A Clinical Trial Comparing the Efficacy and Safety of Different Doses of BL0175 Injection in Treating Postmenopausal Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Locally Advanced or Metastatic Breast Cancer

The goal of this clinical trial is to compare the objective response rates (ORR) of different doses of BL0175 in patients with postmenopausal hormone receptor (HR)-positive, human epidermal growth factor receptor 2 (HER2)-negative locally advanced or metastatic breast cancer. The main questions it aims to answer are: 1. Which dose level of BL0175 demonstrates the optimal ORR? 2. What is the recommended phase 3 dose (RP3D) based on efficacy and safety data across the tested dose levels?

Inclusion Criteria: 1. Voluntary participation in the trial, understanding the specific procedures of the trial, and willing to sign a written informed consent form. 2. Age ≥ 18 years. 3. HR positive, HER2 negative, with locally advanced or metastatic breast cancer in postmenopausal patients during or after endocrine therapy. 4. The blood pregnancy test result during the screening period must be negative (if the postmenopausal state is caused by GnRH agonists). 5. If the postmenopausal state is caused by GnRH agonists, the trial participants must agree to take effective contraceptive measures from the time of enrollment until at least 2 years after the last administration of the study treatment. For patients with breast cancer, the use of estrogen-based hormonal contraceptive methods (including hormone-type intrauterine devices) is not allowed, while the efficacy of progestin-based hormonal contraceptive methods is currently unclear. Effective contraceptive measures include: a. Complete abstinence (if it is their preferred regular lifestyle); b. Intrauterine device; c. Bilateral tubal ligation; d. The partner has undergone vasectomy and confirmed no sperm; e. Dual barrier method (male condom combined with cervical cap, vaginal diaphragm, or contraceptive sponge in combination with spermicidal agent). 6. At least one measurable lesion exists. 7. The Eastern Cooperative Oncology Group (ECOG) status score at screening must be ≤ 1. 8. Life expectancy ≥ 12 weeks. Exclusion Criteria: 1. Participants with symptomatic central nervous system (CNS) metastases or cancerous meningitis; 2. Have a history of other primary malignant tumors (except for participants who have been cured of skin basal cell carcinoma, skin squamous cell carcinoma, or cervical carcinoma in situ, and participants with other primary tumors that have no evidence of disease for 2 years or more and do not require treatment). 3. Patients whose pericardial effusion, pleural effusion or ascites remain uncontrollable after intervention. 4. Participants who have a history of allogeneic transplantation (including but not limited to allogeneic organ, bone marrow, or stem cell transplantation). 5. A history of allergic to fulvestrant, or prone to allergic reactions (such as prone to angioedema, urticaria, asthma, rash, etc.). 6. Severe cardiovascular or cerebrovascular diseases, including: Heart failure classified as New York Heart Association (NYHA) functional class III-IV (assessment only for patients with a history of heart disease), or left ventricular ejection fraction (LVEF) \ 480 ms, or congenital long QT syndrome; Myocardial infarction, severe or unstable angina, congestive heart failure, cerebrovascular accident (including transient ischemic attack), symptomatic pulmonary embolism, or other clinically significant thromboembolic events occurring within 6 months prior to the first administration of the investigational drug, or coronary artery bypass graft surgery performed within 6 months prior to the first administration of the investigational drug; Clinically symptomatic bradycardia as assessed by the investigator; Other clinically significant cardiovascular diseases that, in the opinion of the investigator, make the patient unsuitable for participation in the trial. 7. Human immunodeficiency virus (HIV) infection or positive HIV antibody test at screening. 8. Active hepatitis B (HBV DNA ≥2000 IU/mL) or active hepatitis C (HCV RNA ≥200 IU/mL). Additionally, eligible participants with hepatitis B or C must agree to receive antiviral treatment according to established guidelines; otherwise, they will not be enrolled. 9. Active infection requiring intravenous antibiotic therapy within 1 week prior to the first administration of the investigational drug. 10. Moderate or severe hepatic impairment, defined as meeting Child-Pugh classification criteria B or C. 11. Insufficient organ function reserve at baseline, as defined by any of the following criteria (no blood products \[including platelets or red blood cells\] or colony-stimulating factors \[including granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, or recombinant erythropoietin\] administered within 7 days prior to testing): Absolute neutrophil count (ANC) \1.5×ULN; ALT or AST \>3×ULN if no liver metastasis is present; ALT or AST \>5×ULN if liver metastasis is present; Hemoglobin (Hb) \