Interfacial thickness of liposomes containing poly(ethylene glycol)-cholesterol from electrophoresis

Fluorescent-Probe Characterization for Pore-Space Mapping with Single-Particle Tracking

Porous solids often contain complex pore networks with pores of various sizes. Tracking individual fluorescent probes as they diffuse through porous materials can be used to characterize pore networks at tens of nanometers resolution. However, understanding the motion behavior of fluorescent probes in confinement is crucial to reliably derive pore network properties. Here, we introduce well-defined lithography-made model pores developed to study probe behavior in confinement. We investigated the influence of probe-host interactions on diffusion and trapping of confined single-emitter quantum-dot probes. Using the pH-responsiveness of the probes, we were able to largely suppress trapping at the pore walls. This enabled us to define experimental conditions for mapping of the accessible pore space of a one-dimensional pore array as well as a real-life polymerization-catalyst-support particle.

Rapid characterization of neutral polymer brush with a conventional zetameter and a variable pinch of salt

The fundamental and practical importance of particle stabilization has motivated various characterization methods for studying polymer brushes on particle surfaces. In this work, we show how one can perform sensitive measurements of neutral polymer coating on colloidal particles using a commercial zetameter and salt solutions. By systematically varying the Debye length, we study the mobility of the polymer-coated particles in an applied electric field and show that the electrophoretic mobility of polymer-coated particles normalized by the mobility of non-coated particles is entirely controlled by the polymer brush and independent of the native surface charge, here controlled with pH, or the surface-ion interaction. Our result is rationalized with a simple hydrodynamic model, allowing for the estimation of characteristics of the polymer coating: the brush length L, and the Brinkman length ξ, determined by its resistance to flows. We demonstrate that the Debye layer provides a convenient and faithful probe to the characterization of polymer coatings on particles. Because the method simply relies on a conventional zetameter, it is widely accessible and offers a practical tool to rapidly probe neutral polymer brushes, an asset in the development and utilization of polymer-coated colloidal particles.

Game-based interventions for neuropsychological assessment, training and rehabilitation: Which game-elements to use? A systematic review

Game-based interventions (GBI) have been used to promote health-related outcomes, including cognitive functions. Criteria for game-elements (GE) selection are insufficiently characterized in terms of their adequacy to patients' clinical conditions or targeted cognitive outcomes. This study aimed to identify GE applied in GBI for cognitive assessment, training or rehabilitation. A systematic review of literature was conducted. Papers involving video games were included if: (1) presenting empirical and original data; (2) using video games for cognitive intervention; and (3) considering attention, working memory or inhibitory control as outcomes of interest. Ninety-one papers were included. A significant difference between the number of GE reported in the assessed papers and those composing video games was found (p < .001). The two most frequently used GE were: score system (79.2% of the interventions using video games; for assessment, 43.8%; for training, 93.5%; and for rehabilitation, 83.3%) and narrative context (79.2% of interventions; for assessment, 93.8%; for training, 73.9% and for rehabilitation, 66.7%). Usability assessment was significantly associated with six of the seven GE analyzed (p-values between p ≤ 0.001 and p. = 027). The use of GE that act as extrinsic motivation promotors (e.g., numeric feedback system) may jeopardize patients' long-term adherence to interventions, mainly if associated with progressive difficulty-increase of gaming experience. Lack of precise description of GE and absence of a theoretical framework supporting GE selection are important limitations of the available clinical literature.

