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Synthesis and characterization of dextran coated magnetite nanoparticles for diagnostics and therapy

Introduction: Expansion of efficacious theranostic systems is of pivotal significance for medicine and human healthcare. Magnetic nanoparticles (MNPs) are known as drug delivery system and magnetic resonance imaging (MRI) contrast agent. MNPs as drug carriers have attracted significant attention because of the delivery of drugs loaded onto MNPs to solid tumors, maintaining them in the target site by an external electromagnetic field, and subsequently releasing drugs in a controlled manner. On the other hand, it is believed that MNPs possess high potential as MRI contrast agents. The aim of this work was to payload curcumin into dextran coated MNPs and investigate their potential as theranostic systems for controlled drug delivery and MRI imaging. Methods: MNPs were synthesized as a core and coated with dextran as polymeric shell to provide steric stabilization. Curcumin as anticancer drug was selected to be loaded into NPs. To characterize the synthesized NPs, various techniques (e.g., DLS, FESEM, FT-IR, XRD, and VSM) were utilized. In vitro drug release of curcumin was evaluated at 37˚C at the pH value of 5.4 and 7.4.The feasibility of employment of dextran coated MNPs as MRI contrast agents were also studied. Results: Formulations prepared from dextran coated MNPs showed high loading (13%) and encapsulation efficiency (95%). In vitro release study performed in the phosphate-buffered saline (PBS, pH= 7.4, 5.4) revealed that the dextran coated MNPs possess sustained release behavior at least for 4 days with the high extent of drug release in acidic media. Vibrating sample magnetometer (VSM) analysis proved the superparamagnetic properties of the dextran coated MNPs with relatively high-magnetization value indicating that they were sufficiently sensitive to external magnetic fields as magnetic drug carriers. Furthermore, dextran coated MNPs exhibited high potential as T2 contrast agents for MRI. Conclusion: Based on our findings, we propose the dextran coated MNPs as promising nanosystem for the delivery of various drugs such as curcumin and MRI contrast agent.


Introduction
Nanoparticles (NPs) composed of organic building blocks have widely been studied for the delivery drugs and genes. For example, liposomes, polymersomes, dendrimers, [1][2][3] polymeric NPs and micelles 4,5 are in various steps of preclinical and clinical development. 6 Inorganic NPs have obtained intensive attention in recent years due to their specific physicochemical properties which are not sometimes feasible in organic NPs. Particularly unique optical properties, stability, magnetic properties, possibility of functionalization, and other physical properties make inorganic NPs attractive systems for imaging and drug delivery purposes in comparison to organic nanoparticles. Amongst, superparamagnetic iron oxide nanoparticles (SPIONs) are especially interesting because of their unique magnetic properties, biocompatibility, and good bioadaptability. 7,8 MNPs have been employed in many technologies such as hyperthermia treatments, drug delivery, 9 MRI contrast agents, magnetic separation, 10 and recyclable catalysts. 11 However, in order to have the benefit of both organic and inorganic materials, nanocomposites have recently been introduced. 12, 13 For instance, in order to improve the MNPs characteristics for drug delivery goals such as colloidal stability, it has been shown that their coatings with hydrophilic organic polymers 14 such as dextran can be considered as a valuable strategy. The synthesis of magnetite/polymer NPs can generally be achieved through three methods; (a) assembling MNPs and polymer nanoparticles after being prepared separately via physicochemical or physical interactions between two compounds, 15 (b) in situ precipitation of MNPs in the presence of polymer, and (c) in situ polymerization of monomers in the presence of MNPs. MNPs have been widely studied as T 2 contrast agents in MRI because of enhancement of the negative contrast (darkness) in phantom images by T 2 relaxivity of water protons. Dextran as a naturally made polysaccharide possesses poly-cationic and hydrophilic properties and could be found in certain lactic acid bacteria, Leuconostoc mesenteroides and Streptococcus mutans, holding α-d (1)(2)(3)(4)(5)(6) and α-d (1-3) linked glucose branch units. 16,17 Curcumin, a natural polyphenolic phytoconstituent, is derived from Curcuma longa Linn (Zingiberaceae) and has various pharmacological properties. 18 The bis-α, β-unsaturated β-diketone is the chemical structure of curcumin ( Fig. 1). 19 Curcumin is a well-known natural anti-inflammatory agent owing to the inhibitory properties of cyclooxygenase-2 (COX-2), lipoxygenase, and persuadable nitric oxide synthase (iNOS) enzymes. [20][21][22][23][24] In addition, it displays high potential for the medicinal therapy of different illnesses, particularly cancer. 25,26 However to attain the maximum efficacy of curcumin, low solubility is a major challenge. To avoid the low solubility and consequently low bioaccessibility of curcumin, nanoscaled drug delivery approaches have been proposed, 27 including liposomes, 28 solid lipid nanoparticles (SLNs), 29 chitosan-based NPs, 30 or NPs complexed with phospholipids, 31 and cyclodextrin. 32 In the present study, the aim was to investigate the applicability of MNPs for both drug delivery and noninvasive MRI monitoring. Dextran coated MNPs were prepared and loaded with curcumin with the purpose of simultaneous controlled drug delivery and MRI imaging.

