Rolf Schubert

University of Freiburg, Freiburg, Baden-Württemberg, Germany

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Publications (67)308.68 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: To detect unstable atherosclerotic plaques early and noninvasively would be of great clinical interest. Activated platelets are an interesting molecular target for detecting early lesions or unstable plaques. We therefore developed an MRI contrast agent consisting of magnetoliposomes (ML) linked to an antibody (anti-LIBS) specifically targeting the ligand-induced binding site of the activated GPIIb/IIIa receptor of platelets. ML were prepared by dual centrifugation (DC). ML pegylation up to a total PEG content of 7.5 mol% positively influenced the stability and amount of entrapped SPIOs, and also reduced SPIO-membrane interactions, while higher PEG contents destabilized PEG-ML. Stable anti-LIBS-ML with high amounts of entrapped SPIOs (∼86%, ∼0.22 mol Fe/mol liposomal lipid) and high MRI sensitivity (relaxivity r2 = 422 s(-1) mM(-1) and r2(∗) = 452 s(-1) mM(-1)) were obtained by coupling anti-LIBS to ML in a two-step post-insertion technique. We confirmed specific binding to the GPIIb/IIIa receptor's activated conformation on activated human platelets and cell lines expressing activated GPIIb/IIIa receptor ex vivo. The immuno-ML obtained in this study constitute an important step towards developing a potentially human-compatible MRI contrast agent for the timely detection of plaque rupture by targeting activated platelets. Copyright © 2015 Elsevier Ltd. All rights reserved.
    Biomaterials 06/2015; 53. DOI:10.1016/j.biomaterials.2015.02.088 · 8.31 Impact Factor
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    ABSTRACT: Nanoscale biological materials formed by the assembly of defined block-domain proteins control the formation of cellular compartments such as organelles. Here, we introduce an approach to intentionally ‘program’ the de novo synthesis and self-assembly of genetically encoded amphiphilic proteins to form cellular compartments, or organelles, in Escherichia coli. These proteins serve as building blocks for the formation of artificial compartments in vivo in a similar way to lipid-based organelles. We investigated the formation of these organelles using epifluorescence microscopy, total internal reflection fluorescence microscopy and transmission electron microscopy. The in vivo modification of these protein-based de novo organelles, by means of site-specific incorporation of unnatural amino acids, allows the introduction of artificial chemical functionalities. Co-localization of membrane proteins results in the formation of functionalized artificial organelles combining artificial and natural cellular function. Adding these protein structures to the cellular machinery may have consequences in nanobiotechnology, synthetic biology and materials science, including the constitution of artificial cells and bio-based metamaterials.
    Nature Material 01/2015; 14(1):125-132. DOI:10.1038/nmat4118 · 36.43 Impact Factor
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    ABSTRACT: The aim of the present study was to develop film-coated tablets which release a minor amount of the active pharmaceutical ingredient (API) into the stomach and small intestine, yet show a sharp increase of drug release in the colon. Tablets containing the model drug Diclofenac-Na, microcrystalline cellulose as a filler (MT), as well as tablets consisting of Ludiflash(®) (LT), were both used as tablet cores, respectively. Either chitosan (CHI) alone or different ratios of chitosan and Kollicoat(®) Smartseal 30 D (KCSS) were applied onto these cores. The resulting film-coated tablets were analysed for swelling, drug dissolution and stability. In order to clarify whether the colon release is mainly enzyme-driven or pressure-controlled, the coated tablets were both tested in the colon microflora test (CMT), which simulates the enzyme environment within the colon, and using a bio-relevant dissolution apparatus mimicking the intraluminal pressures and stress conditions present in the gastrointestinal tract (GIT). CHI/KCSS (25:75) coated LTs showed a pressure-controlled site-specific drug release in the large intestine, while remaining intact in the upper GIT. CHI as well as CHI/KCSS (25:75) applied onto MTs, remained stable during the entire simulated bio-relevant dissolution transit of the GIT, but showed enzymatically controlled colon targeting in the CMT. These results could be confirmed for CHI/KCSS (25:75) film-coated MT tablets top-coated with an additional hydroxypropylmethycellulose (HPMC) layer and an Eudragit L 30 D-55 (EUL) layer to avoid the dissolution in the fasting stomach.
