Facile Formation of Gold-Nanoparticle-Loaded γ-Polyglutamic Acid

Communication Cite This: Bioconjugate Chem. XXXX, XXX, XXX-XXX

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Facile Formation of Gold-Nanoparticle-Loaded γ‑Polyglutamic Acid Nanogels for Tumor Computed Tomography Imaging Jianzhi Zhu,†,‡,∥ Wenjie Sun,‡,∥ Jiulong Zhang,†,∥ Yiwei Zhou,‡ Mingwu Shen,‡ Chen Peng,*,† and Xiangyang Shi*,†,‡,§ †

Department of Radiology, Shanghai Tenth People’s Hospital, School of Medicine, Tongji University, Shanghai 200072, People’s Republic of China ‡ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, People’s Republic of China § CQM-Centro de Química da Madeira, Universidade da Madeira, Campus da Penteada, 9000-390 Funchal, Portugal S Supporting Information *

ABSTRACT: The formation of gold nanoparticle (Au NP)-loaded γpolyglutamic acid (γ-PGA) nanogels (NGs) for computed tomography (CT) imaging of tumors is reported. γ-PGA with carboxyl groups activated by 1-ethyl3-[3-(dimethylamino)propyl] carbodiimide hydrochloride is first emulsified to form NGs and then in situ chemically cross-linked with polyethylenimine (PEI)entrapped Au NPs with partial polyethylene glycol (PEG) modification ([(Au0)200−PEI·NH2−mPEG]). The formed γ-PGA−[(Au0)200−PEI·NH2− mPEG] NGs with a size of 108.6 ± 19.1 nm display an X-ray attenuation property better than commercial iodinated small-molecular-contrast agents and can be uptaken by cancer cells more significantly than γ-PGA-stabilized single Au NPs at the same Au concentrations. These properties render the formed NGs with an ability to be used as an effective contrast agent for the CT imaging of cancer cells in vitro and a tumor model in vivo. The developed hybrid NGs may be promising for the CT imaging or theranostics of different biosystems.

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NPs have been demonstrated to be effective contrast agents for CT imaging.12,16−19 For high-performance tumor CT imaging, it is essential to render the particles with targeting specificity or enhanced tumor uptake. In most cases, single polymer-coated Au NPs suffer problems of restricted uptake in cancer cells and, thus, limited tumor penetration, thereby restricting their applications in high-performance CT imaging of tumors. Polymer nanogels (NGs), as a kind of soft particles (diameter smaller than 1000 nm), have been widely used for biomedical applications because they possess both the advantages of bulk hydrogels and features of NPs.20−23 Owing to the softness and admirable fluidity, NGs are able to be easily uptaken by cells, which is particularly beneficial for enhanced drug delivery and medical diagnosis applications.22 In our previous work, PEI-coated superparamagnetic iron oxide (Fe3O4) NPs were employed as a cross-linker to prepare the γpolyglutamic acid (γ-PGA)24 or alginate25 NGs loaded with Fe3O4 NPs for MR imaging of tumors. The formed NGs can be more significantly uptaken by cancer cells than single Fe3O4 NPs with the same surface charge polarity, which rendered them with an ability for enhanced T2-weighted magnetic

n recent years, the great development of nanotechnology has significantly advanced the precision medicines for disease diagnosis and therapy, especially the molecular imaging technology in terms of the scope, precision, and sensitivity.1−8 Computed tomography (CT) imaging, as a reliable medical imaging modality, has gained significant attention due to its unique advantages including deep penetration capability, great density and spatial resolution, and facile image reconstruction process.9 For high CT imaging quality and sensitivity, contrast agents are necessary to improve the X-ray attenuation of the location of diseases.10 Iodine-based small molecules (e.g., Omnipaque) are the generally used clinical contrast agents. However, the CT imaging quality of these kinds of iodine-based small molecules has been restricted by their shortcomings, such as renal toxicity, short imaging time, and nonspecificity.11 A variety of nanoparticulate contrast agents such as gold nanoparticles (Au NPs),12 bismuth sulfide NPs,13 and ytterbium-based NPs14 have been developed for blood pool or tumor CT imaging. In particular, Au NPs have attracted considerable attention in CT imaging due to the higher atomic number of Au than that of iodine, thus having better X-ray attenuation property than iodine-based CT contrast agents. In addition, Au NPs can be easily functionalized to have improved cytocompatibility,15 prolonged blood circulation time,16 higher imaging sensitivity,17 and targeting specificity.18 In our previous work, dendrimer- and polyethylenimine (PEI)-entrapped Au © XXXX American Chemical Society

