Nitroxide-loaded hexosomes provide MRI contrast in vivo

Contrast Agents Based on Human Serum Albumin and Nitroxides for 1H-MRI and Overhauser-Enhanced MRI

In cancer diagnostics, magnetic resonance imaging (MRI) uses contrast agents to enhance the distinction between the target tissue and background. Several promising approaches have been developed to increase MRI sensitivity, one of which is Overhauser dynamic nuclear polarization (ODNP)-enhanced MRI (OMRI). In this study, a macromolecular construct based on human serum albumin and nitroxyl radicals (HSA-NIT) was developed using a new synthesis method that significantly increased the modification to 21 nitroxide residues per protein. This was confirmed by electron paramagnetic resonance (EPR) spectroscopy and matrix-assisted laser desorption/ionization time-of-flight (MALDI ToF) mass spectrometry. Gel electrophoresis and circular dichroism showed no significant changes in the structure of HSA-NITs, and no oligomers were formed during modification. The cytotoxicity of HSA-NITs was comparable to that of native albumin. HSA-NITs were evaluated as potential "metal-free" organic radical relaxation-based contrast agents for 1H-MRI and as hyperpolarizing contrast agents for OMRI. Relaxivities (longitudinal and transversal relaxation rates r1 and r2) for HSA-NITs were measured at different magnetic field strengths (1.88, 3, 7, and 14 T). Phantoms were used to demonstrate the potential use of HSA-NIT as a T1- and T2-weighted relaxation-based contrast agent at 3 T and 14 T. The efficacy of 1H Overhauser dynamic nuclear polarization (ODNP) in liquids at an ultralow magnetic field (ULF, B0 = 92 ± 0.8 μT) was investigated for HSA-NIT conjugates. The HSA-NITs themselves did not show ODNP enhancement; however, under the proteolysis conditions simulating cancer tissue, HSA-NIT conjugates were cleaved into lower-molecular-weight (MW) protein fragments that activate ODNP capabilities, resulting in a maximum achievable enhancement |Emax| of 40-50 and a radiofrequency power required to achieve half of Emax, P1/2, of 21-27 W. The HSA-NIT with a higher degree of modification released increased the number of spin probes upon biodegradation, which significantly enhanced the Overhauser effect. Thus, HSA-NITs may represent a new class of MRI relaxation-based contrast agents as well as novel cleavable conjugates for use as hyperpolarizing contrast agents (HCAs) in OMRI.

Lyotropic liquid crystalline phases: Drug delivery and biomedical applications

Liquid crystal (LC)-based nanoformulations may efficiently deliver drugs and therapeutics to targeted biological sites. Lyotropic liquid crystalline phases (LLCPs) have received much interest in recent years due to their unique structural characteristics of both isotropic liquids and crystalline solids. These LLCPs can be utilized as promising drug delivery systems to deliver drugs, proteins, peptides and vaccines because of their improved drug loading, stabilization, and controlled drug release. The effects of molecule shape, microsegregation, and chirality are very important in the formation of liquid crystalline phases (LCPs). Homogenization of self-assembled amphiphilic lipids, water and stabilizers produces LLCPs with different types of mesophases, bicontinuous cubic (cubosomes) and inverse hexagonal (hexosomes). Moreover, many studies have also shown higher bioadhesivity and biocompatibility of LCs due to their structural resemblance to biological membranes, thus making them more efficient for targeted drug delivery. In this review, an outline of the engineering aspects of LLCPs and polymer-based LLCPs is summarized. Moreover, it covers parenteral, oral, transdermal delivery and medical imaging of LC in targeting various tissues and is discussed with a scope to design more efficient next-generation novel nanosystems. In addition, a detailed overview of advanced liquid crystal-based drug delivery for vaccines and biomedical applications is reviewed.

Remote Loading of Hydrophilic Drug into Cubosomes by Transmembrane pH-Gradient and Characterization of Drug-Loaded Cubosomes Prepared by Different Method

The encapsulation efficiency (EE) of hydrophobic drug into cubosomes was high by conventional methods, while poor for the hydrophilic drug. In this study, a remote loading method based on transmembrane pH-gradient was applied to prepare hydrophilic drug loaded cubosomes. Several hydrophilic drugs were selected and studied. Results showed just part of the investigated drugs were successfully loaded into cubosomes by the remote loading method, whereas all the drugs failed to be encapsulated by the high-pressure homogenization method. The EE based on remote loading method was affected by the solubility, LogP, number of rings, and polarizability of the drug independent of the number of hydrogen acceptor and hydrogen donor. And the drugs that had high EE by remote loading method were BCS class 1 or 2. In addition, the EE base on remote loading method was significantly affected by the external water pH of cubosomes and drug concentration. The size of drug-loaded cubosomes by remote loading method mainly depended on the pre-formed blank cubosomes, which was bigger than that by high-pressure homogenization method. The preparation method affected the liquid crystalline structure of acidic drug loaded cubosomes, while showed no obvious effect on that of basic drug loaded cubosomes. The release of drug was susceptible to the pH of release medium independent of the preparation method. The drug-loaded cubosomes prepared by different method all showed favorable stability during storage. The remote loading method was a promising approach for the efficient encapsulation of hydrophilic drug into cubosomes. This study laid a foundation for the application of remote loading method on the preparation of hydrophilic drug loaded cubosomes.

