This is a Phase II Study of HLX43 in Hormone Receptor Positive，HER-2 negative Locally Advanced or Metastatic Breast Cancer Patients with Disease Progression on Standard of Care. Eligible subjects will be treated with the study drug until the loss of clinical benefit, death, intolerable toxicity, withdrawal of informed consent, or other reasons specified by the protocol (whichever occurs first)

Inclusion Criteria:

1. Male/female who are at least 18 years of age and and no more than 75 years old on the day of signing the informed consent.
2. With histologically confirmed diagnosis of HER2-negative, Hormone Receptor Positive Locally Advanced or Metastatic Breast Cancer.
3. Must have experienced disease progression after receiving at least one line of endocrine therapy for advanced or metastatic disease.
4. Had measurable disease according to the RECIST v1.1, the target lesion must not be a bone metastatic lesion only.
5. ECOG PS: 0-1.
6. Expected survival ≥ 6 months.
7. Had adequate organ function

Exclusion Criteria:

1. Patients with other malignant tumors within 3 years before the randomization
2. Prior documented interstitial lung disease (ILD)/ pneumonitis that required steroids, current ILD/ pneumonitis, or suspected ILD/pneumonitis
3. Lung-specific intercurrent clinically significant illnesses
4. Uncontrolled or significant cardiovascular disease or infection
5. Prior adverse events leading to permanent discontinuation of immunotherapy; or prior immune-related pneumonia or immune-related myocarditis of grade ≥2.