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ABSTRACT: Ultrasound is investigated as a novel drug delivery tool within cancer therapy. Non-thermal ultrasound treatment of solid tumours post i.v.-injection of drug-carrying liposomes may induce local drug release from the carrier followed by enhanced intracellular drug uptake. Recently, ultrasound-mediated drug release of liposomes (sonosensitivity) was shown to strongly depend on liposome membrane composition. In the current study the ultrasound-mediated drug release mechanism of liposomes was investigated. The results showed that differences in ultrasound drug release kinetics obtained for different liposomal compositions were caused by distinctive release mechanisms of the carriers. Two types of liposomes composed of 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) and hydrogenated soy l-α-phosphatidylcholine (HSPC) as main lipids, respectively, were recently shown to vary in sonosensitivity. Here, these liposomes were analyzed prior to and after a given ultrasound-exposure for their mean size, size distribution and morphology. Cryo-transmission electron microscopy, dynamic light scattering and asymmetric flow field-flow fractionation in combination with multi-angle light scattering revealed a significant change in mean size, size distribution and morphology of DOPE-based liposomes after ultrasound, pointing to an irreversible disruption of the vesicles and concomitant drug release. In contrast, the HSPC-based liposomes remained unchanged in size and structure after ultrasound application, indicating pore-mediated release mechanisms. The results show that the release mechanisms and interactions between ultrasound and liposomes depend on the liposome membrane-composition, explaining their sonosensitive properties.
Journal of pharmaceutical and biomedical analysis 02/2013; 78-79C:118-122. · 2.45 Impact Factor
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ABSTRACT: PurposeThe challenge in developing liposomes to be used in active drug targeting is to design a method that can be used for modifying
liposomal membranes that is applicable for a number of different specific ligands. In this study, the post insertion technique
was used with activated sterol-PEG1300 anchors and was evaluated with regard to its effectiveness in active targeting in vitro. The key advantage of these anchors is that the insertion step into the liposomal membrane takes place at room temperature
and is very fast.
Materials and MethodsFor in vitro experiments, neuroblastoma cell lines overexpressing GD2 antigen on their surface as a target structure were chosen. This
allowed the use of anti-GD2 antibodies coupled to the liposomal surface for testing of specific binding. These modified liposomes
were labelled with rhodamine-PE and their cellular association was analyzed by flow cytometry.
ResultsIt was shown that the activated sterol-PEG1300 anchors allow specific and significant interactions of the modified liposomes with GD2 positive cells.
ConclusionCoupling using sterol-PEG1300 anchors is both simple and rapid. It is reproducible and applicable for all ligands bearing amino groups. This method demonstrates
the advantage of a ready-to-use system for the modification of pre-formed liposomes with different ligands.
Pharmaceutical Research 04/2012; 26(3):529-538. · 4.09 Impact Factor
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ABSTRACT: 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.
Journal of Liposome Research 12/2011; 22(2):148-57. · 1.71 Impact Factor
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ABSTRACT: Clostridium difficile toxins A and B bind to eukaryotic target cells, are endocytosed and then deliver their N-terminal glucosyltransferase domain after processing into the cytosol. Whereas glucosyltransferase, autoprocessing and cell-binding domains are well defined, structural features involved in toxin delivery are unknown. Here, we studied structural determinants that define membrane insertion, pore formation and translocation of toxin B. Deletion analyses revealed that a large region, covering amino acids 1501-1753 of toxin B, is dispensable for cytotoxicity in Vero cells. Accordingly, a chimeric toxin, consisting of amino acids 1-1550 and the receptor-binding domain of diphtheria toxin, caused cytotoxic effects. A large N-terminal part of toxin B (amino acids 1-829) was not essential for pore formation (measured by (86) Rb(+) release in mammalian cells). Studies using C-terminal truncation fragments of toxin B showed that amino acid residues 1-990 were still capable of inducing fluorescence dye release from large lipid vesicles and led to increased electrical conductance in black lipid membranes. Thereby, we define the minimal pore-forming region of toxin B within amino acid residues 830 and 990. Moreover, we identify within this region a crucial role of the amino acid pair glutamate-970 and glutamate-976 in pore formation of toxin B.
