Comparative Evaluation of Anti-HER2 Affibody Molecules Labeled with ⁶⁴Cu Using NOTA and NODAGA

Imaging using affibody molecules enables discrimination between breast cancer metastases with high and low expression of HER2, making appropriate therapy selection possible. This study aimed to evaluate if the longer half-life of ⁶⁴Cu (T_1/2 = 12.7 h) would make ⁶⁴Cu a superior nuclide compared to ⁶⁸Ga for PET imaging of HER2 expression using affibody molecules. The synthetic ZHER2:S1 affibody molecule was conjugated with the chelators NOTA or NODAGA and labeled with ⁶⁴Cu. The tumor-targeting properties of ⁶⁴Cu-NOTA-ZHER2:S1 and ⁶⁴Cu-NODAGA-ZHER2:S1 were evaluated and compared with the targeting properties of ⁶⁸Ga-NODAGA-ZHER2:S1 in mice. Both ⁶⁴Cu-NOTA-ZHER2:S1 and ⁶⁴Cu-NODAGA-ZHER2:S1 demonstrated specific targeting of HER2-expressing xenografts. At 2 h after injection of ⁶⁴Cu-NOTA-ZHER2:S1, ⁶⁴Cu-NODAGA-ZHER2:S1, and ⁶⁸Ga-NODAGA-ZHER2:S1, tumor uptakes did not differ significantly. Renal uptake of ⁶⁴Cu-labeled conjugates was dramatically reduced at 6 and 24 h after injection. Notably, radioactivity uptake concomitantly increased in blood, lung, liver, spleen, and intestines, which resulted in decreased tumor-to-organ ratios compared to 2 h postinjection. Organ uptake was lower for ⁶⁴Cu-NODAGA-ZHER2:S1. The most probable explanation for this biodistribution pattern was the release and redistribution of renal radiometabolites. In conclusion, monoamide derivatives of NOTA and NODAGA may be suboptimal chelators for radiocopper labeling of anti-HER2 affibody molecules and, possibly, other scaffold proteins with high renal uptake.