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

The goal of this clinical trial is to test a new PET tracer in patients with HER2-positive breast or gastric cancer. This tracer is made of radioactively labeled trastuzumab, and can show where HER2 is present in the body using a PET-scan. For this research, the investigators make PET-scans in people with HER2-positive, metastasized breast- or gastric cancer. The investigators will investigate if the new HER2-tracer correctly shows all tumor lesions. In the future, this method may be useful to help predict who will benefit from certain HER2-directed therapies. Participants will be injected with the radioactive tracer once. After injection, participants will undergo 3 PET-scans. Each PET-scan will take a maximum of 60 minutes. The PET-scans are on separate days within a week after injection of the tracer (e.g. 1 day, 2 days and 4 days after injection). Furthermore, the investigators will take 7 blood samples (5 mL each). Participants are not required to stay at the hospital. The first 3 participants will undergo an extra PET-scan 1 - 2 hours after injection. The amount of radioactivity injected will be 37 MBq (± 10%).

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.