Dynamic and structural aspects of PEGylated liposomes monitored by NMR

Insight into the liposomal encapsulation of mono and bis-naphthalimides

Mitonafide-loaded liposomes are a promising strategy to overcome the neurotoxicity observed in clinical trials for this drug. This study investigates the influence of loaded mitonafide or a dimer analogue on different liposomal formulations and their therapeutic efficacy in vitro. Physicochemical properties of the liposomes were manipulated using different loading methods (namely bilayer or core loading) and varying the rigidity of the bilayer using distinct phospholipid compositions. Our results demonstrated that the mitonafide dimer analogue had a comparable encapsulation efficiency (EE%) into the liposomes when loaded into rigid or flexible bilayers in contrast to the low mitonafide monomer EE%. A pH gradient core loading method resulted in a more efficient mechanism to load the monomer into the liposomes. DOSY NMR and spectrofluorometric studies revealed key differences in the structure of the vesicles and the arrangement of the monomer or the dimer in the bilayer or the core of the liposomes. The in vitro assessment of the formulations using MDA-MB-231 and RT-112 cells revealed that a flexible lipid bilayer allows a faster drug release, which correlated well with the spectroscopy studies. This study investigated for the first time that the characteristics of the lipid bilayer and the loading method influence the encapsulation efficacy, colloidal properties, photoactivity and stability of mono and bis-naphthalimides loaded in a liposomal carrier, essential factors that will impact the performance of the formulation in a biological scenario.

Bioinspired zwitterionic triblock copolymers designed for colloidal drug delivery: 1 - Synthesis and characterization

This study describes the synthesis and characterization of triblock copolymers composed of poly[2-(methacryloyloxy)ethyl phosphorylcholine]-block-poly(propylene glycol)-block-poly[2-(methacryloyloxy)ethyl phosphorylcholine] (PMPC-b-PPG-b-PMPC) intended for, but not limited to, applications in colloidal drug delivery. Atom transfer radical polymerization led to a library of well-defined PMPC-b-PPG-b-PMPC triblock copolymers with varying overall molecular weight (ranging from ∼5 to ∼25 kDa) and composition (weight fraction of the hydrophobic PPG block ranged from ∼10 to ∼50 wt%). The properties of the synthesized triblock copolymers were linked to the PPG to bioinspired PMPC block(s) ratio, where the more hydrophilic species showed adequate aqueous solubility, surface activity and biocompatibility (non-toxicity) in in vitro cell culture. Their amphiphilic nature makes them adsorb efficiently onto polymer nanoparticles, what improves colloidal stability under stress conditions and, furthermore, depletes proteins from unwanted adsorption to the underlying surface. The current findings strengthen our insights into structure-function relationships of PMPC-based coatings leading to protecting shells on relevant polymer nanoparticle formulations. PMPC-b-PPG-b-PMPC triblock copolymers composed of a hydrophobic PPG block of 2-4 kDa flanked by two hydrophilic PMPC blocks each of 5-10 kDa seem to be most promising to enhance colloidal drug delivery vehicles.

Carbon Dots-Biomembrane Interactions and Their Implications for Cellular Drug Delivery

The effect of carbon dots (CDs) on a model blayer membrane was studied as a means of comprehending their ability to affect cell membranes. Initially, the interaction of N-doped carbon dots with a biophysical liposomal cell membrane model was investigated by dynamic light scattering, z-potential, temperature-modulated differential scanning calorimetry, and membrane permeability. CDs with a slightly positive charge interacted with the surface of the negative-charged liposomes and evidence indicated that the association of CDs with the membrane affects the structural and thermodynamic properties of the bilayer; most importantly, it enhances the bilayer's permeability against doxorubicin, a well-known anticancer drug. The results, like those of similar studies that surveyed the interaction of proteins with lipid membranes, suggest that carbon dots are partially embedded in the bilayer. In vitro experiments employing breast cancer cell lines and human healthy dermal cells corroborated the findings, as it was shown that the presence of CDs in the culture medium selectively enhanced cell internalization of doxorubicin and, subsequently, increased its cytotoxicity, acting as a drug sensitizer.

