Interactions between pH-sensitive liposomes and model membranes

Endosomal escape: A bottleneck for LNP-mediated therapeutics

Lipid nanoparticles (LNPs) have recently emerged as a powerful and versatile clinically approved platform for nucleic acid delivery, specifically for mRNA vaccines. A major bottleneck in the field is the release of mRNA-LNPs from the endosomal pathways into the cytosol of cells where they can execute their encoded functions. The data regarding the mechanism of these endosomal escape processes are limited and contradicting. Despite extensive research, there is no consensus regarding the compartment of escape, the cause of the inefficient escape and are currently lacking a robust method to detect the escape. Here, we review the currently known mechanisms of endosomal escape and the available methods to study this process. We critically discuss the limitations and challenges of these methods and the possibilities to overcome these challenges. We propose that the development of currently lacking robust, quantitative high-throughput techniques to study endosomal escape is timely and essential. A better understanding of this process will enable better RNA-LNP designs with improved efficiency to unlock new therapeutic modalities.

PEGylated Lipid Nanoparticle Formulations: Immunological Safety and Efficiency Perspective

Lipid nanoparticles (LNPs) have been recognized as efficient vehicles to transport a large variety of therapeutics. Currently in the spotlight as important constituents of the COVID-19 mRNA vaccines, LNPs play a significant role in protecting and transporting mRNA to cells. As one of their key constituents, polyethylene glycol (PEG)-lipid conjugates are important in defining LNP physicochemical characteristics and biological activity. PEGylation has proven particularly efficient in conferring longer systemic circulation of LNPs, thus greatly improving their pharmacokinetics and efficiency. Along with revealing the benefits of PEG conjugates, studies have revealed unexpected immune reactions against PEGylated nanocarriers such as accelerated blood clearance (ABC), involving the production of anti-PEG antibodies at initial injection, which initiates accelerated blood clearance upon subsequent injections, as well as a hypersensitivity reaction referred to as complement activation-related pseudoallergy (CARPA). Further, data have been accumulated indicating consistent yet sometimes controversial correlations between various structural parameters of the PEG-lipids, the properties of the PEGylated LNPs, and the magnitude of the observed adverse effects. Detailed knowledge and comprehension of such correlations are of foremost importance in the efforts to diminish and eliminate the undesirable immune reactions and improve the safety and efficiency of the PEGylated medicines. Here, we present an overview based on analysis of data from the CAS Content Collection regarding the PEGylated LNP immunogenicity and overall safety concerns. A comprehensive summary has been compiled outlining how various structural parameters of the PEG-lipids affect the immune responses and activities of the LNPs, with regards to their efficiency in drug delivery. This Review is thus intended to serve as a helpful resource in understanding the current knowledge in the field, in an effort to further solve the remaining challenges and to achieve full potential.

Key Design Features of Lipid Nanoparticles and Electrostatic Charge-Based Lipid Nanoparticle Targeting

Lipid nanoparticles (LNP) have gained much attention after the approval of mRNA COVID-19 vaccines. The considerable number of currently ongoing clinical studies are testament to this fact. These efforts towards the development of LNPs warrant an insight into the fundamental developmental aspects of such systems. In this review, we discuss the key design aspects that confer efficacy to a LNP delivery system, i.e., potency, biodegradability, and immunogenicity. We also cover the underlying considerations regarding the route of administration and targeting of LNPs to hepatic and non-hepatic targets. Furthermore, since LNP efficacy is also a function of drug/nucleic acid release within endosomes, we take a holistic view of charged-based targeting approaches of LNPs not only in the context of endosomal escape but also in relation to other comparable target cell internalization strategies. Electrostatic charge-based interactions have been used in the past as a potential strategy to enhance the drug release from pH-sensitive liposomes. In this review, we cover such strategies around endosomal escape and cell internalization in low pH tumor micro-environments.

Stealth nanoparticles in oncology: Facing the PEG dilemma

Nanoparticles (Nps) have revolutionized the landscape of many treatments, by modifying not only pharmacokinetic properties of the encapsulated agent, but also providing a significant protection of the drug from non-desired interactions, and reducing side-effects of the enclosed therapeutic, enabling co-encapsulation of possibly synergistic compounds or activities, allowing a controlled release of content and improving the therapeutic effect. Nevertheless, in systemic circulation, Nps suffer a rapid removal by opsonisation and the action of Mononuclear phagocyte system (MPS). To overcome this problem, different polymers, in particular Polyethyleneglycol (PEG), have been used to cover the surface of these nanocarriers forming a hydrophilic layer that allows the delay of the removal. These advantages contrast with some drawbacks such as the difficulty to interact with cell membranes and the development of immunological reactions, conforming the known, "PEG dilemma". To address and minimize this phenomenon, different strategies have been applied. Therefore, this review aims to summarize the state of the art of Pegylation strategies, comment in depth on the principal characteristics of PEG and describe the main alternatives, which are the use of cleavable PEG, addition of different polymers or even use other derivatives of cell membranes to camouflage Nps.

Nucleic Acid-Loaded Lipid Nanoparticle Interactions with Model Endosomal Membranes

Lipid nanoparticles (LNPs) are important delivery systems for RNA-based therapeutics, yet the mechanism of their interaction with endosomal membranes remains unclear. Here, the interactions of nucleic acid-loaded LNPs that contain an ionizable lipid with models of the early and late endosomal membranes are studied, for the first time, using different reflectometry techniques. Novel insight is provided with respect to the subphase pH, the stage of the endosome, and the nature of the nucleic acid cargo. It is found that the insertion of lipids from the LNPs into the model membrane is greatest at pH 6.5 and 5.5, whereas at higher pH, lipid insertion is suppressed with evidence instead for the binding of intact LNPs, demonstrating the importance of the pH in the fusion of LNPs undergoing the endosomal pathway. Furthermore, and independently of the pH, the effect of the early- versus late-stage endosomal models is minimal, suggesting that the increased fluidity and anionic nature of the late endosome has little effect on the extent of LNP interaction. Last, there is greater nucleic acid delivery from LNPs containing mRNA than Poly(A), indicating that the extent of interaction can be tuned according to the nature of the nucleic acid cargo. Such new information on the relative impact of factors influencing nucleic acid delivery by LNP interactions with endosomal membranes is important in the design and tuning of vehicles with improved nucleic acid delivery capacities.

