Vitamin E (d-alpha-tocopheryl-co-poly(ethylene glycol) 1000 succinate) micelles-superparamagnetic iron oxide nanoparticles for enhanced thermotherapy and MRI

Abstract

We synthesized vitamin E TPGS (d-α-Tocopheryl-co-poly(ethylene glycol) 1000 succinate) micelles for superparamagnetic iron oxides formulation for nanothermotherapy and magnetic resonance imaging (MRI), which showed better thermal and magnetic properties, and in vitro cellular uptake and lower cytotoxicity as well as better in vivo therapeutic and imaging effects in comparison with the commercial Resovist^® and the Pluronic^®F127 micelles reported in the recent literature. The superparamagnetic iron oxides originally coated with oleic acid and oleylamine were formulated in the core of the TPGS micelles using a simple solvent-exchange method. The IOs-loaded TPGS showed greatest colloidal stability due to the critical micelle concentration (CMC) of vitamin E TPGS. Highly monodisperse and water soluble suspension was obtained which were stable in 0.9% normal saline for a period of 12 days. The micelles were characterized for their size and size distribution. Their morphology was examined through transmission electron microscopy (TEM). The enhanced thermal and superparamagnetic properties of the IOs-loaded TPGS micelles were assessed. Cellular uptake and cytotoxicity were investigated in vitro with MCF-7 cancer cells. Relaxivity study showed that the IOs-loaded TPGS micelles can have better effects for T2-weighted imaging using MRI. T2 mapped images of xenograft grown on SCID mice showed that the TPGS micelle formulation of IOs had ∼1.7 times and ∼1.05 times T2 decrease at the tumor site compared to Resovist^® and the F127 micelle formulation, respectively.

Introduction

The surface coating material strongly affect the colloidal stability of nano-sized iron oxides (IOs) suspensions, which also plays key role in determining the adsorption, distribution, metabolism and excretion (ADME) process of the iron oxides after administration. Various macromolecules for IOs coating include simple sugars such as polysaccharide (Dextran) using epichlohydrin (CLIO, Cross linked iron oxide nanoparticles) [1], [2], hydrophilic compounds such as poly ethylene glycols (PEG) [3], and high molecular weight amphiphilic polymers such as PLGA [4] and PLA-TPGS [5], which are currently under intensive investigation for biomedical applications such as nanothermotherapy and magnetic resonance imaging (MRI).

Amphiphilic macromolecules have the tendency to self-assemble to form nano-sized colloidal micelles in water at a concentration greater than the critical micelle concentration (CMC). These self-assemblies are oriented in such a way that the hydrophobic part of the amphiphile is kept in the core and the hydrophilic part is in contact with the water. A main concern in the various commercial micellar formulations of imaging and therapeutic agents is their stability. They would disassemble in diluted solution below the CMC. Micelles formed from amphiphilic copolymer may have better resistance to disassembly due to the enhanced interaction among the polymer chains in the micelle core [6], [7], [8], [9], [10], [11]. Micelles can be prepared by simply adding the amphiphilic polymer at a concentration above its CMC in water while having higher encapsulation efficiency of the imaging or therapeutic agent in the core.

Miles et al. emphasized the importance and effect of the coating materials on particle formulation in physiological buffer, where the phosphate groups of the buffer solution can interact with the coating, replace them and thus cause instability of the iron oxides [12]. Colloidal stability of iron oxides is a great concern for their biomedical applications. Micellar aggregates can cause blood vessel blockage after administration, which may result in localized hypoxia, necrosis and hypersensitive reactions. Moreover, unstable micelles can be easily recognized by the macrophages, resulting in opsonization. As a result the micelles are readily taken up by the reticuloendothelial system (RES). A possible solution is to make use of surfactant polymers, which can be formulated into nanocapsules, nanospheres and micelles [13]. Nanocapsules contain a hydrophobic core carrying the imaging or therapeutic agent to be encapsulated. The hydrophobic core consists of solvent of high boiling point, which impedes its application in drug delivery due to the toxicity associated with it. Biodegradable nanoparticles, on the other hand, provide an excellent choice for excellent colloidal stability. However, nanoparticles synthesis requires careful optimization and proper selection of solvent and stabilizers. Preparation of micelle is less tedious if the stability problem can be addressed. Diluting the micelle suspension below the CMC, may cause disassembly of the micelle. In many cases, the micelles are cross linked using linker molecules. Unfortunately, most of the linker molecules are not acceptable to be used for biomedical applications [14].

Pluronic^®F127 macromolecule surfactant (Poloxamer 407, HLB of 18–23, CMC of 0.05 wt.%) is a block copolymer, which is intensively used for biological application such as for encapsulation of cells [15], for membrane and membrane protein solubilization as well as for biomedical applications for designing drug delivery vehicle [16] and multifunctional nanoparticles [17], [18], [19]. d-α-Tocopheryl-co-poly (ethylene glycol) 1000 succinate (TPGS, HLB ∼13, CMC 0.02 wt.%), on the other hand, has been an effective surfactant. TPGS is used in combination with chemotherapeutic drug in order to inhibit P-glycoprotein (P-gp) [20], [21], [22], [23] and increase the chemotherapeutic efficacy for cancer treatment. P-gp protein is a class of multi-drug resistance proteins that are present on the cell membrane, which cause increased efflux of drugs, thereby reducing the efficacy of the drug. TPGS has also been used efficiently as an emulsifier for synthesis of nanoparticles of biodegradable polymers, providing a high encapsulation efficiency and cellular uptake of the drug in vivo [5], [24], [25]. TPGS has also been used for pro-drug design for enhanced chemotherapy [26], [27].

In this research, F127 and TPGS are employed to develop micellar formulation of iron oxides for their potential application for nanothermotherapy and magnetic resonance imaging (MRI) in close comparison with the commercial Resovist^®. The magnetic micelles of the two macromolecular surfactants of their various molecular weights and the hydrophilic-lipophilic balance (HLB) ratio are investigated. The polymeric surfactants are assigned a HLB, which determines the surfactant characteristics. For instance a lower HLB value means more hydrophobic and higher HLB value indicates more hydrophilic. The colloidal stability of the two magnetic micelles was attributed to the surfactant HLB. The two micellar formulations of iron oxides were investigated in close comparison for their hyperthermia and magnetic properties, in vitro cellular uptake and cytotoxicity, in vivo biodistribution and MRI imaging on SCID mice of xenograft tumor model.