Molecular design and optimization of ^99mTc-labeled recombinant affibody molecules improves their biodistribution and imaging properties

Affibody molecules are a recently developed class of targeting proteins based on a nonimmunoglobulin scaffold. The small size (7 kDa) and subnanomolar affinity of Affibody molecules enables high-contrast imaging of tumor-associated molecular targets, particularly human epidermal growth factor receptor type 2 (HER2). ^99mTc as a label offers advantages in clinical practice, and earlier studies demonstrated that ^99mTc-labeled recombinant Affibody molecules with a C-terminal cysteine could be used for HER2 imaging. However, the renal retention of radioactivity exceeded tumor uptake, which might complicate imaging of metastases in the lumbar region. The aim of this study was to develop an agent with low renal uptake and preserved tumor targeting. Methods: A series of recombinant derivatives of the HER2-binding Z _HER2:342 Affibody molecule with a C-terminal chelating sequence, -GXXC (X denoting glycine, serine, lysine, or glutamate), was designed. The constructs were labeled with ^99mTc and evaluated in vitro and in vivo. Results: All variants were stably labeled with ^99mTc, with preserved capacity to bind specifically to HER2-expressing cells in vitro and in vivo. The composition of the chelating sequence had a clear influence on the cellular processing and biodistribution properties of the Affibody molecules. The best variant, ^99mTc-Z_HER2:V2, with the C-terminal chelating sequence -GGGC, provided the lowest radioactivity retention in all normal organs and tissues including the kidneys. ^99mTc-Z _HER2:V2 displayed high uptake of radioactivity in HER2-expressing xenografts, 22.6 ± 4.0 and 7.7 ± 1.5 percentage injected activity per gram of tissue at 4 h after injection in SKOV-3 (high HER2 expression) and DU-145 (low HER2 expression) tumors, respectively. In both models, the tumor uptake exceeded the renal uptake. Conclusion: These results demonstrate that the biodistribution properties of recombinant ^99mTc-labeled Affibody molecules can be optimized by modification of the C-terminal cysteine-containing chelating sequence. ^99mTc-Z_HER2:V2 is a promising candidate for further development as a diagnostic radiopharmaceutical for imaging of HER2-expressing tumors. These results may be useful for the development of imaging agents based on other Affibody molecules and, hopefully, other scaffolds.