Affibody PET Imaging of HER2-Expressing Cancers as a Key to Guide HER2-Targeted Therapy

Human epidermal growth factor receptor 2 (HER2) is a major prognostic and predictive marker overexpressed in 15- 20 % of breast cancers. The diagnostic reference standard for selecting patients for HER2-targeted therapy is by analysis of tumor biopsies for HER2 expression and amplification. Previously patients were either defined as HER2-positive or negative, however, with approval of novel treatment options, specifically the antibody-drug conjugate trastuzumab deruxtecan, many breast cancer patients with tumors expressing low levels of HER2 and lacking HER2 amplification have become eligible for HER2-targeted therapy. Such patients will need to be reliably identified by suitable diagnostic methods. Biopsy-based diagnostics are invasive, and repeat biopsies are not always feasible. They cannot visualize heterogeneity of HER2 expression, leading to a substantial number of misdiagnosed patients. An alternative and highly accurate diagnostic method is mo-lecular imaging with radiotracers. In case of HER2, various studies demonstrate the clinical utility and feasibility of such approaches. Affibody-based radiotracers are clinically validated molecules with favorable characteristics for imaging. In this article we summarize the HER2-targeted therapeutic landscape, describe the experience with imaging diagnostics for HER2, and review the currently available clinical data on HER2-Affibody-based molecular imaging as novel diagnostic tool in breast cancer and beyond.