Topological defects in polycrystalline hexosomes from β-cyclodextrin fatty esters

Colloidal nanoparticles were prepared by aqueous self-assembly of amphiphilic β-cyclodextrins (βCDs) acylated on their secondary face with C₁₄ chains to a total degree of substitution of 7.0, via a thermolysin-catalyzed transesterification process. The small-angle X-ray scattering pattern of the nanoparticles was consistent with a reverse hexagonal organization. Cryo-transmission electron microscopy images revealed particles with spectacular tortuous shapes and consisting of misoriented domains with a regular columnar hexagonal structure, separated by sharp interfaces. Edge dislocations as well as a variety of stepped tilt grain boundaries (GBs) composed of symmetrical and asymmetrical sections, together with one twist GB, were identified from axial views of the columnar organization. The tilt GB structure was analyzed using the concepts of coincidence site lattice and structural units developed to describe the atomic structure of interfaces in various types of polycrystals. The tilt GBs were described using sequences of βCD-C₁₄ columns that differed by the number of neighboring columns (5, 6 or 7) and exhibiting distinctive contrasts. To our knowledge, this is the first time that these types of topological defects are described at the nanometric scale by direct observation of colloidal polycrystalline hexosomes of self-organized amphiphiles.

Surface Properties and Liquid Crystal Properties of Alkyltetra(oxyethyl) β-d-Glucopyranoside

Hydrophilic alkyl polyglycosides (APGs) and alkyl glycosides (AGs) with anomeric pure are a class of important substitutes for petroleum-based surfactants. Improving their water solubility should make such hydrophilic glycosurfactants have more excellent potential application value. To solve the inherent problem of poor water solubility of traditional alkyl β-d-glucopyranoside (5), a series of alkyltetra(oxyethyl) β-d-glucopyranosides (4a-4g, n = 7-18) were successfully synthesized by introducing tetra(oxyethylene) fragments to carry out the structural modification. The relationship between the related structure and the physicochemical properties was further investigated, including their hydrophilic-lipophilic balance (HLB), water-solubility, foaming performance, emulsification, hygroscopicity, surface activity, and thermotropic/lyotropic liquid crystal phase behavior. The results showed that the water solubility gradually decreased as the alkyl chain length increased due to the gradual decrease of their HLB number. Octadecyltetra(oxyethyl) β-d-glucopyranoside (4g, n = 18) was found to be insoluble in water at 25 °C. Taken together, long-chain alkyl glycosides had good foaming properties and excellent emulsifying properties. Among them, dodecyltetra(oxyethyl) β-d-glucopyranoside (4d, n = 12) had the best foaming performance. In the rapeseed oil/water system, cetyltetra(oxyethyl) β-d-glucopyranoside (4f, n = 16) had the best emulsifying ability. With the increase of the alkyl chain length, the critical micelle concentration (C_cmc), γ_cmc, Γ_max, and hygroscopicity of this series of glycosides showed a downward trend. Differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) showed that the thermal stability increased with the increase of the alkyl chain length, and alkyltetra(oxyethyl) β-d-glucopyranosides (4d-4g, n = 12-18) had the corresponding melting points and clearing points. Alkyltetra(oxyethyl) β-d-glucopyranosides (4b-4g, n = 8-18) formed a smectic phase with a typical fan-shaped and focal conic texture during the cooling process. In the water contact experiments, it was found that glycosides (4b-4g, n = 8-18) at high concentrations transformed into various lyotropic liquid crystal including hexagonal phase, bicontinuous cubic phase, and lamellar phase phases. Therefore, such green nonionic glycosurfactants alkyltetra(oxyethyl) β-d-glucopyranosides should have potential practical application prospects.

Novel Amphiphilic Block Copolymers for the Formation of Stimuli-Responsive Non-Lamellar Lipid Nanoparticles

Non-lamellar lyotropic liquid crystalline (LLC) lipid nanoparticles contain internal multidimensional nanostructures such as the inverse bicontinuous cubic and the inverse hexagonal mesophases, which can respond to external stimuli and have the potential of controlling drug release. To date, the internal LLC mesophase responsiveness of these lipid nanoparticles is largely achieved by adding ionizable small molecules to the parent lipid such as monoolein (MO), the mixture of which is then dispersed into nanoparticle suspensions by commercially available poly(ethylene oxide)-poly(propylene oxide) block copolymers. In this study, the Reversible Addition-Fragmentation chain Transfer (RAFT) technique was used to synthesize a series of novel amphiphilic block copolymers (ABCs) containing a hydrophilic poly(ethylene glycol) (PEG) block, a hydrophobic block and one or two responsive blocks, i.e., poly(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl acrylate) (PTBA) and/or poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA). High throughput small angle X-ray scattering studies demonstrated that the synthesized ABCs could simultaneously stabilize a range of LLC MO nanoparticles (vesicles, cubosomes, hexosomes, inverse micelles) and provide internal particle nanostructure responsiveness to changes of hydrogen peroxide (H2O2) concentrations, pH and temperature. It was found that the novel functional ABCs can substitute for the commercial polymer stabilizer and the ionizable additive in the formation of next generation non-lamellar lipid nanoparticles. These novel formulations have the potential to control drug release in the tumor microenvironment with endogenous H2O2 and acidic pH conditions.

Bioimaging Applications of Non-Lamellar Liquid Crystalline Nanoparticles

Self-assembling processes of amphiphilic lipids in water give rise to complex architectures known as lyotropic liquid crystalline (LLC) phases. Particularly, bicontinuous cubic and hexagonal LLC phases can be dispersed in water forming colloidal nanoparticles respectively known as cubosomes and hexosomes. These non-lamellar LLC dispersions are of particular interest for pharmaceutical and biomedical applications as they are potentially non-toxic, chemically stable, and biocompatible, also allowing encapsulation of large amounts of drugs. Furthermore, conjugation of specific moieties enables their targeting, increasing therapeutic efficacies and reducing side effects by avoiding exposure of healthy tissues. In addition, as they can be easy loaded or functionalized with both hydrophobic and hydrophilic imaging probes, cubosomes and hexosomes can be used for the engineering of multifunctional/theranostic nanoplatforms. This review outlines recent advances in the applications of cubosomes and hexosomes for in vitro and in vivo bioimaging.

