HER2-PET: Predicting T-DXd Efficacy and HER2 Heterogeneity

Inclusion Criteria: 1. Aged ≥ 18 years. 2. Histologically confirmed unresectable locally advanced or metastatic breast cancer, with immunohistochemistry (IHC) results indicating HER2-positive (IHC 3+; or IHC 2+ and FISH-positive) or HER2-low expression (IHC 1+; or IHC 2+ and FISH-negative). 3. Prior treatment meeting one of the following criteria: 3.1 For HER2-positive breast cancer, patients must have received at least one prior anti-HER2 targeted therapy. 3.2 For HER2-low breast cancer, patients must have received at least one prior line of systemic therapy for metastatic disease, or have relapsed within 6 months during or after completion of adjuvant chemotherapy. 4. ECOG Performance Status of 0 or 1. 5. Evidence of radiographic or objective disease progression at or after the last systemic therapy prior to initiating study treatment. 6. Anticipated life expectancy ≥ 12 weeks at screening. 7. At least one measurable lesion that has not been previously irradiated, with a longest diameter ≥ 10 mm as accurately measured by CT or MRI at baseline (except for lymph nodes, which must have a short axis ≥ 15 mm). Alternatively, if only bone lesions are present, evaluable osteolytic or mixed osteolytic/blastic bone lesions as assessed by CT, MRI, or X-ray are acceptable. 8. Left ventricular ejection fraction (LVEF) ≥ 50% within 28 days prior to randomization. 9. Adequate organ and bone marrow function within 14 days prior to randomization. For all parameters listed below, the most recent results must be used to meet the inclusion criteria: 1. Hemoglobin ≥ 9 g/dL; 2. Absolute Neutrophil Count (ANC) ≥ 1500/mm3; 3. Platelet count ≥ 100,000/mm3; 4. Total bilirubin (TBL) ≤ 1.5 × Upper Limit of Normal (ULN) at baseline in the absence of liver metastases; or \< 3 × ULN in the presence of Gilbert's syndrome (unconjugated hyperbilirubinemia) or liver metastases; 5. ALT and AST ≤ 3 × ULN; or \< 5 × ULN for patients with liver metastases; 6. Serum albumin ≥ 2.5 g/dL; 7. Creatinine clearance ≥ 30 mL/min (calculated using the Cockcroft-Gault formula); 8. International Normalized Ratio (INR) or Prothrombin Time (PT), and Activated Partial Thromboplastin Time (aPTT) ≤ 1.5 × ULN. 10. Female patients must not donate eggs or collect eggs for personal use from the screening period throughout the study treatment period and for 7 months after the last dose of study treatment. Breastfeeding should be avoided during this period. If oocyte preservation is desired, it should be considered prior to randomization in this study. Exclusion Criteria: 1. Uncontrolled concurrent diseases, including but not limited to: persistent or active infections, uncontrolled or significant cardiovascular disease, severe chronic gastrointestinal disease with diarrhea, or psychiatric/social conditions that may limit compliance with study requirements, significantly increase the risk of adverse events (AEs), or impair the patient's ability to provide written informed consent. 2. Uncontrolled or significant cardiovascular disease, including any of the following: 1. History of myocardial infarction or symptomatic congestive heart failure (CHF) (NYHA Class II to IV) within 6 months prior to randomization. Patients with troponin levels above the Upper Limit of Normal (ULN) (as defined by the manufacturer) at screening without any symptoms related to myocardial infarction should undergo cardiac consultation prior to randomization to rule out myocardial infarction; 2. Uncontrolled hypertension; 3. Uncontrolled and/or clinically significant arrhythmias. 3. History of (non-infectious) interstitial lung disease (ILD)/pneumonitis requiring steroid treatment, current ILD/pneumonitis, or suspected ILD/pneumonitis that cannot be ruled out by imaging at screening. 4. Patients who have used immunosuppressive agents within 14 days prior to the first dose of study drug, with the exception of those using intranasal and inhaled corticosteroids, or systemic corticosteroids at a dose less than 10 mg/day prednisone or an equivalent dose. 5. Clinically significant pulmonary-specific comorbidities, including but not limited to any underlying pulmonary disease (e.g., pulmonary embolism within three months prior to randomization, severe asthma, severe chronic obstructive pulmonary disease \[COPD\], restrictive lung disease, significant pleural effusion, etc.) and any autoimmune, connective tissue, or inflammatory disease with associated pulmonary involvement (e.g., rheumatoid arthritis, Sjögren's syndrome, sarcoidosis, etc.), and/or prior lung resection. 6. Uncontrolled infection requiring intravenous antibiotics, antiviral, or antifungal medications. 7. Spinal cord compression or clinically active central nervous system (CNS) metastases, defined as untreated and symptomatic, or requiring corticosteroid or anticonvulsant treatment to control associated symptoms. Subjects with clinically non-active brain metastases may be included in this study. If subjects have received treatment for brain metastases, are no longer symptomatic, do not require corticosteroids or anticonvulsants, and have recovered from acute toxicities of radiotherapy, they may be enrolled in this study. 8. Active primary immunodeficiency, known Human Immunodeficiency Virus (HIV) infection, or active Hepatitis B or C infection. Among patients who are Hepatitis C antibody positive, only those with a negative polymerase chain reaction (PCR) for HCV RNA are eligible for study enrollment. 9. Unresolved toxicities from prior anti-cancer therapy, defined as toxicities not resolved to ≤ Grade 1 or baseline (excluding alopecia). Note: Subjects with chronic, stable Grade 2 toxicities (defined as not worsening to ≥ Grade 2 for at least 3 months prior to enrollment and manageable with standard treatment) that are deemed by the investigator to be related to prior anti-cancer therapy may be included, e.g., chemotherapy-induced neuropathy or fatigue; residual toxicity from prior immunosuppressive therapy: Grade 1 or 2 endocrine disorders. 10. Pregnant or lactating female patients, or patients planning pregnancy. Known history of severe hypersensitivity reaction to the active pharmaceutical ingredient (API), excipients in the drug formulation, or other monoclonal antibodies. 11. History of another primary malignancy within 3 years, with the exception of adequately treated non-melanoma skin cancer, cured in situ disease, other cured solid tumors, or contralateral breast cancer. 12. Substance abuse or any other medical condition that, in the opinion of the investigator, might interfere with the patient's participation in the clinical study or the evaluation of the clinical study results, e.g., psychiatric disorders.