Radiosynthesis and Bioevaluation of Glucose-modified Gold Nanoparticles

Objectives: Multifunctional gold nanoparticles (AuNPs) are highly attractive drug delivery vehicles due to their small size, flexibility, and biocompatibility. The AuNPs with capping agent-free surface used here were produced byfemtosecond pulsed laser ablation of bulk gold target in deionized (DI) water. The conjugation of functional ligands onto the AuNPs can be tunable between 0 and 100%. [¹⁸F]Fluorodeoxyglucose (FDG) is a glucose analog and has been used as the most popular tumor detection PET tracer for decades. Thus, we synthesized AuNPs quantitatively conjugated with a combination of PEG-glucose and various bioactive molecules to enhance the tumor uptake of AuNPs. The biodistribution and microPET imaging of ⁶⁴Cu-labeled AuNPs in tumor-bearing mice are reported.

Methods: Synthesis of AuNP conjugates: Glucose-PEG-SH was synthesized as Scheme 1. Colloidal AuNPs with capping agent-free surface were physically fabricated using femtosecond pulsed laser ablation of a bulk gold target in flowing DI water. For the preparation of tumor-targeting AuNP conjugates with high performance, AuNPs were conjugated with Glu-PEG-SH at a defined surface density first followed by further binding of the2^nd ligand (RGD peptide or methotrexate (MTX)) with predetermined amount too. This quantitatively-controllable surface engineering of AuNPs was achieved by precisely controlling the input molar ratio between the ligands to the colloidal AuNPs. Chelator-free Cu-64 labeling: Sodium ascorbate was dissolved in borate buffer and then ⁶⁴CuCl₂ aqueous solution diluted with ascorbate solution was added into AuNPs. The mixture was allowed to react at room temperature for 1 hour before washed with water by centrifugation. MicroPET study: Two cancer-bearing nude mice were used for each AuNP conjugate.. About 200 µCi of ⁶⁴Cu-AuNPs was injected to each animal (I.v., tail vein). The mice were imaged dynamically for 60 minutes at 22 and 45 hours post-injection. Volumes of interest (VOIs) were drawn over tumor and muscle to generate time-activity curves and tumor to muscle ratios (Figure 1). Biodistribution: After microPET imaging, mice were sacrificed and selected organs were rapidly removed. Tissue samples were weighed and radioactivities were counted. The radioactivity in each organ was calculated as the percent-injected dose per gram of tissue.

Results: Two kinds of i-colloid AuNPs (size 20 nm) were used for the experiments, glucose modified AuNPs conjugated with PEG-MTX or with RGD peptide. The surface modified AuNPs were stable and chelator-free Cu-64 labeling method gave yields of 39% and 41% for AuNP-MTX and AuNP-RGD, respectively. As shown in Figure 1, ⁶⁴Cu-AuNP-MTX-Glu and ⁶⁴Cu-AuNP-RGD-Glu displayed improved tumor uptake compared with our previous reported AuNP conjugates without glucose. Biodistribution also showed good activity retaining in the tumors (Figure 2).

Conclusions: We have successfully synthesized glucose modified gold nanoparticles conjugated with RGD or MTX. The preliminary microPET studies have proven that glucose coat increased tumor uptake indicating theses multifunctional AuNP conjugates have potential for tumor imaging and MTX therapy. Bioevaluation of theragnostic nanoparticles conjugated with various bioactive molecules is underway. Acknowledgement: We thank Jenelle Stauff, Janna Arteaga and Phil Sherman for assistance with animal studies.

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