Recent advances in drug delivery applications of cubosomes, hexosomes, and solid lipid nanoparticles

The use of lipid nanocarriers for drug delivery applications is an active research area, and a great interest has particularly been shown in the past two decades. Among different lipid nanocarriers, ISAsomes (Internally self-assembled somes or particles), including cubosomes and hexosomes, and solid lipid nanoparticles (SLNs) have unique structural features, making them attractive as nanocarriers for drug delivery. In this contribution, we focus exclusively on recent advances in formation and characterization of ISAsomes, mainly cubosomes and hexosomes, and their use as versatile nanocarriers for different drug delivery applications. Additionally, the advantages of SLNs and their application in oral and pulmonary drug delivery are discussed with focus on the biological fates of these lipid nanocarriers in vivo. Despite the demonstrated advantages in in vitro and in vivo evaluations including preclinical studies, further investigations on improved understanding of the interactions of these nanoparticles with biological fluids and tissues of the target sites is necessary for efficient designing of drug nanocarriers and exploring potential clinical applications.

Metal-Free Organo-Theranostic Nanosystem with High Nitroxide Stability and Loading for Image-Guided Targeted Tumor Therapy

The desire for all-organic-composed nanoparticles (NPs) of considerable biocompatibility to simultaneously diagnose and treat cancer is undeniably interminable. Heretofore, metal-based agents dominate the landscape of available magnetic resonance imaging (MRI) contrast agents and photothermal therapeutic agents, but with associated metal-specific downsides. Here, an all-organic metal-free nanoprobe, whose appreciable biocompatibility is synergistically contributed by its tetra-organo-components, is developed as a viable alternative to metal-based probes for MRI-guided tumor-targeted photothermal therapy (PTT). This rationally entails a glycol chitosan (GC)-linked polypyrrole (PP) nanoscaffold that provides abundant primary and secondary amino groups for amidation with the carboxyl groups in a nitroxide radical (TEMPO) and folic acid (FA), to obtain GC-PP@TEMPO-FA NPs. Advantageously, the appreciably benign GC-PP@TEMPO-FA features high nitroxide loading (r₁ = 1.58 mM^-1 s^-1) and in vivo nitroxide-reduction resistance, prolonged nitroxide-systemic circulation times, appreciable water dispersibility, potential photodynamic therapeutic and electron paramagnetic resonance imaging capabilities, considerable biocompatibility, and ultimately achieves a 17 h commensurate MRI contrast enhancement. Moreover, its GC component conveys a plethora of PP to tumor sites, where FA-mediated tumor targeting enables substantial NP accumulation with consequential near-complete tumor regression within 16 days in an MRI-guided PTT. The present work therefore promotes the engineering of organic-based metal-free biocompatible NPs in synergism, in furtherance of tumor-targeted image-guided therapy.

Treatment of Staphylococcus aureus skin infection in vivo using rifampicin loaded lipid nanoparticles

We have previously reported on a novel nanoparticle formulation that was effective at killing Staphylococcus aureus in vitro. Here, we report for the first time, the antibacterial effects of a lipidic nano-carrier containing rifampicin (NanoRIF) which can be used to successfully treat Methicillin-Resistant S. aureus (MRSA) infection at a reduced antibiotic dosage compared to the free drug in a skin wound model in mice. The formulation used contains the lipid monoolein, a cationic lipid N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methyl-sulfate (DOTAP) and the antibiotic. We have shown that rifampicin-loaded nanoparticles are more effective at treating infection in the skin wound model than the antibiotic alone. Cryo-TEM was used to capture for the first time, interactions of the formed nanoparticles with the cell wall of an individual bacterium. Our data strongly indicate enhanced binding of these charged nanoparticles with the negatively charged bacterial membrane. The efficacy we have now observed in vivo is of significant importance for the continued development of nanomedicine-based strategies to combat antibiotic resistant bacterial skin infections.

Pro-organic radical contrast agents ("pro-ORCAs") for real-time MRI of pro-drug activation in biological systems

Nitroxide-based organic-radical contrast agents (ORCAs) are promising as safe, next-generation magnetic resonance imaging (MRI) tools. Nevertheless, stimuli-responsive ORCAs that enable MRI monitoring of prodrug activation have not been reported; such systems could open new avenues for prodrug validation and image-guided drug delivery. Here, we introduce a novel "pro-ORCA" concept that addresses this challenge. By covalent conjugation of nitroxides and drug molecules (doxorubicin, DOX) to the same brush-arm star polymer (BASP) through chemically identical cleavable linkers, we demonstrate that pro-ORCA and prodrug activation, i.e., ORCA and DOX release, leads to significant changes in MRI contrast that correlate with cytotoxicity. This approach is shown to be general for a range of commonly used linker cleavage mechanisms (e.g., photolysis and hydrolysis) and release rates. Pro-ORCAs could find applications as research tools or clinically viable "reporter theranostics" for in vitro and in vivo MRI-correlated prodrug activation.

Human Serum Albumin Labelled with Sterically-Hindered Nitroxides as Potential MRI Contrast Agents

Four albumin-nitroxide conjugates were prepared and tested as metal-free organic radical contrast agents (ORCAs) for magnetic resonance imaging (MRI). Each human serum albumin (HSA) carrier bears multiple nitroxides conjugated via homocysteine thiolactones. These molecular conjugates retain important physical and biological properties of their HSA component, and the resistance of their nitroxide groups to bioreduction was retained or enhanced. The relaxivities are similar for these four conjugates and are much greater than those of their individual components: the HSA or the small nitroxide molecules. This new family of conjugates has excellent prospects for optimization as ORCAs.