Nanostructured Systems Improve the Antimicrobial Potential of the Essential Oil from Cymbopogon densiflorus Leaves

The physicochemical characteristics of nanostructured suspensions are important prerequisites for the success of new drug development. This work aimed to develop nanometric systems containing Cymbopogon densiflorus leaf essential oil and to evaluate their antimicrobial activity. The essential oil was isolated by hydrodistillation from leaves and analyzed by GC-MS. The main constituents were found to be trans-p-mentha-2,8-dien-1-ol, cis-p-mentha-2,8-dien-1-ol, trans-p-mentha-1(7),8-dien-2-ol, cis-piperitol, and cis-p-mentha-1(7),8-dien-2-ol. In silico prediction analysis suggested that this oil possesses antimicrobial potential and the main mechanism of action might be the peptidoglycan glycosyltransferase inhibition. Nanoemulsions were prepared by the phase inversion method, and liposomes were made by the film hydration method. Qualitative evaluation of the antimicrobial activity was performed by the diffusion disk assay with 24 microorganisms; all of them were found to be sensitive to the essential oil. Subsequently, this property was quantified by the serial microdilution technique, where the nanoformulations demonstrated improved activity in comparison with the free oil. Bactericidal action was tested by the propidium iodide method, which revealed that free essential oil and nanoemulsion increased cytoplasmic membrane permeability, while no difference was observed between negative control and liposome. These results were confirmed by images obtained using transmission electron microscopy. This study has shown an optimization in the antimicrobial activity of C. densiflorus essential oil by a nanoemulsion and a liposomal formulation of the active substances.

DePEGylation strategies to increase cancer nanomedicine efficacy

To maximize drug targeting to solid tumors, cancer nanomedicines with prolonged circulation times are required. To this end, poly(ethylene glycol) (PEG) has been widely used as a steric shield of nanomedicine surfaces to minimize serum protein absorption (opsonisation) and subsequent recognition and clearance by cells of the mononuclear phagocyte system (MPS). However, PEG also inhibits interactions of nanomedicines with target cancer cells, limiting the effective drug dose that can be reached within the target tumor. To overcome this dilemma, nanomedicines with stimuli-responsive cleavable PEG functionality have been developed. These benefit from both long circulation lifetimes en route to the targeted tumor as well as efficient drug delivery to target cancer cells. In this review, various stimuli-responsive strategies to dePEGylate nanomedicines within the tumor microenvironment will be critically reviewed.

Liposome-Encapsulated Baicalein Suppressed Lipogenesis and Extracellular Matrix Formation in Hs68 Human Dermal Fibroblasts

The dermis of human skin contains large numbers of fibroblasts that are responsible for the production of the extracellular matrix (ECM) that supporting skin integrity, elasticity and wound healing. Previously, an in vivo study demonstrated that dermal fibroblasts siting in the lower dermis are capable to convert into skin adipose layer and hence fibroblast lipogenesis may vary the structure and elasticity of dermis. In the present study, Hs68 human dermal fibroblasts were utilized as an in vitro model to study the lipogenesis via using adipogenic differentiation medium (ADM). Baicalein, isolated from Scutellaria baicalensis, is one of the flavonoids to inhibit adipocyte differentiation due to high antioxidant activity in vitro. In order to develop a suitable formulation for baicalein (a poorly water-soluble drug), soybean phosphatidylcholine (SPC) was used to prepare baicalein-loaded liposomes to enhance drug bioavailability. Our results demonstrated that liposome-encapsulated baicalein protected cell viability and increased cellular uptake efficiency of Hs68 fibroblasts. Lipid accumulation, triglyceride synthesis and gene expressions of lipogenesis enzymes (FABP4 and LPL) were significantly increased in ADM-stimulated Hs68 fibroblasts but subsequently suppressed by liposome-encapsulated baicalein. In addition, ADM-induced TNF-α expression and related inflammatory factors was down-regulated by liposome-encapsulated baicalein. Through ADM-induced lipogenesis, the protein expression of elastin, type I and type III collagens increased remarkably, whereas liposome-encapsulated baicalein can down-regulate ADM-induced ECM protein synthesis. Taken together, we found that liposome-encapsulated baicalein can inhibit ADM-induced lipid accumulation and ECM formation in Hs68 fibroblasts through the suppression of lipogenesis enzymes and inflammatory responses. Liposome-encapsulated baicalein may have the potential to improve wound healing and restore skin structure after skin injury.

Manufacturing Methods for Liposome Adjuvants

A wide range of studies have shown that liposomes can act as suitable adjuvants for a range of vaccine antigens. Properties such as their amphiphilic character and biphasic nature allow them to incorporate antigens within the lipid bilayer, on the surface, or encapsulated within the inner core. However, appropriate methods for the manufacture of liposomes are limited and this has resulted in issues with cost, supply, and wider scale application of these systems. Within this chapter we explore manufacturing processes that can be used for the production of liposomal adjuvants, and we outline new manufacturing methods can that offer fast, scalable, and cost-effective production of liposomal adjuvants.

Lipid polymer hybrid as emerging tool in nanocarriers for oral drug delivery

The oral route for drug delivery is a widely accepted route. For that reason, many researchers are currently working to develop efficient oral drug delivery systems. Use of polymeric nanoparticles (NPs) and lipid carrier systems, including liposomes, solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLC), has limitations such as drug leakage and high water content of dispersions. Thus, lipid polymer hybrid nanoparticles (LPNs) have been explored by the researchers to provide a better effect using properties of both polymers and lipids. The present review is focused on the challenges, possibilities, and future perspectives of LPNs for oral delivery.

Niosomes from 80s to present: the state of the art

Efficient and safe drug delivery has always been a challenge in medicine. The use of nanotechnology, such as the development of nanocarriers for drug delivery, has received great attention owing to the potential that nanocarriers can theoretically act as "magic bullets" and selectively target affected organs and cells while sparing normal tissues. During the last decades the formulation of surfactant vesicles, as a tool to improve drug delivery, brought an ever increasing interest among the scientists working in the area of drug delivery systems. Niosomes are self assembled vesicular nanocarriers obtained by hydration of synthetic surfactants and appropriate amounts of cholesterol or other amphiphilic molecules. Just like liposomes, niosomes can be unilamellar or multilamellar, are suitable as carriers of both hydrophilic and lipophilic drugs and are able to deliver drugs to the target site. Furthermore, niosomal vesicles, that are usually non-toxic, require less production costs and are stable over a longer period of time in different conditions, so overcoming some drawbacks of liposomes. The niosome properties are specifically dictated by size, shape, and surface chemistry which are able to modify the drug's intrinsic pharmacokinetics and eventual drug targeting to the areas of pathology. This up-to-date review deals with composition, preparation, characterization/evaluation, advantages, disadvantages and application of niosomes.