Rational design of magnetoliposomes for enhanced interaction with bacterial membrane models

There is a growing need for alternatives to target and treat bacterial infection. Thus, the present work aims to develop and optimize the production of PEGylated magnetoliposomes (MLPs@PEG), by encapsulating superparamagnetic iron oxide nanoparticles (SPIONs) within fusogenic liposomes. A Box-Behnken design was applied to modulate size distribution variables, using lipid concentration, SPIONs amount and ultrasonication time as independent variables. As a result of the optimization, it was possible to obtain MLPs@PEG with a mean size of 182 nm, with polydispersity index (PDI) of 0.19, and SPIONs encapsulation efficiency (%EE) around 76%. Cytocompatibility assays showed that no toxicity was observed in fibroblasts, for iron concentrations up to 400μg/ml. Also, for safe lipid and iron concentrations, no hemolytic effect was detected. The fusogenicity of the nanosystems was first evaluated through lipid mixing assays, based on Förster resonance energy transfer (FRET), using liposomal membrane models, mimicking bacterial cytoplasmic membrane and eukaryotic plasma membrane. It was shown that the hybrid nanosystems preferentially interact with the bacterial membrane model. Confocal microscopy and fluorescence lifetime measurements, using giant unilamellar vesicles (GUVs), validated these results. Overall, the developed hybrid nanosystem may represent an efficient drug delivery system with improved targetability for bacterial membrane.

Impact of poly(ethylene glycol) functionalized lipids on ordering and fluidity of colloid supported lipid bilayers

Colloid supported lipid bilayers (CSLBs) are highly appealing building blocks for functional colloids. In this contribution, we critically evaluate the impact on lipid ordering and CSLB fluidity of inserted additives. We focus on poly(ethylene glycol) (PEG) bearing lipids, which are commonly introduced to promote colloidal stability. We investigate whether their effect on the CSLB is related to the incorporated amount and chemical nature of the lipid anchor. To this end, CSLBs were prepared from lipids with a low or high melting temperature (T_m), DOPC, and DPPC, respectively. Samples were supplemented with either 0, 5 or 10 mol% of either a low or high T_m PEGylated lipid, DOPE-PEG₂₀₀₀ or DSPE-PEG₂₀₀₀, respectively. Lipid ordering was probed via differential scanning calorimetry and fluidity by fluorescence recovery after photobleaching. We find that up to 5 mol% of either PEGylated lipids could be incorporated into both membranes without any pronounced effects. However, the fluorescence recovery of the liquid-like DOPC membrane was markedly decelerated upon incorporating 10 mol% of either PEGylated lipids, whilst insertion of the anchoring lipids (DOPE and DSPE without PEG₂₀₀₀) had no detectable impact. Therefore, we conclude that the amount of incorporated PEG stabilizer, not the chemical nature of the lipid anchor, should be tuned carefully to achieve sufficient colloidal stability without compromising the membrane dynamics. These findings offer guidance for the experimental design of studies using CSLBs, such as those focusing on the consequences of intra- and inter-particle inhomogeneities for multivalent binding and the impact of additive mobility on superselectivity.

Understanding molecular mechanisms of biologics drug delivery and stability from NMR spectroscopy

Protein therapeutics carry inherent limitations of membrane impermeability and structural instability, despite their predominant role in the modern pharmaceutical market. Effective formulations are needed to overcome physiological and physicochemical barriers, respectively, for improving bioavailability and stability. Knowledge of membrane affinity, cellular internalization, encapsulation, and release of drug-loaded carrier vehicles uncover the structural basis for designing and optimizing biopharmaceuticals with enhanced delivery efficiency and therapeutic efficacy. Understanding stabilizing and destabilizing interactions between protein drugs and formulation excipients provide fundamental mechanisms for ensuring the stability and quality of biological products. This article reviews the molecular studies of biologics using solution and solid-state NMR spectroscopy on structural attributes pivotal to drug delivery and stability. In-depth investigation of the structure-function relationship of drug delivery systems based on cell-penetrating peptides, lipid nanoparticles and polymeric colloidal, and biophysical and biochemical stability of peptide, protein, monoclonal antibody, and vaccine, as the integrative efforts on drug product design, will be elaborated.

