Quantitative radioimmunoPET imaging of EphA2 in tumor-bearing mice

Purpose EphA2 receptor tyrosine kinase is significantly overexpressed in a wide variety of cancer types. High EphA2 expression has been correlated with increased metastatic potential and poor patient survival. Although many recent reports have focused on blocking the EphA2 signaling pathway in cancer, the in vivo imaging of EphA2 has not yet been investigated. Methods We labeled 1C1, a humanized monoclonal antibody against both human and murine EphA2, with Cu-64 through the chelating agent 1,4,7,10-tetraazacyclododecane N,N',N",N'''-tetraacetic acid (DOTA) and carried out positron emission tomography (PET) imaging of eight tumor models with different EphA2 expression levels. Western blotting of tumor tissue lysate was performed to correlate the EphA2 expression level with Cu-64-DOTA-1C1 uptake in the tumors. Immunofluorescence staining and biodistribution studies were also carried out to validate the in vivo results. Results The radiolabeling yield was 88.9 +/- 9.5% (n = 7) and the specific activity of Cu-64-DOTA-1C1 was 1.32 +/- 0.14 GBq/mg of 1C1 mAb. The antibody retained antigen-binding affinity/specificity after DOTA conjugation as measured by FACS analysis. The uptake of Cu-64-DOTA-1C1 in CT-26 tumors was as high as 25.1 +/- 2.5 %ID/g (n = 3) at 18 h post injection. Cu-64-DOTA-IgG, an isotype-matched control, exhibited minimal non-specific uptake in all eight tumor models. In vivo EphA2 specificity of Cu-64-DOTA-1C1 was confirmed by successful blocking of CT-26 tumor uptake by unlabeled 1C1. Most importantly, the tumor uptake value obtained from PET imaging had excellent linear correlation with the relative tumor tissue EphA2 expression level measured by Western blot, where r(2) equals 0.90 and 0.92 at 18 h and 42 h post injection, respectively. Conclusion The tumor uptake of Cu-64-DOTA-1C1 measured by microPET imaging reflects tumor EphA2 expression level in vivo. This is, to our knowledge, the first report of quantitative radioimmunoPET imaging of EphA2 in living subjects. Future clinical investigation of Cu-64-DOTA-1C1 is warranted.