A Study of SKB264 Versus Investigator's Choice of Chemotherapy in Subjects With Unresectable Locally Advanced, Relapsed, or Metastatic HR+/HER2- Breast Cancer Who Have Previously Failed Endocrine Therapy

Inclusion Criteria: 1. Aged ≥ 18 and ≤ 75 years at the time of signing the ICF, male or female; 2. Histologically and/or cytologically confirmed HR+/HER2- BC based on pathological reports from the most recent biopsy or other pathological specimens; 3. Subjects must have radiologically documented disease progression during or after the most recent treatment prior to enrollment; 4. No prior systemic chemotherapy for locally advanced, relapsed, or metastatic stages. Subjects who previously received adjuvant/neoadjuvant chemotherapy and progressed \>6 months after completion of the last chemotherapy treatment will be allowed for study inclusion; 5. The investigator assessed that the patient could not continue to benefit from endocrine therapy and was suitable for receiving first-line chemotherapy; 6. Able to provide recently newly obtained or archival tumor tissue sections at or after diagnosis of relapsed or metastatic tumor within the recent prior to randomization; 7. At least one measurable lesion per RECIST v1.1； 8. Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 with no worsening within 2 weeks prior to randomization; 9. Life expectancy of ≥ 12 weeks; 10. Suitable to receive one of the chemotherapy regimens listed in the investigator's choice of chemotherapy (paclitaxel, nab-paclitaxel, capecitabine) as assessed by the investigator; 11. Adequate organ and bone marrow function; 12. Having recovered from all toxicities due to prior treatment; 13. Use of effective medical contraception during study treatment and for 6 months after the end of dosing for female subjects of childbearing potential and male subjects with partners of childbearing potential; 14. Willingness to participate in the study, sign the ICF, and comply with the protocol-specified visits and relevant procedures. Exclusion Criteria: 1. Subjects with locally advanced breast cancer suitable for curative therapy at study enrollment; 2. Other malignancies (except those tumors cured by local treatment, such as basal cell carcinoma of skin, squamous cell carcinoma of skin, carcinoma in situ of the cervix) within 3 years prior to randomization; 3. Subiects with known meningeal metastases, brainstem metastases, spinal cord metastases and/or compression or active centralnervous system (CNS) metastases. 4. Presence of any serious cardiovascular and cerebrovascular diseases or cardiovascular and cerebrovascular risk factors; 5. History of (noninfectious) interstitial lung disease (ILD)/noninfectious pneumonitis requiring steroid therapy and current ILD/noninfectious pneumonitis, or suspected ILD/noninfectious pneumonitis at screening that cannot be excluded by imaging; 6. Clinically serious lung injuries caused by lung diseases; 7. Serious infection within 4 weeks prior to randomization, including but not limited to complications requiring hospitalization, sepsis, or severe pneumonia; active infection requiring systemic anti-infective therapy within 2 weeks prior to randomization; 8. Documented severe dry eye syndrome, severe meibomian gland dysfunction and/or blepharitis, or history of severe corneal disorders that prevent/delay corneal healing; 9. History of esophagogastric varices, severe ulcers, gastric perforation, gastrointestinal obstruction, intra-abdominal abscess, or acute gastrointestinal bleeding within 6 months prior to randomization; 10. Active hepatitis B (hepatitis B surface antigen positive and HBV-DNA ≥ 500 IU/mL or above the ULN, whichever is higher) or hepatitis C (hepatitis C antibody positive and HCV-RNA above the ULN); 11. Positive result of human immunodeficiency virus (HIV) test or history of acquired immunodeficiency syndrome (AIDS); known active syphilis infection; 12. 12 Known hypersensitivity to SKB264 or investigator's choice chemotherapy or any of its excipients, including but not limited to polysorbate-20, or history of severe hypersensitivity reaction to other monoclonal antibodies; 13. Known history of allogeneic organ transplantation and allogeneic hematopoietic stem cell transplantation. 14. Pregnant or lactating women; 15. Prior TROP2-targeted therapy or any treatment containing chemotherapeutic agents targeting topoisomerase I (including antibody-drug conjugates \[ADCs\]); 16. Live vaccines within 4 weeks prior to randomization or scheduled to receive live vaccines during study treatment; 17. Receipt of the following therapies prior to randomization: a)Major surgery within 4 weeks prior or expected major surgery during the study; b)Radiation therapy within 2 weeks prior (extensive radiation therapy including radiopharmaceuticals within 4 weeks prior); c)Any immunotherapy, biological therapy, or other investigational drugs within 4 weeks or 5 half-lives of prior drug use (whichever is shorter) (bisphosphonates or RANK-L inhibitors for bone metastases are permitted prior to randomization); or traditional Chinese medicine with approved anti-tumor indications, small molecule targeted therapy, or endocrine therapy within 2 weeks prior. 18. Rapid deterioration of the condition, e.g., significant changes in performance status, etc., during the screening process.