Small animal PET imaging of TAG-72 expressing tumor using 68Ga-NOTA-3E8 Fab radioimmunoconjugate

Objectives The tumor associated glycoprotein-72 (TAG-72) is expressed in the majority of human adenocarcinomas of colon, ovary, pancreas, and breast. 3E8 is anti-TAG-72 humanized antibody. Antibody fragments have some advantages such as improved pharmacokinetics and reduced immunogenicity compared to whole IgG. We investigated the feasibility of ⁶⁸Ga-NOTA-3E8 Fab for tumor imaging in small animal PET.

Methods Fab fragments of 3E8 was conjugated with p-SCN-Bn-NOTA and labeled with ⁶⁸Ga. ⁶⁸Ga labeling was performed in room temperature and labeling yield and serum stability were checked by ITLC. In vitro TAG-72 specific binding of NOTA-3E8 Fab was assessed by cell binding assay. The tumor targeting efficacy of ⁶⁸Ga-NOTA-3E8 Fab was evaluated in LS174T colon cancer xenograft models.

Results ⁶⁸Ga-NOTA-3E8 Fab showed radiolabeling yield with >99% and stable in serum condition during 2 h. ⁶⁸Ga-NOTA-3E8-Fab showed specific binding in TAG-72 positive cell line. ⁶⁸Ga-NOTA-3E8 Fab was showed fast blood clearance and high kidney uptake in normal mice at 1 h post injection. Tumor uptake of ⁶⁸Ga-NOTA-3E8-Fab was 2.1±0.1 %ID/g, and tumor-to-muscle and tumor-to-blood ratio were 3.6±0.1 and 1.1±0.2 at 1 h. Small animal PET images showed that ⁶⁸Ga-NOTA-3E8 Fab have fast and high tumor uptake at 1 h.

Conclusions ⁶⁸Ga labeled Fab fragment antibody showed specific tumor uptake and favorable biodistribution pattern for tumor imaging. ⁶⁸Ga-NOTA-3E8 Fab could be used as a promising radioimmunoconjugate for tumor imaging in TAG-72 expressing tumor.