Enhancement of phospholipase A2 catalyzed degradation of polymer grafted PEG-liposomes: effects of lipopolymer-concentration and chain-length

Enzyme-responsive liposomes for controlled drug release

Compared to other nanovectors, liposomes exhibit unique advantages, such as good biosafety and high drug-loading capacity. However, slow drug release from conventional liposomes makes most payloads unavailable, restricting the therapeutic efficacy. Therefore, in the last ∼20 years, enzyme-responsive liposomes have been extensively investigated, which liberate drugs under the stimulation of enzymes overexpressed at disease sites. In this review, we elaborate on the research progress on enzyme-responsive liposomes. The involved enzymes mainly include phospholipases, particularly phospholipase A2, matrix metalloproteinases, cathepsins, and esterases. These enzymes can cleave ester bonds or specific peptide sequences incorporated in the liposomes for controlled drug release by disrupting the primary structure of liposomes, detaching protective polyethylene glycol shells, or activating liposome-associated prodrugs. Despite decades of efforts, there are still a lack marketed products of enzyme-responsive liposomes. Therefore, more efforts should be made to improve the safety and effectiveness of enzyme-responsive liposomes and address the issues associated with production scale-up.

Strategies for altering lipid self-assembly to trigger liposome cargo release

While liposomes have proven to be effective drug delivery nanocarriers, their therapeutic attributes could be improved through the development of clinically viable triggered release strategies in which encapsulated drug contents could be selectively released at the sites of diseased cells. As such, a significant amount of research has been reported involving the development of stimuli-responsive liposomes and a broad range of strategies have been explored for driving content release. These have included the introduction of trigger groups at either the lipid headgroup or within the acyl chains that alter lipid self-assembly properties of known lipids as well as the rational design of lipid analogs programed to undergo conformational changes induced by events such as binding interactions. This review article describes advances in the design of stimuli-responsive liposome strategies with an eye towards emerging trends in the field.

Impeded Molecular Reorganization by Polyethylene Glycol Conjugation Revealed by X-ray Reflectivity and Diffraction Measurements

Polyethylene glycol (PEG) coatings have been widely applied in pharmaceutical and biomedical systems to prevent nonspecific protein absorption, increase vesicle blood circulation time, and sustain drug release. This study systematically investigated the planar interfacial organization of phospholipid monolayers containing various amounts of PEG conjugations before and after enzyme-catalyzed degradation of the lipids using X-ray reflectivity and grazing incidence X-ray diffraction techniques. Results showed that attaching PEG to the headgroup of the lipids up to 15 mol % had limited effects on molecular packing of the lipid monolayers in the condensed phase at the gas-liquid interface and negligible effects on the enzyme adsorption to the interface. After enzyme-catalyzed degradation, equimolar fatty acids and lyso PC were generated. The fatty acids together with the subphase Ca²⁺ self-assembled into highly organized multilayer domains at the interface. The X-ray measurements unambiguously revealed that the densely packed PEG markedly hindered microphase separation and formation of the palmitic acid-Ca²⁺ complexes.

Enzyme-Responsive Liposomes for the Delivery of Anticancer Drugs

Liposomes are nanocarriers that deliver the payloads at the target site, leading to therapeutic drug concentrations at the diseased site and reduced toxic effects in healthy tissues. Several approaches have been used to enhance the ability of the nanocarrier to target the specific tissues, including ligand-targeted liposomes and stimuli-responsive liposomes. Ligand-targeted liposomes exhibit higher uptake by the target tissue due to the targeting ligand attached to the surface, while the stimuli-responsive liposomes do not release their cargo unless they expose to an endogenous or exogenous stimulant at the target site. In this review, we mainly focus on the liposomes that are responsive to pathologically increased levels of enzymes at the target site. Enzyme-responsive liposomes release their cargo upon contact with the enzyme through several destabilization mechanisms: (1) structural perturbation in the lipid bilayer, (2) removal of a shielding polymer from the surface and increased cellular uptake, (3) cleavage of a lipopeptide or lipopolymer incorporated in the bilayer, and (4) activation of a prodrug in the liposomes.