Nuclear Magnetic Resonance Spectroscopy: A Multifaceted Toolbox to Probe Structure, Dynamics, Interactions, and Real-Time In Situ Release Kinetics in Peptide-Liposome Formulations

Liposomal formulations represent attractive biocompatible and tunable drug delivery systems for peptide drugs. Among the tools to analyze their physicochemical properties, nuclear magnetic resonance (NMR) spectroscopy, despite being an obligatory technique to characterize molecular structure and dynamics in chemistry as well as in structural biology, yet appears to be rather sparsely used to study drug-liposome formulations. In this work, we exploited several facets of liquid-state NMR spectroscopy to characterize liposomal delivery systems for the apelin-derived K14P peptide and K14P modified by Nα-fatty acylation. Various liposome compositions and preparation modes were analyzed. Using NMR, in combination with cryo-electron microscopy and dynamic light scattering, we determined structural, dynamic, and self-association properties of these peptides in solution and probed their interactions with liposomes. Using ³¹P and ¹H NMR, we characterized membrane fluidity and thermotropic phase transitions in empty and loaded liposomes. Based on diffusion and ¹H NMR experiments, we localized and quantified peptides with respect to the interior/exterior of liposomes and changes over time and upon thermal treatments. Finally, we assessed the release kinetics of several solutes and compared various formulations. Taken together, this work shows that NMR has the potential to assist the design of peptide/liposome systems and more generally drug delivery systems.

Plasmonic carriers responsive to pulsed laser irradiation: a review of mechanisms, design, and applications

This review focuses on interactions which govern release from plasmonic carrier systems including liposomes, polymersomes, and nanodroplets under pulsed irradiation.

The structural fate of lipid nanoparticles in the extracellular matrix

Drug-loaded liposomes are the most successful nanomedicine to date, with multiple FDA-approved systems for a myriad of diseases. While liposome circulation time in blood and retention in tissues have been studied in detail, the structural fate of liposomes-and nanoparticles in general-in the body has not been extensively investigated. Here, we explore the interactions of liposomes with synthetic and natural hydrogel materials to understand how the natural extracellular matrix influences liposome structural characteristics. Small angle X-ray scattering, confocal microscopy, and cryogenic transmission electron microscopy data demonstrate that poly(ethylene glycol) (PEG), gelatin, alginate, and Matrigel® hydrogels cause 200-nm liposomes of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) to transform into micrometer-sized aggregates. These aggregates are composed of multilamellar vesicles around 100 nm in diameter with a mean interlamellar separation of 5.5 nm. Protecting the liposomes with a corona of PEG damps this restructuring effect, making the multilamellar vesicles less stable. We attribute this unilamellar to multilamellar transition to an osmotic driving force from the hydrogel environment. This lipid restructuring has broad ramifications in the design and use of nanomedicines, and in understanding the fate and function of natural lipid-based materials within the tissue microenvironment.

Self-Assembly in Ganglioside‒Phospholipid Systems: The Co-Existence of Vesicles, Micelles, and Discs

Ganglioside lipids have been associated with several physiological processes, including cell signaling. They have also been associated with amyloid aggregation in Parkinson’s and Alzheimer’s disease. In biological systems, gangliosides are present in a mix with other lipid species, and the structure and properties of these mixtures strongly depend on the proportions of the different components. Here, we study self-assembly in model mixtures composed of ganglioside GM1 and a zwitterionic phospholipid, 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC). We characterize the structure and molecular dynamics using a range of complementary techniques, including cryo-TEM, polarization transfer solid state NMR, diffusion NMR, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and calorimetry. The main findings are: (1) The lipid acyl chains are more rigid in mixtures containing both lipid species compared to systems that only contain one of the lipids. (2) The system containing DOPC with 10 mol % GM1 contains both vesicles and micelles. (3) At higher GM1 concentrations, the sample is more heterogenous and also contains small disc-like or rod-like structures. Such a co-existence of structures can have a strong impact on the overall properties of the lipid system, including transport, solubilization, and partitioning, which can be crucial to the understanding of the role of gangliosides in biological systems.