Supramolecular and biomacromolecular enhancement of metal-free magnetic resonance imaging contrast agents

Many contrast agents for magnetic resonance imaging are based on gadolinium, however side effects limit their use in some patients. Organic radical contrast agents (ORCAs) are potential alternatives, but are reduced rapidly in physiological conditions and have low relaxivities as single molecule contrast agents. Herein, we use a supramolecular strategy where cucurbit[8]uril binds with nanomolar affinities to ORCAs and protects them against biological reductants to create a stable radical in vivo. We further overcame the weak contrast by conjugating this complex on the surface of a self-assembled biomacromolecule derived from the tobacco mosaic virus.

2-Butyl-2-tert-butyl-5,5-diethylpyrrolidine-1-oxyls: Synthesis and Properties

Nitroxides are broadly used as molecular probes and labels in biophysics, structural biology, and biomedical research. Resistance of a nitroxide group bearing an unpaired electron to chemical reduction with low-molecular-weight antioxidants and enzymatic systems is of critical importance for these applications. The redox properties of nitroxides are known to depend on the ring size (for cyclic nitroxides) and electronic and steric effects of the substituents. Here, two highly strained nitroxides, 5-(tert-butyl)-5-butyl-2,2-diethyl-3-hydroxypyrrolidin-1-oxyl (4) and 2-(tert-butyl)-2-butyl-5,5-diethyl-3,4-bis(hydroxymethyl)pyrrolidin-1-oxyl (5), were prepared via a reaction of the corresponding 2-tert-butyl-1-pyrroline 1-oxides with butyllithium. Thermal stability and kinetics of reduction of the new nitroxides by ascorbic acid were studied. Nitroxide 5 showed the highest resistance to reduction.

The Transition from Metal-Based to Metal-Free Contrast Agents for T1 Magnetic Resonance Imaging Enhancement

Magnetic resonance imaging (MRI) has received significant attention as the noninvasive diagnostic technique for complex diseases. Image-guided therapeutic strategy for diseases such as cancer has also been at the front line of biomedical research, thanks to the innovative MRI, enhanced by the prior delivery of contrast agents (CAs) into patients' bodies through injection. These CAs have contributed a great deal to the clinical utility of MRI but have been based on metal-containing compounds such as gadolinium, manganese, and iron oxide. Some of these CAs have led to cytotoxicities such as the incurable Nephrogenic Systemic Fibrosis (NSF), resulting in their removal from the market. On the other hand, CAs based on organic nitroxide radicals, by virtue of their structural composition, are metal free and without the aforementioned drawbacks. They also have improved biocompatibility, ease of functionalization, and long blood circulation times, and have been proven to offer tissue contrast enhancement with longitudinal relaxivities comparable with those for the metal-containing CAs. Thus, this Review highlights the recent progress in metal-based CAs and their shortcomings. In addition, the remarkable goals achieved by the organic nitroxide radical CAs in the enhancement of MR images have also been discussed extensively. The focal point of this Review is to emphasize or demonstrate the crucial need for transition into the use of organic nitroxide radicals-metal-free CAs-as against the metal-containing CAs, with the aim of achieving safer application of MRI for early disease diagnosis and image-guided therapy.

Nature-Inspired Design and Application of Lipidic Lyotropic Liquid Crystals

Amphiphilic lipids aggregate in aqueous solution into a variety of structural arrangements. Among the plethora of ordered structures that have been reported, many have also been observed in nature. In addition, due to their unique morphologies, the hydrophilic and hydrophobic domains, very high internal interfacial surface area, and the multitude of possible order-order transitions depending on environmental changes, very promising applications have been developed for these systems in recent years. These include crystallization in inverse bicontinuous cubic phases for membrane protein structure determination, generation of advanced materials, sustained release of bioactive molecules, and control of chemical reactions. The outstanding diverse functionalities of lyotropic liquid crystalline phases found in nature and industry are closely related to the topology, including how their nanoscopic domains are organized. This leads to notable examples of correlation between structure and macroscopic properties, which is itself central to the performance of materials in general. The physical origin of the formation of the known classes of lipidic lyotropic liquid crystalline phases, their structure, and their occurrence in nature are described, and their application in materials science and engineering, biology, medical, and pharmaceutical products, and food science and technology are exemplified.

Non-Lamellar Lyotropic Liquid Crystalline Lipid Nanoparticles for the Next Generation of Nanomedicine

Nonlamellar lyotropic liquid crystalline (LLC) lipid nanomaterials have emerged as a promising class of advanced materials for the next generation of nanomedicine, comprising mainly of amphiphilic lipids and functional additives self-assembling into two- and three-dimensional, inverse hexagonal, and cubic nanostructures. In particular, the lyotropic liquid crystalline lipid nanoparticles (LCNPs) have received great interest as nanocarriers for a variety of hydrophobic and hydrophilic small molecule drugs, peptides, proteins, siRNAs, DNAs, and imaging agents. Within this space, there has been a tremendous amount of effort over the last two decades elucidating the self-assembly behavior and structure-function relationship of natural and synthetic lipid-based drug delivery vehicles in vitro, yet successful clinical translation remains sparse due to the lack of understanding of these materials in biological bodies. This review provides an overview of (1) the benefits and advantages of using LCNPs as drug delivery nanocarriers, (2) design principles for making LCNPs with desirable functionalities for drug delivery applications, (3) current understanding of the LLC material-biology interface illustrated by more than 50 in vivo, preclinical studies, and (4) current patenting and translation activities in a pharmaceutical context. Together with our perspectives and expert opinions, we anticipate that this review will guide future studies in developing LCNP-based drug delivery nanocarriers with the objective of translating them into a key player among nanoparticle platforms comprising the next generation of nanomedicine for disease therapy and diagnosis.

