This is a Phase II Study of HLX22 in Combination with Trastuzumab Deruxtecan (T-DXd) in HER2-low, Hormone Receptor Positive Locally Advanced or Metastatic Breast Cancer Patients with Disease Progression or Intolerable Adverse Reaction 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).

experimental group: HLX22(15 mg/kg) + Trastuzumab Deruxtecan( 5.4 mg/kg), once every 3 weeks (Q3W).

Inclusion Criteria:

1. Male/female who are at least 18 years of age on the day of signing the informed consent.
2. With histologically confirmed diagnosis of HER2-low, Hormone Receptor Positive Locally Advanced or Metastatic Breast Cancer.
3. HER2-low defined as IHC 2+/ISH- or IHC 1+ (ISH- or untested) as assessed by a central laboratory on metastatic tumor and was never previously HER2-positive，and was documented HR+ disease in the metastatic setting
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. Previous treatment with any HER2-target therapy
3. Uncontrolled or significant cardiovascular disease or infection
4. Lung-specific intercurrent clinically significant illnesses
5. Prior documented interstitial lung disease (ILD)/ pneumonitis that required steroids, current ILD/ pneumonitis, or suspected ILD/ pneumonitis
6. Patients with spinal cord compression or clinically active central nervous system metastases