Lipid-based Liquid Crystalline Films and Solutions for the Delivery of Cargo to Cells

A major challenge in the delivery of cargo (genes and/or drugs) to cells using nanostructured vehicles is the ability to safely penetrate plasma membranes by escaping the endosome before degradation, later releasing the payload into the cytoplasm or organelle of interest. Lipids are a class of bio-compatible molecules that self-assemble into a variety of liquid crystalline constructs. Most of these materials can be used to encapsulate drugs, proteins, and nucleic acids to deliver them safely into various cell types. Lipid phases offer a plethora of structures capable of forming complexes with biomolecules, most notably nucleic acids. The physichochemical characteristics of the lipid molecular building blocks, one might say the lipid primary structure, dictates how they collectively interact to assemble into various secondary structures. These include bilayers, lamellar stacks of bilayers, two-dimensional (2D) hexagonal arrays of lipid tubes, and even 3D cubic constructs. The liquid crystalline materials can be present in the form of aqueous suspensions, bulk materials or confined to a film configuration depending on the intended application (e.g. bolus vs surface-based delivery). This work compiles recent findings of different lipid-based liquid crystalline constructs both in films and particles for gene and drug delivery applications. We explore how lipid primary and secondary structures endow liquid crystalline materials with the ability to carry biomolecular cargo and interact with cells.

Effect of Pegylation and Targeting Moieties on the Ultrasound-Mediated Drug Release from Liposomes

The use of targeted liposomes encapsulating chemotherapy drugs enhances the specific targeting of cancer cells, thus reducing the side effects of these drugs and providing patient-friendly chemotherapy treatment. Targeted pegylated (stealth) liposomes have the ability to safely deliver their loaded drugs to the cancer cells by targeting specific receptors overly expressed on the surface of these cells. Applying ultrasound as an external stimulus will safely trigger drug release from these liposomes in a controlled manner. In this study, we investigated the release kinetics of the model drug "calcein" from targeted liposomes sonicated with low-frequency ultrasound (20 kHz). Our results showed that pegylated liposomes were more sonosensitive compared to nonpegylated liposomes. A comparison of the effect of three targeting moieties conjugated to the surface of pegylated liposomes, namely human serum albumin (HSA), transferrin (Tf) and arginylglycylaspartic acid (RGD), on calcein release kinetics was conducted. The fluorescent results showed that HSA-PEG and Tf-PEG liposomes were more sonosensitive (showing higher calcein release following the exposure to pulsed LFUS) compared to the control pegylated liposomes, thus adding more acoustic benefits to their targeting efficacy.

Copper complex nanoformulations featuring highly promising therapeutic potential in murine melanoma models

Aim: Preclinical evaluation of a cytotoxic copper (II) complex formulated in long circulating nanoliposomes for melanoma treatment. Materials & methods: Liposomal nanoformulations of the copper complex were characterized in terms of thermodynamic behavior (differential scanning calorimeter), pH-sensitivity (spectrophotometry) and antiproliferative effects against murine melanoma B16F10 cells in vitro. Preclinical studies were performed in a C57BL/6 syngeneic melanoma model. Results: Nanoformulations were thermodynamically stable, and CHEMS-containing nanoliposomes were pH-sensitive and preserved the antiproliferative properties of the copper compound. These nanoformulations significantly impaired tumor progression in vivo, devoid of toxic side effects, compared with control mice or mice treated with the free metallodrug. Conclusion: Copper complex-containing nanoliposomes demonstrate high anticancer efficacy and safety, constituting a step forward to the development of more effective therapeutic strategies against melanoma.

The stabilization of primitive bicontinuous cubic phases with tunable swelling over a wide composition range

In this paper we investigate the pseudo-ternary phase diagram of glycerol monooleate (GMO), a cationic lipid (DOTAP - 1,2-dioleoyl-3-trimethylammonium propane), and a "PEGylated" lipid (DOPE-PEG - 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000 kDa]) in excess water. We use small angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (Cryo-EM) to map out a phase diagram in a regime of low DOPE-PEG content (1-5 mol%), which is pertinent for the application of lipid systems as carriers of biomolecular cargo to cells. Pure GMO is known to self-assemble into bicontinuous cubic phases of the gyroid type at low water content and of the diamond type in excess water. These complex structures have numerous advantages reaching beyond drug delivery, e.g. as protein crystallization matrices, but their formulation is challenging as very small contents of guest molecules can shift the phase behavior towards other geometries such as the lamellar phase. In this work, we show that the ternary GMO/DOTAP/DOPE-PEG system allows the stabilization of bicontinuous cubic phases in excess water over a wide composition range. The symmetry of the phase can be tuned by varying the amount of PEGylated lipid, with the primitive type dominating at low DOPE-PEG content (1-3 mol%) and the diamond phase arising at 5 mol% DOPE-PEG. In addition, we found that the diamond phase is virtually non-responsive to electrostatic swelling. In contrast, primitive bicontinuous cubic lattice dimensions swell up in equilibrium to 650 Å with increased cationic lipid content.