Sulfobetaine-terminated PEG improves the qualities of an immunosensing surface

Poly(ethylene glycol) (PEG) possessing a sulfobetaine (SB) moiety at one end and a pentaethylenehexamine (N6) at the other end (SB-PEG-N6) was newly synthesized as a blocking agent for immunosensing surfaces. The N6 moiety strongly coordinates on gold surfaces, facilitating the tethering of the PEG chain to the sensor chip surface, and leaves the SB moiety free. Non-specific adsorption of bovine serum albumin (BSA) was analyzed on the SB-PEG-N6 tethered surface and compared with the methoxy-PEG-N6 (M-PEG-N6) tethered surface using a surface plasmon resonance (SPR) sensor. Non-specific BSA adsorption decreased with decreasing PEG chain length on the SB-PEG tethered chain surface. Non-specific adsorption of BSA decreased as ionic strength increased on SB-PEG-N6 surfaces; this phenomenon was completely opposite to that observed with an M-PEG-N6 tethered chain surface. The results show that SB moieties located close to the gold surface perform well with regard to protein rejection. Actually, low-molecular weight alkane thiol SB (SB-SH) showed minimum BSA adsorption. To evaluate protein recognition efficacy on a PEGylated surface, an antibody (IgG) immobilized surface was then constructed on a gold sensor chip using SB-PEG-N6 as the blocking agent. The specific protein recognition efficacy of SB-PEG-N6/IgG co-immobilized surfaces was higher than that obtained using SB-SH/IgG co-immobilized surfaces. We conclude that SB-terminated PEG exhibits the optimal qualities of a blocking agent, as it possesses both high suppression efficacy of nonspecific protein adsorption and specific protein recognition ability.

Study of PEGylated lipid layers as a model for PEGylated liposome surfaces: molecular dynamics simulation and Langmuir monolayer studies

We have combined Langmuir monolayer film experiments and all-atom molecular dynamics (MD) simulation of a bilayer to study the surface structure of a PEGylated liposome and its interaction with the ionic environment present under physiological conditions. Lipids that form both gel and liquid-crystalline membranes have been used in our study. By varying the salt concentration in the Langmuir film experiment and including salt at the physiological level in the simulation, we have studied the effect of salt ions present in the blood plasma on the structure of the poly(ethylene glycol) (PEG) layer. We have also studied the interaction between the PEG layer and the lipid bilayer in both the liquid-crystalline and gel states. The MD simulation shows two clear results: (a) The Na(+) ions form close interactions with the PEG oxygens, with the PEG chains forming loops around them and (b) PEG penetrates the lipid core of the membrane for the case of a liquid-crystalline membrane but is excluded from the tighter structure of the gel membrane. The Langmuir monolayer results indicate that the salt concentration affects the PEGylated lipid system, and these results can be interpreted in a fashion that is in agreement with the results of our MD simulation. We conclude that the currently accepted picture of the PEG surface layer acting as a generic neutral hydrophilic polymer entirely outside the membrane, with its effect explained through steric interactions, is not sufficient. The phenomena we have observed may affect both the interaction between the liposome and bloodstream proteins and the liquid-crystalline-gel transition and is thus relevant to nanotechnological drug delivery device design.

Nanoparticles as fluorescence labels: is size all that matters?

Fluorescent labels are often used in bioassays as a means to detect and characterize ligand-receptor binding. This is due in part to the inherently high sensitivity of fluorescence-based technology and the relative accessibility of the technique. There is often little concern raised as to whether or not the fluorescent label itself affects the ligand-receptor binding dynamics and equilibrium. This may be particularly important when considering nanoparticle labels. In this study, we examine the affects of nanoparticle (quantum dots and polymer nanospheres) fluorescent labels on the streptavidin-biotin binding system. Since the nanoparticle labels are larger than the species they tag, one could anticipate significant perturbation of the binding equilibrium. We demonstrate, using fluorescence cross-correlation spectroscopy, that although the binding equilibria do change, the relative changes are largely predictable. We suggest that the nanoparticles' mesoscopic size and surface tension effects can be used to explain changes in streptavidin-biotin binding.

Characterization of Novel Mixed Polyethyleneglycol Modified Liposomes

In our previous paper, mixed polyetheleneglycol (PEG) modification of liposomes by a mixture of 1-monomethoxypolyethyleneglycol-2,3-distearoylglycerol (PEG-DSG) with short polyoxyethylene chain and PEG-DSG with long one was shown to increase fixed aqueous layer thickness (FALT) around the liposomal membrane, and this was useful in vivo. In this study, we investigated the characterization of mixed PEG modification of liposomes with different anchors (PEG2000-DSG and PEG2000-cholesterol (CHO)). When the liposomes was modified by a mixture of PEG2000-DSG and PEG2000-CHO, FALT was increased compared to that of each single PEG-lipids modification and the most suitable mix modification (PEG2000-DSG:PEG2000-CHO = 3:1) showed a maximum FALT. This phenomenon was speculated to be based on the difference in the insertion state of the PEG anchor unit in the liposomal membrane. PEG-CHO-modified liposomes (single or mixed PEG-modified liposomes) were easily incorporated into the liposomal membranes compared with that of single PEG-DSG-modified liposomes. Namely, it was considered that the cholesterol anchor as a single chain was able to be easily introduced, compared with the DSG anchor as two chains, and induced some interaction with both PEG modification. In conclusion, it is expected that novel PEG-modified liposomes with PEG2000-DSG and PEG2000-CHO (3:1) had superior physicochemical properties.

