Magnetoliposomes: versatile innovative nanocolloids for use in biotechnology and biomedicine

Stimuli-responsive liposome-nanoparticle assemblies

INTRODUCTION

Nanoscale assemblies are needed that achieve multiple therapeutic objectives, including cellular targeting, imaging, diagnostics and drug delivery. These must exhibit high stability, bioavailability and biocompatibility, while maintaining or enhancing the inherent activity of the therapeutic cargo. Liposome-nanoparticle assemblies (LNAs) combine the demonstrated potential of liposome-based therapies, with functional nanoparticles. Specifically, LNAs can be used to concentrate and shield the nanoparticles and, in turn, stimuli-responsive nanoparticles that respond to external fields can be used to control liposomal release. The ability to design LNAs via nanoparticle encapsulation, decoration or bilayer-embedment offers a range of configurations with different structures and functions.

AREAS COVERED

This paper reviews the current state of research and understanding of the design, characterization and performance of LNAs. A brief overview is provided on liposomes and nanoparticles for therapeutic applications, followed by a discussion of the opportunities and challenges associated with combining the two in a single assembly to achieve controlled release via light or radiofrequency stimuli.

EXPERT OPINION

LNAs offer a unique opportunity to combine the therapeutic properties of liposomes and nanoparticles. Liposomes act to concentrate small nanoparticles and shield nanoparticles from the immune system, while the nanoparticle can be used to initiate and control drug release when exposed to external stimuli. These properties provide a platform to achieve nanoparticle-controlled liposomal release. LNA design and application are still in infancy. Research concentrating on the relationships among LNA structure, function and performance is essential for the future clinical use of LNAs.

Iron oxide nanoparticle-containing microbubble composites as contrast agents for MR and ultrasound dual-modality imaging

Magnetic resonance (MR) and ultrasound (US) imaging are widely used diagnostic modalities for various experimental and clinical applications. In this study, iron oxide nanoparticle-embedded polymeric microbubbles were designed as multi-modal contrast agents for hybrid MR-US imaging. These magnetic nano-in-micro imaging probes were prepared via a one-pot emulsion polymerization to form poly(butyl cyanoacrylate) microbubbles, along with the oil-in-water (O/W) encapsulation of iron oxide nanoparticles in the bubble shell. The nano-in-micro embedding strategy was validated using NMR and electron microscopy. These hybrid imaging agents exhibited strong contrast in US and an increased transversal relaxation rate in MR. Moreover, a significant increase in longitudinal and transversal relaxivities was observed after US-induced bubble destruction, which demonstrated triggerable MR imaging properties. Proof-of-principle in vivo experiments confirmed that these nanoparticle-embedded microbubble composites are suitable contrast agents for both MR and US imaging. In summary, these magnetic nano-in-micro hybrid materials are highly interesting systems for bimodal MR-US imaging, and their enhanced relaxivities upon US-induced destruction recommend them as potential vehicles for MR-guided US-mediated drug and gene delivery.

Cell death induced by the application of alternating magnetic fields to nanoparticle-loaded dendritic cells

In this work, the capability of primary, monocyte-derived dendritic cells (DCs) to uptake iron oxide magnetic nanoparticles (MNPs) is assessed and a strategy to induce selective cell death in these MNP-loaded DCs using external alternating magnetic fields (AMFs) is reported. No significant decrease in the cell viability of MNP-loaded DCs, compared to the control samples, was observed after five days of culture. The number of MNPs incorporated into the cytoplasm was measured by magnetometry, which confirmed that 1-5 pg of the particles were uploaded per cell. The intracellular distribution of these MNPs, assessed by transmission electron microscopy, was found to be primarily inside the endosomic structures. These cells were then subjected to an AMF for 30 min and the viability of the blank DCs (i.e. without MNPs), which were used as control samples, remained essentially unaffected. However, a remarkable decrease of viability from approximately 90% to 2-5% of DCs previously loaded with MNPs was observed after the same 30 min exposure to an AMF. The same results were obtained using MNPs having either positive (NH(2)(+)) or negative (COOH(-)) surface functional groups. In spite of the massive cell death induced by application of AMF to MNP-loaded DCs, the number of incorporated magnetic particles did not raise the temperature of the cell culture. Clear morphological changes at the cell structure after magnetic field application were observed using scanning electron microscopy. Therefore, local damage produced by the MNPs could be the main mechanism for the selective cell death of MNP-loaded DCs under an AMF. Based on the ability of these cells to evade the reticuloendothelial system, these complexes combined with an AMF should be considered as a potentially powerful tool for tumour therapy.

