Magnetic Nanoparticles for Magnetic Resonance Imaging/Optical Imaging of Cancer Cells

International Journal of Science and Engineering Applications Special Issue Optimal Materials ISSN-2319-7560 (Online)

Magnetic Nanoparticles for Magnetic Resonance Imaging/Optical Imaging of Cancer Cells T. Gayathri, R. Arun Kumar* GRD Centre for Materials Research, PSG College of Technology, Coimbatore, India Abstract: Nanoparticles with the magnetic properties can be combined with the fluorescent nanoparticles in a single platform to form dual modal imaging probes. Dual modal imaging offers several advantages by combining two imaging modalities such as MRI/optical imaging, MRI/CT and MRI/PET. This review article focuses on the recent work done in dual modal imaging, especially MRI/optical imaging. MRI/optical imaging have high spatial resolution and high sensitivity. Magnetic nanoparticles such as gadolinium oxide or iron oxide can be utilized for obtaining MRI. When materials having fluorescent properties are doped into the magnetic nanoparticles, they can be used for dual modal imaging applications. The various dual modal imaging probes developed recently are discussed with their applications.

Keywords: magnetic nanoparticles; magnetic resonance imaging; optical imaging; cancer; dual modal imaging

1. INTRODUCTION Nanotechnology utilizes nanoscale materials with the size of 1-100 nm having unique physical and chemical properties.1 In the field of biomedical technology, nanomaterials are used in imaging and therapy. Nanomaterials are employed in various imaging modalities such as computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), single photon emission computed tomography (SPECT), ultrasound and optical imaging.

Therapeutic methods such as chemotherapy, photodynamic therapy, photothermal therapy and neutron capture therapy also involve the use of nanomaterials.2 Cancer is the second leading cause for death worldwide next to the cardiovascular diseases.3 It is a dreadful disease, which is characterized by uncontrolled growth of cells. The cancer cells invade the normal cells and spread to other cells/organs rapidly.4 Early diagnosis is essential for the reducing the mortality rate and treatment cost.

Nanotechnology emerges to be promising technology in the diagnosis and treatment of cancer. The unique optical, magnetic and electronic properties of the nanoparticles make them ideal for the imaging applications.5 Dual modal imaging combines the use of two imaging modalities. The attracting feature of dual modality is the advantages of the two imaging modalities are shared. For example, when MRI and optical imaging are combined, the high spatial resolution of MRI and high sensitivity of optical imaging can be used simultaneously.6 MRI is a three dimensional non-invasive imaging of soft tissues. It does not use any ionizing radiations or radioisotopes. Hence, it is a preferred and safer diagnostic modality compared to CT, PET and SPECT. MRI has a good spatial resolution with low sensitivity.7 MRI contrast agent improves the sensitivity and helps to differentiate the normal and cancer cells. Magnetic nanoparticles either paramagnetic (gadolinium and manganese oxide) or superparamagnetic (iron oxide) nanoparticles can be used as contrast agents.8

improving their sensitivity.9 The rare earth ions which have luminescent properties can be utilized for optical imaging.

Rare earth ions have narrow emission lines, higher stability and longer lifetimes compared to the organic dyes and semiconductor fluorescent nanocrystals.10 This review article summarizes the recent work on the dual modal imaging probes.

2. GADOLINIUM BASED DUAL MODAL IMAGING PROBES Gadolinium oxide (Gd2O3) nanoparticles are paramagnetic in nature with the magnetic moment of 7.9 đ?? B and longer relaxation time (10-8 to 10-9 s). Gd2O3 nanoparticles are T1 contrast agents which can produce bright contrast images by shortening the proton relaxation time.11 When the rare earth ions such as europium, erbium and terbium are doped into Gd2O3 nanoparticles, the magnetic and optical properties can be combined and can be used for dual modal imaging.

Ningqi Luo et al (2014) synthesized rare earth doped gadolinium oxide nanoparticles by combining laser ablation in liquid (LAL) and solid state reaction techniques. Confocal microscopic images of RAW264.7 cells were taken after 2 h incubation with Gd2O3:Tm3+, Gd2O3:Tb3+ and Gd2O3:Eu3+ nanoparticles. Gd2O3:Tm3+, Gd2O3:Tb3+ and Gd2O3:Eu3+ nanoparticles shows bright emission of blue, green and red respectively, even after swallowing by the cells. This proves that the nanoparticles has a high fluorescent property can be used for fluorescent imaging. Invitro MR images of Gd2O3:Eu3+ nanocrystals with different concentrations were compared with the commercial clinical MRI contrast agent Gd-DTPA. Brighter images were obtained as the concentration was increased, which signifies that Gd2O3:Eu3+ nanocrystals are a potential MRI contrast agents. No significant toxicity was noted when the nanoparticles were evaluated in RAW264.7, S18 and PC12 cell lines. Invitro MRI images of NPC CNE-2 xenografted tumor show a high contrast enhancement of the tumor after injecting the Gd2O3:Eu3+ nanocrystals.12