pH-sensitive liposomes for drug delivery in cancer treatment

In recent years, liposomes have been employed with growing success as pharmaceutical carriers for antineoplastic drugs. One specific strategy used to enhance in vivo liposome-mediated drug delivery is the improvement of intracytoplasmic delivery. In this context, pH-sensitive liposomes (pHSLip) have been designed to explore the endosomal acidification process, which may lead to a destabilization of the liposomes, followed by a release of their contents into the cell cytoplasm. This review considers the current status of pHSLip development and its applicability in cancer treatment, focusing on the mechanisms of pH sensitivity and liposomal composition of pHSLip. The final section will discuss the application of these formulations in both in vitro and in vivo studies of antitumor efficacy.

pH-responsive biodegradable assemblies containing tunable phenyl-substituted vinyl ethers for use as efficient gene delivery vehicles

Novel pH-responsive assemblies (PEG-lipid:DOPE liposomes) containing tunable and bifunctional phenyl-substituted vinyl ether (PIVE) cross-linkers were prepared. The assemblies consisted of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), acid-cleavable poly(ethylene glycol) (PEG)-conjugated lipids, pDNA, and protamine sulfate (PS). The PIVE linkage was designed to hydrolyze under acidic conditions, and the hydrolysis studies of PEG-lipid compounds containing PIVE at pH 4.2, 5.4, and 7.4 indicated that the hydrolysis rates of PIVE linker were influenced by the substitution of electron withdrawing or electron donating groups on the phenyl ring. Acid-catalyzed hydrolysis of PIVE leads to destabilization of the acid labile PEG-PIVE-lipid:DOPE liposomes via dePEGylation, thereby triggering content release. Content release assays showed that dePEGylation was highly pH-dependent and correlated with the PIVE proton affinity of the phenyl group. These results indicated that the dePEGylative triggering based on a new pH-sensitive PIVE linkage can be controlled. In vitro transfection studies on the pH-responsive assemblies containing mPEG-(MeO-PIVE)-conjugated 1,3-dioctadecyl-rac-glycerol lipids (mPEG-(MeO-PIVE])-DOG) showed higher transfection efficiency compared to that of polyethylenimine (PEI), a positive control, on HEK 293 and COS-7 cells. In addition, lower cytotoxicity of PEG-PIVE-lipid:DOPE liposomes/PS/DNA was observed in comparison to PEI. These results suggest that PEG-PIVE-lipid:DOPE liposomes can be considered as nonviral vehicles for drug and gene delivery applications.

Lipid nature and their influence on opening of redox-active liposomes

The pathway for content release from reduction-sensitive liposomes based on a quinone-dioleoylphosphatidylethanolamine lipid conjugate (Q-DOPE) is outlined using results from fluorescent dye content release assays as well as single- and multiple-angle light scattering. Experimental observations are consistent with a shape/size change of the reduced liposomes prior to their aggregation, with subsequent near-quantitative content release achieved only when the lipid membrane experiences conditions favorable to a lamellar to an inverted hexagonal phase transition. Addition of poly(ethyleneglycol)-modified DOPE (PEG-DOPE) to the Q-DOPE liposomal formulation results in stabilization of the lipid bilayer, whereas incorporation of DOPE yields faster content release. At high DOPE concentrations, DOPE/PEG-DOPE/Q-DOPE liposomes exhibit larger content release, indicating a change in pathway for content release. The outcomes here provide a better understanding of the underlying principles of triggered liposomal content release and the potential utility of specific lipid properties for the rational design of drug delivery systems based on the novel Q-DOPE lipid.

Acid-labile mPEG-vinyl ether-1,2-dioleylglycerol lipids with tunable pH sensitivity: synthesis and structural effects on hydrolysis rates, DOPE liposome release performance, and pharmacokinetics

A family of 3-methoxypoly(ethylene glycol)-vinyl ether-1,2-dioleylglycerol (mPEG-VE-DOG) lipopolymer conjugates, designed on the basis of DFT calculations to possess a wide range of proton affinities, was synthesized and tested for their hydrolysis kinetics in neutral and acidic buffers. Extruded ∼100 nm liposomes containing these constructs in ≥90 mol % 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) produced dispersions that retained their calcein cargo for more than 2 days at pH 7.5, but released the encapsulated contents over a wide range of time scales as a function of the electronic properties of the vinyl ether linkage, the solution pH, and the mPEG-VE-DOG composition in the membrane. The in vivo performance of two different 90:10 DOPE:mPEG-VE-DOG compositions was also evaluated for blood circulation time and biodistribution in mice, using (125)I-tyraminylinulin as a label. The pharmacokinetic profiles gave a t(1/2) of 7 and 3 h for 90:10 DOPE:ST302 and 90:10 DOPE:ST502, respectively, with the liposomes being cleared predominantly by liver and spleen uptake. The behavior of these DOPE:mPEG-VE-DOG formulations is consistent with their relative rates of vinyl ether hydrolysis, i.e., the more acid-sensitive mPEG-VE-DOG derivatives produced faster leakage rates from DOPE:mPEG-VE-DOG liposomes, but decreased the blood circulation times in mice. These findings suggest that the vinyl ether-based PEG-lipid derivatives are promising agents for stabilizing acid-sensitive DOPE liposomes to produce formulations with a priori control over their pH responsiveness in vitro. Our data also suggest, however, that the same factors that contribute to enhanced acid sensitivity of the DOPE:mPEG-VE-DOG dispersions are also likely responsible for their reduced pharmacokinetic profiles.

Novel pH-sensitive cationic lipids with linear ortho ester linkers for gene delivery

In an effort to develop pH-sensitive lipoplexes for efficient gene delivery, we report three novel cationic lipids containing a linear ortho ester linker that conjugates either the headgroup (Type I) or one hydrocarbon chain (Type II) with the rest of the lipid molecule. The cationic lipids carry either an iodide or a chloride counterion. Compared to our previously reported cyclic ortho ester linker, the linear ortho ester linker facilitated the construction of cationic liposomes and lipoplexes with different helper lipids. The chloride counterion not only facilitated the hydration of the lipid films during liposome construction, but also enhanced the hydrolysis of the ortho ester linker in the lipoplexes. After incubation at endosomal pH 5.5, the Type I lipoplexes aggregated and destabilized the endosome-mimicking model liposomes, but not the Type II lipoplexes. The helper lipids (DOPE or cholesterol) of the lipoplexes enhanced the pH-sensitivity of the Type I lipoplexes. In CV-1 cells (monkey kidney fibroblast), the Type I ortho ester-based lipoplexes, especially those with the chloride counterion, significantly improved the gene transfection efficiency, in some cases by more than 100 fold, compared to their pH-insensitive counterparts consisting of DOTAP. The gene transfection efficiency of the ortho ester-based lipoplexes was well correlated with their rate of aggregation and membrane destabilization in response to the endosomal pH 5.5.