    European Journal of Pharmaceutics and Biopharmaceutics 10/2014; DOI:10.1016/j.ejpb.2014.09.006 · 4.25 Impact Factor
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    ABSTRACT: The potential of boron-containing lipids with three different structures, which were intended for use in boron neutron capture therapy, was investigated. All three types of boron lipids contained the anionic dodecaborate cluster as the headgroup. Their effects on two different tumor models in mice following intravenous injection were tested; for this, liposomes with boron lipid, distearoyl phosphatidylcholine, and cholesterol as helper lipids, and containing a polyethylene glycol lipid for steric protection, were administered intravenously into tumor-bearing mice (C3H mice for SCCVII squamous cell carcinoma and BALB/c mice for CT26/WT colon carcinoma). With the exception of one lipid (B-THF-14), the lipids were well tolerated, and no other animal was lost due to systemic toxicity. The lipid which led to death was not found to be much more toxic in cell culture than the other boron lipids. All of the lipids that were well tolerated showed hemorrhage in both tumor models within a few hours after administration. The hemorrhage could be seen by in vivo magnetic resonance and histology, and was found to occur within a few hours. The degree of hemorrhage depended on the amount of boron administered and on the tumor model. The observed unwanted effect of the lipids precludes their use in boron neutron capture therapy.
    International Journal of Nanomedicine 07/2014; 9:3583-90. DOI:10.2147/IJN.S65166 · 4.20 Impact Factor
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    ABSTRACT: Ultrasound is a common tool for clinical diagnosis due to its safety and economic. Especially the addition of ultrasound contrast agents leads to a high diagnostic reliability. In recent years ultrasound has been used as a trigger for directed drug delivery or to enhance thrombolysis. We developed a nanoscaled ultrasound contrast agent (NUSCA) to improve these applications. In the future drugs can be incorporated into this contrast agent to achieve a combination of ultrasound diagnosis and therapy. The aim of the present study is to elucidate the structure of the nanoscaled lipid formulations and a potential dependence of the ultrasound contrast enhancement on this structure. Our NUSCA is based on the phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and the single-chained polyethylene glycol (40) stearate (PEG40S). In this study the effect of increasing concentrations of the single chained PEG40S on the structure of the lipid formulations was characterised using Dynamic Light Scattering, cryo-Transmission Electron Microscopy, Nuclear Magnetic Resonance spectroscopy, lipid monolayer studies and epifluorescence measurements. In addition, the ultrasound contrast enhancement for the formulations was determined in vitro. Dependence between structure and ultrasound contrast was found. All PEG40S concentrations lead to a mixture of liposomes and discoid micelles. With increasing PEG40S content the amount of micelles increased. Certain PEG40S concentrations lead to an ultrasound contrast superior to the contrast of the commercially available ultrasound contrast agent SonoVue(®).
    Colloids and surfaces B: Biointerfaces 05/2014; 117:206–215. DOI:10.1016/j.colsurfb.2014.02.029 · 4.29 Impact Factor
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    ABSTRACT: Since pharmacokinetic and pharmacodynamic activities of drugs are often related to their interactions with biomembranes, it is of high interest to establish an approach for the characterization of these interactions at the molecular level. For the present study beta-blockers (oxprenolol, propranolol, and acebutolol) were selected due to their well described nonspecific membrane effects (NME). Their interactions with model lipid membranes composed of palmitoyloleoylphosphatidylcholine (POPC) were studied using Time-Dependent Fluorescence Shift (TDFS) and Generalized Polarization (GP) as well as molecular dynamics (MD) simulations. Liposomal vesicles were labeled with fluorescent membrane polarity probes (Laurdan, Prodan, and Dtmac). Increasing beta-blocker concentrations (0-10 mM for acebutolol and oxprenolol, and 0-1.5 mM for propranolol) significantly rigidifies the lipid bilayer at the glycerol and headgroup level, which was detected in the steady-state and in the time-resolved fluorescence data. The effects of propranolol where considerably stronger than those of the two other beta-blockers. The addition of fluorescent probes precisely located at different levels within the lipid bilayer revealed the insertion of the beta-blockers into the POPC bilayer at the glycerol backbone level, which was further confirmed by MD simulations in the case of propranolol.
