A Study of RC48-ADC for the Treatment of Locally Advanced or Metastatic Breast Cancer With Low Expression of HER2

Inclusion Criteria: * Voluntarily agree to participate in the study and sign the informed consent; * Subjects aged 18-70 years (including 18 years and 70 years) and not reaching the 71st birthday were all considered to be ≤70 years old; * Expected survival ≥12 weeks; * Eastern Cooperative Oncology Group(ECOG) physical condition 0 or 1; * For female subjects of child-bearing age women agreed to study during treatment and experimental subjects within 6 months after the end of the treatment period using an approved by the medical contraception (e.g. intrauterine device, the pill or condoms), before the study drug delivery within 7 days of pregnancy blood test must be negative (sterilization surgery or age 60 or more subjects can choose no pregnancy blood test), and must be an lactation. For male subjects: should be sterilized surgically, or agree to use a medically approved contraceptive method during the study period and for 6 months after the end of the treatment period. Control subjects after the end of the treatment period according to the choice of control drugs to determine the length of contraception. * Able to understand the study requirements and be willing and able to follow the study and follow-up procedures. * Bone marrow function: hemoglobin ≥9g/dL; absolute neutrophil count ≥1.5×109/L; white blood cell count ≥3.0×109/L platelet ≥100 ×109/L; * Liver function (according to the normal value of the clinical trial center) : serum total bilirubin ≤1.5 times the upper limit of normal value (ULN); alanine aminotransferase (ALT), aspartate aminotransferase(AST) and Alkaline phosphatase（ALP） were ≤2.5 × ULN in the absence of liver metastasis, and ALT, AST and Alkaline phosphatase(ALP) were ≤5 × ULN in the presence of liver metastasis * Renal function (according to the normal value of the clinical trial center) : serum creatinine ≤1.5×ULN, or calculated by Cockcroft-Gault formula, the creatinine clearance rate (CrCl) ≥60 mL/min; * Cardiac function: American New York college of cardiology (NYHA) grade \< 3; left ventricular ejection fraction ≥50%; * Breast cancer subjects diagnosed by histology and / or cytology are currently at a locally advanced or metastatic stage and cannot be radically removed; * The low expression of HER2 confirmed by the IHC and FISH results of the central laboratory (defined as: IHC 2+ and no amplification of FISH); the subject can provide a specimen of the primary or metastatic tumor site (paraffin wax) for HER2 detection Block, paraffin-embedded section or fresh tissue section can be used); * Previous use of anthracycline drugs; * Received 1 or 2 systemic chemotherapy treatments after relapse / metastasis. Subjects who relapsed during adjuvant chemotherapy or within 12 months after the end of adjuvant chemotherapy were considered to have failed first-line chemotherapy after relapse / metastasis. * Hormone receptors are negative or positive. Hormone receptor-positive subjects need to progress after receiving endocrine therapy after relapse / metastasis or relapse after less than 2 years. Patients who are not suitable for endocrine therapy can be included in this study after undergoing chemotherapy treatment (first-line or second-line); * The imaging evidence confirmed by the investigator that the tumor disease progressed during or after the most recent treatment is required; * There has been no diagnosis of HER2 positive (HER2 IHC 3+ or FISH amplification) * Have not used drugs targeting HER2 (including antibodies, small molecule Tyrosine kinase inhibitor（TKIs） and antibody drug conjugates). * According to the RECIST 1.1 standard, there is at least one measurable lesion. Exclusion Criteria: * Received chemotherapy within 4 weeks before the start of study administration (treatment with nitrosourea and mitomycin C within 6 weeks, oral fluorouracil within 2 weeks), radiotherapy (palliative for bone metastases Local radiotherapy is within 2 weeks before study administration), immunotherapy; received endocrine therapy for breast cancer within 2 weeks before study administration; * The research drug was used within 4 weeks before the start of study administration; * Have undergone major surgery within 4 weeks before the start of study administration; * Have received a live vaccine within 4 weeks before the start of study administration or plan to receive any vaccine during the study period; * Serious cardiovascular and cerebrovascular events occurred within 12 months, including but not limited to unstable angina, myocardial infarction, cerebral hemorrhage, and cerebral infarction (except for asymptomatic and untreated lacunar infarction)； * Those who are suffering from heart disease are not suitable for enrollment, including but not limited to arrhythmia and heart failure requiring medical treatment or accompanied by symptoms; * There are other lung diseases requiring treatment or serious, including but not limited to active pulmonary tuberculosis, interstitial lung disease, etc ; * Suffering from active infection requiring systemic treatment; * Have active autoimmune diseases (such as the use of corticosteroids or immunosuppressive drugs, etc.) that require systemic treatment within the past 2 years, allowing related alternative treatments (such as thyroxine, insulin, or the physiology of adrenal or pituitary insufficiency Corticosteroid replacement therapy); * The toxicity of the previous anti-tumor therapy has not been restored to the 0 to 1 level defined by CTCAE version 5.0, of which the neurotoxicity has not been restored to 0; except for hair loss, pigmentation or other researches that do not increase the risk of medication Happening; * Have a clear past or current history of neurological or mental disorders, including epilepsy or dementia; * According to the investigator's judgment, there are concomitant diseases that seriously endanger the safety of the subject or affect the completion of the clinical study； * Positive HIV test results; patients with active hepatitis B or C (HBsAg positive and hepatitis B virus（HBV） DNA titers above the upper limit of normal; Hepatitis C Virus Antibody(HCVAb) positive hepatitis C virus （HCV） RNA titers above the upper limit of normal)； * There is a third interstitial fluid that cannot be controlled by drainage or other methods (including a large amount of pleural effusion or ascites); * Known hypersensitivity or delayed allergic reaction to certain components of RC48-ADC or similar drugs; * Subjects who are not suitable for using any of the alternative control drugs; * The presence of brain metastases and / or cancerous meningitis; * Have other malignant tumors within 5 years before signing the informed consent form (except for non-melanoma skin cancer, cervical carcinoma in situ or other tumors that have been effectively treated, except malignant tumors that are considered cured); * Subjects who are estimated to be inadequate for patients to participate in this clinical study or other factors that the investigator believes are inappropriate to participate in this study;