Characterization and cytotoxicity of mixed polyethyleneglycol modified liposomes containing doxorubicin

Liposomes are recognized as one of the useful drug carriers, but have many problems to overcome before their clinical application. Liposomes, bonding peculiarly with serum protein (opsonization), are taken up by reticuloendothelial system (RES) cells in the liver and spleen. It is known that polyethyleneglycol (PEG) modification of the liposome surface induces the formation of a fixed aqueous layer around the liposomes due to the interaction between the PEG-polymer and water molecule, and thus prevents the attraction of opsonins. Namely, PEG-modified liposomes are able to escape trapping by the RES cells, and have a prolonged circulation time. In this study, the effects of different anchors with the same PEG molecular weight on the cell uptake and cytotoxicity of mixed PEG-modified liposomal doxorubicin (DOX) were examined. The fixed aqueous layer thickness (FALT) of liposomes covered with mixtures of PEG-molecules which differ in their chain length were increased, compared to that of the single PEG2000-modified liposome. Mixed PEG-modification of liposomes with different anchors (PEG2000-(1-monomethoxypolyethyleneglycol-2,3-distearoylglycerol (DSG): cholesterol (CHO)=1:1)-modified liposome) led to an increase in the FALT, compared to that of each single PEG-modification. The uptake of DOX into Ehrlich ascites carcinoma cells by the liposomes covered with PEG-CHO was higher than the other liposomes. Thus, liposomes covered with PEG-DSG and PEG-CHO have an enhanced cytotoxicity. In conclusion, it was confirmed that mix-modified liposomes using PEG-lipid with different anchors were superior.

Enhanced efficacy of anti-tumor liposomal doxorubicin by hyperthermia

The efficiency of novel tumor chemotherapeutics could be increased using targeted drug delivery by hyperthermia. In this paper, the 3D liposomal doxorubicin distribution in the tumor tissue enhanced by local hyperthermia was quantitatively studied in real time using laser confocal microscopy. Results showed that the thermally induced liposomal doxorubicin extravasation was non-uniform and more excessive in the peripheral region than that in the tumor center. The effect of the thermally targeted drug delivery was also investigated. On the 1st, 3rd day after the thermally targeted drug treatment, histological examination showed that many nucleolus were condensed and collapsed in the peripheral region. But, in the tumor center, there were no such changes found until the 3rd day. While on the 6th day, tumor cells in both the peripheral and center region were found necrotic. The enhancement of the nanoparticle anti-tumor drug effect was significant. A theoretical analysis of liposomal doxorubicin diffusion to the tumor cells in vivo was performed. Results showed that it took more than 40 hrs for the doxorubicin to get into the tumor cells in the center region from the periphery region. The theoretical results well explained the experimental observations.

Liposomalization of SN-38 as active metabolite of CPT-11

Although many drugs have been developed for the treatment of disease, some drugs have complications such as adverse effects, and antitumor agents should target tumors or cells more selectively. It is therefore necessary to develop drug delivery systems, and liposomes are reportedly useful as an effective drug carrier. An antitumor agent, CPT-11, inhibits DNA synthesis by the inhibition of topoisomerase1 and has a strong antitumor activity. SN-38 is converted from CPT-11 as an active metabolite by carboxylesterase in the liver. As SN-38 is insoluble, it has not been applied at the clinical stage as an injection. It is expected that SN-38 liposomalization may increase its usefulness in cancer chemotherapy. Our purpose is to have a clinical application of SN-38 by a novel method of liposomalization to expand the application for the other insolubility drugs. As SN-38 is hydrophobic, SN-38-trapped liposome preparation was attempted using the Bangham method, which is effective for general preparation. However, a high ratio of SN-38 trapped in liposome was not achieved, and this was not improved by the freezing-thawing method or the freeze-drying method. On the other hand, the ratio of SN-38 trapped in liposome by the modified remote loading method was about 4 times that by the Bangham method, and the ratio by the film loading method, novel method of liposomal preparation, reached 2 times and 8 times that by the modified remote loading method and Bangham method, respectively, showing a remarkable increase. In conclusion, it was suggested that the preparation of SN-38 liposome using the film loading method effectively entraps SN-38. Thus, it is expected that SN-38 liposome can be applied as an injection. This preparation method is useful if application is possible in the other insolubility drugs.