Size-tunable MRI-visible nitroxide-based magnetic mixed micelles: preparation, stability, and theranostic application

Metal-free magnetic mixed micelles (mean diameter: 16 nm) composed of biocompatible surfactant Tween 80 and hydrophobic pyrrolidine-N-oxyl radical were prepared by mixing them in phosphate-buffered saline. The magnetic mixed micelles were characterized by dynamic light scattering and small angle neutron scattering measurements. The stability of the micelles is found to depend on the length of alkyl side chain in the nitroxide compounds and degree of unsaturation in the hydrophobic chain in the surfactant. The size of the mixed micelle can be tuned by changing the molar ratio of Tween 80 and nitroxyl radical. In view of theranostic application of the micelle, the cytotoxicity and stability in a physiological environment was investigated; the mixed micelle exhibited no cytotoxicity, high colloidal stability and high resistance towards reduction by large excess ascorbic acid. The in vitro and in vivo magnetic resonance imaging (MRI) revealed sufficient contrast enhancement in the proton longitudinal relaxation time (T ₁) weighted images. In addition, hydrophobic fluorophores and an anticancer drug are stably encapsulated in the mixed micelles and showed fluorescence (FL) upon reduction by ascorbic acid and cytotoxicity to cancer cells, respectively. For example, the paclitaxel-loaded mixed micelles efficiently suppressed cancer cell growth. Furthermore, they were found to give higher MRI contrast (higher r ₁ value) in vitro than the micelles without paclitaxel. The magnetic mixed micelles presented here are promising theranostic agents in nanomedicine due to their high biocompatibility and high resistivity towards reduction as well as functioning as a drug carrier in therapy and MR or FL imaging probe in diagnosis.

Metallo-Cubosomes: Zinc-Functionalized Cubic Nanoparticles for Therapeutic Nucleotide Delivery

Development of an effective and potent RNA delivery system remains a challenge for the clinical application of RNA therapeutics. Herein, we describe the development of an RNA delivery platform derived from self-assembled bicontinuous cubic lyotropic liquid crystalline phases, functionalized with zinc coordinated lipids. These metallo-cubosomes were prepared from a series of novel lipidic zinc(II)-bis(dipicolylamine) (Zn₂BDPA)) complexes admixed with glycerol monooleate (GMO). The zinc metallo-cubosomes showed the high affinity to siRNA through interaction between Zn₂BDPA and the phosphate groups of RNA molecules. Using a combination of dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM), we demonstrated that a variety of Zn₂BDPA lipid derivatives can be loaded into GMO cubosomes and the introduction of Zn₂BDPA lipids effected an internal cubic phase transition of the resulting metallo-cubosomes. The findings of this study lay the foundations for the development of a new class of noncationic lipid-based encapsulation systems, metallo-cubosomes for RNA therapeutic delivery.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

Triply Loaded Nitroxide Brush-Arm Star Polymers Enable Metal-Free Millimetric Tumor Detection by Magnetic Resonance Imaging

Nitroxides occupy a privileged position among plausible metal-free magnetic resonance imaging (MRI) contrast agents (CAs) due to their inherently low-toxicity profiles; nevertheless, their translational development has been hindered by a lack of appropriate contrast sensitivity. Nanostructured materials with high nitroxide densities, where each individual nitroxide within a macromolecular construct contributes to the image contrast, could address this limitation, but the synthesis of such materials remains challenging. Here, we report a modular and scalable synthetic approach to nitroxide-based brush-arm star polymer (BASP) organic radical CAs (ORCAs) with high nitroxide loadings. The optimized ∼30 nm diameter "BASP-ORCA3" displays outstanding T2 sensitivity with a very high molecular transverse relaxivity ( r2 > 1000 mM-1 s-1). BASP-ORCA3 further exhibits excellent stability in vivo, no acute toxicity, and highly desirable pharmacokinetic and biodistribution profiles for longitudinal detection of tumors by MRI. When injected intravenously into mice bearing subcutaneous plasmacytomas, BASP-ORCA3 affords distinct in vivo visualization of tumors on translationally relevant time scales. Leveraging its high sensitivity, BASP-ORCA3 enables efficient mapping of tumor necrosis, which is an important biomarker to predict therapeutic outcomes. Moreover, BASP-ORCA3 allows for detection of millimetric tumor implants in a disseminated murine model of advanced-stage human ovarian cancer that possess genetic, histological, and vascular characteristics that are similar to those seen in patients. This work establishes BASP-ORCA3 as a promising metal-free spin contrast agent for MRI.

Nitroxyl Modified Tobacco Mosaic Virus as a Metal-Free High-Relaxivity MRI and EPR Active Superoxide Sensor

Superoxide overproduction is known to occur in multiple disease states requiring critical care; yet, noninvasive detection of superoxide in deep tissue remains a challenge. Herein, we report a metal-free magnetic resonance imaging (MRI) and electron paramagnetic resonance (EPR) active contrast agent prepared by "click conjugating" paramagnetic organic radical contrast agents (ORCAs) to the surface of tobacco mosaic virus (TMV). While ORCAs are known to be reduced in vivo to an MRI/EPR silent state, their oxidation is facilitated specifically by reactive oxygen species-in particular, superoxide-and are largely unaffected by peroxides and molecular oxygen. Unfortunately, single molecule ORCAs typically offer weak MRI contrast. In contrast, our data confirm that the macromolecular ORCA-TMV conjugates show marked enhancement for T1 contrast at low field (<3.0 T) and T2 contrast at high field (9.4 T). Additionally, we demonstrated that the unique topology of TMV allows for a "quenchless fluorescent" bimodal probe for concurrent fluorescence and MRI/EPR imaging, which was made possible by exploiting the unique inner and outer surface of the TMV nanoparticle. Finally, we show TMV-ORCAs do not respond to normal cellular respiration, minimizing the likelihood for background, yet still respond to enzymatically produced superoxide in complicated biological fluids like serum.

