Design and construction of multifunctional hyperbranched polymers coated magnetite nanoparticles for both targeting magnetic resonance imaging and cancer therapy

Abstract

Magnetic drug targeting is a drug delivery strategy that can be used to improve the therapeutic efficiency on tumor cells and reduce the side effects on normal cells and tissues. The aim in this study is designing a novel multifunctional drug delivery system based on superparamagnetic nanoparticles for cancer therapy. Magnetic nanoparticles were synthesized by coprecipitation of iron oxide followed by coating with poly citric acid (PCA) dendritic macromolecules via bulk polymerization strategy. It was further surface-functionalized with poly(ethylene glycol) (PEG) and then to achieve tumor cell targeting property, folic acid was further incorporated to the surface of prepared carriers via a facile coupling reaction between the hydroxyl end group of the PEG and the carboxyl group of folic acid. The so prepared nanocarriers (Fe₃O₄@PCA-PEG-FA) were characterized by X-ray diffraction, TEM, TGA, FT-IR, DLS and VSM techniques. The room temperature VSM measurements showed that magnetic particles were superparamagnetic. Transmission electron microscopy and dynamic light scattering were also performed which revealed that size of nanocarriers was lying in the range of 10–49 nm. Quercetin loading and release profiles of prepared nanocarriers showed that up to 83% of loaded drug was released in 250 h. Fluorescent microscopy showed that the cellular uptake by folate receptor-overexpressing HeLa cells of the quercetin-loaded Fe₃O₄@PCA-PEG-FA nanoparticles was higher than that of non-folate conjugated nanoparticles. Thus, folate conjugation significantly increased nanoparticle cytotoxicity. Also, T₂-weighted MRI images of Fe₃O₄@PCA-PEG-FA nanoparticles showed that the magnetic resonance signal is enhanced significantly with increasing nanoparticle concentration in water and they also served as MRI contrast agents with relaxivities of 3.4 mM⁻¹ s⁻¹ (r₁) and 99.8 mM⁻¹ s⁻¹ (r₂). The results indicate that this multifunctional nanocarrier is a significant breakthrough in developing a drug delivery vehicle that combines drug targeting as well as sensing and therapy at the same time.

Introduction

In the past few decades, nanomaterials and nanotechnology have developed to synthesize and characterize magnetic nanoparticles for biomedical applications such as hyperthermia, magnetic resonance imaging contrast agent, targeted drug and gene delivery, tissue engineering, cell tracking, biosensing and bioseparation [1], [2], [3], [4], [5], [6]. Drug delivery systems based on magnetic nanoparticle carriers have improved pharmacotherapy significantly by increase the half-life of drugs; reduce toxicity and extension drug retention time in circulatory system [7], [8]. In the presence of an external magnetic field, magnetic nanoparticle carriers (comprising of coated magnetic nanoparticles loaded with anti-cancer drug) deliver the drug at the targeted area [9], [10].

Magnetic iron oxide nanoparticles (MNPs), especially Fe₃O₄ nanoparticles, have been widely used in the field of biotechnology because of biocompatible, potentially non-cytotoxic, small size and interesting superparamagnetism properties [11], [12]. Additionally, spherical magnetite NPs with diameters less than approximately 20 nm will exhibit superparamagnetic behavior, a property that is exploited to enhance contrast in magnetic resonance imaging (MRI) [13], [14]. In vivo applications, magnetic nanoparticles must be covered in order to protect iron oxide nanoparticles against core agglomeration, protect their surface from oxidation, colloidal stability and biocompatibility, provide functional groups for the conjugation of drug molecules, targeting ligands and limit nonspecific cell interactions [7], [8]. Magnetic nanoparticles are usually grafting of or coated with organic ligands, including surfactants, polymers, biomolecules, dendrimers, silica, metal or nonmetal elementary substance [15], [16], [17], [18], [19], [20], [21], [22], [23]. One of the most widely used shielding polymers for this purpose is PEG [24], [25]. It has very good biocompatibility, immunogenicity and very low toxicity. Therefore, PEGylation also increases target site localization by extension nanocarrier retention time in circulatory system due to enhanced permeability and stability [26], [27], [28], [29], [30], [31], [32], [33]. These advantages have caused which PEG is used to modify the surface of the magnetite nanoparticles in the present study.

Dendrimers are a new group of polymeric materials, which are nano-sized architecture, highly branched and monodispersed macromolecules with numerous functional groups and internal cavities. Dendrimers are composed of three separate architectural components, namely; a central core, branching units and surface functional groups that produce a spherical branching structure [34]. The large numbers of functional end groups make the dendrimers suitable for many biomedical applications like drug and gene delivery, biochemistry and nanomedicine [34], [35], [36], [37], [38].

In targeted drug delivery systems, using of targeting ligands is very important. The common ligands used in this field include folic acid (FA) [39], [40], polysaccharides [41], peptides [42] and monoclonal antibodies [43], [44]. FA is a targeting group which is extensively used to drug delivery [45], [46], compared to the above different ligands, because folic acid receptors (FAR) are up-regulated on a variety of human cancers. FA has a high affinity to FAR on the cell surface. The lack of folic acid receptors in normal cells differentiates them from tumor cells [47]. Hence, in order to increase the internalization of nanocarriers into cancer cells, the nanocarrier surface is modified with folic acid.

In this study, we report the fabrication of folate-decorated and stable magnetic nanocarrier, based on Fe₃O₄ nanoparticle modified by PCA dendrimer and PEG as an integrated platform for tumor-targeted delivery and study controlled release of quercetin under in vitro conditions. Firstly, the Fe₃O₄ nanoparticles are synthesized according to the co-precipitation method. Then, to avoid aggregation and opsonization of nanoparticles the surface of Fe₃O₄ nanoparticle is coated with biodegradable PCA dendrimer. Using of PCA dendrimer for initial coverage surface of magnetic nanoparticles was performed for the first time. Citric acid is a cheap and biocompatible compound that is used on a large scale in the food and drug industries. Despite the large-scale production and intrinsic importance of this material, there are apparently no reports of the production of polymeric materials of this compound based on magnetic nanoparticles especially in drug delivery systems. PEG was selected as the hydrophilic agent owing to its excellent biocompatibility. Fe₃O₄@PCA-PEG was formed by reaction between carboxyl-terminated Fe₃O₄@PCA and OH end groups of PEG. The OH end groups of Fe₃O₄@PCA-PEG were further modified with FA to achieve the function of targeted drug delivery. To assess the suitability of Fe₃O₄@PCA-PEG-FA as a drug carrier, drug loading and in vitro release studies are performed using quercetin as a hydrophobic model drug [48], [49], [50]. To evaluate the feasibility of Fe₃O₄@PCA-PEG-FA as MRI contrast agents, the MRI contrast is monitored in vitro.