Development of a cell-based bioassay for phospholipase A2-triggered liposomal drug release

The feasibility of exploiting secretory phospholipase A₂ (sPLA₂) enzymes, which are overexpressed in tumors, to activate drug release from liposomes precisely at the tumor site has been demonstrated before. Although the efficacy of the developed formulations was evaluated using in vitro and in vivo models, the pattern of sPLA₂-assisted drug release is unknown due to the lack of a suitable bio-relevant model. We report here on the development of a novel bioluminescence living-cell-based luciferase assay for the monitoring of sPLA₂-triggered release of luciferin from liposomes. To this end, we engineered breast cancer cells to produce both luciferase and sPLA₂ enzymes, where the latter is secreted to the extracellular medium. We report on setting up a robust and reproducible bioassay for testing sPLA₂-sensitive, luciferin remote-loaded liposomal formulations, using 1,2-distearoyl-sn-glycero-3-phosphatidylcholine/1,2-distearoyl-sn-glycero-3-phosphatidylglycerol (DSPC/DSPG) 7:3 and DSPC/DSPG/cholesterol 4:3:3 as initial test systems. Upon their addition to the cells, the liposomes were degraded almost instantaneously by sPLA₂ releasing the encapsulated luciferin, which provided readout from the luciferase-expressing cells. Cholesterol enhanced the integrity of the formulation without affecting its susceptibility to sPLA₂. PEGylation of the liposomes only moderately broadened the release profile of luciferin. The provided bioassay represents a useful tool for monitoring active drug release in situ in real time as well as for testing and optimizing of sPLA₂-sensitive lipid formulations. In addition, the bioassay will pave the way for future in-depth in vitro and in vivo studies.

Phospholipase A2-responsive antibiotic delivery via nanoparticle-stabilized liposomes for the treatment of bacterial infection

Adsorbing small charged nanoparticles onto liposome surfaces to stabilize them against fusion and payload leakage has resulted in a new class of liposomes capable of environment-responsive drug delivery. Herein, we engineered a liposome formulation with a lipid composition sensitive to bacterium-secreted phospholipase A2 (PLA2) and adsorbed chitosan-modified gold nanoparticles (AuChi) onto the liposome surface. The resulting AuChi-stabilized liposomes (AuChi-liposomes) showed prohibited fusion activity and negligible drug leakage. However, upon exposure to either purified PLA2 enzyme or PLA2 secreted by Helicobacter pylori (H. pylori) bacteria in culture, AuChi-liposomes rapidly released the encapsulated payloads and such responsive release was retarded by adding quinacrine dihydrochloride, a PLA2 inhibitor. When loaded with doxycycline, AuChi-liposomes effectively inhibited H. pylori growth. Overall, the AuChi-liposomes allowed for smart "on-demand" antibitoic delivery: the more enzymes or bacteria present at the infection site, the more drug will be released to treat the infection. Given the strong association of PLA2 with a diverse range of diseases, the present liposomal delivery technique holds broad application potential for tissue microenvironment-responsive drug delivery.

Nanomedicine for drug targeting: strategies beyond the enhanced permeability and retention effect

The growing research interest in nanomedicine for the treatment of cancer and inflammatory-related pathologies is yielding encouraging results. Unfortunately, enthusiasm is tempered by the limited specificity of the enhanced permeability and retention effect. Factors such as lack of cellular specificity, low vascular density, and early release of active agents prior to reaching their target contribute to the limitations of the enhanced permeability and retention effect. However, improved nanomedicine designs are creating opportunities to overcome these problems. In this review, we present examples of the advances made in this field and endeavor to highlight the potential of these emerging technologies to improve targeting of nanomedicine to specific pathological cells and tissues.