Preparation of dextran coated MNPs
The MNPs were synthesized by co-precipitation method. Briefly, first N 2 gas was bubbled into 45 mL distilled water. Then, 1mmol of FeCl 2 ·4H 2 O, 2mmol of FeCl 3 ·6H 2 O, and 5 mL of 0.5% dextran were added to the solution, then NH 4 OH was added drop-wise by syringe. After stirring of mixture at 60°C under N 2 for 30 min, a black suspension was formed. The black precipitate was separated by centrifuge at 1500 rpm for 15 min. The product was dried at room temperature.

Preparation of curcumin-loaded dextran coated MNPs
Briefly, dextran coated MNPs (50 mg) was added into 10 mg curcumin (preliminarily solubilized in 5 mL ethanol) and stirred for 20 h. The curcumin loaded dextran MNPs were separated by centrifugation at 14000 rpm and then washed (3×) with deionized water. The curcumin entrapped nanoparticles were dried in vacuum oven at 30 º C for 12 h. The unloaded curcumin was determined by measuring the concentration of drug in the supernatant by UV-Vis spectrophotometry at wavelength of 425 nm. The entrapment efficiency was calculated using the equation (1)

Drug release study
The release of drug molecules from NPs were studied in phosphate-buffered saline (PBS) containing 0.5% (w/v) Tween TM 80 in physiological pH of 7.4 and acidic media with the pH value of 5.3. Typically, 10 mg of NPs were placed into a dialysis bag (cut off 12 kDa) and introduced to 15 mL of PBS with desired pH under stirring at 100 rpm at 37°C. At predetermined time intervals, in order to determine the drug concentration in dialysate and thereby time dependent drug release profile, 1.0 mL of dialysate  was taken out and replaced with 1.0 mL of fresh buffer solution maintained at 37°C and assayed by UV-Vis spectroscopy at wavelength of 425 nm.

Characterization methods
Fourier transformed infrared (FT-IR) spectroscopy was recorded with Mattson 1000 FT-IR spectrometer using KBr pellets in the region of 400-4000 cm -1 . The morphology of the dextran coated MNPs was studied by field-emission scanning electron microscopy (FESEM, Mira 3-XMU). The magnetic properties of MNPs and the dextran coated MNPs were measured in vibrating sample magnetometer (VSM, Lake Shore 7400) with an applied field between 20 to 20 kOe at 25°C. The particle size distribution of the prepared core-shell was determined by dynamic light scattering (DLS) (Malvern Instruments, UK, model Nano ZS). The structure and crystal phase of the dextran coated MNPs were measured by a Bruker D8 X-ray diffractometer with monochromated high-intensity Cu Ka radiation (k =1.5418 Å) operated at 40 kV and 30 mA.

Magnetic resonance imaging (MRI)
The feasibility of employment of dextran coated MNPs as MRI contrast agents was studied by plotting T 1 relaxation rate (1/T 1 ) and T 2 relaxation rate (1/T 2 ) as a function of Fe concentration which their slope subsequently results in calculation of the longitudinal relaxivity (r 1 ) and transverse relaxivity (r 2 ), respectively. To achieve this goal, T 1 where, I is the signal intensity and M 0 is constant.