Received: September 21, 2017 Revised: October 26, 2017 Published: October 30, 2017 A

DOI: 10.1021/acs.bioconjchem.7b00571 Bioconjugate Chem. XXXX, XXX, XXX−XXX

Communication

Bioconjugate Chemistry Scheme 1. Schematic Illustration of the Preparation of the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs

Figure 1. (a) UV−vis spectra of [(Au0)200−PEI·NH2−mPEG] NPs and γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs. (b) A typical TEM image of the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs. (c) CT images of the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs under varying Au concentrations (0.005, 0.01, 0.02, and 0.04 M, respectively) and the X-ray attenuation intensity of the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs (1) and Omnipaque (2) as a function of Au or I concentration.

knowledge, no previous reports regarding the formation of Au NP-loaded γ-PGA NGs for CT imaging applications have been published.

resonance (MR) imaging of tumors. Alternatively, synthetic poly(N-vinylcaprolactam) (PVCL) NGs synthesized via precipitation polymerization can be covalently linked with chelators for Gd(III) loading, thereby enabling enhanced T1weighted MR imaging of tumors due to the improved r1 relaxivity.26 These studies highlight the importance to use NGs to load imaging agents for enhanced tumor imaging. It is reasonable to hypothesize that NGs loaded with Au NPs can also be utilized for enhanced tumor CT imaging due to the better cellular uptake of the NGs than single inorganic NPs by combination of both advantages of Au NPs and polymer NGs.24,25 In this present work, Au NP-loaded γ-PGA NGs were synthesized by a double emulsion approach for tumor CT imaging. γ-PGA was first reacted with 1-ethyl-3-[3(dimethylamino)propyl] carbodiimide hydrochloride (EDC) to activate its carboxyl group and then emulsified. After in situ cross-linking by PEI-coated Au NPs ([(Au0)200−PEI·NH2− mPEG]) synthesized via NaBH4 reduction method, the hybrid NGs were formed and thoroughly characterized. The NGs were subjected to cytotoxicity and cellular uptake tests before they were used for CT imaging of cancer cells in vitro and the xenografted tumor model in vivo. To the best of our

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RESULTS AND DISCUSSION Via a similar double-emulsion method reported in our previous work,24 γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs were prepared when the [(Au0)200−PEI·NH2−mPEG] NPs formed by NaBH4 reduction were used as a cross-linker (Scheme 1). The prepared hybrid NGs were then systematically characterized. The surface potential and hydrodynamic size of the synthesized [(Au0)200−PEI·NH2−mPEG] NPs and γ-PGA− [(Au0)200−PEI·NH2−mPEG] NGs in aqueous solution were characterized by dynamic light scattering and ζ potential measurements (Table S1). [(Au0)−γ-PGA] NPs were also characterized for comparison. The [(Au0)200−PEI·NH2− mPEG] NPs show a positive surface potential of 21.5 ± 0.44 mV, in line with the literature data.27 The formed γ-PGA− [(Au0)200−PEI·NH2−mPEG] NGs display an inversed negative surface potential of −13.1 ± 0.32 mV. In this study, the feeding molar ratio of PEI amine to γ-PGA carboxyl was set at 1:1, and the negative surface potential of NGs may be attributed to the fact that a portion of the PEI amines required to stabilize the B

DOI: 10.1021/acs.bioconjchem.7b00571 Bioconjugate Chem. XXXX, XXX, XXX−XXX

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Bioconjugate Chemistry

Figure 2. (a) MTT assay of HeLa cells treated with the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs at varying Au concentrations for 24 h. (b) Uptake of Au in HeLa cells treated with the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs and [(Au0)−γ-PGA] NPs at different Au concentrations for 4 h. HeLa cells treated with PBS were used as a control. (c) CT images and CT values (HU) of HeLa cells treated with the γ-PGA−[(Au0)200−PEI· NH2−mPEG] NGs at Au concentrations of 0, 0.025, 0.05, 0.1, 0.2, and 0.4 mM for 4 h.

