PEGylated crushed gold shell-radiolabeled core nanoballs for in vivo tumor imaging with dual positron emission tomography and Cerenkov luminescent imaging

Background Radioactive isotope-labeled gold nanomaterials have potential biomedical applications. Here, we report the synthesis and characterization of PEGylated crushed gold shell-radioactive iodide-124-labeled gold core nanoballs (PEG-124I-Au@AuCBs) for in vivo tumor imaging applications through combined positron emission tomography and Cerenkov luminescent imaging (PET/CLI). Results PEG-124I-Au@AuCBs showed high stability and sensitivity in various pH solutions, serum, and in vivo conditions and were not toxic to tested cells. Combined PET/CLI clearly revealed tumor lesions at 1 h after injection of particles, and both signals remained visible in tumor lesions at 24 h, consistent with the biodistribution results. Conclusion Taken together, the data provided strong evidence for the application of PEG-124I-Au@AuCBs as promising imaging agents in nuclear medicine imaging of various biological systems, particularly in cancer diagnosis. Electronic supplementary material The online version of this article (10.1186/s12951-018-0366-x) contains supplementary material, which is available to authorized users.

AuNPs in healthy rats. Although these studies have demonstrated successful tumor visualization and biodistribution of these compounds in living subjects using positron emission tomography (PET), the loss of radioactive radioisotopes on AuNPs cannot be prevented because of the absence of rigid protective nanostructures, which leads to a decrease in the sensitivity and transchelation of radioactive isotopes to biological molecules and its sequential accumulation in nontarget organs, thereby increasing the risk of image misinterpretation. Therefore, the continued development of techniques is required to effectively protect the radioisotope embedded in AuNPs.
Recently, it has been documented that Cerenkov luminescence imaging (CLI) is a useful optical imaging modality that can be conducted using luminescent radionuclides, such as 64 Cu, 68 Ga, and 124 I. The introduction of positron emitting radioisotopes into gold nanomaterials has facilitated CLI in several preclinical and clinical settings [23][24][25][26], overcoming the limitations of nuclear medicine imaging.
Well-designed gold nanostructures with optimal sizes can be accumulated in tumors through enhanced permeability and retention (EPR) effects [27,28]. Furthermore, many attempts have been made to improve tumor targeting of imaging particles by changing the size or shape of nanostructures [29][30][31][32][33] or by modifying particle surfaces with hydrophilic polymers such as polyethylene glycol (PEG), which can interfere with aggregation and recognition by the reticuloendothelial system (RES) [34][35][36].
We previously used tannic acid-coated gold nanoparticles (TA-AuNPs) to develop a novel synthetic approach for production of highly sensitive and stable radiolabeled gold nanoparticles with gold shells ( 124 I-Au@AuNPs), and demonstrated the feasibility of this method for in vivo tracking of immunotherapeutic cells [37]. This method showed excellent sensitivity, stability, and biocompatibility, suggesting potential applications in disease diagnosis, particularly tumor diagnosis. On the basis of our previous findings, we postulated that 124 I-Au@AuNPs could be useful for the detection of various types of cancers in living subjects. Thus, we explored whether 124 I-Au@AuNPs could be targeted to tumor lesions by EPR effects following intravenous injection. To evaluate tumor targeting efficacy, 124 I-Au@AuNPs were modified with PEG polymers (PEG-124 I-Au@AuNPs) and administered into mice with breast cancer via tail vein injection, followed by biodistribution analysis at 24 h post-injection. As shown in Additional file 1: Figure S1, most injected radioactive particles accumulated in RES organs, including the liver and spleen [38], and radioactivity (0.015% ± 0.01% ID/g) was rarely found in breast tumors. This finding indicated that PEG-124 I-Au@AuNPs were not suitable for in vivo tumor imaging via passive targeting owing to potential long-term toxicity [39,40], thereby hampering their clinical use [41]. Therefore, these findings encouraged us to identify a new approach for enhancement of the tumor targeting ability of our novel imaging agents. Herein, through the introduction of changes in gold shell shape via modulation of pH concentrations, we developed novel PET/CLI imaging agents ( 124 I-Au@AuCBs) consisting of radioactive 124 I-labeled gold cores or crushed/PEGylated gold shells for visualization of tumor lesions in living mice. These materials were characterized, and the passive targeting capability of currently developed imaging agents were evaluated via EPR effects in a mouse model of breast cancer by biodistribution studies and combined PET/CLI imaging (Fig. 1).

