Comparison of ConjugationStrategies of Cross-BridgedMacrocyclic Chelators with Cetuximab for Copper-64 Radiolabeling andPET Imaging of EGFR in Colorectal Tumor-Bearing Mice

摘要

Epidermal growth-factor receptor (EGFR) is overexpressed in a wide variety of solid tumors and has served as a well-characterized target for cancer imaging and therapy. Cetuximab was the first mAb targeting EGFR approved by the FDA for the treatment of metastatic colorectal and head and neck cancers. Previous studies showed that ⁶⁴Cu (T1/2 = 12.7 h; β⁺ (17.4%)) labeled DOTA–cetuximab showed promise for PET imaging of EGFR-positive tumors; however the in vivo stability of this compound has been questioned. In this study, two recently developed cross-bridged macrocyclic chelators (CB-TE1A1P and CB-TE1K1P) were conjugated to cetuximab using standard NHS coupling procedures and/or strain-promoted azide–alkyne cycloaddition (SPAAC) methodologies. The radiolabeling and in vitro/vivo evaluation of the resulting cetuximab conjugates were compared. Improved Cu-64 labeling efficiency and high specific activity (684 kBq/μg, decay corrected to the end of bombardment) were obtained with the CB-TE1K1P-PEG4-click-cetuximab conjugate. Saturation binding assays indicated that the prepared cetuximab conjugates had comparable affinity (1.32–2.00 nM) in the HCT116 human colorectaltumor cell membranes. In the subsequent in vivo evaluation, ⁶⁴Cu-CB-TE1K1P-PEG4-click-cetuximab demonstratedmore rapid renal clearance with a higher tumorontumor ratio thanother ⁶⁴Cu-labeled cetuximab conjugates, and it shows thegreatest promise for imaging and therapy of EGFR-positive tumors.