Effective targeted gene delivery to dendritic cells via synergetic interaction of mannosylated lipid with DOPE and BCAT

The efficient delivery of plasmids encoding antigenic determinants into dendritic cells (DCs) that control immune response is a promising strategy for rapid development of new vaccines. In this study, we prepared a series of targeted cationic lipoplex based on two synthetic lipid components, mannose-poly(ethylene glycol, MW3000)-1,2-distearoyl-sn-glycero-3-phosphoethanolamine (Mannose-PEG3000-DSPE) and O-(2R-1,2-di-O-(1'Z-octadecenyl)-glycerol)-3-N-(bis-2-aminoethyl)-carbamate (BCAT), that were formulated with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) for evaluation as nonviral vectors for transgene expression in DCs. First, we optimized the N/P ratio for maximum transfection and then screened the effects of mannose targeting for further enhancement of transfection levels. Our results indicate that efficient delivery of gWIZ GFP plasmid into DCs was observed for mannose compositions of ∼10%, whereas low transfection efficiencies were observed with nontargeted formulations. Mannose-targeted lipofectamine complexes also showed high GFP expression levels in DCs relative to nontargeted lipofectamine controls. The best transfection performance was observed using 10 mol % Mannose-PEG3000-DSPE, 60 mol % BCAT, and 30 mol % DOPE, indicating that the most efficient delivery into DCs occurs via synergistic interaction between mannose targeting and acid-labile, fusogenic BCAT/DOPE formulations. Our data suggest that mannose-PEG3000-DSPE/BCAT/DOPE formulations may be effective gene delivery vehicles for the development of DC-based vaccines.

Fusogenic activity of PEGylated pH-sensitive liposomes

The aim of this study was to investigate the fusogenic properties of poly(ethylene glycol) (PEG)ylated dioleoylphosphatidylethanolamine/cholesteryl hemisuccinate (DOPE/CHEMS) liposomes. These pH-sensitive liposomes were prepared by incorporating two different PEG lipids: distearoylphosphatidylethanolamine (DSPE)-PEG₂₀₀₀ was mixed with the liposomal lipids using the conventional method, whereas sterol-PEG₁₁₀₀ was inserted into the outer monolayer of preformed vesicles. Both types of PEGylated liposomes were characterized and compared for their entrapment efficiency, zeta potential and size, and were tested in vitro for pH sensitivity by means of proton-induced leakage and membrane fusion activity. To mimic the routes of intracellular delivery, fusion between pH-sensitive liposomes and liposomes designed to simulate the endosomal membrane was studied. Our investigations confirmed that DOPE/CHEMS liposomes were capable of rapidly releasing calcein and of fusing upon acidification. However, after incorporation of DSPE-PEG₂₀₀₀ or sterol-PEG₁₁₀₀ into the membrane, pH sensitivity was significantly reduced; as the mol ratio of PEG-lipid was increased, the ability to fuse was decreased. Comparison between two different PEGylated pH-sensitive liposomes showed that only vesicles containing 0.6 mol% sterol-PEG₁₁₀₀ in the outer monolayer were still capable of fusing with the endosome-like liposomes and showing leakage of calcein at pH 5.5.

Heterogeneous liposome membranes with pH-triggered permeability enhance the in vitro antitumor activity of folate-receptor targeted liposomal doxorubicin

The killing efficacy of doxorubicin from liposome-based delivery carriers has been shown to correlate strongly with its intracellular trafficking and, in particular, its fast and extensive release from the delivery carrier. However, previously explored pH-triggered mechanisms that were designed to become activated during liposome endocytosis have also been shown to interfere with the liposome stability in vivo. We have designed pH-triggered gel-phase liposomes with heterogeneous membranes for the delivery of doxorubicin. These liposomes are triggered to form "leaky" interfacial boundaries between gel-gel phase separated domains on the membrane bilayer with lowering pH. The pH-triggered mechanism does not compromise liposome stability in vivo and results in superior in vitro killing efficacy of delivered doxorubicin when liposomes are endocytosed by a clathrin-mediated pathway. In the present work, we evaluate the general applicability of these liposomes when targeted to the folate receptor (FR) of KB cancer cells in vitro and become endocytosed by a less acidic pathway: the caveolae pathway. FR-targeting liposomes exhibit almost 50% decrease in cell association for increase in liposome size from 120 to 280 nm in diameter after relatively short incubation times (up to 4 h). The fraction of internalized vesicles, however, is approximately 60% of the cell associated vesicles independent of their size. Our findings demonstrate that, for the same doxorubicin uptake per cancer cell, the killing effect of doxorubicin delivered by pH-triggered lipid vesicles is greater (IC(50) = 0.032 mM for a 6 h incubation) than when delivered by a conventional non-pH-responsive composition (IC(50) = 0.194 mM). These findings suggest higher bioexposure of cells to the therapeutic agent possibly via faster and more extensive release from the carrier. Animal studies of FR-targeting non-pH-responsive liposomal doxorubicin report stronger therapeutic potential for the targeted approach relative to nontargeted liposomes and to free doxorubicin. The findings of the present study suggest that the targeted pH-triggered liposomes could potentially further enhance the therapeutic outcomes of doxorubicin in vivo.

Nanovesicle aerosols as surfactant therapy in lung injury

Acute lung injury causes inactivation of pulmonary surfactant due to leakage of albumin and other markers. Current surfactants are ineffective in this condition and are instilled intratracheally. Nanovesicles of 300 ± 50 nm composed of nonlamellar phospholipids were developed as pulmonary surfactant aerosols for therapy in acid-induced lung injury. A combination of dipalmitoyl phosphatidylcholine and dioleoyl phosphatidylethanolamine was used. The size and composition of the nanovesicles were optimized for an improved airway patency in the presence of albumin and serum. In an acid-induced lung injury model in mice, on treatment with nanovesicle aerosols at a dose of 200 mg/kg, the alveolar protein leakage decreased from 8.62 ± 0.97 μg/mL to 1.94 ± 0.74 μg/mL, whereas the airway patency of the bronchoalveolar lavage fluid increased from 0.6 ± 0.0% to 91.7 ± 1.05%. Nanovesicle aerosols of nonlamellar lipids improved the resistance of pulmonary surfactants to inhibition and were promising as a noninvasive aerosol therapy in acute lung injury.

FROM THE CLINICAL EDITOR

In acute lung injury, intrinsic surfactants are inactivated via albumin leakage and other mechanisms. Currently existing intratracheal surfactants are ineffective in this condition. The authors demonstrate that novel nanovesicle aerosols of nonlamellar lipids improved the resistance of pulmonary surfactants to inhibition and are promising as a noninvasive aerosol therapy in acute lung injury.