formed (Figures 1b and S1c), and the size of each NG was calculated to be 108.6 ± 19.14 nm. Fourier transform infrared (FTIR) spectroscopy was then conducted to identify the structure of the hybrid NGs (Figure S3). The new peak of NGs at 1737 cm−1 could be assigned to the vibrations of amide bond, and the peaks at around 2925 and 1377 cm−1 could be due to the vibrations of −CH2− symmetric stretching and −NH2 scissoring, respectively. All of the results indicate the successful cross-linking of the γ-PGA and [(Au0)200−PEI·NH2−mPEG] NPs. It should be noted that we chose the [(Au0)200−PEI·NH2−mPEG] NPs as a crosslinker instead of [(Au0)200−PEI·NH2] NPs without partial PEGylation. This is because [(Au0)200−PEI·NH2] NPs without partial PEGylation have much stronger covalent interaction with the γ-PGA carboxyl groups, thereby leading to precipitation of the NGs. The composition of the hybrid NGs was analyzed by thermal gravimetric analysis (TGA) (Figure S4). By the subtraction of the weight losses of the [(Au0)200−PEI·NH2−mPEG] NPs from γ-PGA−[(Au0)200− PEI·NH2−mPEG] NGs at 700 °C, the content of γ-PGA within the hybrid NGs was estimated to be 8.24%. We next performed CT phantom studies to investigate the X-ray attenuation property of the γ-PGA−[(Au0)200−PEI· NH2−mPEG] NGs (Figure 1c). The X-ray attenuation intensity of the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs increases with the Au concentration, in parallel with the case of Omnipaque, an iodine-based conventional CT contrast agent. However, at the same concentration of the radiodense element (Au versus iodine), the CT value of the hybrid NGs is much higher than that of Omnipaque. At the concentration of

Au NPs is unable to be cross-linked, corroborating our previous work of dendrimer-entrapped28 and PEI-entrapped Au NPs.27 The hydrodynamic sizes of the NGs were recorded within a period of 7 days to determine the stability of the NGs (Figure S1a). The hydrodynamic size of the NGs does not appreciably change, indicating their good colloidal stability. Their good water dispersibility and stability were also demonstrated by occasionally monitoring the γ-PGA−[(Au0)200−PEI·NH2− mPEG] NGs in water solution after stored for 7 days at room temperature (Figure S1b). The crystalline structure of the synthesized γ-PGA− [(Au0)200−PEI·NH2−mPEG] NGs was then confirmed by Xray diffraction (XRD) patterns (Figure S2). It is clear that the lattice spacing of the NPs and NGs at 2θ of 38.1, 44.2, 64.6, 77.5 and 81.8° matches well with the [111], [200], [220], [311], and [222] planes of Au crystals, respectively, in agreement with the literature.18,27 This suggests the successful immobilization of Au NPs within the NGs. UV−vis spectra data reveal that the NGs exhibit a featured surface plasmon resonance peak at 520 nm (Figure 1a), correlating well with the single [(Au0)200−PEI·NH2−mPEG] NPs. Our data indicate that the Au NPs within the NGs are not aggregated. The XRD and UV−vis data demonstrate the successful formation of AuNP-incorporated γ-PGA NGs. We used transmission electron microscopy (TEM) to characterize the morphology of the synthesized γ-PGA− [(Au0)200−PEI·NH2−mPEG] NGs. It can be seen that typical spherical γ-PGA NGs with clustered [(Au0)200−PEI·NH2− mPEG] NPs incorporated within the entire NG volume can be C

DOI: 10.1021/acs.bioconjchem.7b00571 Bioconjugate Chem. XXXX, XXX, XXX−XXX

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Bioconjugate Chemistry

We next used the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs for the CT imaging of a xenografted HeLa tumor model in vivo (Figure 3a). The tumor region becomes slightly

