Key Inclusion Criteria:

          -  Patients must be at least 18 years of age

          -  Male or female patients who have pathologically documented breast cancer that:

               1. Has a history of HER2-low expression, defined as IHC 2+/ISH- or IHC 1+ (ISH- or
                  untested) with a validated assay

               2. Is documented as HR+ (either ER and/or PgR positive [ER or PgR ≥1%]) or ER and
                  PgR negative (ER and PgR <1%) per ASCO/CAP guidelines in the metastatic setting

          -  Patient must have adequate tumor sample for biomarker assessment

          -  ECOG Performance Status of 0 or 1

        For patients with HR+ disease:

        Part 1: At least 1 prior treatment line of ET with or without a targeted therapy (such as
        CDK4/6, mTOR or PI3-K inhibitors), and at least 1 prior line of chemotherapy for MBC are
        required.

        Part 2: Only 1 prior treatment line of ET with or without a targeted therapy (such as
        CDK4/6, mTOR or PI3-K inhibitors) for MBC is allowed. No prior chemotherapy in the
        metastatic setting is allowed. Note there are no patients with HR+ disease in Part 2 of
        Modules 2 and 3.

        For patients with HR- disease:

        Part 1: At least 1 prior line of chemotherapy for MBC is required. Note there are no
        patients with HR- disease in Part 1 of Modules 4 and 5.

        Part 2: For Module 2, no prior lines of therapy for MBC are allowed, and for Modules 1 and
        3, only 1 prior line of chemotherapy for MBC is allowed. Note there are no patients with
        HR- disease in Part 2 of Modules 4 and 5.

        Key Exclusion Criteria:

          -  Uncontrolled intercurrent illness

          -  Uncontrolled or siginificant cardiovascular disease

          -  History of (non-infectious) ILD/pneumonitis that required steroids, has current
             ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at
             screening.

          -  Lung-specific intercurrent clinically significant illnesses

          -  Has spinal cord compression or clinically active central nervous system metastases

          -  Active primary immunodeficiency

          -  Uncontrolled infection requiring IV antibiotics, antivirals, or antifungals

          -  Prior treatment with ADC that comprises of an exatecan derivative that is a
             topoisomerase I inhibitor.