Temperature-dependent associating property of liposomes modified with a thermosensitive polymer

Acoustic Forceps Based on Focused Acoustic Vortices with Different Topological Charges

For enhanced energy concentration with improved flexibility for object manipulation, a focused acoustic vortex (FAV) is designed using a sector planar piston transducer array and acoustic lens that can produce the effective concentration of the acoustic field to perform the focusing function. Compared to the Gaussian beam, which tends to cause the object to deviate from the axis of acoustic propagation, FAVs can form a central valley region to firmly bind the objects, thus preventing off-target effects. The heat energy in the paraxial region is transferred to the vortex center in the form of heat transfer so that the temperature-sensitive liposomes captured can quickly release drugs, which has a good effect on targeted drug administration. The focused acoustic wave stopped acting on the tissue (gel) for 2 s, the temperature of the vortex center continued to rise, reaching 41.5 °C at the moment of 3.7 s, at which point the liposomes began to release the drug. The FAVs capture the drug and use its thermal effect to achieve accurate and rapid treatment. The simulation results show that the drug release temperature of temperature-sensitive liposomes can be achieved by controlling the action time of the vortices. This study provides a reliable theoretical basis for the clinical application of targeted drugs.

Temperature-sensitive polymers to promote heat-triggered drug release from liposomes: Towards bypassing EPR

Heat-triggered drug release from temperature-sensitive nanocarriers upon the application of mild hyperthermia is a promising approach to achieve site-specific delivery of drugs. The combination of mild hyperthermia (41-42 °C) and temperature-sensitive liposomes (TSL) that undergo lipid phase-transition and drug release has been studied extensively and has shown promising therapeutic outcome in a variety of animal tumor models as well as initial indications of success in humans. Sensitization of liposomes to mild hyperthermia by means of exploiting the thermal behavior of temperature-sensitive polymers (TSP) provides novel opportunities. Recently, TSP-modified liposomes (TSPL) have shown potential for enhancing tumor-directed drug delivery, either by triggered drug release or by triggered cell interactions in response to heat. In this review, we describe different classes of TSPL, and analyze and discuss the mechanisms and kinetics of content release from TSPL in response to local heating. In addition, the impact of lipid composition, polymer and copolymer characteristics, serum components and PEGylation on the mechanism of content release and TSPL performance is addressed. This is done from the perspective of rationally designing TSPL, with the overall goal of conceiving efficient strategies to increase the efficacy of TSPL plus hyperthermia to improve the outcome of targeted anticancer therapy.

Theoretical and in vivo experimental investigation of laser hyperthermia for vascular dermatology mediated by liposome@Au core-shell nanoparticles

The 1064 nm Nd:YAG laser shows a good prospect for the treatment of port-wine stain (PWS), but it is necessary to enhance the blood absorption to laser energy by exogenous chromophore. Owing to the conjunction effect of local surface plasmon resonance (LSPR) by gold nanoparticle and drug delivery as well as lumen blockage abilities by liposome, liposome@Au core-shell nanoparticles are used as exogenous chromophore, and the efficiency of photothermal therapy is studied systematically. In this work, theoretical simulations were conducted to investigate the electric field and solid heat conduction of liposome@Au core-shell nanoparticles with various size and particles distance, aiming to achieve maximum photothermal conversion efficiency during the laser irradiation. Thereafter, liposome@Au core-shell nanoparticles with optimal size and structure were prepared, and in vivo experiments were conducted to evaluate the thermal damage of blood vessels enhanced by liposome@Au core-shell nanoparticles. Theoretical results imply that maximum temperature rise (167 K) is obtained when radius is 45 nm and shell thickness is 5 nm with distance of 4 nm. Liposome@Au core-shell nanoparticles were prepared with diameter of 101 nm and shell thickness of 5 nm according to the finite element simulation of electric field and solid heat conduction. When the molar ratio of chloroauric acid to phospholipid is 2.25, the LSPR absorption peak is about 981 nm, which is close to the wavelength of Nd:YAG laser. In vivo experiments show that injecting liposome@Au core-shell nanoparticles into the blood vessels can effectively reduce the number of laser pulses and the corresponding energy density required for obvious vasoconstriction.

