A Phase 1b Study of T-DXd Combinations in HER2-low Advanced or Metastatic Breast Cancer

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.