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.