Magnetoliposomes as multimodal contrast agents for molecular imaging and cancer nanotheragnostics

In the emerging field of molecular and cellular imaging, flexible strategies to synthesize multimodal contrast agents with targeting ligands are required. Liposomes have the ability to combine with a large variety of nanomaterials, including superparamagnetic iron oxide nanoparticles, to form magnetoliposomes (MLs). MLs can be used as highly efficient MRI contrast agents. Owing to their high flexibility, MLs can be associated with other imaging modality probes to be used as multimodal contrast agents. By using a thermosensitive lipid bilayer in the ML structure, these biocompatible systems offer many possibilities for targeting and delivering therapeutic agents for ‘theragnostics’, a coincident therapy and diagnosis strategy. This article deals with the fast-growing field of MLs as biomedical diagnostic tools. Different kinds of MLs, their preparation methods, as well as their surface modification with different imaging probes, are discussed. ML applications as multimodal contrast agents and in theragnostics are reviewed. Some important issues for the biomedical uses of magnetic liposomes, such as toxicity, are summarized.

Engineering nanomedicines for improved melanoma therapy: progress and promises

Once metastatic, melanoma remains one of the most aggressive and morbid malignancies. Moreover, in past decades, the overall survival for advanced unresectable melanoma exhibited a constancy of poor prognosis. Low response rates and serious adverse effects have been characteristic of standard therapy based on a combination of chemotherapeutic agents or immunotherapy with IL-2. For example, the chemotherapy including dacarbazine, carmustin, cisplatin and tamoxifen is known as 'Dartmouth regimen' while the CVD regimen comprises carmustine, vinblastine and dacarbazine. Thus, there is an urgent and critical need to reformulate these bioactive agents using nanoscience and nanotechnology as alternative strategies. This article overviews current design and evaluation of nanomedicine undertaken to address this unmet medical need. The nanomedicines studied include polymeric nanoparticles, liposomes, polymersomes, dendrimers, cubosomes, niosomes and nanodiamonds. In this preclinical article, nanotechnology provides hope for effective treatment of this aggressive and largely treatment-resistant disease.

Multicomponent folate-targeted magnetoliposomes: design, characterization, and cellular uptake

Folate-targeted cationic magnetoliposomes (FTMLs) have been prepared with coencapsulated doxorubicin (DOX) and anionic superparamagnetic iron oxide (SPIO) nanoparticles (NPs) with 5 nm γ-Fe(2)O(3) cores and 16 nm hydrodynamic diameters. NP encapsulation (89%) was confirmed by cryogenic transmission electron microscopy (TEM), and the presence of the oppositely charged NPs did not cause liposome aggregation. The FTMLs had an average diameter of 174 ± 53 nm and existed as unilamellar and cup-shaped liposomes, which was attributed to dissimilar lipid packing parameters and the presence of PEG-lipids. A 3-fold increase in DOX release was achieved over 2 hours when the encapsulated SPIO NPs were heated by an alternating current electromagnetic field operating at radio frequencies (RF). Results with human cervical cancer cells (HeLa), which have been shown to exhibit high folate receptor (FR) expression, confirmed FTML surface binding and cellular uptake. In contrast, no uptake was observed for lower FR-expressing human breast carcinoma cells (ZR-75-1).

FROM THE CLINICAL EDITOR

This study discusses the design and cellular uptake of multifunctional folate-targeted cationic magnetoliposomes enabling doxorubicin delivery and SPIO labeling.

