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

Inclusion Criteria: * Age ≥ 18 years; pregnancy test (-) for premenopausal and perimenopausal patients, promising to use reliable contraception during treatment. * Patients who were be diagnosed with invasive breast cancer according to the eighth edition of American Joint Committee on Cancer (AJCC) staging system, and develop disease progression after anti-HER2 therapy (TKIs) for stage IV disease at initial diagnosis or within one year of adjuvant anti-HER2 therapy (HP) after surgery for early breast cancer. * At least one measurable lesion according to RECIST 1.1. * ECOG score of 0 or 1. * The organ function is still good and meets the following indicators: hemoglobin ≥ 90g/L, white blood cell ≥ 3.5×10\^9/L, platelet ≥ 100×10\^9/L, neutrophil ≥ 1.5×10\^9/L, aspartate aminotransferase or alanine aminotransferase ≤ 3×ULN, total bilirubin ≤ 1.5×ULN, serum creatinine value ≤ 1.5×ULN. * Without myocardial ischemia in ECG. * NYHA grade I; Echocardiography LVEF ≥55%; Cardiac markers: cardiac troponin (cTnI) and brain natriuretic peptide (BNP) within normal range. * Complete all necessary baseline laboratory and radiological tests prior to treatment. * Complete clinical data. Exclusion Criteria: * male breast cancer or inflammatory breast cancer. * Patients who have other malignant tumors or have contracted malignant tumors other than breast cancer in the past 5 years, except for basal cell carcinoma of the skin or flat cell carcinoma and carcinoma in situ of the cervix that have been adequately treated and controlled. * Accompanying other anti-tumor treatments or participating in other clinical trials. * Serious diseases that will affect the patient's compliance or put the patient at risk. * Major surgical procedures performed within 4 weeks prior to the commencement of study treatment or anticipated major surgical procedures during the course of the study. * Patients who have used ADC drugs at present or before this study. * History of allergic reactions or contraindications to use of any drug ingredient in this study. * Patients with chronic diarrhea and intestinal obstruction, as well as other diseases that affect drug administration and absorption. * Patients who have clinical cardiac symptoms or diseases that are not well controlled, such as: heart failure above NYHA 2; unstable angina; myocardial infarction occurred within one year; clinically significant supraventricular or ventricular arrhythmias requiring treatment or intervention. * Dementia, intellectual abnormality, or any mental illness that interferes with the understanding of informed consent.