Crushed Gold Shell Nanoparticles Labeled with Radioactive Iodine as a Theranostic Nanoplatform for Macrophage-Mediated Photothermal Therapy

Highlights Crushed gold shell radionuclide nanoballs (124I-Au@AuCBs) were fabricated as unique photothermal therapeutics and nuclear medicine imaging nanoplatforms. Macrophage-mediated delivery of 124I-Au@AuCBs and their potential photothermal therapy applications were demonstrated in mice with colon cancer. Electronic supplementary material The online version of this article (10.1007/s40820-019-0266-0) contains supplementary material, which is available to authorized users.

Several approaches have been adopted to effectively deliver photothermal agents to tumor lesions, via passive or active targeting [1,[20][21][22][23][24][25][26]. For passive targeting, via enhanced permeability retention (EPR) effects, several types of coating materials (e.g., polyethylene glycol polymer, chitosan) have been introduced to the surface of AuNPs. To actively target tumor lesions, tumor-specific antibodies or ligands have been adopted to functionalize the AuNPs. Although these approaches have yielded promising results in delivering photothermal agents to tumors, nanoparticles can be entrapped in organs capable of EPR, such as the liver, kidney, and spleen, thereby inducing severe toxicity in normal organs. As an alternative delivery approach, intratumoral injection is useful to treat breast cancer and melanoma, owing to the accurate targeted delivery of PTT agents. However, most injected particles are retained in tumor lesions proximal to the site of injection and cannot penetrate deep into the tumors, thereby resulting in poor therapeutic outcomes. Thus, new approaches should be investigated for the effective delivery of photothermal agents to tumor lesions to elicit drastic therapeutic responses and to reduce toxicity in vital organs.
Macrophages are suitable transporters of various nanoparticles because they are present in the circulation and are easy to harvest from patients. Furthermore, owing to their unique properties, they easily engulf nanoparticles, such that each cell behaves as a "Trojan horse" delivery system, enabling the infiltration of otherwise inaccessible tumor lesions. Furthermore, several studies have reported the successful macrophage-mediated delivery of theranostic biomaterials to tumor lesions and resulting therapeutic effects in glioma, liver, and lung cancer models [27][28][29][30][31], revealing its great potential for cancer treatment.
We recently developed highly stable, biocompatible, and sensitive radioiodine-labeled AuNPs with crushed gold shells ( 124 I-Au@AuCBs) as positron emission tomography/computed tomography (PET/CT) imaging agents for in vivo tumor imaging [32] and suggested their possible use in various biological applications. However, despite interesting findings regarding 124 I-Au@AuCBs as useful biomaterials, we have not yet investigated the possibility of using 124 I-Au@AuCBs as PTT agents. The generation of effective photothermally converted biomaterials is facilitated by the nanogap between the gold core and the gold shell, thus transforming these nanomaterials into promising theranostic biomaterials.
This study aimed to investigate the feasibility of using 124 I-Au@AuCBs as photothermal therapeutics and imaging nanoplatforms in mice with colon cancer (as shown in Fig. 1) and the possibility of macrophage-mediated delivery of 124 I-Au@AuCBs for PTT applications in mice with colon cancer.

Materials and Instruments
All chemical reagents and tannic acid-capped AuNPs were purchased form Ted Pella, Inc. (Redding, CA, USA), and Na 124 I (half-life 4.2 days, emission type: high-energy γ and positron, energy 811 keV) was provided by KIRAMS (Seoul, South Korea). HAuCl 4 was purchased from Sigma-Aldrich (St Louis, MO, USA).
PET/CT imaging was performed with a PET/CT scanner (LabPET8; Gamma Medica-Ideas, Waukesha, WI, USA). Bioluminescence imaging (BLI) was performed using an IVIS Lumina III instrument (PerkinElmer, Waltham, MA, USA). Photothermal images were obtained using a digital thermometer (TES Electrical Electronic Corp, Taipei, Taiwan) and an NIR imaging camera (FLIR Systems, Wilsonville, OR, USA).