Tunable Surface Properties of Temperature-Responsive Polymer-Modified Liposomes Induce Faster Cellular Uptake

Drug delivery by nanoparticle carriers has been limited by inefficient intracellular drug delivery. Temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm)-modified liposomes can release their content following heating. In this study, we synthesized the temperature-responsive polymer poly(N-isopropylacrylamide)-co-N,N'-dimethylaminopropylacrylamide (P(NIPAAm-co-DMAPAAm)) and investigated the properties of liposomes modified with P(NIPAAm-co-DMAPAAm) for intracellular drug carriers. The copolymer displayed a thermosensitive transition at a lower critical solution temperature (LCST) that is higher than body temperature. Above the LCST, the temperature-responsive liposomes started to aggregate and release. The liposomes showed a fixed aqueous layer thickness (FALT) at the surface below the LCST, and the FALT decreased with increasing temperature. Above 37 °C, cytosolic release from the temperature-responsive liposomes was higher than that from the PEGylated liposomes, indicating intracellular uptake. Here, we showed that the tunable surface properties of the temperature-responsive polymer-modified liposomes possibly enabled their dehydration by heating, which likely induced a faster cellular uptake and release. Therefore, the liposomes could be highly applicable for improving intracellular drug-delivery carriers.

PAMAM Dendron Lipid Assemblies That Undergo Structural Transition in Response to Weakly Acidic pH and Their Cytoplasmic Delivery Capability

Dendron lipids designed to consist of amine-terminated polyamidoamine G1 dendron and two octadecyl chains were used for the preparation of pH-responsive molecular assemblies having phase structures that are changed through their dynamic molecular shape. The dendron lipid contains two primary amines and two tertiary amines in the dendron moiety, changing its charged state in the pH region between pH 10 and pH 4. The assemblies were shown to take a vesicle structure at neutral and alkaline pHs, but their structure changed to a micelle-like structure below pH 6.4. Because this pH region corresponds to one in which tertiary amines of the dendron lipid became protonated, protonation of tertiary amines in addition to primary amines in the dendron moiety might affect its dynamic molecular shape, resulting in a sharp pH response of the assemblies. The assemblies tended to form aggregates when taking on a vesicle form with a gel phase, but incorporation of a poly(ethylene glycol)-lipid provided dendron lipid vesicles with both sharp pH response and high colloidal stability. The poly(ethylene glycol)-incorporated dendron lipid vesicles tightly retained ovalbumin molecules in their internal aqueous space but released them almost completely at pH 6.0. In addition, the vesicles were shown to achieve efficient ovalbumin delivery into cytosol of DC2.4 cells (mouse dendritic cell line) after internalization through endocytosis.

Temperature-dependent drug release from DPPC:C12H25-PNIPAM-COOH liposomes: control of the drug loading/release by modulation of the nanocarriers' components

Novel polymer-modified thermosensitive liposomes were developed for the delivery of indomethacin in order to control its release profile. When attached to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes, the end functionalized C12H25-poly(N-isopropylacrylamide)-COOH (C12H25-PNIPAM-COOH) polymer was membrane-disruptive in a temperature-dependent manner. The interest for this polymer is driven by its famous lower critical solution temperature (LCST) behavior, where heating an aqueous solution of PNIPAM above 32°C induces nanophase separation and polymer chain aggregation. The physicochemical/structural behavior of these polymer-modified thermosensitive liposomes was found to depend on the PNIPAM:lipid molar ratio and the composition of the polymeric guest. The incorporation of PNIPAM has caused alterations in the thermotropic behavior of DPPC liposomes, as the differential scanning calorimetry (DSC) experiments revealed. The drug loading and the release were found to be strongly dependent on the thermotropic characteristics of the PNIPAM grafted DPPC liposomes. Namely, the in vitro release is immediate at 37°C (>LCST) ("burst" effect), while the prepared mixed nanocarriers did not release the encapsulated bioactive substance at <32°C (<LCST). Thus, the thermosensitivity and the drug loading/release properties of the prepared formulations can be modulated by varying the ratio of DPPC/PNIPAM components, as well as the molecular characteristics of the polymeric guest.

