Robust chelating stability under biological condi-tions is critical for the design of copper-based radiopharmaceuticals. In this study, the stabilities of 64Cu-DOTA and diamsar (two bifunctional Cu-64 chelators (BFCs)) conjugated DGEA peptides were evaluated. The in vitro stabilities of 64Cu-DOTA-DGEA, 64Cu-DOTA-Ahx-DGEA, and 64Cu-Z-E(diamsar)-Ahx-DGEA were evaluated in PBS. A carboxyl-protected DOTA-DGEA was also synthesized to study the potential inter- and intramolecular interactions between DOTA and the carboxylate groups of DGEA peptide. microPET imaging of 64Cu-DOTA-DGEA and 64Cu-Z- E(diamsar)-Ahx-DGEA were performed in PC-3 prostate tumor model to further investigate the in vivo behavior of the tracers. DOTA-DGEA, DOTA-Ahx-DGEA, Z-E(diamsar)-Ahx-DGEA, and protected DOTA-DGEA peptides were readily obtained, and their identities were confirmed by MS. 64Cu2+ labeling was performed with high radiochemical yields (>98%) for all tracers after 1 h incubation. Stability experiments revealed that 64Cu-DOTA-DGEA had unexpectedly high 64Cu2+ dissociation when incubated in PBS (>55% free 64Cu2+ was observed at 48 h time point). The 64Cu2+ dissociation was significantly reduced in the carboxyl-protected 64Cu-DOTA-DGEA complex but not in the 64Cu-DOTA-Ahx-DGEA complex, which suggests the presence of competitive binding for 64Cu2+ between DOTA and the carboxyl groups of the DGEA peptide. In contrast, no significant 64Cu2+ dissociation was observed for 64Cu-Z- E(diamsar)-Ahx-DGEA in PBS. For microPET imaging, the PC-3 tumors were clearly visualized with both 64Cu-DOTA-DGEA and 64Cu-Z-E(diamsar)- Ahx-DGEA tracers. However, 64Cu-DOTA-DGEA demonstrated 5× higher liver uptake than 64Cu-Z-E(diamsar)-Ahx-DGEA. This biodistribution variance could be attributed to the chelating stability difference between these two tracers, which correlated well with the PBS stability experiments. In summary, the in vitro and in vivo evaluations of 64Cu-Z-E(diamsar)-Ahx-DGEA and 64Cu-DOTA-DGEA have demonstrated the significantly superior Cu-chelation stability for the diamsar derivative compared with the established DOTA chelator. The results also suggest that diamsar may be preferred for Cu chelation especially when multiple carboxylic acid groups are present. Free carboxyl groups may naturally compete with DOTA for 64Cu2+ binding and therefore reduce the complex stability.
|Original language||English (US)|
|Number of pages||8|
|State||Published - Feb 16 2011|
ASJC Scopus subject areas
- Biomedical Engineering
- Pharmaceutical Science
- Organic Chemistry