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

Characterization of Crushed Gold Shell Radionuclide Nanoballs
UV-Vis spectroscopy was performed using a Cary 60 UV-Vis spectrometer (Agilent Technologies, Santa Clara, CA, USA). Transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) mapping were performed using an FEI Tecnai F20 transmission electron microscope (FEI Company, Eindhoven, the Netherlands). The hydrodynamic size of the nanoparticles was measured using ζ-potentials (ELS-Z, Otsuka, Japan). Fourier transform infrared (FT-IR) spectra were recorded on a PerkinElmer spectrometer in the range between 4000 and 400 cm −1 .

Photothermal Conversion In Vitro
The photothermal conversion was evaluated using an 808 nm NIR laser (LVI, Anyang, South Korea). 124 I-Au@AuNPs and 124 I-Au@AuCBs were suspended in glass vials (1 mL) at different concentrations (0.5, 1, 2, and 4 nM) and exposed to the NIR laser at different power settings (3, 6, and 9 W cm −2 ). The temperatures of the suspensions were recorded using a digital thermometer (TES Electrical Electronic Corp, Taipei, Taiwan), with an accuracy of ± 0.1 °C. To determine whether exposure to the NIR laser could damage the morphology of the gold nanomaterials, they were examined using TEM, before and after irradiation at 9 W cm −2 for 5 min.

In Vitro Photothermal Therapy
The following experiments were performed to evaluate the photothermal therapeutic effects of 124 I-Au@AuCBs on cancer cells.

Study 1
Either CT26 or CT26/FM cells were seeded in 96-well plates and incubated with 124 I-Au@AuCBs (2 nM) for 3 h, followed by irradiation with an NIR laser (808 nm, 6 W cm −2 ). After 2 days, cell morphology was determined by microscopic imaging (Leica, Wetzlar, Germany). Cell proliferation was evaluated using a Cell Counting Kit (CCK-8; Dojindo Laboratories, Tokyo, Japan), and in vitro bioluminescent imaging was performed using an IVIS Lumina III instrument.
For cell proliferation assays, 10 µL of CCK-8 solution was added to treated cells and the absorbance was measured at 450 nm using a microplate reader (BMG Labtech, Offenburg, Germany).

Study 2
Either CT26 or CT26/FM cells were seeded in 96-well plates at 24 h and incubated with 124 I-Au@AuCB (2 nM)labeled macrophages, followed by exposure to an NIR laser (808 nm, 6 W cm −2 ) for 5 min. Two days later, cell viability was determined by in vitro bioluminescent imaging using an IVIS Lumina III instrument.

Apoptosis Analysis
Treated cells were collected, stained with FITC-conjugated annexin V and propidium iodide (BD Biosciences, San Jose, CA, USA), and analyzed by flow cytometry using a BD Accuri C6 flow cytometer (BD Biosciences).

Study 1
CT26 cells were injected subcutaneously into mice, and tumor-bearing mice were divided into the following two groups when tumor mass was detected by physical inspection and palpation: Group 1, free 124 I-injected group; Group 2, 124 I-Au@AuCB-injected group. Tumor lesions were exposed to an NIR laser (808 nm, 6 W cm −2 , 5-min exposure), and temperature changes in the tumor lesion were monitored using a digital thermometer. Nine days after therapy, the tumor was excised and weighed.
After injection of 124 I-Au@AuCB-labeled macrophages into each tumor, PET/CT imaging was performed to determine the successful delivery of the particles to the tumor site. After image acquisition (3 h after intratumoral injection of 124 I-Au@ AuCB-labeled macrophages), the tumor lesion was exposed to an NIR laser (808 nm, 6 W cm −2 , 5-min exposure) and temperature changes in the tumor lesion were monitored using a digital thermometer. Therapeutic response was evaluated by in vivo fluorescent imaging of the mCherry reporter gene. At day 9 after therapy, tumors were excised and weighed.

