Imaging of Human Epidermal Growth Factor Receptor Type 2 Expression with 18F-Labeled Affibody Molecule ZHER2:2395 in a Mouse Model for Ovarian Cancer

Affibody molecules are small (7 kDa) proteins with subnanomolar targeting affinity. Previous SPECT studies in xenografts have shown that the Affibody molecule In-111-DOTA-Z(HER2:2395) can discriminate between high and low human epidermal growth factor receptor type 2 (HER2)-expressing tumors, indicating that radiolabeled Affibody molecules have potential for patient selection for HER2-targeted therapy. Compared with SPECT, PET with positron-emitting radionuclides, such as F-18, may improve imaging of HER2 expression because of higher sensitivity and improved quantification of PET. The aim of the present study was to determine whether the F-18-labeled NOTA-conjugated Affibody molecule Z(HER2:2395) is a suitable agent for imaging of HER2 expression. The tumor-targeting properties of F-18-labeled Z(HER2:2395) were compared with In-111- and Ga-68-labeled Z(HER2:2395) in mice with HER2-expressing SK-OV-3 xenografts. Methods: Z(HER2:2395) was conjugated with NOTA and radiolabeled with F-18, Ga-68, and In-111. Radiolabeling with F-18 was based on the complexation of (AlF)-F-18 by NOTA. The 50% inhibitory concentration values for NOTA-Z(HER2:2395) labeled with F-19, Ga-69, and In-115 were determined in a competitive cell-binding assay using SK-OV-3 cells. Mice bearing subcutaneous SK-OV-3 xenografts were injected intravenously with radiolabeled NOTA-Z(HER2:2395). One and 4 h after injection, PET/CT or SPECT/CT images were acquired, and the biodistribution was determined by ex vivo measurement. Results: The 50% inhibitory concentration values for F-19-, Ga-69-, and In-115-NOTA-Z(HER2:2395) were 5.0, 6.3, and 5.3 nM, respectively. One hour after injection, tumor uptake was 4.4 +/- 0.8 percentage injected dose per gram (% ID/g), 5.6 +/- 1.6 % ID/g, and 7.1 +/- 1.4 % ID/g for F-18-, Ga-68-, and In-111-NOTA-Z(HER2:2395), respectively, and the respective tumor-to-blood ratios were 7.4 +/- 1.8, 8.0 +/- 1.3, and 4.8 +/- 1.3. Tumor uptake was specific, because uptake could be blocked efficiently by coinjection of an excess of unlabeled Z(HER2:2395). PET/CT and SPECT/CT images clearly visualized HER2-expressing SK-OV-3 xenografts. Conclusion: This study showed that F-18-NOTA-Z(HER2:2395) is a promising new imaging agent for HER2 expression in tumors. Affibody molecules were successfully labeled with F-18 within 30 min, based on the complexation of (AlF)-F-18 by NOTA. Further research is needed to determine whether this technique can be used for patient selection for HER2-targeted therapy.