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

Objectives: The alpha sub(v) beta sub(3) 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 super(64)Cu-labeled peptides were compared. Methods: CB-TE2A-c(RGDyK) and diamsar-c(RGDfD) were labeled with super(64)Cu in 0.1 M NH sub(4)OAc (pH=8) at 95 degree C and 25 degree C, respectively. PET and biodistribution studies were carried out on M21 ( alpha sub(v) beta sub(3)-positive) and M21L ( alpha sub(v)-negative) melanoma-bearing mice. Binding affinity of the Cu-chelator-RGD peptides to alpha sub(v) beta sub(3) integrins was determined by a competitive binding affinity assay. Results: Biological studies showed higher concentration of super(64)Cu-CB-TE2A-c(RGDyK) in M21 tumor compared to M21L tumor at 1 and 4 h pi. Tumor concentration of super(64)Cu-CB-TE2A-c(RGDyK) was higher than that of super(64)Cu-diamsar-c(RGDfD). The difference is not due to differing binding affinities, since similar values were obtained for the agents. Compared to super(64)Cu-diamsar-c(RGDfD), there is more rapid liver and blood clearance of super(64)Cu-CB-TE2A-c(RGDyK), resulting in a lower liver and blood concentration at 24 h pi. Both super(64)Cu-labeled RGD peptides show similar binding affinities to alpha sub(v) beta sub(3). The differences in their biodistribution properties are likely related to different linkers, charges and lipophilicities. The M21 tumor is clearly visualized with super(64)Cu-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. Conclusions: Both super(64)Cu-CB-TE2A-c(RGDyK) and super(64)Cu-diamsar-cCRGDfD) 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.