Molecular Microbiology 01/2011; 79(6):1643-54. · 5.01 Impact Factor
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ABSTRACT: Novel sonosensitive doxorubicin-containing liposomes comprising dioleoylphosphatidylethanolamine (DOPE) as the main lipid constituent were developed and characterized in terms of ultrasound-mediated drug release in vitro. The liposome formulation showed high sonosensitivity; where approximately 95% doxorubicin was released from liposomes after 6min of 40kHz US exposure in buffered sucrose solution. This represented a 30% increase in release extent in absolute terms compared to liposomes comprising the saturated lipid analogue distearoylphosphatidylethanolamine (DSPE), and a 9-fold improvement in release extent when compared to standard pegylated liposomal doxorubicin, respectively. Ultrasound release experiments in the presence of serum showed a significantly reduction in sonosensitivity of DSPE-based liposomes, whilst the release properties of DOPE-based liposomes were essentially maintained. Dynamic light scattering measurements and cryo-transmission electron microscopy of DOPE-based liposomes after ultrasound treatment indicated liposome disruption and formation of various lipid structures, corroborating the high release extent. The results point to the potential of DOPE-based liposomes as a new class of drug carriers for ultrasound-mediated drug delivery.
European journal of pharmaceutical sciences: official journal of the European Federation for Pharmaceutical Sciences 01/2011; 42(4):380-6. · 2.61 Impact Factor
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ABSTRACT: Ultrasound contrast agents are widely used in clinical diagnosis. In recent years, the use of ultrasound contrast agents as therapeutic agents has gained a lot of attention. Of special interest are ultrasound-enhanced gene delivery in various tissues (e.g. cardiac, vascular, skeletal muscle and tumor tissue), ultrasound-enhanced protein delivery (e.g. insulin delivery) and ultrasound-enhanced delivery of small chemicals (e.g. doxorubicin, vancomycin). Commercially available ultrasound contrast agents such as SonoVue® or Optison® are ranged in a size of 2-8 μm. These micronscaled agents show a good ultrasound contrast enhancement and thus they are used for diagnostic imaging. But they are not suitable for targeted drug delivery to tumor tissues or blood clots because for these applications particles smaller than 700 nm are needed. In the present study, we developed new nanoscaled ultrasound contrast agents with a size between 70 and 300 nm. The lipid formulations show excellent contrast intensities using diagnostic ultrasound of about 1.4 MHz. The negatively charged colloidal dispersions are long-time stable under physiological conditions without loss of ultrasound reflectivity. The adjustable supramolecular organization of the carriers depends on the composition and varies from micellar to liposomal structures. The small size and the circulation stability of these systems make them promising for novel diagnostics and controlled drug release applications.
European journal of pharmaceutics and biopharmaceutics: official journal of Arbeitsgemeinschaft fur Pharmazeutische Verfahrenstechnik e.V 12/2010; 77(3):430-7. · 3.15 Impact Factor
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ABSTRACT: We applied protein-mediated lipid transfer using recombinant His-tagged pro-sterol carrier protein 2 (pro-SCP2) to prepare asymmetrical membrane vesicles (AMV) featuring an unequal transmembrane distribution of the negative phospholipid egg-phosphatidylglycerol (EPG). Pure egg-phosphatidylcholine (EPC) vesicles were used as the acceptor and EPC:EPG 90:10 mol % vesicles as the donor populations. The changes in surface charge during EPG transfer were used to quantify the degree of asymmetry by free-flow electrophoresis (FFE). The relative deflection in FFE correlated with EPG content in the outer monolayer (x(EPG)). The initial transfer rates and first order rate constants for the transfer process were determined. The addition of pro-SCP2 at a molar protein-to-lipid ratio R(P/L) of (15-20) x 10(-5) accelerated the EPG transfer to half-times of between 2 and 3 h. Thus, the transmembrane redistribution of EPG by flip-flop, which reduces the degree of asymmetry and occurs at half-times of tens of hours, was minimized during the transfer process. We investigated the influence of membrane curvature on the transfer rate using 50 and 100 nm vesicles with very low size distribution widths (RSD of 13-17%). Transfer occurred with a 55.7% higher initial rate between the smaller vesicles. The use of equally sized acceptor and donor populations of such narrow size distributions was shown to be important for the preparation of AMV with a uniform degree of asymmetry. Under these conditions, AMV were obtained after less than 3 h by preparative FFE separation. In the case of the acceptor vesicles, EPG transfer increased x(EPG) to 3 mol %, whereas it was reduced to 6 mol % in the donor vesicles.