Improved metabolic stability and therapeutic efficacy of a novel molecular gemcitabine phospholipid complex

The aim of the present research is to increase lipid solubility, metabolic stability and therapeutic efficacy of water soluble gemcitabine (GEM) via phospholipid complex (PC) formation. A novel phospholipid complex of GEM was successfully prepared and optimized. Physical interaction of GEM with phospholipid was evaluated by DSC, FT-IR, ¹H NMR, ³¹P-NMR and P-XRD. SEM images of GEM-PC showed rough structure and TEM images of diluted aqueous dispersion of GEM-PC showed micellar structure. In silico study also revealed the significant interaction between drug and phospholipid. GEM-PC demonstrated sustained drug release pattern and high plasma stability (∼2.2 fold) in vitro as compared to GEM. Increased in vitro cytotoxicity and apoptosis were observed with GEM-PC, when incubated with human pancreas adenocarcinoma cell lines. In vivo pharmacokinetics showed the almost 2 fold increase in AUC_0-∞ (area under curve) with phospholipid complex (8983.26ngh/ml) as compared with GEM (4371.18ngh/ml) and GEMITA (4689.29ngh/ml). Toxicity studies signify the safety of GEM-PC over GEMITA. Pharmacodynamics studies in pancreatic tumor model further revealed higher efficacy of GEM-PC than GEMITA. These findings suggested the higher potential of phospholipid based technology for the enhancement of metabolic stability and therapeutic efficacy of GEM.

Role of lipid polymorphism in acoustically sensitive liposomes

Ultrasound (US) triggered drug release is a promising drug delivery method that allows ex vivo modulation of treatment intensity and duration. This method relies on the synergistic interaction between the rupture of sonosensitive particles and enhanced plasma membrane permeability. Conventional liposomal systems where the drug passively diffuses through the membrane show virtually no response to acoustic energy. One method to activate drug transport is to induce a topological restructuring of the lipid membrane (zero intrinsic curvature, H = 0) by puncturing pores (H < 0) through which the drug can readily leak out from the interior of the liposomes. In this work we demonstrate strategies to lower the energy cost of creating such membrane defects by introducing lipid molecules with molecular shapes prone to self-assemble into non-lamellar (negative intrinsic curvature, H < 0) structures. All formulations investigated comprise the relevant components typically required for delivery applications such as stealth moieties, cholesterol, and phospholipids. Small angle X-ray scattering studies of a number of lipid systems at increasing amounts of phosphatidylethanolamine (PE) phospholipids reveal that membranes without PE respond to ultrasound by thinning ca. 10 Å, which concomitantly lowers the bending rigidity quadratically in addition to increasing the passive drug permeability. However, at the appropriate PE content the lipid systems display a classic lamellar structure (H = 0) that undergoes a topological transformation after ultrasound exposure into lipid tubes of the reversed type (H < 0) packed in a 2D hexagonal array. At the dilute regime, Fluorescence Microscopy of giant unilamellar vesicles (GUVs) comprising DOPE also experience ultrasound induced restructuring that can be modulated by DOPE content. In general, smaller vesicles of diverse shape connect and form into a "pearl-necklace" configuration. We argue that the inclusion of DOPE within the GUV membrane may result in curvature-driven lipid sorting, providing the system with local membrane instabilities that drive vesicle pearling when exposed to ultrasound.

Liposomal stabilization using a sugar-based, PEGylated amphiphilic macromolecule

Liposomes are an important class of colloidal drug delivery systems, yet the clinical applications of conventional liposomes can be hampered by poor colloidal and biological stabilities. In this work, a sugar-based, PEGylated amphiphilic macromolecule (AM) was evaluated for its ability to stabilize dipalmitoyl phosphatidylcholine (DPPC)-based liposomes. Compared to unmodified liposomes, AM-stabilized liposomes exhibited enhanced colloidal stability, maintaining relatively constant particle sizes for 5 weeks without aggregation. AM-stabilized liposomes also showed significantly decreased membrane permeability, even in the presence of serum. Finally, AM-stabilized liposomes displayed improved biological stability, significantly inhibiting phagocytosis by macrophages. Overall, the effectiveness of AM to stabilize liposomes was comparable to a conventional stabilizing agent, PEG-modified phosphatidylethanolamine. Based upon these results, AM is a promising stabilizing agent for colloidal drug delivery applications and currently being optimized.