Polymer-free cubosomes for simultaneous bioimaging and photodynamic action of photosensitizers in melanoma skin cancer cells

We designed novel polymer-free cubic bicontinuous liquid crystalline dispersions (cubosomes) using monoolein as molecular building block, phospholipids as stabilizers, propylene glycol as hydrotrope. Their kinetic stability was evaluated by analysing the backscattering profiles upon ageing, and the most stable formulation was chosen as potential photosensitizers delivery vehicle for photodynamic therapy (PDT) of human skin melanoma cells. Morphological and topological features of such formulation alternatively loaded with Chlorin e6 or meso-Tetraphenylporphine-Mn(III) chloride photosensitizing dyes were investigated by cryo-TEM, DLS, and SAXS. Bioimaging studies demonstrated that Me45 and MeWo cell lines effectively internalized these cubosomes formulations. Particularly, photodynamic activity experiments proved both the very low cytotoxicity of the cubosomes formulation loaded with Chlorin e6 dye in the "dark" condition, and its significant cytotoxic effect after photoirradiation. The toxic effect recorded when the photosensitizer was encapsulated within the cubosomes was shown to be one order of magnitude higher than that caused by the free photosensitizer. This is the first report of biocompatible polymer-free cubosomes for potential application in both PDT and bioimaging of skin malignant melanoma.

Liquid Crystalline Nanostructures as PEGylated Reservoirs of Omega-3 Polyunsaturated Fatty Acids: Structural Insights toward Delivery Formulations against Neurodegenerative Disorders

Omega-3 polyunsaturated fatty acids (ω-3 PUFAs) are bioactive lipids with considerable impact in medicine and nutrition. These compounds exert structuring effects on the cellular membrane organization, regulate the gene expression, and modulate various signaling cascades and metabolic processes. The purpose of the present work is to demonstrate the structural features of ω-3 PUFA-containing three-dimensional supramolecular lipid assemblies suitable for pharmaceutical applications that require soft porous carriers. We investigate the liquid crystalline structures formed upon mixing of eicosapentaenoic acid (EPA, 20:5) with the lyotropic nonlamellar lipid monoolein and the formation of multicompartment assemblies. Starting with the monoolein-based lipid cubic phase, double membrane vesicles, cubosome precursors, sponge-type particles (spongosomes), mixed intermediate nonlamellar structures, and multicompartment assemblies are obtained through self-assembly at different amphiphilic compositions. The dispersions containing spongosomes as well as nanocarriers with oil and vesicular compartments are stabilized by PEGylation of the lipid/water interfaces using a phospholipid with a poly(ethylene glycol) chain. The microstructures of the bulk mixtures were examined by cross-polarized light optical microscopy. The dispersed liquid crystalline structures and intermediate states were studied by small-angle X-ray scattering, cryogenic transmission electron microscopy, and quasielastic light scattering techniques. They established that PUFA influences the phase type and the sizes of the aqueous compartments of the liquid crystalline carriers. The resulting multicompartment systems and stealth nanosponges may serve as mesoporous reservoirs for coencapsulation of ω-3 PUFA (e.g., EPA) with water-insoluble drugs and hydrophilic macromolecules toward development of combination treatment strategies of neurodegenerative and other diseases.

Non-lamellar lyotropic liquid crystalline nanoparticles enhance the antibacterial effects of rifampicin against Staphylococcus aureus

The fight against infection in an era of emerging antibiotic resistant bacteria is one of the grandest scientific challenges facing society today. Nano-carriers show great promise in improving the antibacterial activity of antibiotics as they are able to enhance their solubility, provide sustained release and reduce toxic side effects via specifically targeting infection sites. Here, we investigate the antibacterial effect of two lipidic nano-carriers that contain the poorly soluble antibiotic rifampicin in their bilayers. One nanoparticle is assembled solely from the lipid monoolein, thus is neutral at physiological pH and the other contains a mixture of monoolein and the cationic lipid N-[1-(2,3-Dioleoyloxy)propyl]-N,N,N-trimethylammonium methyl-sulfate (DOTAP), thus is positively charged. Our results show that rifampicin-loaded nanoparticles reduce the minimum inhibitory concentration against Staphylococcus aureus compared to rifampicin alone, however this reduction was most pronounced for the positively charged nanoparticles. Fluorescent microscopy revealed binding of all nanoparticles to the bacteria and enhanced binding was observed for the charged nanoparticles. This suggests that the cationic lipids promote electrostatic interactions with the negatively charged bacterial membrane. Förster resonance energy transfer demonstrated that the cationic charged nanoparticles were able to fuse with bacterial membranes whilst atomic force microscopy and transmission electron microscopy revealed structural damage to the bacterial membranes caused by the nanoparticles. Significantly, we identified a concentration window in which the nanoparticles exhibited antibacterial activity while not affecting HeLa and CHO cell viability. This ability to improve the efficacy of antibiotics without affecting their eukaryotic cytotoxicity is of significant importance for future development of nanomedicine based strategies to combat infections.

Preparation of Robust Metal-Free Magnetic Nanoemulsions Encapsulating Low-Molecular-Weight Nitroxide Radicals and Hydrophobic Drugs Directed Toward MRI-Visible Targeted Delivery

With a view to developing a theranostic nanomedicine for targeted drug delivery systems visible by magnetic resonance (MR) imaging, robust metal-free magnetic nanoemulsions (mean particle size less than 20 nm) consisting of a biocompatible surfactant and hydrophobic, low molecular weight 2,2,5-trimethyl-5-(4-alkoxy)phenylpyrrolidine-N-oxyl radicals were prepared in pH 7.4 phosphate-buffered saline (PBS). The structure of the nanoemulsions was characterized by electron paramagnetic resonance spectroscopy, and dynamic light scattering and small-angle neutron-scattering measurements. The nanoemulsions showed high colloidal stability, low cytotoxicity, enough reduction resistance to excess ascorbic acid, and sufficient contrast enhancement in the proton longitudinal relaxation time (T₁ ) weighted MR images in PBS in vitro (and preliminarily in vivo). Furthermore, the hydrophobic anticancer drug paclitaxel could be encapsulated inside the nanoparticles, and the resulting paclitaxel-loaded nanoemulsions were efficiently incorporated into HeLa cells to suppress cell growth.