Animals and cells
Specific pathogen-free immunocompetent 6-week-old BALB/c mice were obtained fromSLC, Inc. (Shizuoka, Japan). All experimental procedures involving animals were conducted in strict accordance with the appropriate institutional guidelines for animal research. The protocol was approved by the Committee on the Ethics of Animal Experiments of the Kyungpook National University (Approval Number: KNU 2012-43).

Preparation of radioactive gold core nanoparticles ( 124 I-AuNPs)
Iodination was performed by adding 100 μL of 222 MBq Na 124 I into 1.0 mL of 1.0 nM AuNPs in the presence of 90 μL of 3 mg/mL chloramine-T and 100 μL of 1% SDS to produce 124 I-AuNPs. The reaction, which was monitored by radioactive thin layer chromatography (radio-TLC: eluent, acetone), was completed within 15 min. After the reaction, the solution was centrifuged to remove free Na 124 I, and precipitates were redispersed in distilled water.

Calculation of 124 I density per AuNPs ( 124 I-AuNPs)
AuNPs concentration was determined by measuring the absorption at 520 nm using a UV-VIS spectrometer. The following parameters were used:   From (1) and (7):

Calculation of the number of AuNP moles
From (5) and (6):

Number of 124 I per AuNP
From (9)

Characterization of nanoparticles
UV-visible spectroscopy was conducted using a Cary 60 UV-Vis spectrophotometer (Agilent Technologies, Santa Clara, CA, USA), and X-ray photoelectron spectroscopy was performed using a Quantera SXM instrument (ULVAC-PHI; Chiasaki, Kanagawa, Japan). Transmission electron microscopy and energy dispersive X-ray mapping were performed using an FEI Tecnai F20 transmission electron microscope (FEI Company, Eindhoven, the Netherlands). X-ray photoelectron spectroscopy (XPS) and crystallography of the products were carried out using a PANalytical X-ray diffractometer. The hydrodynamic sizes of the nanoparticles were measured using a ζ-potential and particle size analyzer (ELS-Z, Otsuka, Japan). Powder XRD patterns were recorded on a Philips X'Pert PRO SUPER X-ray diffract meter system with CuKα radiation (λ = 1.542 Å, 40 kV, 30 mA) source. Fourier transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer spectrometer in the range between 4000 and 400 cm −1 .

Sensitivity tests
Changes in both PET and CLI signals with varying concentrations of PEG-124 I-Au@AuCBs (1.0 × 10 −13 to 1.0 × 10 −10 ) were examined using a PET system and an IVIS Lumina III (Perkin Elmer).

Stability tests in various pH solutions and serums
One hundred microliters of 1.0 nM 124 I-Au@AuCBs was incubated with 900 μL of various pH solutions and serum at 37 °C. The released radionuclide was quantified by TLC using an AR-2000 scanner (Bioscan, Washington, DC, USA).

Cell proliferation assays
Cell proliferation assays were performed using a Cell Counting Kit (CCK-8; Dojindo Laboratories, Tokyo, Japan) and CellTiter-Glo ® Luminescent Cell Viability Assay (Promega, Madison, USA). CHO, DC2.4, and 4T1 cells were seeded at 1 × 10 4 cells/well in 96-well plates. Either ten microliters of CCK-8 solution or CellTiter-Glo reagents was added to each well at 24 and 48 h after incubation with PEG-124 I-Au@AuCBs, and the plates were then incubated 37 °C for 1 h. Absorbance (at 450 nm) and luminescence was measured using a microplate reader (BMG Labtech, Offenburg, Germany).

Apoptosis analysis
CHO, DC2.4, and 4T1 cells were plated in 6-well plates. Cells were collected and stained with a solution of fluorescein isothiocyanate (FITC)-conjugated annexin V and propidium iodide (BD Science) after 24 and 48 h of incubation with PEG-124 I-Au@AuCBs. Flow cytometric analysis was performed using a BD Accuri C6 Flow cytometry (BD Biosicences).

