Key Inclusion Criteria:

          -  Patients must be at least 18 years of age

          -  Pathologically documented breast cancer that:

               1. Is advanced/unresectable (patients that can be treated with curative intent are
                  not eligible) or metastatic

               2. HER2-positive (IHC 3+ or IHC 2+/ISH+) based on local assessment

               3. Is documented as hormone receptor-positive (estrogen or progesterone receptor) or
                  negative in the metastatic setting

          -  Patient must have adequate tumor sample for biomarker assessment

          -  ECOG Performance Status of 0 or 1

          -  Part 1

               1. Disease progression on or after the last systemic therapy prior to starting study
                  treatment

               2. At least 1 prior treatment line in metastatic setting required.

          -  Part 2 (Modules 0 - 5)

             a) No prior lines of therapy for advanced/MBC allowed

          -  Part 2 (Module 6 and 7) a) Zero or one prior lines of therapy for advanced/MBC allowed

        CNS Inclusion

          -  Modules 0 - 5 Patients must have no brain metastases or stable brain metastases.

          -  Module 6 and 7 Patients must have untreated brain metastases not needing local therapy
             or previously treated brain metastases that have progressed since prior local therapy

        Key Exclusion Criteria:

          -  Uncontrolled or significant cardiovascular disease

          -  Active or prior documented (non-infectious) ILD/pneumonitis that required steroids, or
             suspected ILD/pneumonitis that cannot be ruled out by imaging at screening

          -  Lung-specific intercurrent clinically significant illnesses

          -  Uncontrolled infection requiring IV antibiotics, antivirals, or antifungals

          -  Spinal cord compression or a history of leptomeningeal carcinomatosis

          -  Prior treatment with immune checkpoint inhibitors

          -  Prior treatment with an ADC containing a topoisomerase I inhibitor

          -  Prior treatment with tucatinib

        CNS Exclusion

          -  Modules 0 - 5: Has untreated brain metastasis

          -  Module 6 and 7: Ongoing use of systemic corticosteroids for control of symptoms of
             brain metastases or brain lesion thought to require immediate local therapy