Disitamab Vedotin (RC48) in Hormone Receptor Positive, HER2-low Metastatic Breast Cancer (the Rosy Trial)

Inclusion Criteria:

1. Adult female patients (aged 18-70 years, including 18 and 70 years) with metastaticbreast cancer confirmed by pathology or imaging are not suitable for surgicalresection or radiotherapy for the purpose of cure;
2. Pathological examination confirmed that ER and / or PR were positive, and HER-2 waslow expression (ER expression: immunohistochemical staining of tumor cells ≥ 10%; PRexpression: immunohistochemical staining of tumor cells ≥ 10%; HER2-low:immunohistochemical staining of 2 + and FISH is not expanded, IHC 1 +);
3. Patients who have received endocrine therapy ;
4. According to the efficacy evaluation criteria for solid tumors (RECIST) version 1.1,there is at least one evaluable target lesion or only osteolytic bone metastasis;
5. Patients with stable brain metastasis or asymptomatic brain metastasis;
6. ECOG physical condition score ≤ 2 points, and the estimated survival time is not lessthan 3 months;
7. Prior treatment-related toxicity must be relieved to ≤ 1 degree (according to NCICTCAE 5.0) before enrollment (except for hair loss or other toxicity that isconsidered as no risk to the safety of patients according to the judgment of theresearcher);
8. Adequate bone marrow functional reserve: a. WBC ≥ 3.0 × 10 ^ 9 ／ L, b. Neutrophilcount (ANC) ≥ 1.5 × 10 ^ 9 ／ L, c. Platelet count (PLT) ≥ 70 × 10^9／L;
9. Liver, kidney and heart function tests were basically normal within one week beforeenrollment (based on the normal values of laboratories in each research center): A.total bilirubin (TBIL) ≤ 1.5 × upper limit of normal value (ULN), B. alanineaminotransferase (ALT / AST) ≤ 2.5 × ULN (liver metastasis patients ≤ 5xuln), C. serumcreatinine ≤ 1.5 × ULN or creatinine clearance rate (CCR) ≥ 60 ml / min; d. leftventricular ejection fraction (LVEF) ≥ 55%, e. QTcF(Fridericia correction) ≤ 470 ms;
10. Patients understand the research process, voluntarily participate in the research, andsign the informed consent form.

Exclusion Criteria:

1. Patients who had received chemotherapy, radiotherapy, immunotherapy, and endocrinetherapy for breast cancer within 2 weeks before enrollment.;
2. Patients who had performed major surgery within 2 weeks before enrollment.
3. Severe heart disease or discomfort within 12 months, including, but not limited to,the following: unstable angina pectoris, myocardial infarction, cerebral hemorrhageand cerebral infarction (except for silent lacunar cerebral infarction withouttreatment);
4. Have active autoimmune diseases (such as corticosteroids or immunosuppressive drugs)requiring systemic treatment in the past 2 years, excluding those with adrenalinsufficiency requiring corticosteroid replacement therapy;
5. Have a clear history of neurological or mental disorders, including epilepsy ordementia;
6. According to the judgment of the researchers, there are some accompanying diseasesthat seriously endanger the safety of patients or affect patients to complete thestudy.
7. Those who have been known to have allergic history to Disitamab Vedotin or similardrugs;
8. According to the estimation of the investigator , the patient's compliance with theclinical study is insufficient or the researcher believes that there are other factorsthat are not suitable for the study.