The study will evaluate the safety and efficacy of YL202, when compared with treatment of physician's choice (eribulin, capecitabine, vinorelbine, gemcitabine or sacituzumab govitecan) in participants with unresectable locally advanced, recurrent or metastatic hormone receptor-positive and human epidermal growth factor receptor 2-negative (HR+/HER2-) breast cancer who had failed at least one line of chemotherapy.

Inclusion Criteria:

1. Have been informed of the study before the start of the study and voluntarily sign name and date on the informed consent form.
2. Histologically and/or cytologically confirmed locally advanced or metastatic HR+/HER2- breast cancer who are not candidates for curative surgery or radiotherapy.
3. Patients who had failed at least one line of systemic chemotherapy in unresectable locally advanced, recurrent, or metastatic stage.
4. Have at least 1 extracranial measurable lesion as a target lesion per RECIST 1.1.
5. Tumor tissue samples can be provided at the time of diagnosis of locally advanced or metastatic tumors.
6. Eastern Cooperative Oncology Group (ECOG) performance status score of 0 to 1.
7. Have Adequate organ and bone marrow function within 7 days prior to the first dose.
8. Female patients of childbearing potential must agree to use highly effective contraception from screening throughout the duration of the study and for at least 6 months after the last dose of study drug.
9. Have a expected survival ≥ 3 months.
10. Have ability and willingness to comply with protocol-specified visits and procedures.

Exclusion Criteria:

1. Have prior treatment with an agent targeting HER3.
2. Have prior treatment with topoisomerase I inhibitor or an ADC that consists of topoisomerase I inhibitor.
3. Have insufficient washout period for prior anticancer therapy prior to first dose of the study drug.
4. Have major surgery (excluding diagnostic surgery) within 4 weeks prior to the first dose of study drug or anticipation of major surgery during the study.
5. Leptomeningeal metastases or carcinomatous meningitis, spinal cord compression.
6. Have uncontrolled or clinically significant cardiovascular and cerebrovascular disease.
7. Have clinically significant concomitant pulmonary diseases.
8. Have uncontrolled pleural effusion, abdominal effusion.
9. Have serious infection within 4 weeks prior to the first dose.
10. Have a history of severe hypersensitivity reactions to the drug substance, inactive ingredients in the drug product, or other monoclonal antibodies.