Maintenance Therapy in HER2-Positive Unresectable Locally Recurrent or Metastatic Breast Cancer: A Phase II Study

Inclusion Criteria: To be eligible to participate in this trial, an individual must meet ALL the following criteria: 1. Patient must be capable to understand the purpose of the study and have signed written informed consent form (ICF) prior to beginning specific protocol procedures. 2. Female patients ≥ 18 years of age at the time of signing ICF. 3. Breast Cancer Requirements: 1. Histologically or cytologically confirmed HER2-positive(IHC 3+ or ISH+) unresectable locally advanced or metastatic breast cancer (Note: Patients eligible for curative-intent treatment are excluded). 2. Documented hormone receptor (HR) status. 3. No prior systemic anti-tumor therapy for recurrent/metastatic disease (≤1 line of endocrine therapy is permitted\*). 4. For patients who received (neo)adjuvant therapy: \>12 months between the end of systemic treatment (excluding endocrine therapy) and recurrence/metastasis. 4. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1. 5. Adequate Organ Function. 6. Pregnancy and Contraception: Women of childbearing potential (WOCBP) must agree to use highly effective contraception from screening until7 months after the last dose of study treatment and must refrain from breastfeeding. Exclusion Criteria: An individual who meets ANY of the following criteria will be excluded from participation in this trial: 1. Known active CNS metastases not treated with surgery or radiotherapy, except for those who have achieved stable disease for ≥1 month after treatment and have discontinued corticosteroids for \>2 weeks. 2. Received major cancer-related surgery, radiotherapy, chemotherapy, immunotherapy, molecular targeted therapy, biologic therapy, or investigational drug therapy within 4 weeks prior to the first dose of study treatment. 3. Previous treatment with antibody-drug conjugates containing exatecan derivative topoisomerase I inhibitors. 4. Presence of clinically significant cardiovascular conditions, including: 1. Severe/unstable angina 2. Symptomatic congestive heart failure (NYHA class ≥II) 3. Clinically significant supraventricular or ventricular arrhythmias requiring treatment or intervention 4. Myocardial infarction within 6 months before the first dose 5. Has a history of non-infectious interstitial lung disease (ILD)/pneumonitis that required steroids, has current ILD/pneumonitis, or has suspected ILD/pneumonitis that cannot be ruled out by imaging at screening. 6. Known substance abuse or any other concurrent severe and/or uncontrolled medical condition that would, in the investigator's judgment, contraindicate patient participation.