[Characterization and cytotoxicity of mixed PEG-DSG modified liposomes]

It is known that polyethyleneglycol (PEG) modification of the liposome surface leads to the formation of a fixed aqueous layer around the liposomes due to interaction between the PEG polymer and water molecules, which prevents the attraction of opsonins. When a combination of PEG-distearolyglycerol (PEG-DSG) whose characteristics are remarkably different is used, interaction between molecules occurs, leading to increased fixed aqueous layer thickness (FALT). From this speculation, we studied the effect of both modification of PEG900-DSG and PEG2000-DSG modified liposome on FALT, cell uptake and biodistribution. The FALT of mixed PEG modified liposome increased, compared to that of each single PEG modified liposome. In this mixed modification, maximum FALT was shown at liposome modified by added PEG-2000:PEG-900=2:1. This most suitable additional ratio was equal to actual incorporated ratio. On the other hand, cell uptake of mixed modified liposome containing doxorubicin (DOX) was similar with that of PEG2000 modified liposome. Furthermore, mixed PEG modification of liposome was tendency to increase cytotoxicity, compared to that of other modifications. After DOX contained liposome treatment, DOX distribution in the tumor and antitumor activity of DOX increase by mixed PEG modification. In conclusion, it was suggested that mixed PEG liposome (PEG-2000:PEG-900=2:1) was useful for cancer chemotherapy.

Hydrodynamics and electrokinetics of spherical liposomes with coatings of terminally anchored poly(ethylene glycol): numerically exact electrokinetics with self-consistent mean-field polymer

A detailed theoretical model is presented to interpret electrokinetic experiments performed on colloids with uncharged polymer layers. The methodology removes many of the degrees of freedom that otherwise have to be accounted for by adopting multiple empirical fitting parameters. Furthermore, the level of detail provides a firm basis for future studies examining liposome surface chemistry and charge, surface-charge mobility, and the dynamics of adsorbed polymer on fluidlike membranes. The model predictions are compared with experimental measurements of the electrophoretic mobility of stealth liposomes with molecular weights of terminally anchored poly(ethylene glycol) (PEG) in the range 0.35-10 kg mol(-1) [J.A. Cohen and V.A. Khorosheva, Colloids Surf. A 195, 113 (2001)]. The experimental data are interpreted by drawing upon self-consistent mean-field calculations of the polymer segment density distributions and numerically exact solutions of the governing transport equations [R.J. Hill, D.A. Saville, and W.B. Russel, J. Colloid Interface Sci. 258, 56 (2003)]. The approach leads to excellent agreement between theory and experiment with one adjustable parameter--the hydrodynamic size (Stokes radius) a(s) approximately equal to 0.175 A of the statistical PEG segments with (Kuhn) length l=7.1 A . The remarkably small Stokes radius is demonstrated to be consistent with other applications of the well-known Debye-Brinkman model and, consequently, this work reveals important limitations of the mean-field hydrodynamic model. Despite such limitations, the "full" electrokinetic model is robust in its predictive capacity. The molecular weights of the terminally anchored PEG span the range where the coatings undergo a transition from mushroomlike to brushlike conformations, and the hydrodynamic size and electrophoretic mobility of the liposomes are demonstrated to be sensitive to the PEG chain length and the effects of double-layer polarization.