PET/CT Imaging
For computed tomography imaging, a 20-min scan (tumor lesion imaging) was performed using a Triumph II PET/CT system (LabPET8; Gamma Medica-Ideas). For PET/CT imaging, a 15-min scan was performed using the same animal PET/CT system as described above. 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). CT images were reconstructed using filtered back-projections. All mice were anesthetized using 1-2% isoflurane gas during imaging. CT images were reconstructed using the 3D image visualization and analysis software, VIVID (Gamma Medica-Ideas).

In Vivo Fluorescent Imaging
For CT26/FM tumor imaging, in vivo fluorescent imaging (FLI) was performed at the indicated times after PTT, using an IVIS Lumina III instrument with filter settings for mCherry. Grayscale photographic images and fluorescent color images were superimposed using LIVING IMAGE (version 2.12, PerkinElmer) and IGOR Image Analysis FX software (WaveMetrics, Lake Oswego, OR, USA). FLI signals were expressed in units of photon per cm 2 per second per steradian (P cm −2 s −1 sr −1 ).

Statistical Analysis
All data are expressed as the mean ± standard deviation (SD) from at least three representative experiments, and statistical significance was determined by unpaired Student's tests using Prism 5 software (GraphPad, San Diego, CA, USA). Differences with P values less than 0.05 were considered statistically significant.

Characterization of 124 I-Au@AuCBs as Photothermal Agents
We recently developed highly sensitive and stable PET/CT imaging agents, comprising a radioiodine-labeled gold core and a crushed gold shell ( 124 I-Au@AuCBs) [32] or round gold shell ( 124 I-Au@AuNPs) [33]. Several studies have reported the importance of morphological modification of nanomaterials and intra-nanogaps for the induction of photothermal conversion. 124 I-Au@AuCBs and 124 I-Au@AuNPs exhibit a crushed or round gold shell shape, respectively, with an intra-nanogap of 0.21-0.25 nm. These results led us to further examine the possibility of their use as new photothermal nanomaterials. First, we investigated the localized surface plasmon resonance (LSPR) peaks of our imaging agents. The LSPR peaks were approximately 520 and 600 nm for the 124 I-Au@ AuNPs and 124 I-Au@AuCBs, respectively (Fig. S1). Furthermore, we determined the effect of photothermal conversion of 124 I-Au@AuNPs and 124 I-Au@AuCBs after irradiation with an 808 nm NIR laser in micro-glass vials. As shown in Fig. 2a-c, NIR laser beam (3, 6, and 9 W cm −2 ) energy-dependent and AuNP (0.5, 1.0, 2.0, and 4.0 nM) concentration-dependent photothermal conversion effects were observed in 124 I-Au@AuCB solutions, but not in PBS or 124 I-Au@AuNP solutions. To investigate whether heating of the NIR laser influenced the nanostructure of 124 I-Au@ AuCBs, high-resolution transmission electron microscopic (HR-TEM) examination of the gold nanoplatform was performed, before and after irradiation. When compared with its structure prior to irradiation, no obvious morphological changes were detected after irradiation (Fig. 2d, e). These results suggest that the unique structure of 124 I-Au@AuCBs could undergo photothermal conversion, with excellent stability, during PTT.
In addition, we investigated various characteristics of 124 I-Au@AuCBs. The 124 I-Au@AuCBs had bumpy surfaces, as evident from HR-TEM results. Subsequently, Au and I distributions were determined around the AuCBs by X-ray energy distribution mapping analysis (Fig. S2). Zetapotential analysis results revealed that the surface charges of the particles were − 44.73 ± 2.18, − 50.73 ± 5.49, and − 31.66 ± 3.2 mV for AuNPs, 124 I-AuNPs, and 124 I-Au@ AuCBs, respectively (Fig. S3a). FT-IR spectroscopy analysis revealed peaks at 1213 cm −1 for CH 2 , 1651 cm −1 for C=O stretching, 2858 cm −1 for C-H stretching, and 3408 cm −1 for O-H stretching. The FT-IR spectrum of AuCB products was the same as the spectra of the products of chemical reactions of their respective gold nanomaterials (Fig. S3b).