"Smart" liposomal nanocontainers in biology and medicine

The perspectives of using liposomes for delivery of drugs to desired parts of the human body have been intensively investigated for more than 30 years. During this time many inventions have been suggested and different kinds of liposomal devices developed, and a number of them have reached the stages of preclinical or clinical trials. The latest techniques can be used to develop biocompatible nano-sized liposomal containers having some abilities of artificial intellect, such as the presence of sensory and responsive units. However, only a few have been clinically approved. Further improvements in this area depend on our knowledge of the interactions of drugs with the lipid bilayer of liposomes. Further studies on liposomal transport through the human body, their targeting of cells requiring therapeutic treatment, and finally, the development of techniques for controlled drug delivery to desired acceptors on cell surfaces or in cytoplasm are still required.

New pH-sensitive liposomes containing phosphatidylethanolamine and a bacterial dirhamnolipid

Phosphatidylethanolamine-based pH-sensitive liposomes of various compositions have been described as efficient systems for cytoplasmic delivery of molecules into cells. Incorporation of an amphiphile of appropriate structure is needed for the stabilization and performance of these vesicles. Among the wide variety of interesting activities displayed by Pseudomonas aeruginosa dirhamnolipids (diRL), is their capacity to stabilize bilayer structures in phosphatidylethanolamine systems. In this work, X-ray scattering, dynamic light scattering, fluorescence spectroscopy and fluorescence microscopy have been used to study the structure and pH-dependent behaviour of phosphatidylethanolamine/diRL liposomes. We show that diRL, in combination with dioleoylphosphatidylethanolamine (DOPE), forms stable multilamellar and unilamellar liposomes. Acidification of DOPE/diRL vesicles leads to membrane destabilization, fusion, and release of entrapped aqueous vesicle contents. Finally, DOPE/diRL pH-sensitive liposomes act as efficient vehicles for the cytoplasmic delivery of fluorescent probes into cultured cells. It is concluded that DOPE/diRL form stable pH-sensitive liposomes, and that these liposomes are incorporated into cultured cells through the endocytic pathway, delivering its contents into the cytoplasm, which means a potential use of these liposomes for the delivery of foreign substances into living cells. Our results establish a new application of diRL as a bilayer stabilizer in phospholipid vesicles, and the use of diRL-containing pH-sensitive liposomes as delivery vehicles.

Biological evaluation of pH-responsive polymer-caged nanobins for breast cancer therapy

A series of doxorubicin-loaded polymer-caged nanobins (PCN(DXR)) were evaluated in vivo in a murine MDA-MB-231 xenograft model of triple-negative breast cancer. The cross-linked polymer cage in PCN(DXR) offers protection for the drug payload while serving as a pH-responsive trigger that enhances drug release in the acidic environments commonly seen in solid tumors and endosomes. Varying the degree of cross-linking in the polymer cage allows the surface potential of PCN(DXR), and thus the in vivo circulation lifetime of the nanocarriers, to be tuned in a facile fashion. Given these design advantages, the present study provides the first in vivo evidence that PCN(DXR) can effectively inhibit tumor growth in a murine model of breast cancer. Importantly, PCN(DXR) was well-tolerated by mice, and drug encapsulation attenuated the toxicity of free doxorubicin. Taken together, this study demonstrates the potential utility of the PCN platform in cancer therapy.

Spontaneous formation of densely stacked multilamellar vesicles in dioctadecyldimethylammonium bromide/oleosiloxane mixtures

A double-tailed surfactant, for example, dioctadecyldimethylammonium bromide (DODAB), tends to form bilayer fragments rather than vesicles in aqueous solution upon sonication. This Letter reports on the morphological transformations of the membrane fragments leading to multilamellar vesicle formation. The surfactant membranes were swollen with tetra(11-methoxy-11-oxoundecyl)tetramethylcyclotetrasiloxane (3) (a cyclosiloxane with covalently attached fatty-acid esters) up to 50 mol % based on the surfactant content. That compound was synthesized by hydrosilylation of 1,3,5,7-tetramethylcyclotetrasiloxane with four molecules of methyl undec-10-enoate. Using cryo-transmission electron microscopy (cryo-TEM), it has been shown that densely stacked multilamellar vesicles form spontaneously in the sonicated DODAB dispersion containing 50 mol % of the compound 3 after adjusting pH to the value of 9.5. This occurs as a result of the hydrolysis of ester bonds of the compound 3 inside the membrane with formation of tetra(11-carboxyundecyl)tetramethylcyclotetrasiloxane (4). The carboxylic groups in compound 4 are ionized under experimental conditions. That results in the appearance of the negative charge at the surface which neutralizes the positively charged molecules of DODAB, thus reducing repulsive interactions between the membrane fragments. Additionally, as was shown using Langmuir monolayer measurements, the presence of compound 4 expands the DODAB film and decreases its bending rigidity. That makes the bending process considerably easier and promotes formation of multilamellar vesicles.

Membrane heterogeneities and fusogenicity in phosphatidylcholine-phosphatidic acid rigid vesicles as a function of pH and lipid chain mismatch

The role of pH-dependent lipid heterogeneities on the fusogenicity of membranes was evaluated on model lipid bilayers in the form of unilamellar vesicles composed of lipid pairs at a fixed equimolar ratio of phosphatidylcholine (PC) and phosphatidic acid (PA) headgroups. The pH and the hydrophobic composition (lipid acyl tails) of membranes were systematically altered, and their effect on vesicle aggregation, membrane fusogenicity, content release, and content mixing was evaluated. Lowering pH increases the extent of protonated PA headgroups forming phase-separated PA-rich heterogeneities and giving rise to molecular packing defects originating at the phase boundaries. Phase boundaries within the membrane's hydrophobic portion are affected by the lipid acyl-tail dynamics (fluidity) and the matching or nonmatching lengths of the acyl tails of lipid pairs comprising the membrane. The aggregates' size increases with lowering pH and is independent of the membrane's hydrophobic composition. Contrary to aggregation, the initial rates of lipid mixing are proportional to pH and also depend on membrane's hydrophobic composition. The apparent lipid-mixing rate constants are greater for membranes containing lipid pairs with mismatched acyl-tail lengths, followed by pairs with matching acyl tails in the gel state and by pairs with matching tails where one lipid is close to its transition temperature. Addition of cholesterol conserves the independence of vesicle aggregation from the membrane's hydrophobic composition. However, it decreases the aggregation rates and inverts the tendency for fusion, among the three types of hydrophobic compositions, suggesting a role of cholesterol's preferential partition in different regions of membrane's heterogeneities. We propose a phenomenological model where the membrane phase boundaries containing molecular packing defects act as "sticking points" on the vesicle exterior via which vesicles aggregate upon contact followed by defect merging via intervesicle lipid exchange and mixing. Such heterogeneous bilayers in the form of drug encapsulating liposomes may potentially improve the therapeutic efficacy by fusing with endosomal membranes, thus increasing drug bioavailability.

