Today, the most important treatment methods for cancer are chemotherapy, surgical intervention, and radiotherapy. In recent years, many studies on radioimmunotherapy have been carried out. New drugs and treatment strategies are constantly being researched, and targeted cancer therapy is the treatment with the highest efficacy and negligible side effects compared to other methods. Breast cancer is the most common type of cancer in women. Human epidermal growth factor receptor 2 (HER2) is overexpressed in 20-30% of breast cancer cases. Trastuzumab is a humanized monoclonal antibody of recombinant DNA origin targeting the HER2 protein. In this study, the pharmaceutical part of a new radiopharmaceutical to be used in breast cancer treatment was prepared. For this purpose, molecular insertion studies were conducted to show the binding sites between Trastuzumab and different chelating agents (DOTA, DTPA, EDTA and MAG3) in silico. Then, these chelating agents were complex, and characterization studies were carried out.