Langmuir 03/2010; 26(6):4142-51. · 4.19 Impact Factor
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ABSTRACT: The aim of this study was to investigate the performance of a newly devised high-performance thin-layer chromatography (HPTLC) method in quantifying common liposome membrane components, including the five phospholipids (PLs), phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, as well as cholesterol, cholesteryl hemisuccinate, and linoleic acid. Besides strictly keeping to a standardized procedure, three parameters were particularly critical for proper quantification. First, a relative humidity of higher than 60% caused migration distances to increase and reduced the resolution of the PLs on a silica-gel 60 HPTLC plate. Second, PLs underwent oxidative combustion during storage for 2 or 24 hours on an HPTLC plate, with peak losses of up to 25-44%. These losses could be prevented by storage under nitrogen and, to some extent, by the addition of the antioxidant, DL-alpha-tocopherol. Third, even with automated sample application, the accuracy and consistency of the application volume proved to be an important cause of error and needs routine verification. Considering these parameters, the method was found to accurately and precisely determine the composition of three different liposome preparations. The recovery was 97.2-101.8%, compared to secondary methods, and consistent over different days and with different operators (mean RSD of the recovery: 2.03 +/- 1.16%, n = 9). The working range was determined to be 100-300 ng in the case of the PLs (individual limit of determination between 40 and 80 ng) and 20-60 ng in the case of cholesterol (limit of determination: 16 ng).
Journal of Liposome Research 10/2009; 20(2):124-33. · 1.71 Impact Factor
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ABSTRACT: We have prepared nine new dodecaborate cluster lipids with potential use in boron neutron capture therapy of tumors. This new generation of boron lipids is only singly negatively charged and consists of a pyridinium core with C(12), C(14), and C(16) chains as lipid backbone, connected through the nitrogen atom via a butylene, pentylene, or ethyleneoxyethylene linker to the oxygen atom on the dodecaborate cluster as headgroup. The lipids were obtained by nucleophilic attack of 4-(bisalkylmethyl)pyridine on the tetrahydrofurane, the dioxane, and a newly prepared tetrahydropyrane derivative, respectively, of closo-dodecaborate. All of these boron lipids are able to form closed vesicles in addition to some bilayers in the pure state and in the presence of helper lipids. The thermotropic behavior was found to be increasingly complex and polymorphic with increasing alkyl chain length. Except for two lipids, all lipids have low in vitro toxicity, and longer alkyl chains lead to a significant decrease in toxicity. The choice of the linker plays no major role with respect to their ability to form liposomes and their thermotropic properties, but the toxicity is influenced by the linkers in the case of short alkyl chains.
Bioconjugate Chemistry 10/2009; 20(11):2190-8. · 4.93 Impact Factor
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ABSTRACT: Detergent removal from mixed micelles was combined with preparative size exclusion chromatography (SEC) on Sephacryl S 500 HR to prepare unilamellar and spherical liposomes of defined sizes between 50 and 100 nm with a very narrow size distribution (RSD of vesicle diameter between 13% and 25%). For neutral phosphatidylcholine and negatively charged phosphatidylcholine/phosphatidylglycerol liposome preparations, efficient sizing at the preparative scale was demonstrated by analyzing isolated SEC peak fractions with cryo-transmission electron microscopy and dynamic light scattering. The number-weighted average vesicle diameters obtained using both methods are in very good agreement for fractions of low polydispersity.
Journal of chromatography. A 07/2009; 1216(31):5838-48. · 4.19 Impact Factor
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ABSTRACT: Nanoparticle brushes: Complex nanostructures can be formed by self assembly of amphiphilic CdSe/CdS core-shell nanoparticles that bear a brushlike layer of poly(ethylene oxide) chains. This route is based on controlling the volume fractions of hydrophilic and hydrophobic moieties within the particles and allows the formation of micellar, cylindrical, or vesicular nanoobjects (see picture).
Angewandte Chemie International Edition 02/2009; 48(15):2752-4. · 13.45 Impact Factor
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ABSTRACT: In aqueous solution, poly(2-vinylpyridine-b-ethylene oxide) spontaneously forms bilayer vesicles, the size of which can be tailored by extrusion through polycarbonate membranes. However, their size can be even more precisely influenced by subjecting them to a specific cooling/warming process proceeding through a cylinder-vesicle shape transition. The thermotropic alterations of the polymer aggregates and the topological pathways of the cylinder-vesicle transition were followed by dynamic light scattering (DLS) and cryo-electron microscopy (cryo-TEM). Upon cooling the vesicles to 4degreesC, there is a transition of the vesicles to basketlike aggregates and their further disintegration to wormlike micelles. Rewarming of the dispersion results in the reformation of vesicles via intermediate discoid and octopus-like structures. The variation of incubation times at 4 and 25degreesC, heating rate, polymer concentration, and ionic strength allows tailored preparation of unilamellar and almost monodisperse vesicles with diameters between 60 and 500 nm. Furthermore, fluorescently labeled dextrans, which were used as model drugs of differing molar mass, could be easily and stably encapsulated during the thermotropic formation of vesicles from wormlike micelles.
