⁶⁴Cu-Labeled Repebody Molecules for Imaging of Epidermal Growth Factor Receptor–Expressing Tumors

Abstract

The epidermal growth factor receptor (EGFR) is a member of the erbB family of receptors and is overexpressed in many tumor types. A repebody is a newly designed nonantibody protein scaffold for tumor targeting that contains leucine-rich repeat modules. In this study, 3 ⁶⁴Cu-labeled anti-EGFR repebodies with different chelators were synthesized, and their biologic characteristics were assessed in cultured cells and tumor-bearing mice. <b>Methods:</b> Repebodies were synthesized with the chelators 2-(<i>p</i>-isothiocyanatobenzyl)-1,4,7-triazacyclononane-<i>N,N',N,″-</i>triacetic acid trihydrochloride ([<i>p</i>-SCN-Bn]-NOTA), 2,2',2″-(10-(2-(2,5-dioxopyrrolidin-1-yloxy)-2-oxoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl) triacetic acid (DOTA-<i>N</i>-hydroxysuccinimide ester), or 1-(<i>p</i>-isothiocyanatobenzyl)diethylenetriamine pentaacetic acid trihydrochloride ([<i>p</i>-SCN-Bn]-DTPA) in 1.0 M NaHCO₃ buffer (pH 9.2) for 24 h. Purified NOTA-, DOTA-, and DTPA-conjugated repebody were radiolabeled with ⁶⁴Cu in 0.1 M NH₄OAc buffer (pH 5.5). To compare the EGFR-binding affinities of the repebodies, cellular uptake studies were performed with the human non-small cell lung cancer cell line H1650 (high expression of EGFR) and the human colon adenocarcinoma cell line SW620 (low expression of EGFR). Biodistribution and small-animal PET imaging studies were performed using H1650 tumor-bearing mice. <b>Results:</b> Radiochemical yields of the ⁶⁴Cu-labeled repebodies were approximately 70%-80%. Cellular uptake of the NOTA-, DOTA-, and DTPA-repebodies was over 4-fold higher in H1650 cells than in SW620 cells at 1 h. The 3 repebodies had accumulated specifically in H1650 tumor-bearing nude mice by 1 h after intravenous injection and were retained for over 24 h, as measured by the percentage injected dose per gram of tissue (%ID/g). Tumor uptake of all repebodies increased from 1 to 6 h (at 1 h, 6.28, 8.46, and 6.91 %ID/g for NOTA-, DOTA-, and DTPA-repebody, respectively; at 6 h, 9.4, 8.28, and 10.1 %ID/g, respectively). H1650 tumors were clearly visible after injection of each repebody, with high tumor-to-background ratios (at 1 h, 3.43, 4.89, and 2.38 for NOTA-, DOTA-, and DTPA-repebody, respectively; at 6 h, 3.05, 4.36, and 2.08; at 24 h, 3.81, 4.58, and 2.86). <b>Conclusion:</b> The 3 ⁶⁴Cu-repebody complexes demonstrated specific and rapid uptake in EGFR-expressing tumors within 1 h and may have potential as novel EGFR imaging agents for PET.