Photothermal Therapeutic Effect of 124 I-Au@ AuCBs on Colon Cancer In Vitro and In Vivo
The photothermal therapeutic effect of 124 I-Au@ AuCBs was determined by a cell proliferation assay.  NS not significant unlabeled and labeled groups at the tested doses, indicating the absence of cytotoxicity. Furthermore, to demonstrate the photothermal therapeutic effects of 124 I-Au@AuCBs on colon cancer CT26 cells, cells were incubated with 124 I-Au@AuCBs (2 nM, 3 h) and then irradiated with a laser beam (6 W cm −2 , 5 min), accompanied by photothermal imaging. Irradiated cells were seeded in 96-well plates and further incubated (additional 12 h) to evaluate PTT-mediated cytotoxic effects. As shown in Figs. 3a and S4b, a rapid temporary increase in temperature was observed immediately after irradiation. The CCK-8 assay results (Fig. 3c) showed a marked reduction in cell proliferation in irradiated and 124 I-Au@AuCB-labeled cells, but not in non-irradiated or 124 I-Au@AuCB-labeled cells, which concurred with the results of microscopic analysis (Fig. 3b) and annexin V/PI staining (Fig. 3d). These observations indicate that 124 I-Au@AuCBs induced in vitro PTT-mediated cytotoxic effects via effective photothermal conversion.
Twenty minutes after the intratumoral injection of 124 I-Au@ AuCBs, tumor lesions were exposed to the laser (6 W cm −2 , 808 nm, Fig. S5). Infrared thermal imaging was performed to monitor temperature changes in the tumor lesions during PTT. As shown in Fig. 4b, c, the temperature of the 124 I-Au@ AuCB-injected tumor increased with increasing irradiation time. After irradiation for 5 min, the temperatures were 31 and 52.2 °C in the free 124 I-injected tumor and the 124 I-Au@ AuCB-injected tumor, respectively. Three minutes after irradiation, the temperature increased beyond 43 °C, which is known to damage cancer cells, owing to their relatively low heat tolerance resulting from a poor blood supply [34]. After NIR laser irradiation, the tumor-bearing mice were maintained and PTT-therapeutic outcomes were evaluated 9 days post-treatment, via both CT imaging and tumor weight measurement. CT imaging (Fig. 4a, d) showed a marked reduction in tumor mass in mice treated with 124 I-Au@AuCBs + laser, which concurred with the results of tumor weight measurements (Fig. 4e). These data indicate that 124 I-Au@AuCBs have unique properties to induce PTT-mediated cytotoxic Tumor weight (g) Fig. 4 Evaluation of the effects of photothermal therapy with 124 I-Au@AuCBs in CT26 tumor-bearing mice. a, d Photograph and computed tomographic imaging of CT26 tumor-bearing mice, before and after PTT with 124 I-Au@AuCBs. CT26 tumor-bearing mice were administered either 124 I or 124 I-Au@AuCBs, and tumors were exposed to an NIR laser irradiation. b Temperature changes in either free 124 I-or 124 I-Au@AuCB-injected CT26 tumors after irradiation with an NIR laser (9 W cm −2 , 5 min). c NIR imaging of CT26 tumor-bearing mice, before and after irradiation with an NIR laser. e Measurement of tumor weight after photothermal therapy. Inset: upper, CT26 tumors with 124 I and NIR laser treatment; bottom, CT26 tumors with 124 I-Au@AuCBs and NIR laser treatment. In vivo experiments were performed in duplicate (n = 7 mice per group). ***P < 0.0001 effects via effective photothermal conversion in vitro and in vivo.