Study on the characterization of mixed polyethyleneglycol modified liposomes containing doxorubicin

Previously, we suggested that mixed polyethyleneglycol (PEG)-modification of liposomes with mixture of 1-monomethoxypolyethyleneglycol-2,3-distearoylglycerol (PEG-DSG) with short and long polyoxyethylene chains led to increasing fixed aqueous layer thickness (FALT) and was useful in vivo. FALT is expected to be one of the important factors that influence the pharmacokinetics of liposomes. In this study, we investigated the connection between FALT and some parameters in vitro. In both mixed and single PEG-modified liposomes, the incorporation ratios of PEG-DSGs were the same, the residual amount of PEG-DSGs with serum proteins was not affected by molecular weight. Consequently, the effective properties in vivo of mixed PEG-modification may be the increase in the absolute amount of PEG-DSGs on the liposome membranes, decrease of slipping out action taken PEG-DSGs with long polyoxyethylene chains by serum protein, and thus maintenance of FALT. While, it has been suggested that mixed PEG-modified liposomes are effective in vitro, too, because of the lower leakage and same level of doxorubicin (DOX) uptake as plain liposomal DOX.

Sterol surfactants: from synthesis to applications

An overview is given of sterol surfactants, including raw material aspects, classification and synthesis routes, physico-chemical behaviour and applications in pharmaceuticals and cosmetics.

Biophysical consequences of linker chemistry and polymer size on stealth erythrocytes: size does matter

Immunocamouflaged red blood cells (RBC) are produced by cell surface derivatization with methoxypolyethylene glycol (mPEG). These immunologically attenuated cells may reduce the risk of allosensitization in chronically transfused patients. To characterize the effects of differing linker chemistries and polymer lengths, RBC were modified with cyanuric chloride activated mPEG (C-mPEG 5 kDa), benzotriazole carbonate methoxyPEG (BTC-mPEG; 5 or 20 kDa) or N-hydroxysuccinimidyl ester of mPEG propionic acid (SPA-mPEG; 2, 5 or 20 kDa). Biophysical methods including particle electrophoresis and aqueous two-phase polymer partitioning were employed to compare the PEG derivatives. While C-mPEG was faster reacting, both BTC-mPEG and SPA-mPEG gave comparable findings after 1 h. Both PEG surface density and molecular mass had a large effect on RBC surface properties. Proportional changes in electrophoretic mobility and preferential phase partitioning were achieved by increasing either the quantity of surface PEG or the PEG molecular mass. In addition, two-phase partitioning may provide a means for efficiently removing unmodified or lightly modified (hence potentially immunogenic) RBC in the clinical setting. Furthermore, mPEG modification significantly inhibits cell-cell interaction as evidenced by loss of Rouleaux formation and, consequently, sedimentation rate. Importantly, BTC-mPEG 20 kDa RBC showed normal in vivo survival in mice at immunoprotective concentrations (up to 2 mM).

Relationship between complement activation, cellular uptake and surface physicochemical aspects of novel PEG-modified nanocapsules

The aim of our work was to examine the relationship between modifications of the surface of nanocapsules (NC) by adsorption or covalent grafting of poly(ethylene oxide) (PEG), and changes in their phospholipid (PL) content on complement activation (C3 cleavage) and on uptake by macrophages. The physicochemical characterization of the NC included an investigation of their properties, such as surface charge, size, hydrophilicity, morphology and homogeneity. This is the first time that such properties have been correlated with biological interactions for NC, a novel carrier system with a structure more complex than nanospheres. C3 crossed immunoelectrophoresis revealed the reduced activation for NC with longer PEG chain and higher density, although all formulations induced C3 cleavage to a lesser or greater extent. NC bearing PEG covalently bound to the surface were weaker activators of complement than plain PLA [poly(D,L-lactide)] NC or nanospheres (NS). Furthermore, the fluorescent/confocal microscopy of J774A1 cells in contact with NC reveal a dramatically reduced interaction with PEG-bearing NC. However, the way in which PEG was attached (covalent or adsorbed) seemed to affect the mechanism of uptake. Taken together, these results suggest that the low level of protein binding to NC covered with a high density of 20kDa PEG chains is likely to be due to the steric barriers surrounding these particles, which prevents protein adsorption and reduces their interaction with macrophages.