Fluorescent materials such as rare earth ions, organic dyes, dye doped silica, quantum dots and semiconductor fluorescent nanocrystals can be doped into magnetic nanoparticles. Rare earth ions such as europium, erbium and terbium can be doped into the magnetic nanoparticles for

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International Journal of Science and Engineering Applications Special Issue Optimal Materials ISSN-2319-7560 (Online)

Figure 1. MRI evaluation of Gd2O3:Eu3+ nanoparticles. (a) In vitro T1-weighted MR images of the Gd2O3:Eu3+ compared to Gd-DTPA. (b) Plots of the relaxation rate (1/T1) as a function of Gd3+ concentration, whose slopes provide the longitudinal relaxivity (r1) of the contrast agents. (c) In vivo MRI color images of a NPC CNE-2 xenografted tumor before, 10 min, 20 min, 35 min, 50 min, and 70 min after Gd 2O3:Eu3+ nanoparticles (15 mmol kg-1) injection. (d) Signal intensity of the tumor changes with different injection time of the nanoparticles, indicating the tumor get the best imaging enhancement at 35 minutes after the particles injection.12 Chaohui Zhou and his co-workers (2014) synthesized Eu3+ doped mesoporous gadolinium oxide (MS-Gd2O3:Eu) nanorods for multimodal imaging and drug delivery applications. The nanorods were functionalized with polyethylene glycol (PEG) to improve the biocompatibility.

Doxorubicin hydrochloride (DOX) was chosen as the model drug and loaded on the pores of MS-Gd2O3:Eu@PEG nanorods. Nitrogen adsorption desorption isotherm shows that the synthesized nanorods exhibit mesoporous structure. The surface area of Eu3+-doped Gd(OH)3 precursor without and with the silica layer was 15.66 m2g−1 and 106 m2g−1 respectively, and average pore size was found to be 4.11 nm as shown in the figure 2.

The loading and releasing of drug in the pore channels of MS-Gd2O3:Eu@PEG nanorods was evaluated by using UV– vis absorption spectroscopy at a wavelength of 480 nm. The drug loading DOX loading capacity increased with enhancement of pH values from 5.0, 7.4, to 8.0. The maximal DOX loading capacity was about 12.07. The drug releasing behavior of DOX-loaded MS-Gd2O3:Eu@PEG particles at pH 7.4 and 5.0 in phosphate buffers were studied. At pH 5, approximately 80% of drug was quickly released within 6 h, but only 40% of drug was released in pH 7.4. pH sensitive release of DOX in MS-Gd2O3:Eu@PEG particles makes them ideal for use in the cancer therapy. In the T1-weighted MR images, contrast brightening is enhanced with the increasing of Gd3+ ion concentration.13 Graziella Goglio et al (2015) prepared Eu3+-doped Gd2O3 nanoparticles by glycine-nitrate process. In the synthesis procedure, the glycine/nitrate molar ratio was optimized to 1.1 and heat treated at 800 °C for 30 min. TEM images confirms that highly crystalline particles with the size of 20-30 nm were formed. The photoluminescence emission spectra of Eu3+ ions shows that maximum emission intensity occurs at λ = 611.2 nm which is due to the 5D0-7F2 transitions. The internalization of Gd2O3 and Eu:Gd2O3 inside the Hela cells was studied for

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Figure 2. A) Nitrogen adsorption–desorption isotherm and pore size distribution (inset) of MS-Gd2O3:Eu nanorods. B) Nitrogen adsorption–desorption isotherms of Gd2O3:Eu nanorods obtained from heat treatment on the Eu 3+-doped Gd(OH)3 precursor without the silica layer parceled.13

2 h and 6 h. The results show that Eu:Gd2O3 nanoparticles internalize faster than the Gd2O3 nanoparticles at a time duration of 2 h. After 6 h, both types of nanoparticles have an uptake of 6X106 nanoparticles per cell. Cell viability was assessed by the adenosine triphosphate (ATP) production.

