Trastuzumab is a monoclonal antibody used primarily in the treatment of HER2-positive breast cancer and HER2-overexpressing gastric or gastroesophageal junction adenocarcinoma. It works by selectively binding to the extracellular domain of the human epidermal growth factor receptor 2 (HER2), inhibiting HER2-mediated signaling pathways that drive uncontrolled cell proliferation and survival in cancer cells.

Its main therapeutic applications include adjuvant and neoadjuvant treatment in early-stage HER2-positive breast cancer, as well as first-line therapy in metastatic settings, often in combination with chemotherapy agents like paclitaxel or docetaxel. In gastric cancer, trastuzumab is used in combination with cisplatin and a fluoropyrimidine for advanced disease.

Beyond oncology, trastuzumab serves as a reference standard in laboratory research and diagnostic assay development, particularly in validating HER2 detection methods such as immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH). It is also used in the development of biosimilars and in pharmacokinetic or pharmacodynamic studies to assess antibody behavior in biological systems.