Effects of PTT with 124 I-Au@AuCB-Labeled Macrophages on Colon Cancer In Vitro and In Vivo
The development of nanoparticle systems with a high uptake efficiency is vital for successful cell-mediated photothermal therapy. Macrophages can be easily labeled with nanoparticles owing to their unique phagocytic activity [35]. Thus, several studies have attempted to load therapeutic or imaging particles into macrophages for cell tracking and effective in vivo delivery of cytotoxic agents (doxorubicin) throughout tumor lesions [29,31,36,37].
In the present study, 124 I-Au@AuCBs were directly introduced into murine macrophage Raw 264.7 cells and microscopic examination was performed to confirm the presence of the particles within the macrophages. Concurrent with previous reports, 124 I-Au@AuNPs were easily taken up by macrophages without any assistance and labeled cells were darkened owing to the presence of AuNPs (data not shown). When laser irradiation was applied to 124 I-Au@AuNPlabeled macrophages, effective photothermal conversion was observed in 124 I-Au@AuNP-labeled macrophages, but not in unlabeled macrophages (Fig. 5b, c). 124 I-Au@AuNP-labeled macrophages reached temperatures greater than 43 °C after irradiation, which is essential for successful PTT.
Thereafter, we evaluated whether 124 I-Au@AuNP-labeled macrophages resulted in the death of colon cancer cells in the following groups: control group, NIR laser group, 124 I-Au@ AuNP-labeled macrophages group, 124 I-Au@AuNP-labeled macrophages + NIR laser treatment group, and unlabeled macrophages + NIR laser treatment group. To visualize cell viability in vitro and in vivo, we engineered CT26 cells to co-express firefly luciferase and mCherry as optical reporter genes (CT26/FM cells). Briefly, CT26/FM cells were co-incubated with 124 I-Au@AuNP-labeled macrophages and their respective cells were irradiated. After an additional incubation for 12 h, the in vitro effects of PTT were confirmed using BLI and apoptosis analysis. In vitro BLI (Fig. 5d) and apoptosis assays (Fig. 5e) showed the lowest BLI signals and the highest number of annexin V/PI-positive cells in the 124 I-Au@ AuNP-labeled macrophages + NIR laser treatment group. These findings indicate that 124 I-Au@AuCBs engulfed by macrophages retain the potential for photothermal conversion, thereby effectively eliminating colon cancer cells surrounding the 124 I-Au@AuNP-labeled macrophages.
Finally, the in vivo effects of PTT using 124 I-Au@AuCBlabeled macrophages were evaluated in CT26/FM tumorbearing mice (Fig. 6a). When the tumors approached approximately 100-120 mm 3 , mice were divided into the following five groups: control, laser-treated, 124 I-Au@AuNP-labeled macrophages, 124 I-Au@AuNP-labeled macrophages + lasertreated, and unlabeled macrophages + laser-treated groups. PET/CT imaging was performed 5 min after the intratumoral injection of 124 I-Au@AuNP-labeled macrophages. After image acquisition, NIR laser irradiation (6.0 W cm −2 , 808 nm) was administered to the tumor site, 3 h after macrophage injection. Temperature changes in tumor lesions were monitored using a digital thermometer. The therapeutic effects of PTT were determined by in vivo fluorescent imaging of the mCherry reporter gene. Zhibin et al. [31] also reported the therapeutic potential of the delivery of PTT agents via macrophages. They demonstrated more effective distribution around the tumor site and could overcome the extracellular matrix barrier and penetrate deeper into tumor sites, resulting in enhanced tumor inhibition compared with intratumoral injection of the PTT agent alone. The present study differed from the study by Zhibin et al., in terms of the irradiation time points. In the previous study, tumors were irradiated 48 h after the intratumoral injection of labeled macrophages. In our study, tumors sites were directly irradiated by an NIR laser 3 h after transferring the 124 I-Au@ AuNP-labeled macrophages. Using our current approach, 124 I-Au@AuCB-labeled macrophages were observed to be evenly distributed in tumor lesions upon PET/CT imaging, with intense signals in tumor lesions receiving 