Stimuli-responsive electrospun fibers and their applications

Stimuli-responsive electrospun nanofibers are gaining considerable attention as highly versatile tools which offer great potential in the biomedical field. In this critical review, an overview is given on recent advances made in the development and application of stimuli-responsive fibers. The specific features of these electrospun fibers are highlighted and discussed in view of the properties required for the diverse applications. Furthermore, several novel biomedical applications are discussed and the respective advantages and shortcomings inherent to stimuli-responsive electrospun fibers are addressed (136 references).

Bilayer heating in magnetite nanoparticle-liposome dispersions via fluorescence anisotropy

Temperature measurements have been made within magnetite (Fe(3)O(4)) nanoparticle-liposome dispersions subjected to electromagnetic field at radiofrequency (RF) heating based on the fluorescence anisotropy of diphenylhexatriene (DPH) embedded within the bilayer. Incorporating cholesterol within dipalmitoylphosphatidylcholine (DPPC) bilayers broadened the anisotropy window associated with lipid melting. Cryogenic transmission electron microscopy showed that the dispersions contained magnetoliposomes with nanoparticle aggregates at both low and high encapsulation densities. RF heating results demonstrated the ability to measure the temperature of the ML bilayer with on/off RF cycles using DPH anisotropy. These measurements reflected the temperature of the bulk aqueous phase, which is consistent with previous work showing rapid heat dissipation from a nanoparticle surface during RF heating and a negligible difference between surface and bulk temperature.

Magnetoliposomes as magnetic resonance imaging contrast agents

Among the wide variety in iron oxide nanoparticles which are routinely used as magnetic resonance imaging (MRI) contrast agents, magnetoliposomes (MLs) take up a special place. In the present work, the two main types (large and small MLs) are defined and their specific features are commented. For both types of MLs, the flexibility of the lipid coating allows for efficient functionalization, enabling bimodal imaging (e.g., MRI and fluorescence) or the use of MLs as theranostics. These features are especially true for large MLs, where several magnetite cores are encapsulated within a single large liposome, which were found to be highly efficient theranostic agents. By carefully fine-tuning the number of magnetite cores and attaching Gd(3+) -complexes onto the liposomal surface, the large MLs can be efficiently optimized for dynamic MRI. A special type of MLs, biogenic MLs, can also be efficiently used in this regard, with potential applications in cancer treatment and imaging. Small MLs, where the lipid bilayer is immediately attached onto a solid magnetite core, give a very high r2 /r1 ratio. The flexibility of the lipid bilayer allows the incorporation of poly(ethylene glycol)-lipid conjugates to increase blood circulation times and be used as bone marrow contrast agents. Cationic lipids can also be incorporated, leading to high cell uptake and associated strong contrast generation in MRI of implanted cells. Unique for these small MLs is the high resistance the particles exhibit against intracellular degradation compared with dextran- or citrate-coated particles. Additionally, intracellular clustering of the iron oxide cores enhances negative contrast generation and enables longer tracking of labeled cells in time.

Intracellular nanoparticle coating stability determines nanoparticle diagnostics efficacy and cell functionality

Iron oxide nanoparticles (NPs) are frequently employed in biomedical research as magnetic resonance (MR) contrast agents where high intracellular levels are required to clearly depict signal alterations. To date, the toxicity and applicability of these particles have not been completely unraveled. Here, we show that endosomal localization of different iron oxide particles results in their degradation and in reduced MR contrast, the rate of which is governed mainly by the stability of the coating. The release of ferric iron generates reactive species, which greatly affect cell functionality. Lipid-coated NPs display the highest stability and furthermore exhibit intracellular clustering, which significantly enhances their MR properties and intracellular persistence. These findings are of considerable importance because, depending on the nature of the coating, particles can be rapidly degraded, thus completely annihilating their MR contrast to levels not detectable when compared to controls and greatly impeding cell functionality, thereby hindering their application in functional in vivo studies.

