Theranostic PSMA-Targeted Gold Nanoparticles using X-ray Fluorescence Imaging Guided Prostate Tumor Radiotherapy

Abstract

Gold nanoparticles (GNPs) are promising as radiosensitizers, owing to their strong absorption of ionizing radiation and capacity to intensify the production of secondary electrons and reactive oxygen species. Although GNPs do passively leak into the tumor interstitium through enhanced permeability and retention (EPR) effect, its efficiency is low and whether the EPR effect is relevant in humans is still controversial. One purpose of our study is to design PSMA-targeted GNPs, to enhance targeting efficiency for prostate tumors, as well as to bring nanoparticles closer to the cellular nucleus for greater DNA damage following radiotherapy. We also developed an x-ray fluorescence computed tomography (XFCT) system to noninvasively monitor and assess the delivery of the targeted GNPs. Thiol-terminated polyethylene glycol is used to functionalize the GNP surface, to keep GNPs from aggregating and to avoid uptake by the reticular endothelial system. Simultaneously, the anti-PSMA antibodies were conjugated using 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxy sulfosuccinimide. GNP imaging was carried out on an in-house-developed dual-modality transmission CT and XFCT system. The performance of XFCT, in terms of quantitative and sensitive GNP imaging, was firstly evaluated on water phantoms containing GNP inserts of various concentration (0.02∼0.16 wt.%). Then in vivo imaging was conducted on LNCaP prostate tumor model to compare the tumor GNP concentration between active-targeting (GNP-anti-PSMA, n = 3) and passive-targeting (GNP-IgG, n = 3) groups. After GNPs (15 mg/ml, 200μL) were intravenous injected, tumor x-ray fluorescence signals were collected at various timepoints (5 min∼30 hr) for pharmacokinetics analysis, and in vivo 3D tumor XFCT imaging was conducted 24 hr post GNP injection. In vitro fluorescent microscopic imaging confirms the enhanced (∼4 times) targeting efficiency of PSMA-targeted GNPs. Phantom experiments validate that XFCT could accurately localize GNP targets with <0.6 mm offset, and sensitively quantify GNPs at a concentration of 0.02 wt.%. In vivo x-ray fluorescence-based tumor pharmacokinetics demonstrates the higher accumulation and longer retention of PSMA-targeted GNPs compared to passively administered GNPs. Active-targeting GNPs reached their peak accumulation in tumor at ∼24 hr after intravenous injection. In vivo tumor XFCT imaging shows its unique capabilities to simultaneously determine the heterogeneous spatial distribution and concentration of GNPs, wherein the maximum concentration of PSMA-targeted GNPs was ∼4 times higher than that of passive-targeting GNPs. PSMA-targeted GNPs exhibit enhanced prostate tumor targeting efficiency, showing their potential as targeted radiosensitizers. XFCT imaging localizes GNP targets and accurately quantifies GNP concentration, which is helpful to elucidate the mechanism of GNP-induced radiosensitization.