In vitro radical scavenging activity of curcumin loaded MNPs
In order to measure free radical scavenging activity, decolorization property of the stable DPPH was used in the presence of antioxidants, that is, curcumin and curcumin loaded dextran coated MNPs. DPPH radical scavenging activity of nanoparticle was determined as follows: Briefly, the different concentrations of curcumin were prepared in methanol (0.006-0.06 mM). Curcumin solution was protected from light. Equal volumes of each of the concentrations of curcumin (1 mL) were added to the test containers containing DPPH (100 mM) and following the incubation for 30 min, absorbance was read at 517 nm. Triplicate experiments were performed for each sample. Scavenging activity was calculated using the formula (4): Where, A C is the absorbance of the control, and A S is the absorbance of extract or standard sample. Finally, IC 50 value was calculated which was defined as the concentration of the sample required to decrease the absorbance at 517 nm by 50%.

Physicochemical characterization of curcumin loaded MNPs
The FT-IR spectra were recorded to prove the successful preparation of curcumin loaded MNPs and possible interaction between various constituents of nanoparticles.  a), and dextran coated MNPs (spectrum b) which are characteristic peaks of MNPs. The intensities of MNPs diffraction peaks for dextran coated MNPs were weaker than those of the naked MNPs, which indicate the presence of dextran on the surface of MNPs. The average particle size calculated using Debye Scherer's formula for naked and dextran coated MNPs, were 10.48 and 12.83 nm, respectively. Fig. 4 shows the magnetization curves of the naked MNPs and dextran coated MNPs. The results showed that both samples exhibited superparamagnetic properties. The saturation magnetization values (M s ) of the MNPs and dextran coated MNPs were 80.152 and 70.572 emu/g, respectively. As shown in Fig. 4, the saturation magnetization was reduced following the coating of MNPs with dextran. The finding can be explained by the presence of dextran on the surface of MNPs. It was shown that the thickness of shell was determinant parameter in the extent of magnetization decrease; whatever the density of shell increased, the magnetism decreased. The size of MNPs before and after surface modification was investigated by DLS analysis. In the case of naked MNPs, the size of nanoparticles was about 126 nm (Fig. 5A). The particle size distribution curves exhibited only one peak with a relative high polydispersity index indicating the MNPs aggregation in solution. The size of dextran coated MNPs was also analyzed by DLS. The results showed that the particle size of dextran coated MNPs decreased to 58 nm (Fig. 5B). Decrease in particle size after dextran coating can be related to the impact of dextran coating on protecting MNPs from aggregation which consequently results in high dispersion capability of MNPs. Morphology of the MNPs was studied by FESEM technique. Fig. 5 (panels C and D)

In vitro drug release treatment
The cumulative release of curcumin loaded MNPs was studied in buffer solutions with two different pH values of 5.3 and 7.4 at the physiological temperature of 37 º C. The cumulative drug release was expressed as the percent of drug liberation as a function of time. Fig. 6 shows the cumulative release of curcumin from dextran coated MNPs. Of particular note was that drug release at both pHs showed comparatively sustained release behavior. However, as shown in Fig. 6, there was high amount of burst drug release in acidic pH compared to the one at neutral media probably to the high solubility of curcumin in acidic solution. The maximum drug release attainable at neutral and acidic media after 72 h was up to 70% and 40%, respectively (Fig. 6).

Drug release kinetic
To comprehend the mechanisms that control the drug release from dextran coated MNPs, the release data were fitted with some widely used mathematical models including Zero-order, First-order, Gompertz, Korsmeyer-Peppas, Higuchi, Hixson-Crowell, Peppas-Sahlin, and Weibull. 38 The model with the highest determination coefficient (R 2 adjusted) and the minimum Akaike information criterion (AIC) was chosen as the best fit. Data analysis was carried out using the Excel add-in DDSolver program. The findings revealed that in both release condition (neutral and acidic media) release data were adequately fitted to the Peppas-Sahlin model. According to the Peppas-Sahlin model, the values of R 2 for curcumin loaded MNPs in the media with the pH value of 5.3 and 7.4 were found to be 0.9815 and 0.9769, respectively (Table 1). Based upon the structure of engineered NPs, it is believed that the release rate of drug is mainly controlled by the rate of drug diffusion.

