Small-animal PET Imaging of human epidermal growth factor receptor type 2 expression with site-specific 18F-labeled protein scaffold molecules

Human epidermal growth factor receptor type 2 (HER2) is a well-established tumor biomarker that is overexpressed in a wide variety of cancers and that serves as a molecular target for therapeutic intervention. HER2 also serves as a prognostic indicator of patient survival and as a predictive marker of the response to antineoplastic therapy. The development of F-18-labeled biomolecules for PET imaging of HER2 (HER2 PET) is very important because it may provide a powerful tool for the early detection of HER2-positive tumor recurrence and for the monitoring of HER2-based tumor treatment. Methods: In this study, anti-HER2 monomeric and dimeric protein scaffold molecules [Z(HER2:477) and (Z(HER2:477))(2), respectively] were radiofluorinated at a reasonable radiochemical yield (13%-18%) by use of site-specific oxime chemistry. The resulting radiofluorinated protein scaffold molecules were then evaluated as potential molecular probes for small-animal HER2 PET by use of a SKOV3 tumor-bearing mouse model. Results: The 4-F-18-fluorobenzaldehyde conjugated aminooxy-protein scaffolds [F-18-N-(4-fluorobenzylidene)oxime (FBO)-Z(HER2:477) and F-18-FBO-(Z(HER2:477))(2)] both displayed specific HER2-binding ability in vitro. Biodistribution and small-animal PET imaging studies further revealed that F-18-FBO-Z(HER2:477) showed rapid and high SKOV3 tumor accumulation and quick clearance from normal tissues, whereas F-18-FBO-(Z(HER2:477))(2) showed poor in vivo performance (low tumor uptake and tumor-to-normal tissue ratios). The specificity of F-18-FBO-Z(HER2:477) for SKOV3 tumors was confirmed by its lower uptake on pretreatment of tumor-bearing mice with the HER2-targeting agents ZHER2 and trastuzumab. Moreover, small-animal PET imaging studies revealed that F-18-FBO-Z(HER2:477) produced higher-quality tumor imaging than F-18-FBO-(Z(HER2:477))(2). F-18-FBO-Z(HER2:477) could clearly identify HER2-positive tumors with good contrast. Conclusion: Overall, these data demonstrate that F-18-FBO-Z(HER2:477) is a promising PET probe for imaging HER2 expression in living mice. It has a high potential for translation to clinical applications. The radiofluorination method developed can also be used as a general strategy for the site-specific labeling of other proteins with F-18. The protein scaffold molecules used here are attractive for the further development of PET probes for other molecular targets.