pH-Sensitive fusogenic polymer-modified liposomes as a carrier of antigenic proteins for activation of cellular immunity

By modification of liposomes with poly(glycidol) derivatives such as succinylated poly(glycidol) and 3-methylglutarylated poly(glycidol), we have developed functional liposomes that generate fusion ability at mildly acidic pH. We investigated the feasibility of these polymer-modified liposomes as a carrier of antigenic proteins for induction of cellular immunity. These pH-sensitive fusogenic liposomes encapsulating ovalbumin (OVA) were applied to DC2.4 cells, a murine dendritic cell line. Observation with confocal laser scanning microscopy showed that these polymer-modified liposomes were taken up efficiently by the cells, thereafter delivering their contents into the cytosol, probably through fusion with endosomal membranes. Murine bone marrow-derived dendritic cells treated with polymer-modified liposomes encapsulating OVA stimulated CD8-OVA1.3 cells more strongly than OT4H.1D5 cells, indicating that the liposomes induced MHC class I-restricted presentation. Furthermore, administration of the polymer-modified, OVA-loaded liposomes from nasal cavities of mice induced stronger cellular immune responses than the OVA-loaded plain liposomes. Because the ability of the polymer-modified liposomes to activate cellular immunity was comparable to that of Freund's complete adjuvant, which is a widely used adjuvant, they potentially have use in production of efficient vaccines for immunotherapy.

Encapsulated cargo internalized by fusogenic liposomes partially overlaps the endoplasmic reticulum

Few endocytosed ligands, including bacterial toxins and simian virus 40 (SV40) have been shown to reach the endoplasmic reticulum (ER) in mammalian cells. Using calcein and fluorescently labelled lactoferrin encapsulated in fusogenic liposomes we found that the cargo uses a microtubule-based pathway with ER delivery. Endocytic uptake of the lipid vesicles was cholesterol dependent in all cell lines tested, including the caveolin-1-deficient human hepatoma 7 cell line. The ligand was transported in non-caveosome organelles requiring acidic pH for maturation, but able to escape the lysosomal route. These organelles were not recycling endosomes either, as shown by the lack of co-localization with recycling transferrin. Co-localization with the ER-tracker, orange fluorescent protein with KDEL signal retention and cholera toxin in live microscopy revealed an ER distribution of the fluorescent ligand. Brefeldin A, which prevents Golgi-dependent retrograde trafficking, does not disrupt the cargo delivery to the ER. This new endocytic pathway making use of acidic endosome-like organelles is an alternative to the reported SV40 caveolae pathways. Exploiting a cellular route linking the cell surface to the ER, fusogenic liposomes may become efficient drug delivery vehicles for ER stress and diseases.

Cytoplasmic delivery of liposomal contents mediated by an acid-labile cholesterol-vinyl ether-PEG conjugate

An acid-cleavable PEG lipid, 1'-(4'-cholesteryloxy-3'-butenyl)-omega-methoxy-polyethylene[112] glycolate (CVEP), has been developed that produces stable liposomes when dispersed as a minor component (0.5-5 mol %) in 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). Cleavage of CVEP at mildly acidic pHs results in dePEGylation of the latently fusogenic DOPE liposomes, thereby triggering the onset of content release. This paper describes the synthesis of CVEP via a six-step sequence starting from the readily available precursors 1,4-butanediol, cholesterol, and mPEG acid. The hydrolysis rates and release kinetics from CVEP/DOPE liposome dispersions as a function of CVEP loading, as well as the cryogenic transmission electron microscopy and pH-dependent monolayer properties of 9:91 CVEP/DOPE mixtures, also are reported. When folate receptor-positive KB cells were exposed to calcein-loaded 5:95 CVEP/DOPE liposomes containing 0.1 mol % folate-modified 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-polyethylene[76] glycolamide (folate-PEG-DSPE), delivery of the calcein cargo to the cytoplasm of the cells was observed as determined by fluorescence microscopy and flow cytometry. Fluorescence resonance energy transfer analysis of lipid mixing in these cells was consistent with membrane-membrane fusion between the liposome and endosomal membranes.

Heterogeneous domains and membrane permeability in phosphatidylcholine-phosphatidic acid rigid vesicles as a function of pH and lipid chain mismatch

Heterogeneous lipid membranes tuned by pH were evaluated at 37 degrees C in the form of PEGylated vesicles composed of lipid pairs with dipalmitoyl ( n = 16) and distearoyl ( n = 18) chain lengths. One lipid type was chosen to have the titratable moiety phosphatidic acid on its headgroup, and the other lipid type was chosen to have a phosphatidylcholine headgroup. The effect of pH on the formation of lipid heterogeneities and on membrane permeability was studied on vesicles composed of lipid pairs with matching and nonmatching chain lengths. The formation of lipid heterogeneities increases with decreasing pH in membranes composed of lipid pairs with either matching or nonmatching chain lengths. Increased permeability with decreasing pH was exhibited only by membranes composed of lipid pairs with nonmatching chain lengths. Permeability rates correlate strongly with the predicted extent of interfacial boundaries of heterogeneities, suggesting defective packing among nonmatching acyl chains of lipids. In heterogeneous mixtures with one lipid type in the fluid state ( n = 12), the dependence of membrane permeability on pH is weaker. In the presence of serum proteins, PEGylated gel-phase vesicles containing lipid pairs with nonmatching chain lengths exhibit faster release rates with decreasing pH compared to measured release rates in phosphate buffer, suggesting a second mechanism of formation of separated phases. PEGylated vesicles composed of lipid pairs with nonmatching chain lengths labeled with internalizing anti-HER2/neu antibodies that target overexpressed antigens on the surface of SKOV3-NMP2 ovarian cancer cells exhibit specific cancer cell targeting, followed by extensive internalization (more than 84% of bound vesicles) and fast release of contents intracellularly. These PEGylated vesicles composed of rigid membranes for long blood circulation times that exhibit pH-dependent release of contents intracellularly could become potent drug delivery carriers for the targeted therapy of solid tumors.