Preparation of long-circulating immunoliposomes using PEG-cholesterol conjugates: effect of the spacer arm between PEG and cholesterol on liposomal characteristics

Poly(ethylene glycol)-coated liposomes were prepared with two new synthesised pegylated cholesterol (Chol) derivatives linked via carbamate bond. Poly(ethylene glycol) (PEG) was directly linked to Chol (PEG-Chol) or through a space arm of diaminebutane (PEG-L-Chol). In buffer, the physicochemical properties of PC/Chol liposomes (2/1, molar ratio) containing up to 10 mol% of pegylated Chol derivatives did not change significantly and the PEG layer at liposome surface inhibited the agglutination of biotin-liposomes induced by streptavidin. On the other hand, in serum, PEG-L-Chol seemed to reduce the interactions of liposomes with serum proteins, much more than PEG-Chol. The low steric hindrance of PEG-Chol derivative may be due to the slow conformational transition rate of the polymer, since PEG may be deeper located in the membrane. The coupling efficiency of the ligand to the functionalised amino group at the polymer end was also affected, but, its antigen-binding activity was preserved. The basic physical-chemical characteristics studied in this work are relevant to assess the application of pegylated Chol liposomes as drug delivery systems.

Determination of properties of individual liposomes by capillary electrophoresis with postocolumn laser-induced fluorescence detection

Individual liposome measurements by capillary electrophoresis with postcolumn laser-induced fluorescence detection facilitated the determination of liposome property distributions, two-dimensional plots, and an improved characterization of a liposomal preparation. This advancement in liposome analysis was feasible by using a high-sensitivity postcolumn laser-induced fluorescence detector wired for millisecond response. For each individual liposome containing fluorescein, peak height and migration time were determined. From these measurements the individual entrapped volumes and electrophoretic mobilities were determined. Distribution analysis of these properties facilitated comparison of various liposome dilutions and indicated that the method is reproducible and unaffected by the density of liposomes (10(7)-10(9) liposomes/mL) in the suspension. Furthermore, liposomes showed entrapped volumes that vary from 0.3 to 13 fL with apparent radius varying from 370 nm to 1.8 microns. Two-dimensional plots of reduced mobility versus kappa R (Debye parameter x liposome radius) revealed that the liposomes resuspended from a dried film of phospholipids are heterogeneous in regard to the surface charge density of individual liposomes. The described method has the potential of becoming a new tool for characterization of commercial liposomal preparations and theoretical studies.

Unusual electrostatic effects on binding of C1q to anionic liposomes: role of anionic phospholipid domains and their line tension

The binding of 125I-C1q to anionic liposomes was studied as a function of protein concentration, pH, ionic strength, and anionic lipid composition. The maximum amount of protein bound per micromole of lipid was very sensitive to electrostatic factors, increasing strongly with decreased pH and ionic strength or increased anionic lipid content. The apparent association constant was independent of these electrostatic factors, however, in marked contrast to studies on basic peptide binding to anionic lipid vesicles. Microscopic observations of large unilamellar liposomes containing fluorescently labeled C1q or phosphatidylglycerol demonstrated, under conditions causing strong electrostatic interactions, that C1q and anionic lipids colocalized into domains whose radii of curvature were higher than that of the surrounding lipid. These domains were observed to bud and pinch off into brightly fluorescent vesicles. We propose a model for all of these observations in which the line tension or edge energy at the boundary of the domain resists its increase in circumference as the domain grows by electrostatic effects on binding, eventually resulting in vesiculation. We propose that under favorable electrostatic conditions, as larger domains form the edge energy balances the increases in the electrostatic contribution to binding, resulting in a net binding energy independent of electrostatic factors.

