Trastuzumab Deruxtecan (T-DXd) in Patients Who Have Hormone Receptor-negative and Hormone Receptor-positive HER2-low or HER2 IHC 0 Metastatic Breast Cancer

Inclusion Criteria:

• Sign and date the main informed consent form

• Must agree to provide a newly obtained or archival baseline biopsy from primaryand/or metastatic lesion.

• Pathologically documented Breast Cancer (BC) tumor

• Is unresectable and/or metastatic.

• Is hormone receptor-negative or hormone receptor-positive.

• Must include percentage of positively stained cells to characterize ifhormone receptor-positive or -negative.

• Has confirmed HER2 IHC 1+ or IHC 2+/ISH- (HER2-low) status or HER2 IHC 0 statusas determined according to ASCO CAP 2018 guidelines1 based on sample collectedduring Tissue Screening as described above.

• Was never previously HER2-positive (IHC 3+ or IHC 2+/ISH+) on prior pathologytesting (per ASCO CAP guidelines).

• Was never previously treated with anti-HER2 therapy in the metastatic setting.

• Has had at least one and up to two prior lines of therapy in the metastatic setting.

• In participants with hormone receptor-positive HER2-low metastatic BC (Cohort 3):

• Has recurrent disease <2 years from the initiation of adjuvant ET OR

• Has disease progression on CDK4/6 inhibitor-based regimen within 12 monthsof completion of adjuvant therapy with a CDK4/6 inhibitor OR

• Has disease progression within the first 12 months of CDK4/6 in the firstline metastatic setting

• Presence of at least one measurable lesion based on computed tomography or magneticresonance imaging.

• Participants with brain metastases are allowed in the study. The brain lesion(s)should be small (<2 cm), untreated, asymptomatic, not requiring urgent medicalintervention, and are asymptomatic and clinically stable.

• Has an Eastern Cooperative Oncology Group performance status of 0 or 1.

• Has a minimum life expectancy of 12 weeks at Screening.

• Has a left ventricular ejection fraction ≥50% within 28 days before enrollment.

• Has adequate organ and bone marrow function within 28 days before enrollment.

• Has adequate treatment washout period before enrollment.

• Male and female subjects of reproductive/childbearing potential must agree to use ahighly effective form of contraception.

Exclusion Criteria:

• Prior treatment with an antibody drug conjugate (ADC).

• Uncontrolled or significant cardiovascular disease.

• Has a corrected QT interval prolongation.

• Has a history of (non-infectious) interstitial lung disease (ILD)/pneumonitis thatrequired steroids, has current ILD/pneumonitis, or where suspected ILD/pneumonitiscannot be ruled out by imaging at Screening.

• Has spinal cord compression or clinically active central nervous system metastases.

• Has multiple primary malignancies within 3 years, except adequately resectednon-melanoma skin cancer, curatively treated in situ disease, other solid tumorscuratively treated, or contralateral BC.

• Has a history of severe hypersensitivity reactions to either the drug substances orinactive ingredients in the drug product.

• Has a history of severe hypersensitivity reactions to other monoclonal antibodies.

• Has an uncontrolled infection requiring intravenous (IV) antibiotics, antivirals, orantifungals.

• Active primary immunodeficiency, known uncontrolled active human immunodeficiencyvirus (HIV) infection, or active hepatitis B or C infection.

• Has history of receiving a live, attenuated vaccine (messenger RNA andreplication-deficient adenoviral vaccines are not considered attenuated livevaccines) within 30 days prior to the first exposure to study drug.

• Has unresolved toxicities from previous anticancer therapy, defined as toxicities (other than alopecia) not yet resolved to Grade ≤1 or baseline.

• Is pregnant or breastfeeding or planning to become pregnant.

• Lung-specific intercurrent clinically significant illnesses.

• Any autoimmune, connective tissue, or inflammatory disorders.

• Prior complete pneumonectomy.