New small 99m Tc-labeled peptides for HER2 receptor imaging.

The high expression of the human epidermal growth factor receptor 2 (HER2) and the accessibility of its extracellular domain make it an ideal target for the targeted delivery of anti-tumor drugs as well as imaging agents. In this study, the heptapeptide leucine-threonine-valine-serine-proline-tryptophan-tyrosine (LTVSPWY) as a new small peptide for an anti-HER2 target was labeled by incorporating 99m Tc to the cysteine-based ligands CGGG (Cys-Gly-Gly-Gly) and CSSS (Cys-Ser-Ser-Ser) linked to this peptide. Both 99m Tc-labeled peptides were evaluated for HER2 bindings as well as pharmacokinetics and tumor targeting. CGGG- and CSSS-LTVSPWY peptides were labeled with 99m Tc using a gluconate ligand exchange. Cellular specific binding, affinities, and internalization of both peptides to the HER2 receptor were evaluated in the SKOV-3 cell line. Specific targeting of both peptides to the HER2 receptor was assessed in three cell lines with different levels of HER2 expression. Studies were performed in SKOV-3 tumor bearing mice for tumor targeting. Both peptides were labeled with 99m Tc with more than 99% efficiency and showed favorable stability in solution and serum. The HER2 binding affinities of both the radiolabeled peptides were inhibited up to 60% by the unlabeled peptide, as well as with trastuzumab antibody. We observed nanomolar binding affinities for both radiolabeled peptides. The tumor uptakes were 4.95 ± 4.84% and 3.84 ± 2.53% for the CSSS and CGGG chelators, respectively, at 1 h after injection. However, tumor uptakes were similar for both peptides at 4 h postinjection, although a higher tumor to background ratio and lower radioactivity retention in the kidney were observed for CSSS, leading to a clearer tumor image with injection of this peptide. These small new peptides were selectively targeted to the HER2 receptor, and introduction of a serine residue into the chelator improved the pharmacokinetics of 99m Tc labeled LTVSPWY for clear tumor imaging in animals.