Assessing iron oxide nanoparticle toxicity in vitro: current status and future prospects

The in vitro labeling of stem or therapeutic cells with engineered nanoparticles with the aim of transplanting these cells into live animals and, for example, noninvasively monitoring their migration, is a hot topic in nanomedicine research. It is of crucial importance that cell–nanoparticle interactions are studied in depth in order to exclude any negative effects of the cell labeling procedure. To date, many disparate results can be found in the literature regarding nanoparticle toxicity due to the great versatility of different parameters investigated. In the present work, an overview is presented of different types of nanomaterials, focusing mostly on iron oxide nanoparticles, developed for biomedical research. The difficulties in assessing nanoparticle-mediated toxicity are discussed, an overview of some of the problems encountered using commercial (dextran-coated) iron oxide nanoparticles is presented, several key parameters are highlighted and novel methods suggested – emphasizing the importance of intracellular nanoparticle degradation and linking toxicity data to functional (i.e., cell-associated) nanoparticle levels, which could help to advance any progress in this highly important research topic.

Cytotoxic effects of iron oxide nanoparticles and implications for safety in cell labelling

The in vitro labelling of cultured cells with iron oxide nanoparticles (NPs) is a frequent practice in biomedical research. To date, the potential cytotoxicity of these particles remains an issue of debate. In the present study, 4 different NP types (dextran-coated Endorem, carboxydextran-coated Resovist, lipid-coated magnetoliposomes (MLs) and citrate-coated very small iron oxide particles (VSOP)) are tested on a variety of cell types, being C17.2 neural progenitor cells, PC12 rat pheochromocytoma cells and human blood outgrowth endothelial cells. Using different NP concentrations, the effect of the NPs on cell morphology, cytoskeleton, proliferation, reactive oxygen species, functionality, viability and cellular homeostasis is investigated. Through a systematic study, the safe concentrations for every particle type are determined, showing that MLs can lead up to 67.37 ± 5.98 pg Fe/cell whereas VSOP are the most toxic particles and only reach 18.65 ± 2.07 pg Fe/cell. Using these concentrations, it is shown that for MRI up to 500 cells/μl labelled with VSOP are required to efficiently visualize in an agar phantom in contrast to only 50 cells/μl for MLs and 200 cells/μl for Endorem and Resovist. These results highlight the importance of in-depth cytotoxic evaluation of cell labelling studies as at non-toxic concentrations, some particles appear to be less suitable for the MR visualization of labelled cells.

Background migration of USPIO/MLs is a major drawback for in situ labeling of endogenous neural progenitor cells

MR-labeling of endogenous neural progenitor cells (NPCs) to follow up cellular migration with in vivo magnetic resonance imaging (MRI) is a very promising tool in the rapidly growing field of cellular imaging. To date, most of the in situ labeling work has been performed using micron-sized iron oxide particles. In this work magnetoliposomes (MLs), i.e. ultrasmall superparamagnetic iron oxide cores (USPIOs), each individually coated by a phospholipid bilayer, were used as the MR contrast agent. One of the main advantages of MLs is that the phospholipid bilayer allows easy modification of the surface, which creates the opportunity to construct a wide range of MLs optimized for specific biomedical applications. We have investigated the ability of MLs to label endogenous NPCs after direct injection into the adult mouse brain. Whereas MRI revealed contrast relocation towards the olfactory bulb, our data strongly imply that this relocation is independent of the migration of endogenous NPCs but represents background migration of MLs along a white matter tract. Our findings suggest that the small size of USPIOs/MLs intrinsically limits their potential for in situ labeling of NPCs.

PEGylated lipids impede the lateral diffusion of adsorbed proteins at the surface of (magneto)liposomes

Protein binding to nanoparticles is a crucial issue in biomedicine, as it triggers their clearance from the bloodstream after intravenous injection. Many techniques are available for measuring strong protein binding interactions, but weak dynamic interactions are more difficult to assess. To tackle the latter problem, in the present work, cytochrome c was chosen as a representative model of a water-soluble protein and the adsorbing particulates were either small unilamellar phospholipid vesicles or 14 nm diameter solid superparamagnetic iron oxide cores onto which a phospholipid bilayer was strongly chemisorbed (so-called magnetoliposomes). Incorporation of cytochrome c oxidase into the phospholipid bilayer allowed the association of cytochrome c with the surface of the particles to be measured with high sensitivity by VIS-spectrophotometry. The impact of enzyme density as well as some of the physical features of the PEG corona (degree of PEGylation and PEG chain length) adjacent to the surface of the lipid structures on the overall kinetics was also investigated.