In vivo imaging of combined PET and CLI
Breast cancer cell line (4T1) tumors were established by subcutaneous injection of 1 × 10 6 cells to right upper flank. When tumor volume is detectable by inspection and palpation, in vivo study was done.
For the PET/CT study, a 20-min (tumor imaging) scan was performed using the Triumph II PET/CT system (LabPET8; Gamma Medica-ideas, Wausha, WI, USA). The PET imaging system has the following characteristics: ring diameter, 162 mm; FOV, 60 mm; crystals, 3072; spatial resolution, 1.35 mm FWHM FOV; and noiseequivalent counts, 37 kcps at 245 MBq (250-650 keV). CT scans were performed with an X-ray detector (fly acquisition; number of projections: 512; binning setting: 2 × 2; frame number: 1; X-ray tube voltage 75 kVp; focal spot size 50 μm; magnification factor 1.5; matrix size 512) immediately following the acquisition of PET images. PET images were reconstructed using 3D-OSEM iterative image reconstruction. CT images were reconstructed using filtered back-projections. All mice were anesthetized using 1-2% isoflurane gas during imaging. PET images were co-registered with anatomical CT images using three-dimensional image visualization and analysis software (VIVID; Gamma Medica-ideas, Northridge, CA, USA). To measure the uptake (counts) for the volumes of interest (VOIs), each image was manually segmented from co-registered CT images using VIVID, and the radioactivity in the ROI was determined.
For in vivo CLI, images were acquired using an IVIS Lumina III imaging system (PerkinElmer). All luminescence images were measured from a bioluminescence channel with no excitation light or emission filters using IVIS Lumina III. The scan time was varied from 5 s to 5 min depending on the intensity of the emitted luminescence signal. The luminescence image was thresholded to maximize the visualization of the region of interest and to minimize background. Grayscale photographic images and bioluminescent color images were superimposed using LIVINGIMAGE version 2.12 (PerkinElmer) and IGOR Image Analysis FX software (WaveMetrics, Lake Oswego, OR, USA). CLI signals were expressed in units of photons per cm 2 per second per steradian (P cm −2 s −1 sr −1 ).

Biodistribution study
Mice (n = 5) were sacrificed at 1, 6, and 24 h post-injection of PEG-124 I-Au@AuCBs. Tissues were weighed and counted on a gamma counter. Uptake in each tissue was expressed as the percentage injected dose per gram of tissue (%ID g −1 ).

Ex-vivo PET and CLI
At 24 h postinjection of particles, mice were killed. Organs of interest, including tumors, were excised and placed in 6-well plates, followed by ex vivo PET/CT and CLI.

Histological examination
4T1 tumors were excised, fixed in formalin, embedded in paraffin, sectioned, stained with hematoxylin and eosin (H&E), and analyzed by light microscopy. The presence of gold particles in tumor lesions was determined by a histological specialist.

Statistical analysis
All data are expressed as means ± standard deviations, from at least three repeated experiments. Statistical significance was determined using unpaired Student's t tests with Graph Pad Prism version 5 statistical software (GraphPad Software, Inc.) Differences with p values of less than 0.05 were considered statistically significant.