MRI contrast enhancement
The proton relaxivity measurements was performed to study the possibility of dextran coated MNPs as MRI contrast agent. The effect of NPs on the longitudinal (T 1 ) and transverse (T 2 ) proton relaxation was evaluated using a clinical 1.5 T whole body magnetic resonance (MR) scanner. Fig. 7 shows T 2 -weighted MR images of dextran coated MNPs with different Fe concentrations. It is shown that the T 2 -weighted images' brightness decreases significantly with the increase of Fe concentration. The observation reveals that dextran coated MNPs can be considered as a good MR T 2 contrast agent under the T 2 -imaging sequences. Longitudinal relaxivity (r 1 ) values were calculated by the linear fit of the equation (5): where, R i is the relaxation rate, T i0 is the relaxation time in the pure water, C is the concentration of the contrast agent, and r i is relaxivity. As shown in Fig. 8, there is a linear relationship between the T 1 relaxation rate (1/T 1 ) and Fe concentration. Based on the slope of the line, longitudinal relaxivity was found to be 12.79 mM −1 s −1 for the dextran coated MNPs. 39 On the other hand, r 2 value was calculated from the slope of linear plots of 1/T 2 versus Fe concentration as given in Fig. 9. Dextran coated MNPs demonstrated transversal relaxivity value of 220.20 mM −1 s −1 . The calculated r 1 , r 2, and r 2 /r 1 values are shown in Table 2. The relaxivity ratio, r 2 /r 1 , was calculated to be 17.21 which is higher than that of Resovist, commercially available MRI contrast agent 40 indicating that dextran coated MNPs are feasible to be used as negative MRI contrast agents. In other words, dextran coated MNPs can be used as T 2 MRI contrast agents and consequently they are able to decrease the MR signal intensity by dephasing of proton spins.

In vitro radical scavenging activity of curcumin loaded MNPs
Determination of scavenging activity with the DPPH stable radical is a common method to evaluate the free radical scavenging ability of various compounds. DPPH is a free radical that is stable at room temperature and produces a purple color when dissolved in ethanol. This solution becomes colourless in the presence of an antioxidant. It should be stated that this method is fast, reliable and easy. In the present study, the scavenging activity of curcumin and curcumin loaded MNPs were determined by DPPH assay. The results on the DPPH scavenging ability from curcumin and curcumin-loaded dextran-coated MNPs are shown in Fig. 10. These results showed that curcumin and curcumin-loaded dextran-coated MNPs have similar power of antioxidant activities. Investigation on the radical scavenging activity showed that curcumin and curcumin-loaded dextran-coated MNPs had similar power of antioxidant activities with an IC 50 value of about 6 μM.

Discussion
MNPs were prepared using the co-precipitation of ferric and ferrous salts in alkaline medium based upon the reactions (6) and (7): 4 4 NH OH NH OH      in comparison with 50% at pH 7.4 after 4 days). On the other hand, dextran loaded MNPs showed a considerable potential as T 2 MR contrast agent with the relaxivity ratio value of 17.21. The IC 50 of both curcumin itself and curcumin loaded MNPs were found to be similar. This indicates that curcumin loaded MNPs display similar radical scavenging activity in comparison with the free curcumin ( Fig. 10), while the drug liberation profile showed controlled release behavior.
In the cytotoxicity assay in human breast cancer MDA-MB-231 cells (Fig. 11), free curcumin showed a significantly higher cytotoxicity than those of the synthesized MNPs and curcumin loaded MNPs. It was expected to face somewhat higher toxic impacts by the curcumin loaded MNPs after 48 h post-treatment. However, we speculate that the liberation of curcumin molecules from the MNPs is largely time-dependent and hence the cytotoxicity studies need to be considered for a longer time period.

Conclusions
Magnetic nanoparticles were successfully synthesized by co-precipitation method. In order to avoid the agglomeration of MNPs, dextran was used as polymeric shell on MNPs. The anticancer curcumin molecules were loaded onto the dextran coated MNPs. Curcumin MNPs were characterized using various techniques including FT-IR, FESEM, and DLS. The size of MNPs was decreased after surface coating, while it did not impose any effect on the morphology of NPs. Study of magnetic property of prepared MNPs proved their superparamagnetic nature. Dextran loaded MNPs showed considerable potential as BioImpacts, 2015, 5(3), 141-150 149 T 2 MR contrast agent with the relaxivity ratio value of 17.21 ( Table 2). The IC 50 of both curcumin and curcumin loaded MNPs via DPPH method was found to be about 6 μM indicating considerable potential of curcumin loaded MNPs as targeted drug delivery and diagnostic agent for cancer therapy. Based upon the cytotoxicity assay, it can be concluded that these surface modified MNPs can be considered as a safe and biocompatible nanocarriers for the drug delivery and diagnostics purposes.