Trastuzumab Mechanism of Action; 20 Years of Research to Unravel a Dilemma

SIMPLE SUMMARY: Overexpression of HER2 receptors have been identified in various types of cancer including breast cancer and ovarian cancer. HER2 overexpression is generally associated with poor clinical outcomes in patients with HER2-positve tumors. Trastuzumab, an antibody specifically targeting HER2 receptors, showed promising clinical benefits for patients with HER2-positive tumors. Studies show that trastuzumab suppresses HER2 receptors’ oncogenic functions in HER2-postive tumors. Moreover, trastuzumab has been shown to provoke immune responses against the HER2-amplified tumors. ABSTRACT: Trastuzumab as a first HER2-targeted therapy for the treatment of HER2-positive breast cancer patients was introduced in 1998. Although trastuzumab has opened a new avenue to treat patients with HER2-positive breast cancer and other types of cancer, some patients are not responsive or become resistant to this treatment. So far, several mechanisms have been suggested for the mode of action of trastuzumab; however, the findings regarding these mechanisms are controversial. In this review, we aimed to provide a detailed insight into the various mechanisms of action of trastuzumab.