Trastuzumab Emtansine(T-DM1) After Trastuzumab Deruxtecan(T-DXd) in HER2-Positive Breast Cancer in Korea

Inclusion Criteria:

• HER2-positive, advanced (unresectable locally advanced or metastatic) breast cancer

• Prior treatment with trastuzumab deruxtecan(T-DXd) in the advanced setting

• Intention to treat with Trastuzumab Emtansine (T-DM1) as first therapy upon T-DXdprogression

Exclusion Criteria:

• Any systemic treatment, other than T-DM1, for breast cancer after trastuzumabderuxtecan (As an exception, patients treated with endocrine agents, not incombination with cytotoxics or antibody-drug conjugates, will be permitted in thestudy)

• Has been previously treated with T-DM1 for advanced breast cancer

• Has been previously treated with 4 or more lines of therapy for advanced breastcancer

• Half (50%) or more patients will be those who were treated with second-linetrastuzumab deruxtecan (i.e. patients at the later line will be capped).