Inclusion Criteria:
\- Age ≥ 18 years; ER/PgR expression ≤10% and low HER2 expression (IHC 1+, or IHC 2+ and ISH-); Advanced breast cancer patients; Prior treatment with ≥1 line of therapy for localized recurrence or metastatic disease; At least one prior treatment with taxane or anthracycline therapy; Allowed to receive immunotherapy during neoadjuvant or adjuvant stages; Measurable lesions according to RECIST 1.1 standards; ECOG PS score: 0-1; Voluntary participation with good compliance; Negative pregnancy test result, childbearing-age participants commit to effective contraception from the study start to 6 months after the last dose; Adequate organ function; Blood routine: ANC ≥1.5×109/L, PLT≥70×109/L, HGB ≥90g/L; Liver function: TBIL ≤1.5×ULN, ALT and AST ≤3×ULN, serum albumin ≥28 g/L, ALP ≤5×ULN; Stable liver function for at least 1 week after routine liver protection treatment, as assessed by the investigator, is eligible for inclusion; Renal function: Cr ≤1.5×ULN, or CrCl ≥50 mL/min (using standard Cockcroft-Gault formula); Coagulation function: INR ≤1.5/PT ≤1.5×ULN, aPTT ≤1.5×ULN; Left ventricular ejection fraction ≥50%
Exclusion Criteria:
\- Active central nervous system metastases or carcinomatous meningitis (stable brain metastasis patients are allowed to participate); Prior treatment with anti-HER2 ADC drugs; Active autoimmune disease or history of autoimmune disease (including but not limited to: autoimmune hepatitis, interstitial pneumonia, uveitis, enteritis, hepatitis, hypophysitis, vasculitis, nephritis, hyperthyroidism; Patients with vitiligo; Childhood asthma completely resolved in adulthood without any intervention can be included, but asthma requiring bronchodilators for medical intervention in adulthood cannot be included); Patients using immunosuppressants or systemic steroid therapy for immunosuppressive purposes (dose\>10mg/day prednisone or equivalent) in the 2 weeks before inclusion; Patients with other malignant tumors in the past or simultaneously; Grade ≥4 adverse reactions after ADC drug treatment, grade ≥3 immune-related adverse reactions; Known allergy to drug components used; Patients with known interstitial pneumonia; Poorly controlled clinical heart symptoms or diseases, such as: (1) NYHA class 2 or above heart failure; (2) unstable angina; (3) myocardial infarction within the past year; (4) patients with clinically significant ventricular or supraventricular arrhythmias requiring treatment or intervention; Active infection or unexplained fever \>38.5°C during the screening period or before the first dose of study medication, depending on the investigator's judgment, patients with fever due to tumor can be included; Not receiving live vaccines within less than 4 weeks before or during the study; Patients known to have a history of psychiatric drug abuse, alcoholism, or drug addiction; Factors that may lead to the premature termination of the study judged by the investigator, such as other serious diseases (including mental illness) requiring concomitant treatment, severe laboratory abnormalities, family or social factors affecting patient safety, data and sample collection