Folic Acid-Targeted Etoposide Cubosomes for Theranostic Application of Cancer Cell Imaging and Therapy

Background

The aim of this study was to develop a novel Poloxamer-based drug delivery system featuring a tumor-targeting folate moiety, which was expected to provide better targeting properties and therapeutic effects compared with the traditional cubosomes (Cubs).

Material/Methods

Both folate-modified Cubs containing etoposide (ETP-Cubs-FA) and normal cubic nanoparticles loaded with etoposide (ETP-Cubs) were prepared through the fragmentation of bulk gels under the homogenization condition of 1500 bar, and a mean particle size of around 180 nm was obtained with a narrow size distribution. The cubosomes were further characterized by differential scanning calorimetry (DSC) and Polarized light microscopy (PLM). The release of ETP in vitro from these nanoparticles was found to be 82.5% at 36 h, showing a sustained release property compared with the free drug administration.

Results

Folate-modified cubosomes exhibited best anti-proliferative activity followed by normal cubosomes and the free drug. A further cell uptake study of Rhodamine B-loaded Cubs-FA (Rh-B-Cubs-FA) showed a marked increase of cellular accumulation compared with free Rh-B and Rh-B-loaded Cubs (Rh-B-Cubs). In vivo Rh-B-based tumor imaging demonstrated that Cubs-FA specifically targeted the tumor tissue.

Conclusions

The folate-modified cubosomes containing ETP may be a promising drug candidate for antitumor treatment.

Self-assembled Lyotropic Liquid Crystalline Phase Behavior of Monoolein-Capric Acid-Phospholipid Nanoparticulate Systems

We report here the lyotropic liquid crystalline phase behavior of two lipid nanoparticulate systems containing mixtures of monoolein, capric acid, and saturated diacyl phosphatidylcholines dispersed by the Pluronic F127 block copolymer. Synchrotron small-angle X-ray scattering (SAXS) was used to screen the phase behavior of a library of lipid nanoparticles in a high-throughput manner. It was found that adding capric acid and phosphatidylcholines had opposing effects on the spontaneous membrane curvature of the monoolein lipid layer and hence the internal mesophase of the final nanoparticles. By varying the relative concentration of the three lipid components, we were able to establish a library of nanoparticles with a wide range of mesophases including at least the inverse bicontinuous primitive and double diamond cubic phases, the inverse hexagonal phase, the fluid lamellar phase, and possibly other phases. Furthermore, the in vitro cytotoxicity assay showed that the endogenous phospholipid-containing nanoparticles were less toxic to cultured cell lines compared to monoolein-based counterparts, improving the potential of the nonlamellar lipid nanoparticles for biomedical applications.

Cubosomes for in vivo fluorescence lifetime imaging

Herein we provided the first proof of principle for in vivo fluorescence optical imaging application using monoolein-based cubosomes in a healthy mouse animal model. This formulation, administered at a non-cytotoxic concentration, was capable of providing both exogenous contrast for NIR fluorescence imaging with very high efficiency and chemospecific information upon lifetime analysis. Time-resolved measurements of fluorescence after the intravenous injection of cubosomes revealed that the dye rapidly accumulated mainly in the liver, while lifetimes profiles obtained in vivo allowed for discriminating between free dye or dye embedded within the cubosome nanostructure after injection.

Theranostic hexosomes for cancer treatments: an in vitro study

Targeted liquid crystalline nanoparticles with a reverse hexagonal inner structure as diagnostic and therapeutic tools in oncology.

Dual-modality NIRF-MRI cubosomes and hexosomes: High throughput formulation and in vivo biodistribution

Engineered nanoparticles with multiple complementary imaging modalities are of great benefit to the rapid treatment and diagnosis of disease in various organs. Herein, we report the formulation of cubosomes and hexosomes that carry multiple amphiphilic imaging contrast agents in their self-assembled lipid bilayers. This is the first report of the use of both near infrared fluorescent (NIRF) imaging and gadolinium lipid based magnetic resonance (MR) imaging modalities in cubosomes and hexosomes. High-throughput screening was used to rapidly optimize formulations with desirable nano-architectures and low in vitro cytotoxicity. The dual-modal imaging nanoparticles in vivo biodistribution and organ specific contrast enhancement were then studied. The NIRF in vivo imaging results indicated accumulation of both cubosomes and hexosomes in the liver and spleen of mice up to 20h post-injection. Remarkably, the biodistribution of the nanoparticle formulations was affected by the mesophase (i.e. cubic or hexagonal), a finding of significant importance for the future use of these compounds, with hexosomes showing higher accumulation in the spleen than the liver compared to cubosomes. Furthermore, in vivo MRI data of animals injected with either type of lyotropic liquid crystal nanoparticle displayed enhanced contrast in the liver and spleen.