Characterization of PEGylated crushed gold shell-radiolabeled core nanoballs (PEG-124 I-Au@AuCBs)
The shape and size of nanoparticles have been reported to be important factors for enhanced blood circulation and improved disease-targeting efficacy through minimizing nonspecific accumulation in RES organs. Thus, we attempted to produce effective tumor imaging probes via a passive approach by modifying the shape of gold shell nanostructures in PEG-124 I-Au@AuNPs. For production of effective tumor imaging agents, radioactive iodine-labeled gold core nanoparticles ( 124 I-AuNPs) were first produced (Fig. 2a (1) and (2)). TA-AuNPs have multiple functional sites that can be labeled with numerous radioactive iodides [42], and 124 I has been extensively investigated for PET imaging in preclinical and clinical settings [43]. Furthermore, many reports have demonstrated that 124 I is a useful radionuclide for CLI [24].
Thus, TA-functionalized gold nanoparticles were reacted with 124 I by simple incubation and stirring at room temperature, and radiolabeling progression was monitored with a radio-TLC scanner. As illustrated in Fig. 3, complete radiolabeling of TA-AuNPs was observed within 15 min, with a final radiochemical yield of 98% (Fig. 3c, black bar). Perrault et al. [44] reported that rodshaped nanomaterials have much higher blood circulation half-lives than spherical nanomaterials. Moreover, Lee et al. [45] reported the successful control of gold shell nanostructures such as a star-shaped shell with irregular nanogap and intra-nanogap distances by pH and NaCl concentrations. Accordingly, we attempted to modulate the shape of gold shell nanostructures through modulations of pH to protect the release of radioiodine from gold core nanoparticles by various biological factors in living subjects and effectively enhance passive tumor targeting efficiency. When additional gold shells were reacted onto 124 I-AuNPs in solution at pH 12, we observed crushed gold shell formation as early as 20 s after the start of the reaction, with a maximum at 50 s (Fig. 2b). The radiochemical yield of 124 I-Au@AuCBs was high (84%; Na 124 I 0.094 GBq, AuCBs 1 nM; Fig. 3c, red bar). To improve the blood circulation of 124 I-Au@AuCBs in vivo, these particles were further functionalized through 10 4 molar excess of PEGylation [MW: 5000; PEG-124 I-Au@AuCBs; Fig. 2a, (4)]. The final radiochemical yield of PEG-124 I-Au@AuCBs was 80% (Na 124 I: 0.089 GBq, mole: 1 nM; Fig. 3c, blue bar). Ultraviolet (UV)-visible spectroscopy of PEG-124 I-Au@AuCBs exhibited an identical wavelength to that of intermediate gold nanoparticles, indicating the absence of nanostructure aggregation during the reaction (Fig. 2c). Dynamic light scattering analysis revealed a hydrodynamic radius of 31.1 ± 1.3, 52.8 ± 2.1, and 87.1 ± 3.5 for 124 I-AuNPs, 124 I-Au@ AuCBs, and PEG-124 I-Au@AuCBs, respectively (Fig. 2d). X-ray photoelectron spectroscopy revealed the presence of Au and iodine in PEG-124 I-Au@AuCBs (Fig. 2e). Furthermore, PEG-124 I-Au@AuCBs had a bumpy surface morphology, as revealed by high-resolution transmission electron microscopy (HR-TEM; Fig. 2f (1)). Subsequently, the distribution of iodine was observed around the AuCBs (Fig. 2f, (2-4)) via X-ray energy dispersive mapping analysis. Zeta-potential (ζ-potential) analysis demonstrated that the surface charges of respective particles were − 46.13 ± 4.6, − 53.20 ± 0.8, − 32.97 ± 1.5, and − 0.62 ± 0.3 mV for AuNPs, 124 I-AuNPs, 124 I-Au@ AuCBs, and PEG-124 I-Au@AuCBs, respectively (Additional file 1: Figure S2b). Fourier transform infrared spectroscopy (FT-IR) analysis confirmed the presence of carbon, hydrogen, and oxygen bonding changes peaks in gold nanoparticles (Additional file 1: Figure S2a). X-ray diffraction (XRD) analysis also clearly showed the gold nanostructure, revealing unique Au reflections at 111, 200, 220, and 311° in the XRD spectra of PEG-124 I-Au@ AuCBs (Additional file 1: Figure S2c). HR-TEM analysis showed that the Au-Au lattice sizes ranged from 0.21 to 0.25 nm (Additional file 1: Figure S3).

