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

Abstract

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 18F-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 (ZHER2:477)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 mousemodel. Results: The 4- 18F-fluorobenzaldehyde conjugated aminooxy-protein scaffolds [ 18F-N-(4-fluorobenzylidene)oxime (FBO)-ZHER2:477 and 18F-FBO-(ZHER2:477)2] both displayed specific HER2-binding ability in vitro. Biodistribution and small-animal PET imaging studies further revealed that 18F-FBO-ZHER2:477 showed rapid and high SKOV3 tumor accumulation and quick clearance from normal tissues, whereas 18F-FBO-(ZHER2:477)2 showed poor in vivo performance (low tumor uptake and tumorto-normal tissue ratios). The specificity of 18F-FBO-ZHER2:477 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 18F-FBO-ZHER2:477 produced higher-quality tumor imaging than 18F-FBO-(Z HER2:477)2. 18F-FBO-ZHER2:477 could clearly identify HER2-positive tumors with good contrast. Conclusion: Overall, these data demonstrate that 18F-FBO-ZHER2:477 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 proteinswith 18F. The protein scaffold molecules used here are attractive for the further development of PET probes for other molecular targets. Copyright © 2008 by the Society of Nuclear Medicine, Inc.