    European journal of pharmaceutics and biopharmaceutics: official journal of Arbeitsgemeinschaft fur Pharmazeutische Verfahrenstechnik e.V 03/2014; DOI:10.1016/j.ejpb.2014.03.013 · 4.25 Impact Factor
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    ABSTRACT: Classical Hodgkin's lymphoma (cHL)-affected lymphoid tissue contains only a few malignant Hodgkin and Reed-Sternberg (HRS) cells, which are disseminated within a massive infiltrate of reactive cells. In particular, the innate immune infiltrate is deemed to support tumour growth by direct cell-cell interaction. Since they are rarely found in close proximity to the malignant cells in situ, we investigated whether cHL-derived extracellular vesicles might substitute for a direct cell-cell contact. We studied the crosstalk of the transmembrane proteins CD30 and CD30 ligand (CD30L) because they are selectively expressed on HRS and innate immune cells, respectively. Here, we showed that HRS cells released both the ectodomain as a soluble molecule (sCD30) and the entire receptor on the surface of extracellular vesicles. The vesicle diameter was 40-800 nm, as determined by cryo- and immune electron microscopy. In addition to CD30, typical extracellular vesicle markers were detected by mass spectrometry and flow cytometry, including tetraspanins, flotillins, heat shock proteins and adhesion molecules. In contrast to sCD30, vesicles caused a CD30-dependent release of interleukin-8 in CD30L(+) eosinophil-like EoL-1 cells and primary granulocytes from healthy donors, underscoring the functionality of CD30 on vesicles. In extracellular matrix (ECM)-embedded culture of HRS cells, a network of actin and tubulin-based protrusions guided CD30(+) vesicles into the micro-environment. This network targeted CD30(+) vesicles towards distant immune cells and caused a robust polarization of CD30L. Confocal laser scanning microscopy of 30 µm sections showed a CD30 vesicle-containing network also in cHL-affected lymphoid tissue of both mixed-cellularity and nodular sclerosing subtypes. This network might facilitate the communication between distant cell types in cHL tissue and allow a functional CD30-CD30L interaction in trans. The tubulin backbone of the network may provide a target for the therapy of cHL with antitubulin-based CD30 antibody constructs. Copyright © 2013 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.
    The Journal of Pathology 03/2014; 232(4):405-14. DOI:10.1002/path.4306 · 7.33 Impact Factor
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    ABSTRACT: The delivery of nucleic acids like DNA or siRNA still represents a major hurdle, especially with regard to possible therapeutic applications in vivo. Much attention has been focused on the development of non-viral gene delivery vectors including liposomes or cationic polymers. Among them, polyethylenimines (PEIs) have been widely explored for the delivery of nucleic acids and show promising results. The combination of cationic polymers and liposomes (lipopolyplexes) for gene delivery may further improve their efficacy and biocompatibility, by combining the favourable properties of lipid systems (high stability, efficient cellular uptake, low cytotoxicity) and PEI (nucleic acid condensation, facilitated endosomal release). In this study, we systematically analyse various conditions for the preparation of liposome-polyethylenimine-based lipopolyplexes with regard to biological activity (DNA transfection efficacy, siRNA knockdown efficacy) and physicochemical properties (size, zeta potential, stability). This includes the exploration of lipopolyplex compositions containing different liposomes and different relevant branched or linear low-molecular weight PEIs. We establish optimal parameters for lipopolyplex generation, based on various PEIs, N/P ratios, lipids, lipid/PEI ratios and preparation conditions. Importantly, we also demonstrate that certain lipopolyplexes retain their biological activity and physicochemical integrity upon prolonged storage even at 37 °C and/or in the presence of serum, thus providing formulations with considerably higher stability as compared to polyplexes. In conclusion, we establish optimal liposome-polyethylenimine lipopolyplexes that allow storage under ambient conditions. This is the basis and an essential prerequisite for novel, promising and easy-to-handle formulations for possible therapeutic applications.
