Affibody molecules are small (7 kDa) proteins with subnanomolar targeting affinity. Previous SPECT studies in xenografts have shown that the Affibody molecule ¹¹¹In-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 ¹⁸F, 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 ¹⁸F-labeled NOTA-conjugated Affibody molecule Z_HER2:2395 is a suitable agent for imaging of HER2 expression. The tumor-targeting properties of ¹⁸F-labeled Z_HER2:2395 were compared with ¹¹¹In- and ⁶⁸Ga-labeled Z_HER2:2395 in mice with HER2-expressing SK-OV-3 xenografts.

Methods:

Z_HER2:2395 was conjugated with NOTA and radiolabeled with ¹⁸F, ⁶⁸Ga, and ¹¹¹In. Radiolabeling with ¹⁸F was based on the complexation of Al¹⁸F by NOTA. The 50% inhibitory concentration values for NOTA-Z_HER2:2395 labeled with ¹⁹F, ⁶⁹Ga, and ¹¹⁵In 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 ¹⁹F-, ⁶⁹Ga-, and ¹¹⁵In-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 ¹⁸F-, ⁶⁸Ga-, and ¹¹¹In-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 ¹⁸F-NOTA-Z_HER2:2395 is a promising new imaging agent for HER2 expression in tumors. Affibody molecules were successfully labeled with ¹⁸F within 30 min, based on the complexation of Al¹⁸F by NOTA. Further research is needed to determine whether this technique can be used for patient selection for HER2-targeted therapy.