In vitro evaluation of the sensitivity, stability, and cytotoxicity of PEG-124 I-Au@AuCBs
The sensitivity and stability of newly developed imaging agents are essential for selective detection and follow-up in various diseases [18]. Thus, we examined the sensitivity of PEG-124 I-Au@AuCBs with combined PET/CLI following serial dilution of the particles. Radioactivity and CLI signals increased in a dose-dependent manner, with good linearity between both imaging signals and particles (Fig. 4a-c, Additional file 1: Figure S4; R 2 = 0.72 for PET, R 2 = 0.77 for CLI). Radioactivity and CLI signals were detected as low as 0.1 pM, indicating high sensitivity. Furthermore, we observed a good correlation between PET signals and CLI signals (R 2 = 0.98 for PET and CLI; Fig. 4d).
In addition, we tested the stability of PEG-124 I-Au@ AuCBs in solutions with different pH and in serum. Notably, PEG-124 I-Au@AuCBs exhibited high stability (more than 90%) in solutions ranging from pH 1 to 14 (Fig. 5a, b, Additional file 1: Figures S5, S6). In various types of serum, PEG-124 I-Au@AuCBs also showed strong stability over 48 h (Fig. 5d-f, Additional file 1: Figure S5).
Next, we determined the biocompatibility of our PEG-124 I-Au@AuCBs by examining the cytotoxicity of these agents in various cell types. As shown in Additional file 1:  Figure S7, no differences in cell viability were observed in unlabeled and labeled groups at the tested concentrations of PEG-124 I-Au@AuCBs. Consistent with the results of cell proliferation assays, there were no differences in the level of apoptosis between unlabeled and labeled groups at all tested concentrations of PEG-124 I-Au@AuCBs (Additional file 1: Figure S8). These results suggest that PEG-124 I-Au@AuCBs had essential features of in vivo bioimaging probes.  Au@AuCBs (a, b). Quantification of radioactivity and Cerenkov luminescence signal of PEG-124 I-Au@AuCBs as a function of concentration. c Correlation between PET signal and CLI signal of PEG-124 I-Au@AuCBs. Multiwell plates containing solutions and PEG-124 I-Au@ AuCB particles were exposed to PET instruments and an IVIS imaging system, and radioactivity and CLI signals were quantified. Data are presented as mean ± standard deviation (SD)

In vivo detection of breast cancer with dual PET and CLI imaging
The feasibility of PEG-124 I-Au@AuCBs for in vivo detection of tumor lesions was then assessed in a mouse model of breast cancer. As shown in Additional file 1: Figure  S9, in vivo experiments were carried out, and PET/CT imaging showed the distinct uptake of PEG-124 I-Au@ AuCBs at tumor sites as early as 1 h post-injection, with a strong radioactive signal detectable at 24 h (Fig. 6a, c), consistent with the biodistribution examination (Additional file 1: Figure S10). The uptake value of particles in the tumor region (% injected dose per gram (%ID/g)) were 5.38 ± 3.57, 3.42 ± 2.15, and 1.81 ± 1.30 at 1, 6, and 24 h post-injection, respectively. Consistent with PET/ CT scanning, in vivo CLI clearly visualized the accumulation of PEG-124 I-Au@AuCBs in breast tumor lesions at 24 h after particle injection (Fig. 7a), a finding which was consistent with ex vivo CLI of excised organs (Fig. 7c). Accordingly, ex vivo imaging of combined PET/CLI clearly showed intensive signals in tumor lesions (Figs. 6b and 7b, Additional file 1: Figure S11). PET and CLI imaging of breast cancer revealed great linearity between the PET and CLI signals at 24 h (R 2 = 0.85, Additional file 1: Figure S12). As shown in Figs. 6 and 7, histological analysis demonstrated the accumulation of black nanocrushed ball-laden angiogenic vascular wall regions of tumors (Additional file 1: Figure S13), indicating the feasibility of applying PEG-124 I-Au@AuCBs as in vivo tumor imaging agents. Future efforts are needed to reduce RES accumulation and enhance passive tumor targeting efficiency through the introduction of various coating systems, including hyaluronic acid, human chorionic gonadotrophin, and antibodies.

Conclusions
In this study, we described the synthesis of PEG-124 I-Au@ AuCBs as novel oncological radionuclide biomedical imaging agents for in vivo tumor detection via passive tumor targeting. PEG-124 I-Au@AuCBs showed good sensitivity, stability, and biocompatibility under various biological conditions. Importantly, PEG-124 I-Au@ AuCBs facilitated rapid detection of breast carcinoma with a low dose of imaging agents through a passive targeting system with combined PET/CLI. Thus, our imaging agents may hold great promise for applications as inorganic nanomedicines in clinical practice. Although PEG-124 I-Au@AuCBs exhibit remarkable potential as tumor detection agents, major hurdles such as significant pathophysiological heterogeneity and the PEG dilemma, need to be overcome prior to their adoption in clinical practices via the RES system.