Electrostatically mediated interactions between cationic lipid-DNA particles and an anionic surface

In an effort to model the interaction of lipid-based DNA delivery systems with anionic surfaces, such as a cell membrane, we have utilized microelectrophoresis to characterize how electrokinetic measurements can provide information on surface charge and binding characteristics. We have established that cationic lipids, specifically N-N-dioleoyl-N,N-dimethylammonium chloride (DODAC), incorporated into liposomes prepared with 1, 2-dioleoyl-i-glycero-3-phosphoethanolamine (DOPE) or 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) at 50 mol%, change the inherent electrophoretic mobility of anionic latex polystyrene beads. Self-assembling lipid-DNA particles (LDPs), prepared at various cationic lipid to negative DNA phosphate charge ratios, effected no changes in bead mobility when the LDP charge ratio (+/-) was equal to or less than 1. Increasing the LDP concentration in a solution of 0.1% (w/v) anionic beads resulted in a charge reversal effect when a net charge of LDP to total bead charge ratio (+/-) of 1:1 was observed. LDP formulations, utilizing either DOPE or DOPC, showed similar titration profiles with a charge reversal observed at a 1:1 net LDP to bead charge ratio (+/-). It was confirmed through centrifugation studies that the DNA in the LDP was associated with the anionic latex beads through electrostatic interactions. LDP binding, rather than the binding of dissociated cationic lipids, resulted in the observed electrophoretic mobility changes of the anionic latex beads.

C1q binding to liposomes is surface charge dependent and is inhibited by peptides consisting of residues 14-26 of the human C1qA chain in a sequence independent manner

Complement activation by anionic liposomes proceeds by antibody-independent, C1q-initiated activation of the classical pathway. Purified C1q bound to anionic liposomes in an acidic lipid concentration-dependent manner. Saturation binding, but not the apparent association constant, was enhanced by increasing the cardiolipin content of the liposomes or decreasing either the pH or ionic strength of the reaction mixture. These observations indicate the involvement of electrostatic factors in the binding. A highly cationic region in the collagen-like domain of C1q comprised of residues 14-26 of the C1qA polypeptide chain was assessed for involvement in liposome binding. This region has previously been shown to mediate C1q binding to other immunoglobulin-independent activators of the classical pathway of complement. Peptides containing residues 14-26 of C1qA, denoted C1qA14-26, inhibited C1q binding to and complement activation by anionic liposomes. The inhibitory capacity of these cationic peptides had no sequence or conformation specificity. Rather, the amount of positive charge on the peptides was the determining factor. When present in excess, peptides with five cationic residues inhibited C1q binding and complement activation; however, C1q peptides with only two cationic residues did not. In addition to the C1qA14-26 region, other parts of C1q that contain cationic residues may also be involved in C1q binding to anionic liposomes.

Inhibition of liposome-induced complement activation by incorporated poly(ethylene glycol)-lipids

Complement activation causes opsonization of foreign particles leading to particle elimination from the blood. Complement-mediated opsonization of charged and large liposomes presents a fundamental problem in their use to deliver therapeutic agents in vivo. To prolong the circulation half-lives of such liposomes, complement activation must be curtailed. The aim of this study was to assess the ability of poly(ethylene glycol)-lipids (PEG-lipids) to inhibit the in vitro activation of the classical pathway of complement in human serum by anionic liposomes. Incorporation of cholesterol-PEG600 (CH-PEG600), cholesterol-PEG1000 (CH-PEG1000), or phosphatidylethanolamine-PEG2000 (PE-PEG2000) resulted in dose-dependent inhibition of C1q binding and complement activation. The dose of PEG-lipid at which complement activation was blocked was inversely related to the PEG chain length. Complement activation was strongly inhibited when 15 mole% of CH-PEG600, 10 mole% CH-PEG1000, or 5 mole% PE-PEG2000 was incorporated into 100-nm anionic liposomes. PEG-lipid incorporation into larger liposomes (240 nm) was also successful in blocking C1q binding and complement activation. Radiolabeled cholesterol-PEG approximately 1400 was prepared and used to determine both the percentage of CH-PEG incorporated into the liposomes and the percentage maintained in the liposomes in the presence of 50% human serum at 37 degrees C for up to 24 h.

Controlling liposome blood clearance by surface-grafted polymers

The incorporation of polymer-lipid conjugates, initially using PEG and subsequently other selected flexible, hydrophilic polymers, into lipid bilayers gives rise to sterically stabilized liposomes that exhibit reduced blood clearance and concomitant changes in tissue distribution largely because of reduced, but not eliminated, phagocytic uptake. Changes in tissue distribution includes 'passive' targeting localization into sites of tumors, infection, inflammation characterized by presence of a 'leaky' vasculature which represent useful applications for drug delivery. The polymer forms a surface coating which has been characterized by physical measurements and it appears to function through steric inhibition of the protein binding and cellular interactions leading to phagocytic uptake. The current understanding of the physical and biological properties are reviewed. Ongoing work in the field involves interests to increase complexity such as addition of (1) selective targeting ligands by chemical conjugation to the exterior surface of the polymer coating, (2) capabilities for intracellular release of encapsulated agents into the cytoplasm, and (3) both simultaneously.

