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 (18)F-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-(18)F-fluorobenzaldehyde conjugated aminooxy-protein scaffolds [(18)F-N-(4-fluorobenzylidene)oxime (FBO)-Z(HER2:477) and (18)F-FBO-(Z(HER2:477))(2)] both displayed specific HER2-binding ability in vitro. Biodistribution and small-animal PET imaging studies further revealed that (18)F-FBO-Z(HER2:477) showed rapid and high SKOV3 tumor accumulation and quick clearance from normal tissues, whereas (18)F-FBO-(Z(HER2:477))(2) showed poor in vivo performance (low tumor uptake and tumor-to-normal tissue ratios). The specificity of (18)F-FBO-Z(HER2:477) for SKOV3 tumors was confirmed by its lower uptake on pretreatment of tumor-bearing mice with the HER2-targeting agents Z(HER2) and trastuzumab. Moreover, small-animal PET imaging studies revealed that (18)F-FBO-Z(HER2:477) produced higher-quality tumor imaging than (18)F-FBO-(Z(HER2:477))(2). (18)F-FBO-Z(HER2:477) could clearly identify HER2-positive tumors with good contrast.


CONCLUSIONS
Overall, these data demonstrate that (18)F-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 (18)F. The protein scaffold molecules used here are attractive for the further development of PET probes for other molecular targets.
Cheng
Zhen
Z
Molecular Imaging Program at Stanford, Department of Radiology, Bio-X Program, Stanford University, Stanford, California 94305, USA.
De Jesus
Omayra Padilla
OP
Namavari
Mohammad
M
De
Abhijit
A
Levi
Jelena
J
Webster
Jack Matt
JM
Zhang
Rong
R
Lee
Brian
B
Syud
Faisal A
FA
Gambhir
Sanjiv Sam
SS
D000818:Animals; D045744:Cell Line, Tumor; D019281:Dimerization; D066246:ErbB Receptors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D005462:Fluorine Radioisotopes; D005786:Gene Expression Regulation; D054879:Halogenation; D006801:Humans; D051379:Mice; D009369:Neoplasms / Q000000981:diagnostic imaging; D049268:Positron-Emission Tomography / Q000379:methods*; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D011854:Radiochemistry; D011993:Recombinant Fusion Proteins; D013379:Substrate Specificity; D014018:Tissue Distribution
D005462:Fluorine Radioisotopes; D011993:Recombinant Fusion Proteins; D066246:ErbB Receptors