0.04 M, the HU of the NGs is approximately 60% higher than that of Omnipaque. In addition, it should be noticed that the Xray attenuation property of the γ-PGA−[(Au0)200−PEI·NH2− mPEG] NGs is similar to that of the acetylated [(Au0)200−PEI· NH2−mPEG] NPs developed in our previous work27 under the same Au concentrations, suggesting that the incorporation of Au NPs within the γ-PGA NGs does not impact the X-ray attenuation property of the Au NPs. Our results indicate that the formed NGs have a better X-ray attenuation property than Omnipaque, in accordance with the literature.18,27 MTT viability assay of HeLa cells was executed to assess the cytocompatibility of the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs (Figure 2a). Clearly, HeLa cells incubated with NGs after 24 h at the given Au concentrations (0.025, 0.05, 0.1, 0.2, and 0.4 mM) own an approximately similar viability to the PBS control. This suggests that the formed NGs do not display obvious cytotoxicity in the studied Au concentration range, in line with the cytotoxicity data of the γ-PGA-coated Fe3O4 NPs,29 Fe3O4-NPs-loaded γ-PGA NGs,24 or γ-PGA-coated MnFe2O4 NPs.30 The cytocompatibility of the formed NGs was then visually determined by phase-contrast microscopic observation of HeLa cells incubated with the NGs at the different Au concentrations (Figure S5). Clearly, HeLa cells treated with the hybrid NGs do not show apparent changes in morphology compared to the PBS control. Taken together, our results validate that the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs possess an acceptable cytocompatibility at the given Au concentrations. For cancer cell CT imaging in vitro, it is essential to test if the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs can be uptaken by cells. For comparison, [(Au0)−γ-PGA] NPs were also prepared using γ-PGA as a stabilizer. The formed [(Au0)−γPGA] NPs display a negative surface potential (−22.9 mV, Table S1), ensuring the meaningful comparison with the hybrid NGs with the same charge polarity. The formation of [(Au0)−γ-PGA] NPs was confirmed by UV−vis spectroscopy and TEM (Figure S6), in which the typical SPR band at 527 nm can be assigned to the Au NPs and the Au NPs display a relatively uniform spherical morphology with a size of 6.5 ± 1.6 nm. Quantitative evaluation by inductively coupled plasma optical emission spectroscopy (ICP-OES; Figure 2b) shows that for both the [(Au0)−γ-PGA] NPs and the γ-PGA− [(Au0)200−PEI·NH2−mPEG] NGs, the Au uptake in HeLa cells gradually increases with the Au concentration. However, the cellular Au uptake for the hybrid NGs is significantly higher than that for the [(Au0)−γ-PGA] NPs. This implies that the softness and admirable fluidity of the NGs are able to facilitate their enhanced cellular Au uptake possibly through phagocytosis and diffusion via cell membranes.24 The excellent cellular Au uptake for the hybrid NGs is essential for their further cancer cell CT imaging applications. The γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs were then employed as a contrast agent for cancer cell CT imaging in vitro (Figure 2c). Clearly, as shown in Figure 2c (upper panel), the hybrid NGs afford an increasing brightness of the CT images of cells with the Au concentration, corroborating the quantitative analysis of the HU values, and the HU value of the cells treated with the NGs increases from 32.8 to 203.6 with the increase of Au concentration (lower panel of Figure 2c). Our data suggest that the developed γ-PGA−[(Au0)200−PEI·NH2− mPEG] NGs have a tremendous potential to be adopted as a contrast agent for cancer cell CT imaging.

Figure 3. (a) CT images and (b) tumor CT values (Hounsfield units, HU) of the nude mice bearing xenografted HeLa tumor at different time points postintravenous injection of the γ-PGA−[(Au0)200−PEI· NH2−mPEG] NGs ([Au] = 0.05 M in 0.1 mL of PBS).