Receptor mediated binding of avidin to polymer covered liposomes

Fluoresence technique involving a receptor-mediated fluorescence increase of bodipy-labeled avidin upon binding to biotinylated lipids has been used to investigate the steric barrier effect of submicellar concentrations of poly(ethylene glycol)-phospholipids (PE-PEG(2000) and PE-PEG(5000)) incorporated into pure DPPC liposomes as well as PE-PEG(5000) incorporated into DPPC liposomes containing 20 mol% cholesterol. It is found that the incorporation of PE-PEG lipopolymers into DPPC lipid bilayers lowers the receptor-mediated adhesion of avidin to the biotinylated liposomes. The most pronounced screening effect is observed at surface densities corresponding to the mushroom conformation of the polymer. Furthermore, the results show that the steric baric effect induced by the surface-grafted polymers becomes stronger when the length of the polymer chain increases. In addition it is found that cholesterol improves the barrier effect of PE-PEG(5000) at low lipopolymer concentrations while no effect is observed at higher concentrations. The results reveal that both the surface density and the polymer length of the PE-PEG lipopolymers play a major role for the accessibility of avidin to biotin surface receptors. However, none of the lipopolymers were capable of completely preventing avidin from reaching the surface bound ligands. Cholesterol only affected the barrier effect at lipopolymer concentrations below the mushroom to brush transition. Consequently, from a steric stabilization viewpoint there is no rationale for incorporating cholesterol into liposomes when the PE-PEG lipopolymer concentration exceeds the mushroom to brush transition. The results presented in this study are of importance in relation to a deeper understanding of the interaction of liposome degrading enzymes and proteins with polymer covered liposomes as well as for the receptor-based targeting and interaction of liposomes with cell surface receptors.

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.

Lipids, curvature, and nano-medicine

The physical properties of the lamellar lipid-bilayer component of biological membranes are controlled by a host of thermodynamic forces leading to overall tensionless bilayers with a conspicuous lateral pressure profile and build-in curvature-stress instabilities that may be released locally or globally in terms of morphological changes. In particular, the average molecular shape and the propensity of the different lipid and protein species for forming non-lamellar and curved structures are a source of structural transitions and control of biological function. The effects of different lipids, sterols, and proteins on membrane structure are discussed and it is shown how one can take advantage of the curvature-stress modulations brought about by specific molecular agents, such as fatty acids, lysolipids, and other amphiphilic solutes, to construct intelligent drug-delivery systems that function by enzymatic triggering via curvature.Practical applications: The simple concept of lipid molecular shape and how it impacts on the structure of lipid aggregates, in particular the curvature and curvature stress in lipid bilayers and liposomes, can be exploited to construct liposome-based drug-delivery systems, e.g., for use as nano-medicine in cancer therapy. Non-lamellar-forming lysolipids and fatty acids, some of which may be designed to be prodrugs, can be created by phospholipase action in diseased tissues thereby providing for targeted drug release and proliferation of molecular entities with conical shape that break down the permeability barrier of the target cells and may hence enhance efficacy.

Enzyme-triggered nanomedicine: drug release strategies in cancer therapy

Nanomedicine as a field has emerged from the early success of nanoparticle-based drug delivery systems, in particular for treatment of cancer, and the advances made in nano- and biotechnology over the past decade. A prerequisite for nanoparticle-based drug delivery systems to be effective is that the drug payload is released at the target site. A large number of drug release strategies have been proposed that can be classified into certain areas. The simplest and most successful strategy so far, probably due to relative simplicity, is based on utilizing certain physico-chemical characteristics of drugs to obtain a slow drug leakage from the formulations after accumulation in the cancerous site. However, this strategy is only applicable to a relatively small range of drugs and cannot be applied to biologicals. Many advanced drug release strategies have therefore been investigated. Such strategies include utilization of heat, light and ultrasound sensitive systems and in particular pH sensitive systems where the lower pH in endosomes induces drug release. Highly interesting are enzyme sensitive systems where over-expressed disease-associated enzymes are utilized to trigger drug release. The enzyme-based strategies are particularly interesting as they require no prior knowledge of the tumour localization. The basis of this review is an evaluation of the current status of drug delivery strategies focused on triggered drug release by disease-associated enzymes. We limit ourselves to reviewing the liposome field, but the concepts and conclusions are equally important for polymer-based systems.

