Trastuzumab Emtansine (T-DM1) in HER2-positive Breast Cancer Patients With Progressive Disease After TKIs or HP Therapy

This trial is studying the drug T-DM1 (trastuzumab emtansine, a targeted therapy) in women with HER2-positive breast cancer whose disease got worse after receiving other targeted treatments such as tyrosine kinase inhibitors (TKIs) or a combination of trastuzumab and pertuzumab (HP therapy). **You may be eligible if:** - You are a woman aged 18 or older with HER2-positive invasive breast cancer - Your cancer progressed after anti-HER2 targeted therapy (TKIs or HP) either for stage IV disease or within one year of completing adjuvant treatment - You have at least one measurable lesion - Your blood counts, liver, and kidney function are within acceptable ranges - Your heart function is normal (LVEF at least 55%) **You may NOT be eligible if:** - You are male or have inflammatory breast cancer - You have had another cancer in the past 5 years (except certain skin or cervical in-situ cancers) - You are currently on another cancer treatment or clinical trial - You have previously used ADC (antibody-drug conjugate) drugs - You have heart disease above NYHA class 2 - You have dementia, cognitive problems, or a mental illness that prevents giving informed consent Talk to your doctor to see if this trial is right for you.