brighter with the time post-injection, and the tumor region is the brightest at 2 h post-injection and then starts to recover probably due to the clearance process of the NGs in mice, which was also confirmed by quantitative tumor CT value measurements (Figure 3b). The tumor CT value at 2 h postinjection is significantly higher than that before injection and at 1 h post-injection (p < 0.05). In vivo CT imaging results indicate that the developed NGs are able to be accumulated in the tumor region probably through the passive enhanced permeability and retention effect, affording effective tumor CT imaging. In contrast, the control material of Omnipaque does not lead to any significant CT contrast enhancement under the same experimental conditions possibly due to the rapid renal clearance (Figure S7). The in vivo biodistribution of the fabricated γ-PGA− [(Au0)200−PEI·NH2−mPEG] NGs was then quantitatively analyzed by ICP-OES (Figure S8a). We can see that the Au concentration in all major organs and tumors of the mice injected with the NGs is apparently higher than that before injection at 1, 2, 3, and 4 h post-injection (p < 0.05), and most of the NGs are accumulated in the liver and spleen. The Au uptake in the tumor starts to decrease at 2 h post-injection, further corroborating the CT imaging data. It is also crucial to further explore the in vivo biocompatibility of the γ-PGA−[(Au0)200−PEI·NH2−mPEG] NGs. Healthy mice intravenously injected with the NGs were subjected to biodistribution studies at 7 and 14 days (Figure S8b). Obviously, the Au contents in the major organs (heart, liver, spleen, lung, and kidney) at 7 and 14 days post-injection D

DOI: 10.1021/acs.bioconjchem.7b00571 Bioconjugate Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (grant nos. 81761148028, 21773026, and 81401458), the Science and Technology Commission of Shanghai Municipality (grant nos. 15520711400 and 17540712000), and the Fundamental Research Funds for the Central Universities (For W.S., M.S., and X.S.). X.S. also acknowledges the support by FCTFundaçaõ para a Ciência e a Tecnologia (project no. PEst-OE/ QUI/UI0674/2013, CQM, Portuguese Government funds) and by ARDITI-Agência Regional para o Desenvolvimento da Investigaçaõ Tecnologia e Inovaçaõ through the project no. ́ M1420-01-0145-FEDER-000005-Centro de Quimica da Madeira-CQM+ (Madeira 14-20).

show no appreciable difference when compared with the control mice injected with PBS (p > 0.05), implying that the developed hybrid NGs are able to be ultimately cleared out of the body after 7 days, and hence, the NGs do not display any possible in vivo toxicity to the animals. Hematoxylin and eosin staining was further used to validate the biocompatibility of the NGs (Figure S9). We can see that the histological sections of all mouse major organs including the heart, liver, spleen, lung, and kidney after NGs were injected display similar morphologies when compared to the PBS-treated control mice. Taken together with both the biodistribution and histological data, we can safely conclude that the fabricated hybrid NGs possess good in vivo biocompatibility.

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CONCLUSIONS In conclusion, we synthesized γ-PGA−[(Au0)200−PEI·NH2− mPEG] NGs via a facile double-emulsion method for tumor CT imaging. Through the in situ EDC-mediated cross-linking of the amine groups of [(Au0)200−PEI·NH2−mPEG] NPs and carboxyl groups of γ-PGA, hybrid NGs with a size of 108.6 ± 19.14 nm can be prepared. The developed γ-PGA−[(Au0)200− PEI·NH2−mPEG] NGs with a relative uniform spherical morphology display good water dispersibility, colloid stability, and cytocompatibility in a studied Au concentration range. With good X-ray attenuation properties and the ability to be significantly taken up by cancer cells, the formed hybrid NGs have great potential to be employed as an efficient contrast agent for CT imaging of cancer cells in vitro and the xenografted tumor model in vivo. Owing to the abundant carboxyl groups on the surface of the γ-PGA, which can be further functionalized with other ligands, the developed γPGA−[(Au0)200−PEI·NH2−mPEG] NGs may be promising to be used as a multifunctional platform for CT imaging or theranostics of different biosystems.

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ASSOCIATED CONTENT

* Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.bioconjchem.7b00571. Additional details on experimental methods, materials characterization data, in vitro cell morphology observation, and in vivo animal CT imaging, biodistribution, and histological examination data. A table showing ζ potentials and hydrodynamic sizes. Figures showing hydrodynamic size, digital images, XRD patterns, FTIR spectra, TGA curves, phase-contrast microscopic images, a UV−vis spectrum, a size-distribution histogram, a correlation function, CT images and values, biodistribution of Au, and hematoxylin and eosin staining. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiangyang Shi: 0000-0001-6785-6645 Author Contributions ∥

REFERENCES

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J.Z., W.S., and J.Z. contributed equally

Notes

The authors declare no competing financial interest. E

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DOI: 10.1021/acs.bioconjchem.7b00571 Bioconjugate Chem. XXXX, XXX, XXX−XXX