Tumor Imaging Using a Picomolar Affinity HER2 Binding Affibody Molecule

TLDR

Detection of cell‑bound proteins produced by aberrant gene expression in malignant tumors can inform patient management, and small radiolabeled targeting proteins could enable high‑contrast radionuclide imaging of such cancers if they possess sufficient affinity and specificity. Here, we describe a HER2‑specific 6 kDa Affibody molecule with 22 pmol/L affinity that can be used for visualization of HER2 expression in tumors in vivo using a gamma camera. A library for affinity maturation was created by re‑randomizing key positions identified from first‑generation nanomolar variants, enabling selection of high‑affinity binders. The affinity‑matured Affibody ZHER2:342.

Abstract

Abstract The detection of cell-bound proteins that are produced due to aberrant gene expression in malignant tumors can provide important diagnostic information influencing patient management. The use of small radiolabeled targeting proteins would enable high-contrast radionuclide imaging of cancers expressing such antigens if adequate binding affinity and specificity could be provided. Here, we describe a HER2-specific 6 kDa Affibody molecule (hereinafter denoted Affibody molecule) with 22 pmol/L affinity that can be used for the visualization of HER2 expression in tumors in vivo using gamma camera. A library for affinity maturation was constructed by re-randomization of relevant positions identified after the alignment of first-generation variants of nanomolar affinity (50 nmol/L). One selected Affibody molecule, ZHER2:342 showed a >2,200-fold increase in affinity achieved through a single-library affinity maturation step. When radioiodinated, the affinity-matured Affibody molecule showed clear, high-contrast visualization of HER2-expressing xenografts in mice as early as 6 hours post-injection. The tumor uptake at 4 hours post-injection was improved 4-fold (due to increased affinity) with 9% of the injected dose per gram of tissue in the tumor. Affibody molecules represent a new class of affinity molecules that can provide small sized, high affinity cancer-specific ligands, which may be well suited for tumor imaging. (Cancer Res 2006; 66(8): 4339-48)