Evaluation of the first 44 Sc-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 non-immunoglobulin scaffold. A clinical study has demonstrated that PET imaging using Affibody molecules labeled with 68 Ga (T½ =68min) 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 4h post injection. Due to longer half-life, a positron-emitting radionuclide 44 Sc (T½ =4.04h) might be a preferable label for Affibody molecules for imaging at several hours after injection.

METHODS: A synthetic second-generation anti-HER2 Affibody molecule ZHER2:2891 was labeled with 44 Sc via a DOTA-chelator conjugated to the N-terminal amino group. Binding specificity, affinity and cellular processing 44 Sc-DOTA-ZHER2:2891 and 68 Ga-DOTA-ZHER2:2891 were compared in vitro using HER2-expressing cells. Biodistribution and imaging properties of 44 Sc-DOTA-ZHER2:2891 and 68 Ga-DOTA-ZHER2:2891 were evaluated in Balb/c nude mice bearing HER2-expression xenografts.

RESULTS: The labeling yield of 98±2% and specific activity of 7.8GBq/μmol 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 3h post injection was similar for 44 Sc-DOTA-ZHER2:2891 and 68 Ga-DOTA-ZHER2:2891 , but the blood clearance of the 44 Sc-labeled variant was slower and the tumor-to-blood ratio was reduced (15±2 for 44 Sc-DOTA-ZHER2:2891 vs 46±9 for 68 Ga-DOTA-ZHER2:2891 ). At 6h after injection of 44 Sc-DOTA-ZHER2:2891 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 44 Sc-DOTA-ZHER2:2891 provides specific and high-contrast imaging of HER2-expressing xenografts.

CONCLUSION: The 44 Sc- DOTA-ZHER2:2891 Affibody molecule is a promising probe for imaging of HER2-expression in malignant tumors.