Thermoresponse improvement of poly(N-isopropylacrylamide) hydrogels via formation of poly(sodium p-styrenesulfonate) nanophases

The block copolymer networks composed of poly(N-isopropylacrylamide) (PNIPAM) and poly(sodium p-styrenesulfonate) were synthesized via sequential reversible addition-fragmentation chain transfer (RAFT) polymerization with α,ω-didithiobenzoate-terminated poly(sodium p-styrenesulfonate) (PSSNa) as the macromolecular chain transfer agent. It was found that the block copolymer networks were microphase-separated as evidenced by means of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). In the block copolymer networks, spherical or cylindrical PSSNa microdomains were finely dispersed into continuous PNIPAM matrixes. In comparison with unmodified PNIPAM hydrogel, the nanostructured hydrogels displayed improved thermoresponsive properties. In addition, the swelling ratios of the PSSNa-modified PNIPAM hydrogels were significantly higher than that of plain PNIPAM hydrogel. The improvement of thermoresponse was attributable to the formation of the PSSNa nanophases, which promoted the transportation of water molecules in the cross-linked networks.

Materials Science and Engineering of the Low Temperature Sensitive Liposome (LTSL): Composition-Structure-Property Relationships That Underlie its Design and Performance

This chapter presents the material science and materials engineering concepts that went into the design and testing of the Low Temperature-Sensitive Liposome (LTSL), including: the roles of each of the components that make up the composite membrane; how the molecular and nanostructures that they form might influence the already anomalous permeability at the phase transition of the bilayer; and how this thermally sensitive “Smart Drug Delivery System” leads to ultrafast release of a loaded doxorubicin drug, triggered and controlled in the micro-vasculature of tumors by applied mild hyperthermia. This formulation approach, as ThermoDox®, has been used in a completed 700-patient Phase III human clinical trial in liver cancer (HEAT study), is in a Phase II trial in chest wall recurrence of cancer (DIGNITY study) and has been used in a Phase I trial of patients with colorectal liver metastases (ABLATE study). With additional research and preclinical studies underway, and a range of other drugs, imaging agents and biological modifiers poised for encapsulation, the LTSL could provide a new paradigm for drug and agent delivery for the treatment of localized tumors: rapid triggered drug release in the tumor bloodstream and deep penetration of drug into the tumor tissue.

Nanoscale Drug Delivery and Hyperthermia: The Materials Design and Preclinical and Clinical Testing of Low Temperature-Sensitive Liposomes Used in Combination with Mild Hyperthermia in the Treatment of Local Cancer

The overall objective of liposomal drug delivery is to selectively target drug delivery to diseased tissue, while minimizing drug delivery to critical normal tissues. The purpose of this review is to provide an overview of temperature-sensitive liposomes in general and the Low Temperature-Sensitive Liposome (LTSL) in particular. We give a brief description of the material design of LTSL and highlight the likely mechanism behind temperature-triggered drug release. A complete review of the progress and results of the latest preclinical and clinical studies that demonstrate enhanced drug delivery with the combined treatment of hyperthermia and liposomes is provided as well as a clinical perspective on cancers that would benefit from hyperthermia as an adjuvant treatment for temperature-triggered chemotherapeutics. This review discusses the ideas, goals, and processes behind temperature-sensitive liposome development in the laboratory to the current use in preclinical and clinical settings.

Chapter 7 - Preparation of complexes of liposomes with gold nanoparticles

Liposomes have been widely used as drug carriers. Visible liposomes have recently become more attractive as drug carriers in personalized medicine. Gold nanoparticles (Au NPs) have unique size- and shape-dependent properties based on their surface plasmon resonance. They can be visualized by computed tomography (CT) and laser optoacoustic imaging. In addition, their photothermogenic properties are useful for photothermal therapy and photoresponsive drug release from liposomes. Therefore, complexation of liposomes with Au NPs is of considerable interest. There are three types of complex: Liposomes containing Au NPs in the inner phase, liposomes with Au NPs at the lipid membrane, and liposomes modified with Au NPs on the surface. This chapter focuses on the preparation and characterization of the third type of complex that is prepared by direct mixing of a Au NP dispersion with a liposome suspension.

Flexible macromolecules attached to lipid bilayers: impact on fluidity, curvature, permeability and stability of the membranes

This review summarizes recent investigations on the association of macromolecules on lipid bilayers. Hydrophilic and flexible polymers can form soft coronae tenuously adsorbed or anchored on the lipid membrane. Other synthetic macromolecules are embedded in the apolar region of the membrane. