To evaluate whether serial 68Ga-MY6349 PET/CT imaging can serve as a noninvasive biomarker to predict the therapeutic efficacy of Trop2-targeted antibody-drug conjugate (Trop2-ADC) therapy in breast cancer patients.

Patients diagnosed with metastatic breast cancer will undergo 68Ga-MY6349 PET/CT imaging at baseline and after two cycles of Trop2-ADC therapy. Tumor uptake will be quantified using both maximum standardized uptake value (SUVmax) and mean SUV (SUVmean). Imaging changes will be correlated with treatment response to assess the utility of 68Ga-MY6349 PET/CT as a predictive biomarker.

Inclusion Criteria:

• adult patients (aged 18 years or order)
• histologically or cytologically confirmed metastatic breast cancer previously treated with systemic therapy, supported by imaging (e.g., MRI, CT), tumor markers, or pathology reports
• presence of at least one measurable lesion according to the Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1
• willingness to undergo serial 68Ga-MY6349 PET/CT scans before and during Trop2-ADC therapy
• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1
• Ability to provide written informed consent and, where applicable, assent in accordance with the requirements of the Clinical Research Ethics Committee

Exclusion Criteria:

• Evidence of significantly impaired hepatic or renal function
• Estimated life expectancy of less than 3 months

Adult female patients with previously treated metastatic breast cancer (supporting evidence may include magnetic resonance imaging (MRI), CT, tumor markers and pathology report).