in the metastatic setting

          -  Has protocol-defined adequate organ and bone marrow function

        Key Exclusion Criteria:

          -  Ineligible for all options in the investigator's choice chemotherapy arm

          -  Lung-specific intercurrent clinically significant illnesses

          -  Uncontrolled or significant cardiovascular disease or infection

          -  Active or prior documented interstitial lung disease (ILD)/pneumonitis or suspected
             ILD/pneumonitis that cannot be ruled out by imaging at screening

          -  Patients with spinal cord compression or clinically active central nervous system
             metastases

          -  Prior randomization or treatment in a previous trastuzumab deruxtecan study regardless
             of treatment arm assignment