Controlled release from bilayer-decorated magnetoliposomes via electromagnetic heating

Nanoscale assemblies that can be activated and controlled through external stimuli represent a next stage in multifunctional therapeutics. We report the formation, characterization, and release properties of bilayer-decorated magnetoliposomes (dMLs) that were prepared by embedding small hydrophobic SPIO nanoparticles at different lipid molecule to nanoparticle ratios within dipalmitoylphosphatidylcholine (DPPC) bilayers. The dML structure was examined by cryogenic transmission electron microscopy and differential scanning calorimetry, and release was examined by carboxyfluorescein leakage. Nanoparticle heating using alternating current electromagnetic fields (EMFs) operating at radio frequencies provided selective release of the encapsulated molecule at low nanoparticle concentrations and under physiologically acceptable EMF conditions. Without radio frequency heating, spontaneous leakage from the dMLs decreased with increasing nanoparticle loading, consistent with greater bilayer stability and a decrease in the effective dML surface area due to aggregation. With radio frequency heating, the initial rate and extent of leakage increased significantly as a function of nanoparticle loading and electromagnetic field strength. The mechanism of release is attributed to a combination of bilayer permeabilization and partial dML rupture.

Magneto-mechanical mixing and manipulation of picoliter volumes in vesicles

Superparamagnetic beads in giant unilamellar vesicles are used to facilitate magnetic manipulation, positioning, agitation and mixing of ultrasmall liquid volumes. Vesicles act as leakproof picoliter reaction vessels in an aqueous bulk solution and can be deliberately conveyed by an external magnetic field to a designated position. Upon application of an external magnetic field the beads align to form extended chains. In a rotating magnetic field chains break up into smaller fragments caused by the interplay of viscous friction and magnetic attraction. This process obeys a simple relationship and can be exploited to enhance mixing of the vesicle content and the outer solution or adjacent vesicle volumes exactly at the position of release.

The role of nanoparticle concentration-dependent induction of cellular stress in the internalization of non-toxic cationic magnetoliposomes

Magnetoliposomes (MLs), built up of ultrasmall iron oxide cores each individually surrounded by a lipid bilayer, have emerged as highly biocompatible nanoparticles and promising tools in many biomedical applications. To improve cell uptake, cationic amphiphiles are inserted into the ML coat, but this often induces cytotoxic effects. In the present work, we synthesized and tested a cationic peptide-lipid conjugate (dipalmitoylphosphatidylethanolamine-succinyl-tetralysine [DPPE-succ-(Lys)4]) which is entirely composed of biodegradable moieties and specifically designed to exert minimal cytotoxic effects. Uptake studies with both murine 3T3 fibroblasts and C17.2 neural progenitor cells shows 95.63 +/- 5.83 pg Fe and 87.46 +/- 5.62 pg Fe per cell after 24 h, respectively, for 16.66% DPPE-succ-(Lys)4-containing MLs, with no effect on cell viability. However, these high intracellular nanoparticle concentrations transiently affect actin cytoskeleton architecture, formation of focal adhesion complexes and cell proliferation, returning to control levels after approximately 7 days post ML-incubation in both cell types. This study points out the great need for thorough characterization of cell-nanoparticle interactions as subtle time-dependent effects are hard to monitor and commonly used viability and functionality assays are not sufficient to address the broad spectrum of possible interferences of the nanoparticle with normal cell functioning.

