Evaluation of the first Sc-44-labeled Affibody molecule for imaging of HER2-expressing tumors

Introduction: Affibody molecules are small (58 amino acids) high-affinity proteins based on a tri-helix nonimmunoglobulin scaffold. A clinical study has demonstrated that PET imaging using Affibody molecules labeled with Ga-68 (T-1/2 = 68 min) can visualize metastases of breast cancer expressing human epidermal growth factor receptor type 2 (HER2) and provide discrimination between tumors with high and low expression level. This may help to identify breast cancer patients benefiting from HER2-targeting therapies. The best discrimination was at 4 h post injection. Due to longer half-life, a positron-emitting radionuclide Sc-44 (T-1/2 = 4.04 h) might be a preferable label for Affibody molecules for imaging at several hours after injection. Methods: A synthetic second-generation anti-HER2 Affibody molecule Z(HER2:2891) was labeled with Sc-44 via a DOTA-chelator conjugated to the N-terminal amino group. Binding specificity, affinity and cellular processing Sc-44-DOTA-Z(HER2:2891) and Ga-68-DOTA-Z(HER2:2891) were compared in vitro using HER2-expressing cells. Biodistribution and imaging properties of Sc-44-DOTA-Z(HER2,2891) and Ga-68-DOTA-Z(HER2:2891) were evaluated in Balb/c nude mice bearing HER2-expression xenografts. Results: The labeling yield of 98 +/- 2% and specific activity of 7.8 GBq/mu mol were obtained. The conjugate demonstrated specific binding to HER2-expressing SKOV3.ip cells in vitro and to SKOV3.ip xenografts in nude mice. The distribution of radioactivity at 3 h post injection was similar for Sc-44-DOTA-Z(HER2:2891) and Ga-68-DOTA-Z(HER2:2891), but the blood clearance of the Sc-44-labeled variant was slower and the tumor-to-blood ratio was reduced (15 +/- 2 for (SC)-S-44-DOTA-Z(HER2:2891) vs 46 +/- 9 for Ga-68-DOTA-Z(HER2.2891)). At 6 h after injection of Sc-44-DOTA-Z(HER2,2891) the tumor uptake was 8 +/- 2% IA/g and the tumor-to-blood ratio was 51 +/- 8. Imaging using small-animal PET/CT demonstrated that (SC)-S-44-DOTA-ZHER2,2891 provides specific and high-contrast imaging of HER2-expressing xenografts. Conclusion: The Sc-44- DOTA-Z(HER2:2891) Affibody molecule is a promising probe for imaging of HER2-expression in malignant tumors.