64Cu-Labeled CB-TE2A and diamsar-conjugated RGD peptide analogs for targeting angiogenesis: comparison of their biological activity

ObjectivesThe αvβ3 integrin is a cell adhesion molecule known to be involved in stages of angiogenesis and metastasis. In this study, the chelators CB-TE2A and diamsar were conjugated to cyclic RGDyK and RGDfD and the biological properties of 64Cu-labeled peptides were compared.MethodsCB-TE2A-c(RGDyK) and diamsar-c(RGDfD) were labeled with 64Cu in 0.1 M NH4OAc (pH=8) at 95°C and 25°C, respectively. PET and biodistribution studies were carried out on M21 (αvβ3-positive) and M21L (αv-negative) melanoma-bearing mice. Binding affinity of the Cu-chelator–RGD peptides to αvβ3 integrins was determined by a competitive binding affinity assay.ResultsBiological studies showed higher concentration of 64Cu-CB-TE2A-c(RGDyK) in M21 tumor compared to M21L tumor at 1 and 4 h pi. Tumor concentration of 64Cu-CB-TE2A-c(RGDyK) was higher than that of 64Cu-diamsar-c(RGDfD). The difference is not due to differing binding affinities, since similar values were obtained for the agents. Compared to 64Cu-diamsar-c(RGDfD), there is more rapid liver and blood clearance of 64Cu-CB-TE2A-c(RGDyK), resulting in a lower liver and blood concentration at 24 h pi. Both 64Cu-labeled RGD peptides show similar binding affinities to αvβ3. The differences in their biodistribution properties are likely related to different linkers, charges and lipophilicities. The M21 tumor is clearly visualized with 64Cu-CB-TE2A-c(RGDyK) by microPET imaging. Administration of c(RGDyK) as a block significantly reduced the tumor concentration; however, the radioactivity background was also decreased by the blocking dose.ConclusionsBoth 64Cu-CB-TE2A-c(RGDyK) and 64Cu-diamsar-c(RGDfD) are potential candidates for imaging tumor angiogenesis. For diamsar-c(RGDfD), a linker may be needed between the Cu-chelator moiety and the RGD peptide to achieve optimal in vivo tumor concentration and clearance from nontarget organs.