Langmuir 02/2009; 25(3):1337-44. · 4.19 Impact Factor
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ABSTRACT: The luciferase gene expression of lipoplexes, a liposome containing luciferase plasmid (pCMVLuc), in HeLa cell lines, was investigated. Cationic liposomes were prepared by the chloroform film method with sonication. The lipoplex was formed by loading the liposome with pCMVLuc. The lipoplex with an optimal weight ratio of dimethyl dioctadecyl ammonium bromide (DDAB)/pCMVLuc protected from DNaseI was determined by an agarose gel electrophoresis. The selected lipoplexes were assayed for luciferaase activity by using a luminometer. The effect on cell proliferation was evaluated by WST-1 assay. The highest luciferase activity of 1.5 x 10(6) RLU was observed in the cholesterol (Chol)/DDAB (2:1 molar ratio) lipoplex at the DDAB/pCMVLuc weight ratio of 10:1 at 48 hours, which was about 10, 100, and 1,000 times higher than the DDAB, L-alpha-dipalmitoyl phosphatidylcholine (DPPC)/Chol/DDAB (1:2:1 molar ratio), and DPPC/Chol/DDAB (2:2:1 molar ratio) lipoplexes, respectively. The liposome with the smallest particle size was obtained from the cationic liposome composed of DPPC/Chol/DDAB (7:1:1 molar ratio) with the zeta potential of 7.17 +/- 0.73. The optimal weight ratio of DDAB/pCMVLuc that protected pCMVLuc from DNaseI digestion was 4:1 in the DDAB formulation. The Chol/DDAB (2:1 molar ratio) lipoplex with the DDAB/pCMVLuc of 10:1 showed the highest luciferase activity of 1.5 x 10(6) RLU and the highest cytotoxicity as well. DPPC/Chol/DDAB (1:1:1 molar ratio)-lipoplex (DDAB/pCMVLuc = 14:1), which had the amount of DPPC and cholesterol not exceeding 33 and 50% mol, respectively, gave the lower gene expression of about 4 times, but lower cytoxicity of about 14 times, than the Chol/DDAB lipoplex (2:1 molar ratio) and was considered to be the most suitable formulation. The results from this study can be applied as a model for the development of a gene-therapeutic dosage form.
Journal of Liposome Research 02/2009; 19(2):131-40. · 1.71 Impact Factor
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ABSTRACT: The challenge in developing liposomes to be used in active drug targeting is to design a method that can be used for modifying liposomal membranes that is applicable for a number of different specific ligands. In this study, the post insertion technique was used with activated sterol-PEG(1300) anchors and was evaluated with regard to its effectiveness in active targeting in vitro. The key advantage of these anchors is that the insertion step into the liposomal membrane takes place at room temperature and is very fast.
For in vitro experiments, neuroblastoma cell lines overexpressing GD2 antigen on their surface as a target structure were chosen. This allowed the use of anti-GD2 antibodies coupled to the liposomal surface for testing of specific binding. These modified liposomes were labelled with rhodamine-PE and their cellular association was analyzed by flow cytometry.
It was shown that the activated sterol-PEG(1300) anchors allow specific and significant interactions of the modified liposomes with GD2 positive cells.
Coupling using sterol-PEG(1300) anchors is both simple and rapid. It is reproducible and applicable for all ligands bearing amino groups. This method demonstrates the advantage of a ready-to-use system for the modification of pre-formed liposomes with different ligands.
Pharmaceutical Research 12/2008; 26(3):529-38. · 4.09 Impact Factor
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ABSTRACT: The antimicrobial activity of poly(2-methyl-1,3-oxazoline)s (PMOX) with the antimicrobial N,N-dimethyldodecylammonium (DDA) end group is greatly dependent on the nature of the group at the distal end of the polymer, the satellite group. Three comparable PMOX with a DDA end group and different satellite groups (methyl, decyl, hexadecyl) were investigated with respect to the reasons for the huge differences in their biocidal behavior. Static light scattering (SLS) and pulsed field gradient diffusion NMR measurements revealed that the samples show comparable aggregation conduct, thus, not being responsible for the varying biological activity. Experiments using different liposomal systems as models for bacterial cell membranes have been performed. It was found that differential interactions between the respective polymers and the phospholipid membranes constitute the reason for the varying effectiveness observed in antimicrobial susceptibility determinations.