124 I-Au@ AuCB-labeled macrophages; this indicated the successful transfer of 124 I-Au@AuCBs into tumor lesions, with macrophages acting as Trojan horses (Fig. 6c). As seen in Fig. 6d, e, tumors injected with 124 I-Au@AuCB-labeled macrophages approached 55.2 °C within 5 min, at which point cancer cells were effectively eliminated. Consistent with this observation, fluorescent signals were markedly reduced in the 124 I-Au@AuNP-labeled macrophages + lasertreated group at post-therapy day 1, and the signals then gradually decreased until day 9 (Fig. 6b, f). Accordingly, tumor weight was also the lowest in the macrophages + lasertreated group (Fig. 6g). However, the temperature in the tumors of the control + laser-treated and unlabeled  AuCB-labeled macrophages after irradiation with an NIR laser for 5 min. The red line indicates temperature changes in media irradiated with an NIR laser. d, e Evaluation of cell proliferation and apoptosis levels in CT26/FM alone (black), CT26/FM + NIR laser (red), CT26/FM + 124 I-Au@AuCB-labeled macrophages (blue), CT26/FM + 124 I-Au@AuCB-labeled macrophages + NIR laser (green), and unlabeled macrophages alone + laser (purple) using in vitro bioluminescent imaging and annexin V/PI staining, respectively. **P < 0.001, ***P < 0.0001 (1) (2)  Fig. 6 Ablation of colon cancer by 124 I-Au@AuCB-labeled macrophage-mediated PTT. a Scheme for 124 I-Au@AuCB-labeled macrophagemediated PTT in vivo. In vivo PET/CT imaging of CT26/FM tumor-bearing mice, intratumorally injected with 124 I-Au@AuCB-labeled macrophages. Experimental groups: (1) control, (2) laser alone, (3) 124 I-Au@AuCB-labeled macrophages, (4) 124 I-Au@AuCB-labeled macrophages + laser-treated, and (5) macrophages + laser-treated. b In vivo FLI showing the therapeutic response to 124 I-Au@AuCB-labeled macrophage-mediated PTT. c PET/CT imaging of CT26/FM tumor-bearing mice administered either unlabeled or labeled macrophages and subsequently irradiated with an NIR laser (6 W cm −2 , 5 min). d Temperature and e temperature changes (Δ temperature) in CT26/FM tumors, before and after irradiation with an NIR laser (6 W cm −2 , 5 min). f Quantification of FLI signals in CT26/FM tumors, before and after PTT. g Photograph of an excised CT26/FM tumor and determination of tumor weight from respective groups. In vivo experiments were performed in duplicate (n = 7 mice per group). ***P < 0.0001 macrophage + laser-treated groups was less than 43 °C. Accordingly, we did not observe significant tumor reduction in the control + laser-treated, 124 I-Au@AuNP-labeled macrophages, or unlabeled macrophages + laser-treated groups. Recently, Baek et al. [22] reported that peritumoral injection of macrophages loaded with gold nanoshells is more effective than intratumoral injection for PTT, due to complete migration of the macrophages throughout the tumor. Of note, we also need to further investigate the effect of 124 I-Au@AuNP-loaded macrophages on PTT, when they are administered via another injection route, such as peritumoral injection.

Conclusion
This study evaluated the possibility of using 124 I-Au@ AuCBs as a novel theranostic nanoplatform for in vivo PTT. 124 I-Au@AuCBs exhibited great photothermal conversion ability, and macrophage labeling was achieved via a simple incubation, with no associated cytotoxicity. Furthermore, the effects of 124 I-Au@AuCBs on PTT were retained upon macrophage uptake, thereby inducing discernible cell death in tumors in vitro. More importantly, it is possible to visualize the successful delivery of 124 I-Au@AuCBs to tumor lesions by using macrophages as Trojan horses, owing to the presence of 124 I, which can be detected by PET imaging. Finally, 124 I-Au@AuCB-labeled macrophages resulted in significant tumor ablation, suggesting that this is a potentially effective photothermal approach for optimizing nanoplatform delivery in vivo. Further studies are required to examine the tracking of 124 I-Au@AuNP-loaded macrophages to tumor lesions after intravenous injection and to assess the potential for translation of our experimental findings to clinical applications in cancer therapy.