On the Selection of a Tracer for PET Imaging of HER2-Expressing Tumors: Direct Comparison of a 124I-Labeled Affibody Molecule and Trastuzumab in a Murine Xenograft Model

Human epidermal growth factor receptor type 2 (HER2) is a tyrosine kinase, which is often overexpressed in many carcinomas. Imaging HER2 expression in malignant tumors can provide important prognostic and predictive diagnostic information. The use of anti-HER2 tracers labeled with positron-emitting radionuclides may increase the sensitivity of HER2 imaging. The goal of this study was to compare directly 2 approaches for developing anti-HER2 PET tracers: a 124I-labeled monoclonal antibody and a small (7-kDa) scaffold protein, the Affibody molecule. Methods: The anti-HER2 Affibody ZHER2:342 and humanized monoclonal antibody trastuzumab were labeled with 124/125I using p-iodobenzoate (PIB) as a linker. Cellular processing of both tracers by HER2-expressing cells was investigated. The biodistributions of 124I-PIB-ZHER2:342 and 125I-PIB-trastuzumab were compared in BALB/C nu/nu mice bearing HER2-expressing NCI-N87 xenografts using paired labels. Small-animal PET of 124I-PIB-ZHER2:342 and 124I-PIB-trastuzumab in tumor-bearing mice was performed at 6, 24, and 72 h after injection. Results: Both radioiodinated ZHER2:342 and trastuzumab bound specifically to HER2-expressing cells in vitro and specifically targeted HER2-expressing xenografts in vivo. Radioiodinated trastuzumab was more rapidly internalized and degraded, which resulted in better retention of radioactivity delivered by ZHER2:342. Total uptake of trastuzumab in tumors was higher than that of 124I-PIB-ZHER2:342. However, tumor-to-organ ratios were appreciably higher for 124I-PIB-ZHER2:342 due to the more rapid clearance of radioactivity from blood and normal organs. The ex vivo results were confirmed by small-animal PET. Conclusion: The use of the small scaffold targeting Affibody provides better contrast in HER2 imaging than does the monoclonal antibody.

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