Protein release from biodegradable dextran nanogels

The use of drugs with intracellular targets will strongly depend on the availability of delivery systems that are able to deliver them to specific intracellular sites at an optimal rate. Biodegradable dextran nanogels were prepared using liposomes as a nanoscaled reactor.1,2 These nanogels were obtained by UV polymerization of dextran hydroxyethylmethacrylate (dex-HEMA) containing 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) liposomes. We found the encapsulation efficiency of bovine serum albumin (BSA) and lysozyme in the dextran nanogels to be about 50%. Specifically, the release of BSA and lysozyme from the dextran nanogels was clearly governed by the cross-link density of the tiny gels. Depending on the size of the encapsulated protein, the cross-link density of the dextran network, and the presence or absence of a lipid coating, proteins were released from the nanogels over days to weeks. Interestingly, when sufficiently diluted, dextran nanogels did not aggregate in human serum, which is of major importance when one considers intravenous administration of such nanogels. Also, reconstitution of lyophilized dextran nanogels seemed perfectly possible, which is also an important finding since dextran nanogels will have to be stored in dry form. Because dextran nanogels can be taken up by cells, they are promising materials for controlled intracellular release of proteins.

Unsaturated Cationic Ortho Esters for Endosome Permeation in Gene Delivery

Two cleavable cationic lipids were designed to trigger the fusogenicity and membrane permeation of their lipoplexes in endosomes via the formation of inverted hexagonal phases (HII). Both lipids contain a cationic head group and an unsaturated hydrophobic dioleylglycerol moiety joined together by a linear or a cyclic ortho ester linker. At pH 7.4, the lipids formed stable complexes with plasmid DNA together with the conelike helper lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). The decrease of pH enhanced the hydrolysis of the ortho ester linkers, which removed the cationic head groups and caused the aggregation of the lipoplexes. At pH 5.5, the cationic lipid N-[2-methyl-2-(1',2'-dioleylglyceroxy)dioxolan-4-yl]methyl-N,N,N-trimethylammonium iodide (2) with a cyclic ortho ester linker showed exceptional pH-sensitivity and triggered its lipoplex to permeate model biomembranes within the time span of endosome processing prior to lysosomal degradation. Lipid 2 significantly improved gene transfection in cultured cells compared to the pH-insensitive control lipid 1,2-dioleoyl-3-trimethylammoniopropane (DOTAP).

Sheddable coatings for long-circulating nanoparticles

Nanoparticles, such as liposomes, polymeric micelles, lipoplexes and polyplexes are frequently studied as targeted drug carrier systems. The ability of these particles to circulate in the bloodstream for a prolonged period of time is often a prerequisite for successful targeted delivery. To achieve this, hydrophilic ‘stealth’ polymers, such as poly(ethylene glycol) (PEG), are used as coating materials. Such polymers shield the particle surface and thereby reduce opsonization by blood proteins and uptake by macrophages of the mononuclear phagocyte system. Yet, after localizing in the pathological site, nanoparticles should deliver their contents in an efficient manner to achieve a sufficient therapeutic response. The polymer coating, however, may hinder drug release and target cell interaction and can therefore be an obstacle in the realization of the therapeutic response. Attempts have been made to enhance the therapeutic efficacy of sterically stabilized nanoparticles by means of shedding, i.e. a loss of the coating after arrival at the target site. Such an ‘unmasking’ process may facilitate drug release and/or target cell interaction processes. This review presents an overview of the literature regarding different shedding strategies that have been investigated for the preparation of sterically stabilized nanoparticulates. Detach mechanisms and stimuli that have been used are described.

Ligand-receptor binding on nanoparticle-stabilized liposome surfaces

We explore the access of receptor (streptavidin) to liposome-immobilized ligand (biotin) in cases where the liposomes are stabilized against fusion by allowing nanoparticles to adsorb. It is found that receptor binding persists over that range of nanoparticle surface coverage where liposome fusion and large-scale aggregation are prevented. This indicates that liposome outer surfaces, in the presence of stabilizers, remain biofunctionalizable, and may have bearing on explaining the long circulation time of stabilized liposomes as drug delivery vehicles.

Synthesis of a polymer skeleton at the inner leaflet of liposomal membranes: polymerization of membrane-adsorbed pH-sensitive monomers

We describe the synthesis of liposomes with an artificial membrane skeleton as a model of the native cellular cytoskeleton. Similar to natural conditions, a flat polymer network is coupled to the inner membrane leaflet like a suspended ceiling via membrane-inserted anchor monomers with a spacer. The polymer is composed of DMAPMA (N-(3-N,N-dimethylaminopropyl) methacrylamide) and TEGDM (tetraethylene glycol dimethacrylate) as a linker and is coupled to the membrane anchor DOGM (1,2-distearyl-3-octaethylene glycol glycerol ether methacrylate). In the first step of the synthesis, DMAPMA and TEGDM are encapsulated into liposomes composed of egg phosphatidylcholine (EPC), and free monomers are removed by gel chromatography. At pH 10, DMAPMA adsorbs to the inner membrane surface, as demonstrated in parallel studies with lipid monolayers using a Langmuir film balance. The polymerization by UV irradiation was initiated with DEAP (2,2-diethoxyacetophenone) as the initiator and was shown to be complete after 15 min. At pH 6, polymer was desorbed from the inner membrane surface to form a lamellar structure similar to that of the cellular cytoskeleton, as shown by electron microscopy. In comparison to NIPAM (N-isopropylacrylamide), which was used as a monomer in a recent study (Stauch, O.; Uhlmann, T.; Frohlich, M.; Thomann, R.; El-Badry, M.; Kim, Y.-K.; Schubert, R. Biomacromolecules 2002, 3, 324-32), DMAPMA shows much slower membrane permeation leading to an essential restriction of the formed polymer to the liposomal interior. The DMAPMA-based composite structure stabilizes the lipid membrane against sodium cholate by a factor of 2.5 as compared to plain EPC liposomes. This is discussed in the context of the situation in the liver, where the cytoskeleton probably plays a crucial role in the stabilization of the membrane against high bile salt concentration.

pH-induced release from P2VP-PEO block copolymer vesicles

The pH-induced release of hydrophilic dyes from poly(2-vinylpyridine-b-ethylene oxide) (P2VP-PEO) block copolymer vesicles is investigated. The structure of the vesicles is characterized using small-angle neutron scattering (SANS) and cryo-electron microscopy (cryo-TEM). A decrease of the pH below 5 leads to protonation and dissolution of the poly-2-vinylpyridine blocks which induces rupture and dissolution of the vesicle membrane. Details of the rupture, dissolution, and release process are studied by fluorescence video microscopy, gel electrophoresis, and high-performance ultrafiltration.

