HER2 Expression of CTC to Predict Response in HER2-positive Advanced Breast Cancer Patients Treated With ADC

Inclusion Criteria:

• Has pathologically documented breast cancer that:

1. Is unresectable or metastatic
2. has confirmed HER2-positive expression defined as IHC 2+/ISH+ or IHC 3+
3. was previously treated with trastuzumab and taxane in the advanced setting orprogressed within 6 months after neoadjuvant or adjuvant treatment involving aregimen including trastuzumab and taxane

• Has adequate cardiac, bone marrow, renal, hepatic and blood clotting functions
• Receives anti-HER2 ADC treatment
• Life expectancy > 3 months
• Evidence of measurable or evaluable disease (RECIST 1.1 criteria)in the 28 days beforeselection
• ECOG ≤2

Exclusion Criteria:

• Has uncontrolled or significant cardiovascular disease
• Has history of (noninfectious) interstitial lung disease (ILD)/pneumonitis thatrequired steroids, has current ILD/pneumonitis, or suspected ILD/pneumonitis thatcannot be ruled out by imaging at screening
• Has any medical history or condition that per protocol or in the opinion of theinvestigator is inappropriate for the study