Patients must be ≥18 years of age

Pathologically documented breast cancer that:

• is advanced or metastatic
• has a history of HER2-low or negative expression defined as IHC 2+/ISH- or IHC 1+ (ISH- or untested) or HER2 IHC 0 (ISH- or untested)
• has HER2-low or HER2 IHC >0 <1+ expression
• was never previously HER2-positive
• is documented HR+ disease in the metastatic setting.
• No prior chemotherapy for advanced or metastatic breast cancer.
• Has adequate tumor samples for assessment of HER2 status

Must have either:

• disease progression within 6 months of starting first line metastatic treatment with an endocrine therapy combined with a CDK4/6 inhibitor
• disease progression on at least 2 previous lines of endocrine therapy with or without a targeted therapy in the metastatic setting

Of note with regards to the ≥2 lines of previous ET requirement: disease recurrence while on the first 24 months of starting adjuvant ET, will be considered a line of therapy; these patients will only require 1 line of ET in the metastatic setting

Has protocol-defined adequate organ and bone marrow function

• Ineligible for all options in the investigator’s choice chemotherapy arm
• Lung-specific intercurrent clinically significant illnesses
• Uncontrolled or significant cardiovascular disease or infection
• Active or prior documented interstitial lung disease (ILD)/pneumonitis or suspected ILD/pneumonitis that cannot be ruled out by imaging at screening
• Patients with spinal cord compression or clinically active central nervous system metastases
• Prior randomization or treatment in a previous trastuzumab deruxtecan study regardless of treatment arm assignment

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