    Acta biomaterialia 02/2014; 10(6). DOI:10.1016/j.actbio.2014.02.037 · 5.68 Impact Factor
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    ABSTRACT: Abstract Poly(n-butyl-cyanoacrylate)-nanocapsules filled by perfluorodecalin (PFD) are proposed as potential oxygen carriers for blood substitute. The capsule dispersion is prepared via interfacial polymerisation from a PFD emulsion in water which in turn is generated by spontaneous phase separation. The resulting dispersion is capable of carrying approximately 10% of its own volume of gaseous oxygen, which is approximately half of the capacity of human blood. The volumes of the organic solvents and water are varied within a wide range, connected to a change of the capsule radius between 200 and 400 nm. The principal suitability of the capsule dispersion for intravenous application is proven in first physiological experiments. A total amount of 10 ml/kg body weight has been infused into rats, with the dispersion supernatant and a normal saline solution as controls. After the infusion of nanocapsules, the blood pressure as well as the heart rate remains constant on a normal level.
    Journal of Microencapsulation 10/2013; DOI:10.3109/02652048.2013.843600 · 1.88 Impact Factor
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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. DOI:10.1016/j.jpba.2013.01.043 · 2.83 Impact Factor
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    ABSTRACT: Highly ordered two or three dimensional mesophases in aqueous solution could be usefully obtained by using monoolein (MO) or diolein (DO) monomers. Nanostructures (also indicated as nanoparticles, NPs) of MO or DO containing different amounts (1%, 5%, 10% and 20%) of the synthetic amphiphilic gadolinium complex (C18)2DTPA(Gd) have been prepared and characterized for their relaxometric and structural behaviors. The nanostructure is found in the 110–200 nm range for all investigated systems, while the presence of the gadolinium containing monomer produces a partial loss of the cubic symmetry, as shown by Cryo-TEM images of NPs doped with 10% w/w of (C18)2DTPA(Gd). Gadolinium containing nanostructures display high relaxivity values (in the 10–15 mM−1 s−1 range at 25° and 20 MHz, with a further increase at 37 °C for DO based NPs), and interesting relaxometric properties for their possible use as MRI contrast agents. NPs containing 10% w/w of (C18)2DTPA(Gd) (MO3-NPs and DO3-NPs) have been also derivatized by introducing 3% wt of (C18)2–Peg3000–FA to obtain targeted aggregates (MO3-NP–FA, DO3-NP–FA). A preferential uptake efficiency of DO3-NP–FA in IGROV-1 cells with respect to DO-NPs without folic acid is observed, especially when cells are incubated with low concentrations of nanostructures or at short incubation times, thus indicating its potential use as a target-selective delivery system for MRI contrast agents on tumor cells overexpressing the folate receptor.
    01/2013; 1(5):617-628. DOI:10.1039/C2TB00329E
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    ABSTRACT: The T-cell antigen receptor (TCR) exists in monomeric and nanoclustered forms independently of antigen binding. Although the clustering is involved in the regulation of T-cell sensitivity, it is unknown how the TCR nanoclusters form. We show that cholesterol is required for TCR nanoclustering in T cells, and that this clustering enhances the avidity but not the affinity of the TCR-antigen interaction. Investigating the mechanism of the nanoclustering, we found that radioactive photocholesterol specifically binds to the TCRβ chain in vivo. In order to reduce the complexity of cellular membranes, we used a synthetic biology approach and reconstituted the TCR in liposomes of defined lipid composition. Both cholesterol and sphingomyelin (SM) were required for the formation of TCR dimers in phosphatidylcholine-containing large unilamellar vesicles. Further, the TCR was localized in the liquid disordered phase in giant unilamellar vesicles. We propose a model in which cholesterol- and SM-binding to the TCRβ chain causes TCR dimerization. The lipid-induced TCR nanoclustering enhances the avidity to antigen, and thus might be involved in enhanced sensitivity of memory compared to naive T cells. Our work contributes to the understanding of the function of specific nonannular lipid-membrane protein interactions.
