The purpose of this study is to evaluate the efficacy and safety of SKB264 in patients with unresectable locally advanced, recurrent, or metastatic HR+/HER2- breast cancer.

This is a randomized, open-label, multicenter Phase 3 clinical study to evaluate SKB264 monotherapy versus Treatment of Physician's Choice (TPC) in subjects with unresectable locally advanced, recurrent, or metastatic HR+/HER2- breast cancer who had failed at least one line of systemic chemotherapy.

Inclusion Criteria:

• Males or females aged ≥ 18 to ≤ 75 years at the time of signing the ICF;
• Histologically and/or cytologically confirmed HR+/HER2- breast cancer based on pathology reports on recent biopsy specimens or other pathological samples;
• Patients who had failed at least one line of systemic chemotherapy in unresectable locally advanced, recurrent, or metastatic stage;
• Patients with at least one measurable lesion per RECIST 1.1 criteria; those with only skin or bone lesions cannot be included;
• Eastern Cooperative Oncology Group (ECOG) performance status score of 0 to 1;
• Expected survival ≥ 12 weeks;
• Adequate organ and bone marrow function;
• Patients who are eligible for a chemotherapy regimen in the control group;
• Female patients of childbearing potential and male patients with partners of childbearing potential who use effective medical contraception from the time of signing the informed consent form until 6 months after the last dose;
• Patients who voluntarily participate in the study and sign the ICF, and able to comply with the visit and related procedures stipulated in the plan.

Exclusion Criteria:

• Patients with a history of central nervous system (CNS) metastases or current CNS metastases；
• Patients with other malignancies (except cured basal or squamous cell skin cancer or carcinoma in situ of the cervix) within 3 years prior to the first dose;
• Patients with any cardio cerebral Vascular disease or cardio cerebral vascular risk factors may affect investigational treatment;
• Uncontrollable systemic diseases assessed by the investigator;
• History of (noninfectious) interstitial lung disease (ILD)/noninfectious neumonitis requiring steroid therapy and current ILD/noninfectious pneumonitis, or ILD/noninfectious pneumonitis at screening that cannot be excluded by imaging;
• Clinically serious lung injuries caused by lung diseases;
• Patients with active chronic Inflammatory bowel disease, gastrointestinal obstruction, severe ulcer, gastrointestinal perforation, abdominal abscess or acute gastrointestinal bleeding;
• Toxicities from prior anti-tumor therapy not recovering to ≤ Grade 1;
• Active hepatitis B or hepatitis C;
• Human immunodeficiency virus (HIV) antibody test positive or a history of acquired immunodeficiency syndrome (AIDS), known active syphilis infection;
• Known allergy or hypersensitivity to SKB264, or the excipients of SKB264;
• Prior TROP2 targeted therapy, prior topoisomerase I inhibitor therapy, including antibody-drugconjugate(ADC) therapy;
• Patients who received major surgeries 4 weeks prior to the first dose of study treatment or planned to receive major surgeries during the study ;
• Patients with concomitant infections requiring systemic antibiotic therapy within 2 week prior to the first dose of study treatment;
• Patients who have received live vaccines within 30 days prior to the first dose, or are scheduled to receive live vaccines during the study;
• Pregnant or lactating women.