The Efficacy and Safety of Trastuzumab Rezetecan (SHR-A1811) in HER2-Low Unresectable/Metastatic Breast Cancer Patients With Visceral Crisis

Inclusion Criteria: * Age: ≥18 years old and ≤75 years old; * Pathologically confirmed HER2-Low unresectable or metastatic breast cancer; * Investigators assess that the patient is at risk of rapid disease progression and presents signs, clinical symptoms, or laboratory abnormalities of visceral metastasis, including：a）Hepatic crisis: Rapid elevation of bilirubin \> 1.5 × upper limit of normal (ULN) in the absence of Gilbert syndrome or biliary obstruction；b）Pulmonary crisis: Rapid exacerbation of dyspnea at rest, which is not relieved by pleural effusion drainage；c）Other visceral crises: Symptomatic visceral metastasis. * Patients whom investigators deem to have indications for single-agent chemotherapy. * Sufficient bone marrow function, defined as follows：a) Neutrophil count (ANC) ≥ 1,500/mm³ (1.5 × 10⁹/L)；b) Platelet count (PLT) ≥ 100,000/mm³ (100 × 10⁹/L)；c) Hemoglobin (Hb) ≥ 80 g/L； * Patients who have received ≤ 1 line of chemotherapy in the advanced stage are permitted. * Patients who have received endocrine therapy in the advanced stage are permitted. * Previous treatment with CDK4/6 inhibitors is permitted. * Previous or concurrent local treatment for symptom relief is permitted. * For female subjects who are premenopausal or not surgically sterilized: A serum pregnancy test must be performed within 7 days before the first dose of study drug, with a negative result. They must agree to either abstain from sexual activity or use a medically approved highly effective contraceptive method from the time of signing the informed consent form, throughout the study period, and for 1 year after the last dose of study drug. * Patients must voluntarily participate in this study, sign the informed consent form, have good compliance, and be willing to cooperate with follow-up. Exclusion Criteria: * Have received treatment with new antibody-drug conjugates (ADCs) at any stage of breast cancer. * Are deemed by the investigator as unsuitable for systemic chemotherapy. * Have had other malignant tumors within the past 5 years, excluding cured carcinoma in situ of the cervix, cutaneous basal cell carcinoma, or cutaneous squamous cell carcinoma (patients with other malignant tumors that occurred more than 5 years before randomization and were cured solely by surgery are eligible for enrollment). * Have undergone major surgical procedures or suffered significant trauma within 4 weeks prior to randomization, or are expected to undergo major surgical treatment. * Have severe heart disease or cardiac discomfort, including but not limited to the following conditions：a) A history of heart failure or systolic dysfunction (left ventricular ejection fraction \[LVEF\] \< 50%)；b) High-risk angina pectoris requiring treatment or cardiac arrhythmias；c) Clinically significant valvular heart disease；d) ECG findings indicating transmural myocardial infarction；e) Poorly controlled hypertension. * Have a known history of allergy to any component of the drugs in this protocol. * Have a history of immunodeficiency, including HIV infection, or other acquired or congenital immunodeficiency diseases. * Have a known history of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation. * Have a known history of psychotropic substance abuse or drug addiction. * Are pregnant or lactating women, or women of childbearing age who are unwilling to use effective contraceptive measures throughout the trial period and within 7 months after the last administration of the study drug. * Have severe diseases, other comorbidities that would interfere with the planned treatment, or any other conditions that make them unsuitable for participation in this study (e.g., active hepatitis B, pulmonary infection requiring treatment).