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; * Visceral crisis is defined as severe organ dysfunction as assessed by signs, symptoms and laboratory studies, resulting from rapid progression of neoplastic disease and indicative of substantial visceral compromise that may serve as an indication for more aggressive therapeutic intervention, including：a）Liver visceral crisis: rapidly increasing bilirubin \>1.5 × ULN in the absence of Gilbert's syndrome or biliary tract obstruction；b）Lung visceral crisis: rapidly increasing dyspnoea at rest, not alleviated by drainage of pleural effusion；c）Other visceral crises: the evaluation is conducted by the clinician based on the definition of visceral crisis specified in the ABC7 guideline. * 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) ≥ 80 ×10\^9/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 b…