Novel ⁶⁴Cu- and ⁶⁸Ga-labeled RGD conjugates show improved PET imaging of ανβ3 integrin expression and facile radiosynthesis

PET with ¹⁸F-labeled arginine-glycine-aspartic acid (RGD) peptides can visualize and quantify α_νβ₃ integrin expression in patients, but radiolabeling is complex and image contrast is limited in some tumor types. The development of ⁶⁸Ga-RGD peptides would be of great utility given the convenience of ⁶⁸Ga production and radiolabeling, and ⁶⁴Cu-RGD peptides allow for delayed imaging with potentially improved tumor-to-background ratios.Methods:We used the chelators DOTA,1,4,7- triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA), and 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2] hexadecane (CB-TE2A) to radiolabel the cyclic pentapeptide c(RGDfK) with ⁶⁸Ga or ⁶⁴Cu. NODAGA-c(RGDfK) was labeled at room temperature with both radionuclides within 10 min. Incubation at 95°C for up to 30 min was used for the other conjugates. The affinity profile of the metallopeptides was evaluated by a cell-based receptor-binding assay. Small-animal PET studies and biodistribution studies were performed in nude mice bearing subcutaneous U87MG glioblastoma xenografts. Results: The conjugates were labeled with a radiochemical purity greater than 97% and specific activities of 15-20 GBq/μmol. The affinity profile was similar for all metallopeptides and comparable to the reference standard c(RGDfV). In the biodistribution studies, all compounds demonstrated a relatively similar tumor and normal organ uptake at 1 h after injection that was comparable to published data on ¹⁸F-labeled RGD peptides. At 18 h after injection, however, ⁶⁴Cu-NODAGA-c(RGDfK) and ⁶⁴Cu-CB-TE2A-c(RGDfK) showed up to a 20-fold increase in tumor-to-organ ratios. PET studies demonstrated high-contrast images of the U87MG tumors at 18 h, confirming the biodistribution data. Conclusion: The ease of radiolabeling makes ⁶⁸Ga-NODAGA- c(RGDfK) an attractive alternative to ¹⁸F-labeled RGD peptides. The high tumor-to-background ratios of ⁶⁴Cu- NODAGA-c(RGDfK) and ⁶⁴Cu-CB-TE2A-c(RGDfK) at 18 h warrant testing of ⁶⁴Cu- labeled RGD peptides in patients.