Amphiphilic brush polymers produced using the RAFT polymerisation method stabilise and reduce the cell cytotoxicity of lipid lyotropic liquid crystalline nanoparticles

Self-assembled lipid lyotropic liquid crystalline nanoparticles such as hexosomes and cubosomes contain internal anisotropic and isotropic nanostructures, respectively. Despite the remarkable potential of such nanoparticles in various biomedical applications, the stabilisers used in formulating the nanoparticles are often limited to commercially available polymers such as the Pluronic block copolymers. This study explored the potential of using Reversible Addition-Fragmentation chain Transfer (RAFT) technology to design amphiphilic brush-type polymers for the purpose of stabilising phytantriol and monoolein-based lipid dispersions. The synthesised brush-type polymers consisted of a hydrophobic C12 short chain and a hydrophilic poly(ethylene glycol)methyl ether acrylate (PEGA) long chain with multiple 9-unit poly(ethylene oxide) (PEO) brushes with various molecular weights. It was observed that increasing the PEO brush density and thus the length of the hydrophilic component improved the stabilisation effectiveness for phytantriol and monoolein-based cubosomes. Synchrotron small-angle X-ray scattering (SAXS) experiments confirmed that the RAFT polymer-stabilised cubosomes had an internal double-diamond cubic phase with tunable water channel sizes. These properties were dependent on the molecular weight of the polymers, which were considered in some cases to be anisotropically distributed within the cubosomes. The in vitro toxicity of the cubosomes was assessed by cell viability of two human adenocarcinoma cell lines and haemolytic activities to mouse erythrocytes. The results showed that phytantriol cubosomes stabilised by the RAFT polymers were less toxic compared to their Pluronic F127-stabilised analogues. This study provides valuable insight into designing non-linear amphiphilic polymers for the effective stabilisation and cellular toxicity improvement of self-assembled lipid lyotropic liquid crystalline nanoparticles.

Cubosomes and hexosomes as versatile platforms for drug delivery

Nonlamellar liquid crystalline phases are attractive platforms for drug solubilization and targeted delivery. The attractiveness of this formulation principle is linked to the nanostructural versatility, compatiblity, digestiblity and bioadhesive properties of their lipid constituents, and the capability of solubilizing and sustaining the release of amphiphilic, hydrophobic and hydrophilic drugs. Nonlamellar liquid crystalline phases offer two distinct promising strategies in the development of drug delivery systems. These comprise formation of ISAsomes (internally self-assembled ‘somes’ or particles) such as cubosomes and hexosomes, and in situ formation of parenteral dosage forms with tunable nanostructures at the site of administration. This review outlines the unique features of cubosomes and hexosomes and their potential utilization as promising platforms for drug delivery.

Lipid-PEG conjugates sterically stabilize and reduce the toxicity of phytantriol-based lyotropic liquid crystalline nanoparticles

Lyotropic liquid crystalline nanoparticle dispersions are of interest as delivery vectors for biomedicine. Aqueous dispersions of liposomes, cubosomes, and hexosomes are commonly stabilized by nonionic amphiphilic block copolymers to prevent flocculation and phase separation. Pluronic stabilizers such as F127 are commonly used; however, there is increasing interest in using chemically reactive stabilizers for enhanced functionalization and specificity in therapeutic delivery applications. This study has explored the ability of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine conjugated with poly(ethylene glycol) (DSPE-PEGMW) (2000 Da ≤ MW ≤ 5000 Da) to engineer and stabilize phytantriol-based lyotropic liquid crystalline dispersions. The poly(ethylene glycol) (PEG) moiety provides a tunable handle to the headgroup hydrophilicity/hydrophobicity to allow access to a range of nanoarchitectures in these systems. Specifically, it was observed that increasing PEG molecular weight promotes greater interfacial curvature of the dispersions, with liposomes (Lα) present at lower PEG molecular weight (MW 2000 Da), and a propensity for cubosomes (QII(P) or QII(D) phase) at MW 3400 Da or 5000 Da. In comparison to Pluronic F127-stabilized cubosomes, those made using DSPE-PEG3400 or DSPE-PEG5000 had enlarged internal water channels. The toxicity of these cubosomes was assessed in vitro using A549 and CHO cell lines, with cubosomes prepared using DSPE-PEG5000 having reduced cytotoxicity relative to their Pluronic F127-stabilized analogues.

Understanding the Mechanism of Enzyme-Induced Formation of Lyotropic Liquid Crystalline Nanoparticles

Liquid crystalline nanoparticles have shown great potential for application in fields of drug delivery and agriculture. However, optimized approaches to generating these dispersions have long been sought after. This study focused on understanding the mechanism of formation of cubosomes during the recently reported enzymatic approach and extending the approach to alternative lipid types other than phytantriol. The chain length of digestible lipids was found to influence the effectiveness of triglycerides in disrupting the equilibrium cubic phase structure to form the emulsion precursor. In general, a greater hydrophobicity of the triglyceride required a lower concentration to inhibit liquid crystal structure formation. Selachyl alcohol was also examined due to its nondigestible trait and ability to form the inverted hexagonal phase. Digestion of its precursor emulsion formed hexosomes analogous to the phytantriol-based systems. Finally, the assumption that fatty acids liberated during digestion needed to partition out of the nondigestible lipids for the re-formation of the phase structure was found to be untrue. Their ionization state, however, did have an effect on the resulting nanostructure, and this unique property could potentially provide a useful attribute for oral drug delivery systems.

Sterically stabilized spongosomes for multidrug delivery of anticancer nanomedicines

SAXS patterns of drug-loaded lipid nanocarriers stabilized by polysorbate P80 (left); cryo-TEM image of BAI-BJO-spongosomes-2 (right).

Nanostructure and cytotoxicity of self-assembled monoolein–capric acid lyotropic liquid crystalline nanoparticles

Monoolein–capric acid combinations form into particles with internal nanostructures, including inverse hexagonal and bicontinuous cubic mesophases, with differing cytotoxicity.

Solvatochromic fluorescent BODIPY derivative as imaging agent in camptothecin loaded hexosomes for possible theranostic applications

Tumor cell-targeted hexosomes simultaneously loaded with pyrene-modified BODIPY and camptothecin are easily internalized by HeLa cells.