A 2-helix small protein labeled with 68Ga for PET imaging of HER2 expression.
Affibody molecules are a class of scaffold proteins being developed into a generalizable approach to targeting tumors. Many 3-helix-based Affibody proteins have shown excellent in vivo properties for tumor imaging and therapy. By truncating one alpha-helix that is not responsible for receptor recognition in the Affibody and maturating the protein affinity through synthetic strategies, we have successfully identified in our previous research several small 2-helix proteins with excellent binding affinities to human epidermal growth factor receptor type 2 (HER2). With preferential properties such as faster blood clearance and tumor accumulation, lower immunogenic potential, and facile and economically viable synthetic schemes, we hypothesized that these 2-helix protein binders could become excellent molecular imaging probes for monitoring HER2 expression and modulation.


METHODS
In this study, a 2-helix small protein, MUT-DS, was chemically modified with a metal chelator, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). DOTA-MUT-DS was then site-specifically radiolabeled with an important PET radionuclide, (68)Ga. The resulting radiolabeled anti-HER2 2-helix molecule was further evaluated as a potential molecular probe for small-animal PET HER2 imaging in a SKOV3 tumor mouse model.


RESULTS
The 2-helix DOTA-MUT-DS showed high HER2-binding affinity (dissociation constant, 4.76 nM). The radiolabeled probe displayed high stability in mouse serum and specificity toward HER2 in cell cultures. Biodistribution and small-animal PET studies further showed that (68)Ga-DOTA-MUT-DS had rapid and high SKOV3 tumor accumulation and quick clearance from normal organs. The specificity of (68)Ga-DOTA-MUT-DS for SKOV3 tumors was confirmed by monitoring modulation of HER2 protein on treatment of tumor mice with heat shock protein 90 inhibitor 17-N,N-dimethyl ethylene diamine-geldanamycin in vivo.


CONCLUSIONS
This proof-of-concept research clearly demonstrated that synthetic 2-helix (68)Ga-DOTA-MUT-DS is a promising PET probe for imaging HER2 expression in vivo. The Affibody-derived small 2-helix protein scaffold has great potential for developing targeting agents for a variety of tumor-associated biomarkers.
Ren
Gang
G
Molecular Imaging Program at Stanford (MIPS), Department of Radiology and Bio-X Program, Stanford University, Stanford, California, USA.
Zhang
Rong
R
Liu
Zhe
Z
Webster
Jack M
JM
Miao
Zheng
Z
Gambhir
Sanjiv S
SS
Syud
Faisal A
FA
Cheng
Zhen
Z
D000818:Animals; D002352:Carrier Proteins / Q000737:chemistry / Q000493:pharmacokinetics*; D045744:Cell Line, Tumor; D005260:Female; D005710:Gallium Radioisotopes / Q000737:chemistry / Q000493:pharmacokinetics*; D020869:Gene Expression Profiling / Q000379:methods; D006801:Humans; D008657:Metabolic Clearance Rate; D051379:Mice; D008819:Mice, Nude; D009928:Organ Specificity; D010051:Ovarian Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism*; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000138:chemical synthesis / Q000493:pharmacokinetics; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000378:metabolism*; D014018:Tissue Distribution
D002352:Carrier Proteins; D005710:Gallium Radioisotopes; D019275:Radiopharmaceuticals; D018719:Erb-b2 Receptor Tyrosine Kinases
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