Comparative Preclinical Evaluation of Peptide-Based Chelators for the Labeling of DARPin G3 with 99m Tc for Radionuclide Imaging of HER2 Expression in Cancer.

Non-invasive radionuclide imaging of human epidermal growth factor receptor type 2 (HER2) expression in breast, gastroesophageal, and ovarian cancers may stratify patients for treatment using HER2-targeted therapeutics. Designed ankyrin repeat proteins (DARPins) are a promising type of targeting probe for radionuclide imaging. In clinical studies, the DARPin [ 99m Tc]Tc-(HE) 3 -G3 labeled using a peptide-based chelator His-Glu-His-Glu-His-Glu ((HE) 3 ), provided clear imaging of HER2 expressing breast cancer 2-4 h after injection. The goal of this study was to evaluate if the use of cysteine-containing peptide-based chelators Glu-Glu-Glu-Cys (E 3 C), Gly-Gly-Gly-Cys (G 3 C), and Gly-Gly-Gly-Ser-Cys connected via a (Gly-Gly-Gly-Ser) 3 -linker (designated as G3-(G 3 S) 3 C) would further improve the contrast of imaging using 99m Tc-labeled derivatives of G3. The labeling of the new variants of G3 provided a radiochemical yield of over 95%. Labeled G3 variants bound specifically to human HER2-expressing cancer cell lines with affinities in the range of 1.9-5 nM. Biodistribution of [ 99m Tc]Tc-G3-G 3 C, [ 99m Tc]Tc-G3-(G 3 S) 3 C, and [ 99m Tc]Tc-G3-E 3 C in mice was compared with the biodistribution of [ 99m Tc]Tc-(HE) 3 -G3. It was found that the novel variants provide specific accumulation in HER2-expressing human xenografts and enable discrimination between tumors with high and low HER2 expression. However, [ 99m Tc]Tc-(HE) 3 -G3 provided better contrast between tumors and the most frequent metastatic sites of HER2-expressing cancers and is therefore more suitable for clinical applications.