Addressing the problem of cationic lipid-mediated toxicity: the magnetoliposome model

The high biocompatibility and versatile nature of liposomes made these particles keystone components in many hot-topic research areas. For transfection and cell labelling purposes, synthetic cationic lipids are often added, but in most studies, little attention has been paid to their cytotoxic effects. In the present work, cationic magnetoliposomes (MLs), i.e. iron oxide cores enwrapped by a phospholipid bilayer (dimyristoylphosphatidylcholine or sphingomyelin) doped with cationic lipids (1,2-distearoyl-3-trimethylammonium propane), serve as a model to examine cationic lipid toxicity. Mechanisms of cytotoxic effects were found to be either dependent or independent of actual particle internalisation according to data obtained in the absence or presence of several endocytosis inhibitors. The former seem to be caused by the generation of reactive oxygen species (ROS) leading to a Ca2+ influx at high ROS levels. The latter are due to a destabilisation of the cell plasma membrane upon transfer of the cationic lipid from the ML bilayer into the plasma membrane. However, these adverse effects can be diminished by the use of a ROS scavenger, a Ca(2+)-channel blocker or by modulating the liposome size, lipid bilayer constitution or by stabilising the membrane by anchoring it on a solid core. Careful attention must be paid in terms of assessing cell viability as the effects are highly time dependent and the data suggest the incompatibility of using the well-known MTT assay when high levels of ROS species are generated.

Cell labeling and tracking for experimental models using magnetic resonance imaging

Magnetic Resonance Imaging (MRI), as one of the most powerful methods in clinical diagnosis, has emerged as an additional method in the field of molecular and cellular imaging. Compared to established molecular imaging methods, MRI provides in vivo images with high resolution. In particularly in the field of cell-based therapy, non-invasively acquired information on temporal changes of cell location linked to high-resolution anatomical information is of great interest. Relatively new approaches like responsive contrast agents or MR imaging reporter gene expression are MRI applications beyond temporal and spatial information on labeled cells towards investigations on functional changes of cells in vivo. MRI-based cell monitoring and tracking studies require prior labeling of the cells under investigation for excellent contrast against the background of host tissue. Here, an overview is provided on contrast generation strategies for MRI of cells. This includes MR contrast agents, various approaches of cell labeling and MRI as well as MR spectroscopic methods used for cell tracking in vivo. Advantages and disadvantages of the particular labeling approaches and methods are discussed. In addition to description of the methods, the emphasis is on the potential but also challenges and shortcomings of this imaging technique for applications that aim to visualize cellular processes in vivo.

Lipid-based nanoparticles as pharmaceutical drug carriers: from concepts to clinic

In recent years, various nanotechnology platforms in the area of medical biology, including both diagnostics and therapy, have gained remarkable attention. Moreover, research and development of engineered multifunctional nanoparticles as pharmaceutical drug carriers have spurred exponential growth in applications to medicine in the last decade. Design principles of these nanoparticles, including nanoemulsions, dendrimers, nano-gold, liposomes, drug-carrier conjugates, antibody-drug complexes, and magnetic nanoparticles, are primarily based on unique assemblies of synthetic, natural, or biological components, including but not limited to synthetic polymers, metal ions, oils, and lipids as their building blocks. However, the potential success of these particles in the clinic relies on consideration of important parameters such as nanoparticle fabrication strategies, their physical properties, drug loading efficiencies, drug release potential, and, most importantly, minimum toxicity of the carrier itself. Among these, lipid-based nanoparticles bear the advantage of being the least toxic for in vivo applications, and significant progress has been made in the area of DNA/RNA and drug delivery using lipid-based nanoassemblies. In this review, we will primarily focus on the recent advances and updates on lipid-based nanoparticles for their projected applications in drug delivery. We begin with a review of current activities in the field of liposomes (the so-called honorary nanoparticles), and challenging issues of targeting and triggering will be discussed in detail. We will further describe nanoparticles derived from a novel class of amphipathic lipids called bolaamphiphiles with unique lipid assembly features that have been recently examined as drug/DNA delivery vehicles. Finally, an overview of an emerging novel class of particles (based on lipid components other than phospholipids), solid lipid nanoparticles and nanostructured lipid carriers will be presented. We conclude with a few examples of clinically successful formulations of currently available lipid-based nanoparticles.