This is a randomized, parallel-controlled, open-label, multicenter clinical study to assess the efficacy and safety of D-0502 in the treatment of subjects with ER-positive, HER2-negative locally advanced or metastatic breast cancer with fulvestrant injection as a control drug.

Inclusion Criteria:

• Subjects pathologically confirmed with ER-positive, HER2-negative breast cancer;
• Subjects with locally advanced (unresectable) or metastatic breast cancer who have disease recurrence during or after adjuvant endocrine therapy, or whose disease has progressed after 1-2 lines of systemic endocrine therapy;
• Presence of at least 1 measurable lesion that can be measured by CT or MRI based on RECIST V1.1 criteria; in the absence of measurable lesions, subjects with evaluable bone lesions [osteolytic or mixed (osteolytic + osteogenic) bone lesions] are also acceptable. Lesions that have been previously treated with radiotherapy or other local therapy can be regarded as measurable lesions only if there is disease progression as confirmed by imaging examination;
• Expected survival time ≥ 12 weeks;

Exclusion Criteria:

• Subjects with unstable or symptomatic or progressive central nervous system (CNS) metastases. Subjects with a history of brain metastases who are clinically stable and have no CNS disease progression confirmed by brain MRI or CT (if MRI is not appropriate) can be enrolled (MRI or CT examination must be conducted at least 4 weeks after the last brain radiotherapy);
• Subjects with locally advanced or metastatic breast cancer who have previously received more than 2 prior systemic chemotherapy;
• Subjects are unsuitable for endocrine therapy judged by the investigator, including uncontrolled pleural effusion, ascites or pericardial effusion;
• Subjects with concomitant medical conditions that the investigator believes may increase the risk of toxicity, such as serious cardiovascular, respiratory or neurological diseases;
• Prior treatment with a selective estrogen receptor degrader (SERD)/selective estrogen receptor covalent antagonist (SERCA), such as fulvestrant, GDC-9545, AZD9833, SAR-439859, Zn -c5, LX-039, HS234, etc.;
• Pregnant or lactating females;