Impact of interfacial cholesterol-anchored polyethylene glycol on sterol-rich non-phospholipid liposomes

HYPOTHESIS

Liposomes made of single-chain amphiphiles and a large amount of sterols display several advantages including a limited permeability. In the present paper, we examine the possibility to prepare such non-phospholipid liposomes with interfacial polyethylene glycol (PEG) in order to improve their circulation in the blood stream. Cholesterol (Chol) was chosen as the PEG anchor.

EXPERIMENTS

The phase behavior of mixtures of palmitic acid (PA) and cholesterol including various proportions of PEGylated cholesterol (PEG-Chol) was characterized. In conditions leading to the formation of fluid bilayers, properties of the resulting liposomes were assessed.

FINDINGS

Up to 20 mol% of PEGylated cholesterol could be introduced without significant perturbations in fluid bilayers made of PA and cholesterol. With 10 mol% PEG-Chol, PA/Chol/PEG-Chol liposomes showed a very limited permeability to calcein and doxorubicin. Doxorubicin could be actively loaded in PA/Chol/PEG-Chol liposomes with a high drug loading efficiency and a high drug to lipid ratio. Pharmaco-kinetic experiments in rats indicated that interfacial PEG reduced the clearance of PA/Chol liposomes compared to the naked ones. However the lifetime of these non-phospholipid liposomes in the blood circulation was considerably shorter than that observed for control PEGylated phospholipid liposomes, a phenomenon associated with the negative interfacial charge of the PA/Chol/PEG-Chol liposomes.

Inspired by nature: fundamentals in nanotechnology design to overcome biological barriers

Synergy between nanotechnology and drug delivery has created a multitude of novel drug-delivery systems with great therapeutic potential. However, directing these systems across the biological barriers to the target site has proven difficult. Nanotechnology is looking for inspiration in natural systems that have evolved to overcome such barriers. Here, we review nature-inspired strategies and fundamental features common to successful drug-delivery systems across biological barriers.

Phospholipase A(2)-susceptible liposomes of anticancer double lipid-prodrugs

A novel approach to anticancer drug delivery is presented based on lipid-like liposome-forming anticancer prodrugs that are susceptible to secretory phospholipase A(2) (sPLA(2)) that is overexpressed in several cancer types. The approach provides a selective unloading of anticancer drugs at the target tissues, as well as circumvents the necessity for "conventional" drug loading. In our attempts to improve the performance of the liposomes in vivo, several PEGylated and non-PEGylated liposomal formulations composed of a retinoid prodrug premixed with the sPLA(2)-hydrolyzable DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) were prepared. Besides favorably modifying the physicochemical properties of the liposomes, the incorporation of DPPC and PEG-lipids in the liposomes should substantially enhance the enzymatic activity, as concluded from literature. In addition, one can reap benefits from the presumed permeability enhancing effect of the liberated fatty acids and lysolipids. The size distribution of the prepared liposomes as well as their phase behavior, enzymatic hydrolysis, and cytotoxicity, in the presence and absence of sPLA(2), were determined. The liposomes were around 100nm in diameter and in the gel/fluid coexistence region at 37°C. The enzymatic hydrolysis of the prodrug was pronouncedly accelerated upon the premixing with DPPC, and the hydrolysis was further enhanced by PEGylation. Interestingly, the faster hydrolysis of the prodrug and the released fatty acids and lysolipids from DPPC did not improve the cytotoxicity of the mixture; the effect of combining the prodrug with DPPC was additive and not synergistic. The data presented here question the significance of the permeability enhancing effects claimed for fatty acids and lysolipids at the target cell membrane, and whether these effects can be achieved using physiologically achievable concentrations of fatty acids and lysolipids.

