Noninvasive Evaluation of EGFR Expression of Digestive Tumors Using Tc-MAG-Cet-F(ab')-Based SPECT/CT Imaging.

PURPOSE

This study is aimed at investigating the feasibility of cetuximab (Cet) F(ab') fragment- (Cet-F(ab')-) based single photon emission tomography/computed tomography (SPECT/CT) for assessing the epidermal growth factor receptor (EGFR) expression in digestive tumor mouse models.

METHODS

Cet-F(ab') was synthesized using immunoglobulin G-degrading enzyme of (IdeS) protease and purified with protein A beads. The product and its in vitro stability in normal saline and 1% bovine serum albumin were analyzed with sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The EGFR expression in the human colon tumor cell line HT29 and the human stomach tumor cell line MGC803 were verified using western blotting and immunocytochemistry. Cet-F(ab') was conjugated with 5(6)-carboxytetramethylrhodamine succinimidyl ester to demonstrate its binding ability to the MGC803 and HT29 cells. Cet-F(ab') was conjugated with NHS-MAG for Tc radiolabeling. The best imaging time was determined using a biodistribution assay at 1, 4, 16, and 24 h after injection of the Tc-MAG-Cet-F(ab') tracer. Furthermore, Tc-MAG-Cet-F(ab') SPECT/CT was performed on MGC803 and HT29 tumor-bearing nude mice.

RESULTS

HT29 cells had low EGFR expression while MGC803 cell exhibited the high EGFR expression. Cet-F(ab') and intact cetuximab showed similar high binding ability to MGC803 cells but not to HT29 cells. Cet-F(ab') and Tc-MAG-Cet-F(ab') showed excellent in vitro stability. The biodistribution assay showed that the target to nontarget ratio was the highest at 16 h (17.29 ± 5.72, = 4) after tracer injection. The Tc-MAG-Cet-F(ab')-based SPECT/CT imaging revealed rapid and sustained tracer uptake in MGC803 tumors rather than in HT29 tumors with high image contrast, which was consistent with the results in vitro.

CONCLUSION

SPECT/CT imaging using Tc-MAG-Cet-F(ab') enables the evaluation of the EGFR expression in murine EGFR-positive tumors, indicating the potential utility for noninvasive evaluation of the EGFR expression in tumors.