89Zr-DFO*-Trastuzumab PET in Patients With Gastric or Breast Cancer - a Pilot Study

Positron emission tomography (PET) imaging with 89Zr-labeled trastuzumab can potentially discriminate between human epidermal growth factor 2 (HER2) positive and HER2-negative lesions in cancer patients. The obvious advantage over tumor biopsies is that PET imaging provides an overview regarding the heterogeneity of HER2-positivity of all lesions in the patient noninvasively at any given time. The use of a tracer with high specificity and low background signal is critical for accurately identifying positive lesions in patients. Earlier studies in patients with HER2-positive (HER2+) breast cancer and gastric cancer using 89Zr-trastuzumab led to the identification of HER2-positive tumor lesions. However, also lesions were missed and false-positive lesions were seen. Previous work has shown that organs like liver and spleen have significant uptake of 89Zr-trastuzumab. In addition, HER2-directed therapy is most effective in HER2-positive tumors, which are defined as immunohistochemistry (IHC) HER2 expression 3+ or HER2 2+ with gene amplification. Tumors with IHC 0, 1+ or 2+ without amplification are considered HER2-negative. To be able to discriminate between these expression levels with PET imaging, an excellent signal to background ratio is required. Recently, an improved 89Zr-labeled trastuzumab tracer has been developed using a different chelator for 89Zr binding, DFO\*, which further improves stability of the 89Zr-labeled trastuzumab tracer. 89Zr-DFO\*-trastuzumab preclinically shows improved specificity in uptake of tumor lesions while non-tumor related uptake in bone, liver and spleen is reduced, potentially improving the discrimination of HER2-positive tumor lesions in patients. The investigators hypothesize that the improved 89Zr-DFO\*-trastuzumab tracer will lead to a reduced background uptake and thus a better discrimination of HER2-positive tumor lesions.