Biomacromolecules 08/2008; 9(7):1764-71. · 5.48 Impact Factor
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ABSTRACT: Cationic liposomes with high stability and low cytotoxicity for gene therapy have been developed. Luciferase plasmid DNA (pLuc) was used as a model gene. The empty liposomes and niosomes were prepared by freeze dried empty liposomes (FDEL) method. The entrapment of pLuc in the liposomes was by reconstitution of the lyophilized dried vesicles with the plasmid solution. The morphology of the vesicles showing multilamellar structure was characterized by transmission electron microscope (TEM) and cryo-TEM. Cytotoxicity of the vesicular formulations was investigated on mouse melanoma cell lines (B16F10) by MTT assay. Cationic lposomes and niosomes containing the cationic lipid DDAB were less cytotoxic than other bilayer vesicular formulations. The pLuc entrapped in the cationic DPPC/Chol/DDAB liposomes (at 1:1:1 molar ratio) exhibited higher stability than other vesicular formulations and the pLuc in solution when stored at 4, 30 and 50 degrees C for 8 weeks. The entrapment efficiency determined by gel electrophoresis and gel documentation of the pLuc in this liposomal formulation was 100%. Luciferase gene expression of pLuc-loaded in cationic liposomes (lipoplexes) in HeLa cell lines was evaluated from luciferase activity determined by a luminometer at 24 and 48 h incubation. Percentages of cell proliferation of the lipoplexes on HeLa cell line at 24 and 48 h incubation were evaluated by the WST-1 assay. When the amount of DPPC or cholesterol was increased in the lipoplexes, the higher amount of DDAB was needed to protect pLuc from enzymatic degradation. However, DPPC and cholesterol exceeded 33 and 50% mol, respectively gave no gene expression. The DPPC/Chol/DDAB (at 1:1:1 molar ratio) lipoplex has demonstrated moderately lower luciferase gene expression and low cytoxicity. This lipoplex with the DDAB/pLuc weight ratio of 14:1 was the most desirable formulation for gene therapy because of its high stability, high luciferase gene expression and low cytotoxicity.
Arzneimittel-Forschung 02/2008; 58(10):485-92. · 0.72 Impact Factor
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ABSTRACT: The molecular exchange of tracer molecules through the membranes of dispersed vesicles of the block copolymer poly(2-vinylpyridine-block-ethylene oxide) was studied by using NMR spectroscopy combined with pulsed field gradients. The hydrodynamic radius of the tracer molecules was varied systematically to obtain a permeability profile of the vesicle membrane. In addition, the effect of system parameters, such as temperature, pH value, vesicle size, and thickness of the vesicle membrane, was studied. In the case of rapid exchange with average residence times significantly smaller than 10 s, the permeation is observed under equilibrium conditions and the data are analyzed by using a simple analytical approach. For slow exchange processes with average residence times above 10 s, the permeation is monitored in a time-resolved measurement under nonequilibrium conditions. Generally, the transmembrane exchange rate of the tracer clearly depends on its hydrodynamic radius. The characteristics of this dependence indicate the presence of two different mechanisms of membrane penetration, one dominating for smaller and one for larger tracer molecules, respectively. The exchange rate also shows a significant dependence on the bilayer thickness and on the vesicle diameter. By contrast, no variation of the membrane permeability with the temperature or the pH value could be detected as long as the vesicles remain stable.
Small 07/2007; 3(6):1074-83. · 8.35 Impact Factor
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ABSTRACT: This study examines a new method for the remote loading of doxorubicin into liposomes. It was shown that doxorubicin can be loaded to a level of up to 98% into large unilamellar vesicles composed of egg phosphatidylcholine/cholesterol (7/3 mol/mol) with a transmembrane phosphate gradient. The different encapsulation efficiencies which were achieved with ammonium salts (citrate 100%, phosphate 98%, sulfate 95%, acetate 77%) were significantly higher as compared to the loading via sodium salts (citrate 54%, phosphate 52%, sulfate 44%, acetate 16%). Various factors, including pH-value, buffer capacity, solubility of doxorubicin in different salt solutions and base counter-flow, which likely has an influence on drug accumulation in the intraliposomal interior are taken into account. In contrast to other methods, the newly developed remote loading method exhibits a pH-dependent drug release property which may be effective in tumor tissues. At physiological pH-value doxorubicin is retained in the liposomes, whereas drug release is achieved by lowering the pH to 5.5 (approximately 25% release at 25 degrees C or 30% at 37 degrees C within two h). The DXR release of liposomes which were loaded via a sulfate gradient showed a maximum of 3% at pH 5.5.
Biochimica et Biophysica Acta 11/2006; 1758(10):1633-40. · 4.66 Impact Factor
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ABSTRACT: 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.
Langmuir 07/2006; 22(13):5843-7. · 4.19 Impact Factor
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ABSTRACT: 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.
Biomacromolecules 06/2006; 7(5):1422-8. · 5.48 Impact Factor
