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

Objectives: The α_vβ₃ 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. Methods: CB-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 (α_vβ₃-positive) and M21L (α_v-negative) melanoma-bearing mice. Binding affinity of the Cu-chelator-RGD peptides to α_vβ₃ integrins was determined by a competitive binding affinity assay. Results: Biological 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 α_vβ₃. 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. Conclusions: Both 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.