Transfection mediated by pH-sensitive sugar-based gemini surfactants; potential for in vivo gene therapy applications

In this study, the in vitro and in vivo transfection capacity of novel pH-sensitive sugar-based gemini surfactants was investigated. In an aqueous environment at physiological pH, these compounds form bilayer vesicles, but they undergo a lamellar-to-micellar phase transition in the endosomal pH range as a consequence of an increased protonation state. In the same way, lipoplexes made with these amphiphiles exhibit a lamellar morphology at physiological pH and a non-lamellar phase at acidic pH. In this study, we confirm that the gemini surfactants are able to form complexes with plasmid DNA at physiological pH and are able to transfect efficiently CHO cells in vitro. Out of the five compounds tested here, two of these amphiphiles, GS1 and GS2, led to 70% of transfected cells with a good cell survival. These two compounds were tested further for in vivo applications. Because of their lamellar organisation, these lipoplexes exhibited a good colloidal stability in salt and in serum at physiological pH compatible with a prolonged stability in vivo. Indeed, when injected intravenously to mice, these stable lipoplexes apparently did not substantially accumulate, as inferred from the observation that transfection of the lungs was not detectable, as examined by in vivo bioluminescence. This potential of avoiding 'preliminary capture' in the lungs may, thus, be further exploited in developing devices for specific targeting of gemini lipoplexes.

Effects of phospholipid hydrolysis on the aggregate structure in DPPC/DSPE-PEG2000 liposome preparations after gel to liquid crystalline phase transition

Upon storage of phospholipid liposome samples, lysolipids, fatty acids, and glycerol-3-phosphatidylcholine are generated as a result of acid- or base-catalyzed hydrolysis. Accumulation of hydrolysis products in the liposome membrane can induce fusion, leakage, and structural transformations of the liposomes, which may be detrimental or beneficial to their performance depending on their applications as, e.g., drug delivery devices. We investigated in the present study the influence of phospholipid hydrolysis on the aggregate morphology of DPPC/DSPE-PEG2000 liposomes after transition of the phospholipid membrane from the gel phase to liquid crystalline phase using high performance liquid chromatography (HPLC) in combination with static light scattering, dynamic light scattering, and cryo-transmission electron microscopy (cryo-TEM). The rates of DPPC hydrolysis in DPPC/DSPE-PEG2000 liposomes were investigated at a pH of 2, 4, or 6.5 and temperatures of 22 degrees C or 4 degrees C. Results indicate that following phase transition, severe structural reorganizations occurred in liposome samples that were partially hydrolyzed in the gel phase. The most prominent effect was an increasing tendency of liposomes to disintegrate into membrane discs in accordance with an increasing degree of phospholipid hydrolysis. Complete disintegration occurred when DPPC concentrations had decreased by, in some cases, as little as 3.6%. After extensive phospholipid hydrolysis, liposomes and discs fused to form large bilayer sheets as well as other more complex bilayer structures apparently due to a decreased ratio of lysolipid to palmitic acid levels in the liposome membrane.

Enhanced drug transport from unilamellar to multilamellar liposomes induced by molecular recognition of their lipid membranes

Unilamellar PC-based liposomes bearing a recognizable moiety were loaded either with the hydrophilic drug doxorubicin (DXR) or with the hydrophobic drug tamoxiphen (TMX) and allowed to interact with multilamellar PC-based liposomes bearing complementary recognizable groups. It has been established that, due to molecular recognition of these complementary liposomes, effective and fast transport of the drugs occurs from unilamellar to multilamellar liposomes. The transport of TMX is more effective compared to that of DXR. This behavior was observed for both PEGylated and non-PEGylated unilamellar liposomes, and it was attributed to the different sites of solubilization of the drugs in the unilamellar liposomes. PEGylation reduces the transport of both drugs since it inhibits to some extent the molecular recognition effectiveness of the complementary moieties.

Advanced strategies in liposomal cancer therapy: problems and prospects of active and tumor specific drug release

Tumor specific drug delivery has become increasingly interesting in cancer therapy, as the use of chemotherapeutics is often limited due to severe side effects. Conventional drug delivery systems have shown low efficiency and a continuous search for more advanced drug delivery principles is therefore of great importance. In the first part of this review, we present current strategies in the drug delivery field, focusing on site-specific triggered drug release from liposomes in cancerous tissue. Currently marketed drug delivery systems lack the ability to actively release the carried drug and rely on passive diffusion or slow non-specific degradation of the liposomal carrier. To obtain elevated tumor-to-normal tissue drug ratios, it is important to develop drug delivery strategies where the liposomal carriers are actively degraded specifically in the tumor tissue. Many promising strategies have emerged ranging from externally triggered light- and thermosensitive liposomes to receptor targeted, pH- and enzymatically triggered liposomes relying on an endogenous trigger mechanism in the cancerous tissue. However, even though several of these strategies were introduced three decades ago, none of them have yet led to marketed drugs and are still far from achieving this goal. The most advanced and prospective technologies are probably the prodrug strategies where non-toxic drugs are carried and activated specifically in the malignant tissue by overexpressed enzymes. In the second part of this paper, we review our own work, exploiting secretory phospholipase A2 as a site-specific trigger and prodrug activator in cancer therapy. We present novel prodrug lipids together with biophysical investigations of liposome systems, constituted by these new lipids and demonstrate their degradability by secretory phospholipase A2. We furthermore give examples of the biological performance of the enzymatically degradable liposomes as advanced drug delivery systems.

Effects of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymers on structure and stability of liposomal dioleoylphosphatidylethanolamine

The effects caused by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO; Pluronic) copolymers on the structure and stability of dioleoylphosphatidylethanolamine (DOPE) liposomes were studied by means of turbidity, leakage, and cryo-transmission electron microscopy investigations. The results show that by inclusion of Pluronics in the DOPE dispersion it is possible to stabilize the lamellar Lalpha phase and to produce liposomes that are stable and nonleaky at low pH (pH 5). The stabilizing capacity was observed to depend critically on the molecular composition of the Pluronics. Block copolymers with comparably long PPO and PEO segment lengths, such as F127 and F108, most effectively protected DOPE liposomes prepared at high pH from aggregation and subsequent structural rearrangements induced by acidification. A sufficiently long PPO block was found to be the most decisive parameter in order to obtain adequate coverage of the liposome surface at low Pluronic concentrations. Upon increasing the copolymer concentration, however, Pluronics with comparably short PPO and PEO segment lengths, such as F87 and P85, could also be used to stabilize the DOPE liposomes. Essentially the same trends were observed when the Pluronics were added to preformed DOPE liposomes instead of being included in the preparation mixture. In this case the least effective copolymers failed, however, to completely prevent the DOPE liposomes from releasing encapsulated hydrophilic markers.