Affibody molecules for epidermal growth factor receptor targeting in vivo: aspects of dimerization and labeling chemistry.

UNLABELLED Noninvasive detection of epidermal growth factor receptor (EGFR) expression in malignant tumors by radionuclide molecular imaging may provide diagnostic information influencing patient management. The aim of this study was to evaluate a novel EGFR-targeting protein, the ZEGFR:1907 Affibody molecule, for radionuclide imaging of EGFR expression, to determine a suitable tracer format (dimer or monomer) and optimal label. METHODS An EGFR-specific Affibody molecule, ZEGFR:1907, and its dimeric form, (ZEGFR:1907)2, were labeled with 111In using benzyl-diethylenetriaminepentaacetic acid and with 125I using p-iodobenzoate. Affinity and cellular retention of conjugates were evaluated in vitro. Biodistribution of radiolabeled Affibody molecules was compared in mice bearing EGFR-expressing A431 xenografts. Specificity of EGFR targeting was confirmed by comparison with biodistribution of non-EGFR-specific counterparts. RESULTS Head-to-tail dimerization of the Affibody molecule improved the dissociation rate. In vitro, dimeric forms demonstrated superior cellular retention of radioactivity. For both molecular set-ups, retention was better for the 111In-labeled tracer than for the radioiodinated counterpart. In vivo, all conjugates accumulated specifically in xenografts and in EGFR-expressing tissues. The retention of radioactivity in tumors was better in vivo for dimeric forms; however, the absolute uptake values were higher for monomeric tracers. The best tracer, 111In-labeled ZEGFR:1907, provided a tumor-to-blood ratio of 100 (24 h after injection). CONCLUSION The radiometal-labeled monomeric Affibody molecule ZEGFR:1907 has a potential for radionuclide molecular imaging of EGFR expression in malignant tumors.