Imaging of HER3-expressing xenografts in mice using a ^99mTc(CO) ₃-HEHEHE-Z_HER3:08699 affibody molecule

Purpose: Human epidermal growth factor receptor type 3 (HER3) is a transmembrane receptor tyrosine kinase belonging to the HER (ErbB) receptor family. Membranous expression of HER3 is associated with trastuzumab resistance in breast cancer and the transition to androgen independence in prostate cancer. Imaging of HER3 expression in malignant tumors may provide important diagnostic information that can influence patient management. Affibody molecules with low picomolar affinity to HER3 were recently selected. The aim of this study was to investigate the feasibility of HER3 imaging using radiolabeled Affibody molecules. Methods: A HER3-binding Affibody molecule, Z₀₈₆₉₉, with a HEHEHE-tag on N-terminus was labeled with ^99mTc(CO)₃ using an IsoLink kit. In vitro and in vivo binding specificity and the cellular processing of the labeled binder were evaluated. Biodistribution of ^99mTc(CO)₃-HEHEHE-Z₀₈₆₉₉ was studied over time in mice bearing HER3-expressing xenografts. Results: HEHEHE-Z₀₈₆₉₉ was labeled with ^99mTc(CO)₃ with an isolated yield of >80 % and a purity of >99 %. Binding of ^99mTc(CO) ₃-HEHEHE-Z₀₈₆₉₉ was specific to BT474 and MCF7 (breast cancer), and LS174T (colon cancer) cells. Cellular processing showed rapid binding and relatively quick internalization of the receptor/Affibody molecule complex (70 % of cell-associated radioactivity was internalized after 24 h). The tumor targeting was receptor mediated and the excretion was predominantly renal. Receptor-mediated uptake was also found in the liver, lung, stomach, intestine, and salivary glands. At 4 h pi, tumor-to-blood ratios were 7±3 for BT474, and 6±2 for LS174T xenografts. LS174T tumors were visualized by microSPECT 4 h pi. Conclusions: The results of this study suggest the feasibility of HER3-imaging in malignant tumors using Affibody molecules.