Molecular targeted therapy for breast cancer is a treatment method that uses specific genes or gene expression products (proteins, etc.) that are overexpressed in breast cancer cells but not expressed or under-expressed in normal cells as the targets of drug action, and inhibits or kills tumor cells to a greater extent and causes less harm to normal cells through the combination of drugs and these specific action sites. 1. What is the relationship between human epidermal growth factor receptor 2 and breast cancer? The amplification of human epidermal growth factor receptor 2 gene (HER2 gene) is associated with increased cell differentiation, migration, tumor invasion, local and distant metastasis and other biological activities. Domestic and international studies have found that about 30% of breast cancer patients have HER2 gene amplification and overexpression, and its overexpression is closely associated with the development of breast cancer. Several domestic and international clinical trials have confirmed the feasibility and effectiveness of using this expression site as a target for breast cancer treatment. 2.What are the commonly used drugs for breast cancer targeted therapy? The commonly used drugs for targeted breast cancer treatment include: trastuzumab, patuximab, lapatinib, T-DM1 and so on. Trastuzumab is the first-line drug for targeted breast cancer treatment in China. 3.What is the method of trastuzumab treatment? When trastuzumab is used in combination with chemotherapy drugs, weekly or once every 3 weeks treatment regimen can be adopted. The first dose of trastuzumab in the weekly regimen is 4mg/kg, followed by 2mg/kg every week; the first dose of trastuzumab in the 3-weekly regimen is 8mg/kg, followed by 6mg/kg every 3 weeks, and the duration of treatment is 1 year. 4.What are the adverse effects during trastuzumab treatment? The most common adverse reactions of trastuzumab used in breast cancer treatment are: fever, nausea, vomiting, infusion reaction, diarrhea, infection, cough aggravation, headache, malaise, dyspnea, rash, neutropenia, anemia and myalgia. Adverse reactions requiring interruption or discontinuation of trastuzumab therapy include: congestive heart failure, significant decrease in left ventricular function, severe infusion reactions and pulmonary toxicity. 5.What are the precautions during trastuzumab treatment? The most important thing that we should pay attention to during trastuzumab treatment in China is the cardiotoxicity of trastuzumab. (1) Adequate assessment of the patient’s cardiac function (history, physical examination, ECG, cardiac ultrasound, etc.) and recording of the left ventricular ejection fraction (LVEF) should be performed before treatment. (2) During trastuzumab treatment, cardiac ultrasound was repeated every 3 months to assess LVEF. (3) After the end of trastuzumab treatment, it is recommended to review cardiac ultrasound every 6 months (within 2 years of the end). (4) Trastuzumab treatment should be stopped for at least 4 weeks and LVEF should be measured every 4 weeks if: ① LVEF decreases by ≥ 16% in absolute terms from pre-treatment values. (ii) LVEF is below the normal range of the test center and LVEF has decreased by ≥10% in absolute terms from the pretreatment value. ③Trastuzumab may be resumed if LVEF returns to the normal range within 4-8 weeks or if LVEF decreases by ≤15% in absolute terms from pre-treatment values. (iv) Trastuzumab should be permanently discontinued if LVEF continues to decline (>8 weeks) or if trastuzumab therapy is discontinued more than 3 times due to cardiomyopathy.