ATP production is higher than or equal to the control for the concentrations below 100 μg/mL. Therefore, the nanoparticles are not toxic to the cell line below this concentration. The relaxivity values was found to be r1 = 1.27 ± 0.03 s-1mM-1; r2 = 2.11 ± 0.01 s-1mM-1 at 25 °C and r1 = 1.44 ± 0.04 s-1mM-1; r2 = 2.63 ± 0.01 s-1mM-1 at 37 °C.14 Nabil M Maalej et al (2015) synthesized Gd2O3:Eu nanoplatelets by polyol method. The doping concentration was varied from 2 to 10%. FESEM images show that the thickness of the nanoplatelet is about 15 to 25 nm. It was observed clearly that when the concentration of Eu is increased, the thickness and diameter of the nanoplatelets is also increased. A sharp red emission around 612 nm was noted in the PL emission spectrum which is due to the 5D0-7F2 transitions. CIE color chromaticity diagram of Gd2O3:Eu nanoplatelets gives emission in the red region which can be used for fluorescent imaging. MR images of the gadolinium and Gd2O3:Eu nanoparticles was taken and compared with the commercially available DOTAREM. The addition of Eu reduced the MRI contrast enhancement due to the replacement of Eu in the gadolinium oxide nanoparticles.15

3. IRON BASED DUAL MODAL IMAGING PROBES Chan Wang et al (2015) synthesized fluorescent gold nanoclusters (NCs) and magnetic iron oxide composite (Fe3O4@AuNCs) by using glutathione (GSH) template for dual bioimaging. TEM image shows that the average size of the nanoparticles is 13.5 nm. Fe3O4@AuNCs nanoparticles showed a color of transparent yellow in daylight and an intense red fluorescence under 365 nm UV light. The magnetic hysteresis loop reveals that Fe3O4@AuNCs exhibits superparamagnetism. The cytotoxicity test evaluated in 293T cells confirms that Fe3O4@AuNCs does not affect the cell viability even at the concentration of 100 μg/mL for 24 h.

Bright red fluorescence was observed in confocal fluorescent microscopy images when the 293T cells uptake the Fe3O4@AuNCs. Negative contrast MR images were produced with r2 = 20.4 s-1mM-1 as shown in the figure 3.16

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International Journal of Science and Engineering Applications Special Issue Optimal Materials ISSN-2319-7560 (Online)

Figure 3. (a) T2-weighted MR images at different concentrations; (b) Relaxation rate R2 (1/T2) versus concentrations at room temperature using a 3.0 T MRI scanner.16 Parvin Eghbali and his co-workers (2016) prepared a bimodal contrast agent for MR/optical imaging by conjugating rhodamine B to 3-aminopropyltriethoxysilane (APTES) modified superparamagnetic Fe3O4 nanoparticles through amide linkage.

Rhodamine B-conjugated nanoparticles with the average diameter of 11.6 nm were observed from the TEM images. The magnetization saturation of nanoparticles was calculated to be 69.2 emu/g by the room temperature VSM measurements. From the MR images, it is clear that the MR signal is decreased as the concentration increases. The relaxivity values were calculated to be r1 = 16.93 mM−1s−1 (0.47 T); 7.22 mM−1s−1 (1.41 T) and r2 = 161.21 mM−1s−1 (0.47 T); 154.84 mM−1s−1 (1.41 T). Optical images of rhodamine B-labeled nanoparticles shows brighter images as the rhodamine B concentration is increased.17 Kyung Mo Yang et al (2014) synthesized FITC@USPIO (fluorescein isothiocyanate labelled ultra small PEGylated iron oxide) nanoparticles. USPIO nanoparticles with the average size of 4.2 ± 0.39 nm were synthesized. The cytotoxicity of the FITC@USPIO was evaluated using the SKOV3 cell by MTT analysis at different concentrations. The cell viability of the SKOV3 cell was not affected after 24 h incubation to a concentration of 100 mg Fe per mL. T2weighted image shows that higher concentrations of the FITC@USPIO resulted in higher R2 relaxation rates and the r2 was calculated to be 27.5 mM-1s-1. When the FITC@USPIO is injected intravenously into the tail vein of mouse, enhancement was continued until 3 hours and the nanoparticles were accumulated in the liver, spleen and kidney. After 24 h of injection, no contrast material was visualized in the kidney but contrast still remained in the liver and spleen as shown in the Figure 4. From these results, it is evident that the synthesized nanoparticle can be used for multimodal imaging.18

Figure 4. Serial T2* coronal MR imaging (A and B) of mouse after injection of FITC into the tail vein. Signal intensity of the spleen, kidney, and liver gradually decreased until 3 hours after FITC injection. 24 hours after injection the signal intensity of the kidney became normal but those of the spleen and liver still remained low.18

4. CONCLUSIONS Dual modal imaging probes can provide efficient diagnosis of the cancer cells. However, the biocompatibility, invivo toxicity and the contrasting ability of the imaging probes should be improved for the clinical applications. It is a very big challenge to develop novel dual modal imaging probes for humans. Nanotechnology can provide a solution in the near future.

5. ACKNOWLEDGMENTS The authors state that there are no conflicts of interest. The authors would like to thank Prof. R. Rudramoorthy, Principal, PSG College of Technology and the management for their support and encouragement. One of the authors (Ms. T. Gayathri) is grateful to TEQIP for providing financial assistance.

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