Chelator Free 64Cu-Labeled Gold Nanoparticles for PET imaging

Objectives Multifunctional gold nanoparticles (AuNPs) have been attracting interest for both fundamental scientific and biomedical applications. Surface modifications are essential for chemical stability, biocompatibility, and targeting efficacy. The i-colloid AuNPs used here were produced by femtosecond laser ablation with surfactant-free surfaces. The conjugation of functional ligands to AuNP surfaces is tunable between 0 and 100%. We have proven that there is high correlation between the density of RGD peptide on AuNP surfaces and tumor cell uptake using 125I-labeled AuNPs. However, the radioactive tags of 125I-labeled AuNPs were rapidly disassociated in vivo. We report here chelator-free 64Cu-Integrated AuNPs and preliminary microPET studies.

Methods AuNPs were synthesized by femtosecond laser ablation of metal targets immersed in water and stabilized with thiolated PEG 5K at different densities. 64CuCl2 was diluted in NH4OAc buffer and added to the AuNP solution, followed by N2H4 addition. The mixture was allowed to react at room temperature for 1 hour before being washed by centrifugation. MicroPET studies were performed using well PEGylated AuNPs (2000 PEG molecules on each particle) and bare AuNPs (100 PEGs per particle). About 150 µCi of 64Cu-Integrated AuNPs were injected into each mouse. The mice were imaged for 120 minutes, and then for 30 minutes after 24 hours.

Results 64Cu-integrated AuNPs were prepared by reducing copper-64 onto the surfaces of AuNPs in 60-90% radiochemical yields. MicroPET images showed that bare AuNPs localized in the liver within 5 minutes post injection, while PEGylated AuNPs were distributed throughout the body after a 2 hour imaging period. This indicated that the PEGylated AuNPs still remained in the blood circulation at that time. The radioactivity of PEGylated AuNPs was accumulated in the live after 24 hours. This result was in good agreement with previous studies of AuNPs and provided evidence that the radioactivity well represented the in vivo behaviors of AuNPs.

Conclusions The i-colloid AuNPs have been radiolabeled using a chelator-free copper-64 labeling method with excellent radiochemical yields. 64Cu-integrated AuNPs exhibited significant stability in vivo compared to 125I-labeled AuNPs. This methodology provides a highly efficient tool for monitoring in vivo behaviors of i-colloid AuNPs. Systematic investigation with various combinations of different peptides and densities are underway.