New sterically stabilized vesicles based on nonionic surfactant, cholesterol, and poly(ethylene glycol)-cholesterol conjugates

Monomethoxypoly(ethylene glycol) cholesteryl carbonates (M-PEG-Chol) with polymer chain molecular weights of 1000 (M-PEG1000-Chol) and 2000 (M-PEG2000-Chol) have been newly synthesized and characterized. Their aggregation behavior in mixture with diglycerol hexadecyl ether (C16G2) and cholesterol has been examined by cryotransmission electron microscopy, high-performance gel exclusion chromatography, and quasielastic light scattering. Nonaggregated, stable, unilamellar vesicles were obtained at low polymer levels with optimal shape and size homogeneity at cholesteryl conjugate/ lipids ratios of 10 mol% M-PEG1000-Chol or 5 mol% M-PEG2000-Chol, corresponding to the theoretically predicted brush conformational state of the PEG chains. At 20 mol% M-PEG1000-Chol or 10 mol% M-PEG2000-Chol, the saturation threshold of the C16G2/cholesterol membrane in polymer is exceeded, and open disk-shaped aggregates are seen in coexistence with closed vesicles. Higher levels up to 30 mol% lead to the complete solubilization of the vesicles into disk-like structures of decreasing size with increasing PEG content. This study underlines the bivalent role of M-PEG-Chol derivatives: while behaving as solubilizing surfactants, they provide an efficient steric barrier, preventing the vesicles from aggregation and fusion over a period of at least 2 weeks.

Up-regulation of Ca2+ influx mediated by store-operated channels in HL60 cells induced to differentiate by 1 alpha,25-dihydroxyvitamin D3

The physiologically active form of vitamin D, 1 alpha,25-dihydroxyvitamin D3 (1,25D3), induces promyelocytic HL60 cells to differentiate towards monocyte-like cells. During this differentiation increased cytosolic calcium (Cai2+) and expression of surface receptors for chemotactic factors "prime" the cell for the activation of monocyte functions and the triggering of the respiratory burst pathway. We examined whether the Ca2+ influx mediated by store-operated channels (SOC) contributed to the increased Cai2+ following exposure of HL60 cells to 10(-7) M 1,25D3. Cells treated with 1,25D3 for 72 hr demonstrated a rapid transient rise in Cai2+ followed by a second, phasic, increase in Cai2+ in response to the purinergic agonist ATP. This second Cai2+ transient was blocked by Ni2+, SKF 96365, or withdrawal of extracellular Ca2+. In cells suspended in Ca(2+)-free medium, peak changes (delta) in [Ca2+]i elicited by ATP-induced Ca2+ mobilization occurred with similar EC50 values in differentiated and vehicle (EtOH)-treated cells; however, peak [Ca2+]i was reduced by 55% in 1,25D3-treated cells. Decreased Ca2+ mobilization was associated with a 25-35% reduction in intracellular Ca2+ stores (determined with ionomycin). 1,25D3-treated cells exposed to ATP or thapsigargin (Tg) in Ca(2+)-free medium for 3 min with subsequent addition of 1 mM Ca2+ exhibited a respective 80% or 120% stimulation in peak [Ca2+]i compared to EtOH-treated cells. Enhanced Ca2+ influx mediated by SOC was also seen in these cells as an increase in the rate of Mn2+ entry after exposure to ATP or Tg. At 96 hr after addition of 1,25D3, when differentiated phenotype was established, basal Ca2+i and Ca2+ entry mediated by SOC returned to control values, but Ca2+ store size remained reduced. Up-regulation of Ca2+ influx via the SOC pathway during 1,25D3-induced differentiation may contribute to the functional properties of the maturing monocyte, or to the resetting of molecular programs responsible for the changing phenotype.