Inclusion
- Male or female adults (defined as ≥ 18 years of age or acceptable age according to local regulations at the time of voluntarily signing of informed consent).
- Pathologically documented breast cancer that:
1) Is advanced or metastatic 2) Has HER2-low expression (IHC 1+ or IHC 2+/ISH-) as determined by the central laboratory result.
3) Was never previously reported as HER2-positive (IHC 3+ or ISH+) as per American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines.
4) Is documented as HR+ (either estrogen receptor [ER] and/or progesterone receptor [PgR] positive [ER or PgR ≥1%]) per ASCO/CAP guidelines (Allison et al 2020).
3. Must have an adequate tumor tissue sample available for assessment of HER2 by central laboratory, preferably in formalin fixation and paraffin embedding (FFPE) blocks based on a mandatory FFPE tumor sample obtained at the time of metastatic disease or later;
4. Eastern Cooperative Oncology Group performance status of 0 or 1.
5. Must have had either:
- Disease progression on endocrine therapy + CDK4/6 inhibitor within 6 months of starting first line treatment for metastatic disease and considered appropriate for chemotherapy as the next treatment by the investigator, OR
- Disease progression on at least 2 previous lines of ET with or without a targeted therapy (such as CDK4/6, mammalian target of rapamycin [mTOR] or phosphoinositide 3-kinase [PI3-K] inhibitors) administered for the treatment of metastatic disease.
6. No prior chemotherapy for advanced or metastatic breast cancer. Subjects who have received chemotherapy in the neo-adjuvant or adjuvant setting are eligible, as long as they have had a disease-free interval (defined as completion of systemic chemotherapy to diagnosis of advanced or metastatic disease) of >12 months.
7. Life expectancy ≥12 weeks at screening.
8. Subjects must have at least one measurable lesion as defined per RECIST v1.1 or have non-measurable, bone-only disease that can be assessed by computer tomography (CT) or Magnetic Resonance Imaging (MRI) or X-Ray. Lytic or mixed lytic bone lesions that can be assessed by CT or MRI or X-Ray in the absence of measurable disease as defined above is acceptable; subjects with sclerotic/osteoblastic bone lesions only in the absence of measurable disease are not eligible.
9. Female subjects of childbearing potential who are sexually active with a non-sterilized male partner must use at least one highly effective method of contraception from the time of screening and must agree to continue using such precautions for 7 months after the last dose of study treatment.
10. Non-sterilized male subjects who are sexually active with a female partner of childbearing potential must use a condom with spermicide from screening and throughout the duration of the study treatment and the washout period