Activation of interfacial enzymes at membrane surfaces

A host of water-soluble enzymes are active at membrane surfaces and in association with membranes. Some of these enzymes are involved in signalling and in modification and remodelling of the membranes. A special class of enzymes, the phospholipases, and in particular secretory phospholipase A(2) (sPLA(2)), are only activated at the interface between water and membrane surfaces, where they lead to a break-down of the lipid molecules into lysolipids and free fatty acids. The activation is critically dependent on the physical properties of the lipid-membrane substrate. A topical review is given of our current understanding of the physical mechanisms responsible for activation of sPLA(2) as derived from a range of different experimental and theoretical investigations.

Role of lipid protrusions in the function of interfacial enzymes

Secretory phospholipase A(2) (sPLA(2)) is a class of interfacially active enzymes that selectively hydrolyze lipid molecules organized at interfaces like membranes. We present a simple theoretical model that relates the sPLA(2) action to the protrusions of the lipid molecules. The model explains (1) the observed enhancement of enzymatic activity by lipids with flexible, neutral, water-soluble polymers linked to their head groups and (2) the lag-burst kinetics of sPLA(2). It yields qualitative predictions of the effect of the initial composition of the membrane, the molecular weight of the polymer, and the composition of the hydrolysis products.

Direct determination by capillary electrophoresis of cardiovascular drugs, previously included in liposomes

The lipophilicity of some cardiovascular drugs was determined by capillary electrophoresis (CE). Mexiletine, amlodipine and indapamide, the drugs considered, were in contact with liposomial vescicles for 2, 4 or 6 h. After the contact time the drugs, penetrated into liposomial vesicles, were determined by CE using phosphate buffer (pH 6.3 or 7.4) or borate buffer (pH 9). The lipophilicity of three drugs was determined considering the drug percentage penetrated into liposomial vesicles. The found lipohilicity order was amlodipine > mexiletine > indapamide.

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.

Enzymatic Release of Antitumor Ether Lipids by Specific Phospholipase A2 Activation of Liposome-Forming Prodrugs

An enzymatically activated liposome-based drug-delivery concept involving masked antitumor ether lipids (AELs) has been investigated. This concept takes advantage of the cytotoxic properties of AEL drugs as well as the membrane permeability enhancing properties of these molecules, which can lead to enhanced drug diffusion into cells. Three prodrugs of AELs (proAELs) have been synthesized and four liposome systems, consisting of these proAELs, were investigated for enzymatic degradation by secretory phospholipase A(2) (sPLA(2)), resulting in the release of AELs. The three synthesized proAELs were (R)-1-O-hexadecyl-2-palmitoyl-sn-glycero-3-phosphocholine (1-O-DPPC), (R)-1-O-hexadecyl-2-palmitoyl-sn-glycero-3-phosphoethanolamine poly(ethylene glycol)(350) (1-O-DPPE-PEG(350)), and 1-O-DPPE-PEG(2000) of which 1-O-DPPC was the main liposome component. All three phospholipids were synthesized from the versatile starting material (R)-O-benzyl glycidol. A phosphorylation method, employing methyl dichlorophosphate, was developed and applied in the synthesis of two analogues of (R)-1-O-hexadecyl-2-palmitoyl-sn-glycero-3-phosphoethanolamine poly(ethylene glycol). Differential scanning calorimetry has been used to investigate the phase behavior of the lipid bilayers. A release study, employing calcein encapsulated in non-hydrolyzable 1,2-bis-O-octadecyl-sn-glycero-3-phosphocholine (D-O-SPC) liposomes, showed that proAELs, activated by sPLA(2), perturb membranes because of the detergent-like properties of the released hydrolysis products. A hemolysis investigation was conducted on human red blood cells, and the results demonstrate that proAEL liposomes display a very low hemotoxicity, which has been a major obstacle for using AELs in cancer therapy. The results suggest a possible way of combining a drug-delivery and prodrug concept in a single liposome system. Our investigation of the permeability-enhancing properties of the AEL molecules imply that by encapsulating conventional chemotherapeutic drugs, such as doxorubicin, in liposomes consisting of proAELs, an increased effect of the encapsulated drug might be achievable due to an enhanced transmembrane drug diffusion.