    Journal of Biological Chemistry 10/2012; DOI:10.1074/jbc.M112.386045 · 4.60 Impact Factor
  • S Simon, R Schubert
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    ABSTRACT: Phospholipids are widely used excipients for pharmaceutical formulations, such as for preparing biphasic systems or to solubilize or encapsulate poorly soluble drugs. The present study investigates a new property of this class of substance: its ability to inhibit the efflux transporter Pglycoprotein (P-gp). P-gp is expressed in the intestinal epithelium, thereby significantly impairing the systemic absorption of various pharmaceutically active substances. The phospholipid screening performed in this study involved derivatives with different headgroups and fatty acid residues and a number of experimental parameters. For in vitro studies we carried out transport experiments and calcein accumulation assays in Caco-2- and MDCKII mdr1 and wildtype cell lines. The three compounds which displayed significant P-gp inhibition in both assays and in Caco-2 as well as in MDCKII mdr1, consisted of phosphatidylcholine (PC) and either two saturated fatty acid residues of eight (8:0 PC) or ten carbon atoms (10:0 PC), or of two unsaturated docosahexaeonic acid residues (cis-22:6 PC).Supported by P-gp ATPase activity measurements, 8:0 and 10:0 PC were assumed to function as direct P-gp inhibitors interacting with the transporter probably in their monomeric state, whereas a different, as yet unknown mechanism of action applied for cis-22:6 PC.Because of their proven ability to significantly inhibit P-gp in vitro, these phospholipids shall further be elucidated in vivo, whether they may truly serve to increase the bioavailability of orally applied drugs with a P-gp substrate character.
    Biochimica et Biophysica Acta 06/2012; 1821(9):1211-23. DOI:10.1016/j.bbalip.2012.06.001 · 4.66 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. DOI:10.3109/08982104.2011.633267 · 1.53 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 03/2011; 42(4):380-6. DOI:10.1016/j.ejps.2011.01.002 · 3.01 Impact Factor
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    ABSTRACT: We report the synthesis, characterization and micellization properties of two anionic reactive surfactants based on 3-pentadecyl phenol obtainable from a renewable resource, cardanol. The synthesis is achieved through simple chemical transformations, first converting the phenol to the acrylate that is sulfonated in a second step. The products were characterized by elemental analysis and spectroscopic techniques. The surfactant properties of the sulfonated acrylates were measured and compared with the standard non-reactive anionic surfactant sodium dodecyl sulfonate. The micellization behavior of aqueous solutions was studied using conductivity, surface tension measurements, and the fluorescence probe technique based on diphenyl hexatriene. Characterization by surface tension measurements facilitated the determination of basic surfactant properties like the critical micelle concentration (CMC), the surface tension at the CMC, surface excess and area per surfactant molecule. The Gibbs free energy of micellization showed a negative value suggesting spontaneous micellization in aqueous solution. The micellization of the surfmer with an ethylene spacer between the phenyl ring and the acrylate group seems to be enhanced as indicated by the lower surface excess and lower free energy. Its CMC was also lower.
    Journal of Surfactants and Detergents 03/2011; 15(2). DOI:10.1007/s11743-011-1294-z · 1.35 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. DOI:10.1016/j.ejpb.2010.12.007 · 4.25 Impact Factor
  • Martin Holzer, Joachim Momm, Rolf Schubert
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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. DOI:10.1021/la903386d · 4.38 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. DOI:10.3109/08982100903218884 · 1.53 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. DOI:10.1021/bc900147w · 4.82 Impact Factor

Publication Stats

1k Citations
308.68 Total Impact Points


  • 1997–2015
    • University of Freiburg
      • Institute of Pharmaceutical Sciences
      Freiburg, Baden-Württemberg, Germany
  • 2011
    • University of Zagreb
      • Department of Pharmaceutics
      Zagreb, Grad Zagreb, Croatia
  • 2009
    • Universität Bremen
      • Chemistry Department
      Bremen, Bremen, Germany
  • 1998
    • Humboldt-Universität zu Berlin
      Berlín, Berlin, Germany