Novel Methods of Enhanced Retention in and Rapid, Targeted Release from Liposomes

Liposomes are single bilayer capsules with distinct interior compartments in which hydrophilic drugs, imaging agents, diagnostics, etc. can be sequestered from the exterior environment. The polar parts of the individual lipids face the water compartments, while the hydrophobic parts of the lipid provide a barrier in which hydrophilic or charged molecules are poorly soluble. Hydrophobic molecules can be dissolved within the bilayer. The bilayers are typically from 3 - 6 nm thick and the liposome can range from about 50 nm - 50 microns in diameter. The question asked in this review is if any one bilayer, regardless of its composition, can provide the extended drug retention, long lifetime in the circulation, active targeting to specific tissues and rapid and controllable drug release at the site of interest. As an alternative, we review methods of self-assembling multicompartment lipid structures that provide enhanced drug retention in physiological environments. We also review methods of externally targeting and triggering drug release via the near infrared heating of gold nanoshells attached to or encapsulated within bilayer vesicles.

Development and characterization of new nanoscaled ultrasound active lipid dispersions as contrast agents

Ultrasound contrast agents are widely used in clinical diagnosis. In recent years, the use of ultrasound contrast agents as therapeutic agents has gained a lot of attention. Of special interest are ultrasound-enhanced gene delivery in various tissues (e.g. cardiac, vascular, skeletal muscle and tumor tissue), ultrasound-enhanced protein delivery (e.g. insulin delivery) and ultrasound-enhanced delivery of small chemicals (e.g. doxorubicin, vancomycin). Commercially available ultrasound contrast agents such as SonoVue® or Optison® are ranged in a size of 2-8 μm. These micronscaled agents show a good ultrasound contrast enhancement and thus they are used for diagnostic imaging. But they are not suitable for targeted drug delivery to tumor tissues or blood clots because for these applications particles smaller than 700 nm are needed. In the present study, we developed new nanoscaled ultrasound contrast agents with a size between 70 and 300 nm. The lipid formulations show excellent contrast intensities using diagnostic ultrasound of about 1.4 MHz. The negatively charged colloidal dispersions are long-time stable under physiological conditions without loss of ultrasound reflectivity. The adjustable supramolecular organization of the carriers depends on the composition and varies from micellar to liposomal structures. The small size and the circulation stability of these systems make them promising for novel diagnostics and controlled drug release applications.

Constituent-dependent liposome structure and organization

We have used steady state and time-resolved fluorescence spectroscopy in concert with TEM to study organization and dynamics of molecules comprising liposomes, discoidal micelles, and spherical micelles. The lipid aggregates contained controlled amounts of lipids with headgroups modified with a thiol-terminated polyethylene glycol (thio-PEG lipids) and a small amount of 1-palmitoyl-2-(pyrene-1-yl)decanoyl-sn-glycero-3-phosphocholine (pyrene tethered DPPC), pyrene, or perylene as spectroscopic probes. The maximum diameter of the lipid aggregates was controlled by the polycarbonate filter pore size used in the extrusion process. The concentration of thio-PEG lipid in the aggregates determines not only the shape of the lipid assemblies but also the organization of the molecules within the assembly. Fluorescence lifetime and anisotropy decay data show that the immediate environment of pyrene tethered DPPC changes with the addition of thio-PEG lipid. In contrast, the dynamics of free chromophore (perylene) are insensitive to the addition of thio-PEG lipid. The addition of thio-PEG lipid to the lipid assembly produces changes in organization that are most pronounced in the lipid headgroup region. Reorientation dynamics of perylene show that the organization of the lipid bilayer acyl chain region is affected little by the addition of thio-PEG lipid and consequent macroscopic changes in the morphology of the lipid assemblies.

Influence of the vesicular bilayer structure on the sorption of ethylbenzyl alcohol

The influence of the physicochemical properties of the vesicular bilayer on the sorption of poorly water soluble compounds was investigated with pulsed field gradient 1H nuclear magnetic resonance (PFG-NMR) for the case of phosphatidylcholine and dioctadecyldimethylammonium bromide (DODAB), using 4-ethylbenzyl alcohol as a model compound. Hereby, the effect of bilayer thickness at a constant physicochemical state was studied using a range of phosphatidylcholines of varying chain lengths, whereas DODAB was preferred to check the influence of the bilayer physicochemical state since this cationic lipid is characterized by three different states within the studied temperature range. When the phospholipid alkyl chain length was changed, no differences were observed in the sorption which was linked to the surface-mediated sorption. On the other hand, when the chemical composition was preserved but the temperature and thus the physical state of the bilayer were changed, the sorption in dioctadecyldimethylammonium bromide (DODAB) vesicles changed dramatically. From those experiments, a strong relationship between the ordering of the surfactant molecules and the sorption can be assumed.