The study is being conducted to evaluate the efficacy and safety of HRS-8080 combined with dalpiciclib versus fulvestrant combined with dalpiciclib in patients with locally advanced/metastatic breast cancer who had developed drug resistance to prior adjuvant endocrine therapy.

Inclusion Criteria:

1. Women aged 18 - 75 years old;
2. Eastern Cooperative Oncology Group performance status (ECOG-PS) of 0 to 1;
3. Patients with histologically confirmed locally advanced or metastatic breast cancer;
4. Patients with prior adjuvant endocrine resistance following curative-intent surgery;
5. Menstrual status: postmenopausal, perimenopausal, or premenopausal;
6. Presence of evaluable lesions;
7. Organ function must meet required criteria.

Exclusion Criteria:

1. Patients with rapidly progressing disease, as judged by the investigator to be unsuitable for endocrine therapy;
2. Patients who have previously received fulvestrant or other novel SERMs (excluding tamoxifen and toremifene);
3. Patients with uncontrolled brain metastases, carcinomatous meningitis, or spinal cord compression;
4. Patients with a history of clinically significant cardiovascular disease;
5. Participants who have not recovered from adverse effects caused by prior therapies;
6. Participants with a history of another malignancy within the past 5 years or currently having another malignancy;
7. Known hypersensitivity to HRS-8080, fulvestrant, dalpiciclib, or any of their components, etc.