Different modulation of phospholipase A2 activity by saturated and monounsaturated N-acylethanolamines

The physiological functions of N-acylethanolamines (NAEs) are poorly understood, although many functions were suggested for these naturally occurring membrane components of plants and animals. The binding with cannabinoid receptors CB1 and CB2 was demonstrated for some NAEs, such as anandamide. However, the chemical nature of these molecules suggests that some of their biological effects on biomembranes could be related, at least partially, to physical interactions with the lipid bilayer. The present work studies the effect of saturated and monounsaturated NAEs on phospholipase A2 (PLA2) activity, which is dependent on lipid bilayer features. The present study, performed by 2-dimethylamino-(6-lauroyl)-naphthalene (Laurdan) fluorescence, demonstrates that the acyl chain length and the presence of a single double bond are crucial for the enzymatic activity modulation by NAEs. In fact, saturated NAEs with 10 carbon atoms don't affect the PLA2 activity, while NAEs with 12 and 16 carbon atoms largely activate the enzyme. On the other hand, an acyl chain length of 18 carbon atoms, with or without the presence of a double bond, only slightly affects the enzymatic activity. A structural model for NAE-lipid interactions is proposed in order to explain the differences in PLA2 activity modulation by these fatty acid derivatives.

Secreted phospholipase A(2) as a new enzymatic trigger mechanism for localised liposomal drug release and absorption in diseased tissue

Polymer-coated liposomes can act as versatile drug-delivery systems due to long vascular circulation time and passive targeting by leaky blood vessels in diseased tissue. We present an experimental model system illustrating a new principle for improved and programmable drug-delivery, which takes advantage of an elevated activity of secretory phospholipase A(2) (PLA(2)) at the diseased target tissue. The secretory PLA(2) hydrolyses a lipid-based proenhancer in the carrier liposome, producing lyso-phospholipids and free fatty acids, which are shown in a synergistic way to lead to enhanced liposome destabilization and drug release at the same time as the permeability of the target membrane is enhanced. Moreover, the proposed system can be made thermosensitive and offers a rational way for developing smart liposome-based drug delivery systems. This can be achieved by incorporating specific lipid-based proenhancers or prodestabilisers into the liposome carrier, which automatically becomes activated by PLA(2) only at the diseased target sites, such as inflamed or cancerous tissue.

The presence of PEG-lipids in liposomes does not reduce melittin binding but decreases melittin-induced leakage

Poly(ethyleneglycol) (PEG), anchored at the surface of liposomes via the conjugation to a lipid, is commonly used for increasing the liposome stability in the blood stream. In order to gain a better understanding of the protective properties of interfacial polymers, we have studied the binding of melittin to PEG-lipid-containing membranes as well as the melittin-induced efflux of a fluorescent marker from liposomes containing PEG-lipids. We examined the effect of the polymer size by using PEG with molecular weights of 2000 and 5000. In addition, we studied the role of the anchoring lipid by comparing PEG conjugated to phosphatidylethanolamine (PE) which results in a negatively charged PEG-PE, with PEG conjugated to ceramide (Cer) which provides the neutral PEG-Cer. Our results show that interfacial PEG does not prevent melittin adsorption onto the interface. In fact, PEG-PE promotes melittin binding, most likely because of attractive electrostatic interactions with the negative interfacial charge density of the PEG-PE-containing liposomes. However, PEG-lipids limit the lytic potential of melittin. The phenomenon is proposed to be associated with the change in the polymorphic tendencies of the liposome bilayers. The present findings reveal that the protective effect associated with interfacial hydrophilic polymers is not universal. Molecules like melittin can sense surface charges borne by PEG-lipids, and the influence of PEG-lipids on liposomal properties such as the polymorphic propensities may be involved in the so-called protective effect.

