Key Inclusion Criteria:

          -  Patients must be ≥18 years of age

          -  Pathologically documented breast cancer that:

               1. is advanced or metastatic

               2. is locally assessed and prospectively centrally confirmed as HER2-positive (IHC3+
                  or ISH+)

               3. is documented by local testing as hormone receptor (HR)-positive or HR-negative
                  disease in the metastatic setting

          -  No prior chemotherapy or HER2-targeted therapy for advanced or metastatic breast
             cancer or only 1 previous line of endocrine therapy in the metastatic setting.
             Participants who have received chemotherapy or HER2-targeted therapy in the
             neo-adjuvant or adjuvant setting are eligible if > 6 months from treatment to
             metastatic diagnosis.

          -  Has protocol-defined adequate organ and bone marrow function

        Key Exclusion Criteria:

          -  Ineligible for any of the agents on the study.

          -  Any substance abuse or other medical conditions that, in the investigator's opinion,
             may interfere with subject's participation or study results

          -  Patients with spinal cord compression or clinically active central nervous system
             metastases. Participants with clinically inactive brain metastases or treated brain
             metastases that are no longer symptomatic may be included in the study.

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

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