Phospholipase A(2) activity towards vesicles of DPPC and DMPC-DSPC containing small amounts of SMPC

Phospholipase A(2) (PLA(2)) is an interfacially active enzyme whose hydrolytic activity is known to be enhanced in one-component phospholipid bilayer substrates exhibiting dynamic micro-heterogeneity. In this study the activity of PLA(2) towards large unilamellar vesicles composed of DPPC:SMPC and DMPC:DSPC:SMPC is investigated using fluorescence and HPLC techniques. Phase diagrams of the mixtures are established by differential scanning calorimetry and the PLA(2) activity, monitored by the lag time, is correlated with the phase behavior of the mixtures. In addition, the degree of lipid hydrolysis in the DMPC:DSPC:SMPC lipid mixtures is detected by HPLC. The PLA(2) activity is found to be significantly increased in the temperature range of the coexistence region where the lipid mixtures exhibit lateral gel-fluid phase separation. Furthermore, in the entire temperature range it is demonstrated that PLA(2) preferentially hydrolyzes the short chain DMPC lipid. This discriminative effect becomes less pronounced when the asymmetric lipid SMPC is present in the lipid substrate. Inclusion of SMPC into either DPPC or DMPC:DSPC vesicles prolongs the lag time. The results clearly show that the PLA(2) activity is significantly enhanced by lipid bilayer micro-heterogeneity in both one-component and multi-component lipid bilayer substrates. The PLA(2) activity measurements are discussed in terms of dynamic gel-fluid lipid domain formation due to density fluctuations and static lipid domain formation due to gel-fluid phase separation.

Screening effect of PEG on avidin binding to liposome surface receptors

This study investigates the screening effect of poly(ethylene glycol)-phospholipids (PE-PEG) on the interaction of avidin with PEGylated liposomes containing surface-bound biotin ligands. The influence of grafting density and lipopolymer chain length is examined. A simple fluorescence assay involving a receptor-mediated fluorescence increase of BODIPY-labeled avidin upon binding to biotinylated lipids is employed to study the screening effect of submicellar concentrations of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine-N-[poly(ethylene glycol)-2000] (PE-PEG(2000)) and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine-N-[poly(ethylene glycol)-5000] (PE-PEG(5000)) incorporated into 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) liposomes. The results show that incorporation of lipopolymers into DPPC lipid bilayers reduces binding of avidin to the biotinylated liposomes, and it is found that the screening effect of PE-PEG(5000) is stronger than that for PE-PEG(2000). Thus, the results reveal that both the grafting density and the polymer length of the PE-PEG lipopolymers are of importance for the ability of water-soluble macromolecules to reach the surface of PEG liposomes. Furthermore, it is found that none of the lipopolymers completely prevents avidin from reaching the surface-bound biotin ligands.

Drug delivery by phospholipase A(2) degradable liposomes

The effect of poly(ethylene glycol)-phospholipid (PE-PEG) lipopolymers on phospholipase A(2) (PLA(2)) hydrolysis of liposomes composed of stearoyl-oleoylphosphatidylcholine (SOPC) was investigated. The PLA(2) lag-time, which is inversely related to the enzymatic activity, was determined by fluorescence, and the zeta-potentials of the liposomes were measured as a function of PE-PEG lipopolymer concentration. A significant decrease in the lag-time, and hence an increase in enzymatic activity, was observed with increasing amounts of the negatively charged PE-PEG lipopolymers incorporated into the SOPC liposomes. The enhancement of the PLA(2) enzymatic activity might involve a stronger PLA(2) binding affinity towards the negatively charged and polymer covered PEG liposomes.

Enzymatic degradation of polymer covered SOPC-liposomes in relation to drug delivery

Polyethylenoxide (PEG) covered liposomes are used as lipid-based drug-delivery systems. In comparison to conventional liposomes the polymer-covered liposomes display a long circulation half-life in the blood stream. We investigate the influence of polyethyleneoxide-distearoylphosphatidylethanolamine (DSPE-PEG750) lipopolymer concentration on phospholipase A2 (PLA2) catalyzed hydrolysis of liposomes composed of stearoyloleoylphosphatidylcholine (SOPC). The characteristic PLA2 lag-time was determined by fluorescence and the degree of lipid hydrolysis was followed by HPLC analysis. Particle size and zeta-potential were measured as a function of DSPE-PEG750 lipopolymer concentration. A significant decrease in the lag-time, and hence an increase in enzyme activity, was observed with increasing concentrations of the anionic DSPE-PEG750 lipopolymer lipids. The observed decrease in lag-time might be related to changes in the surface potential and the PLA2 lipid membrane affinity.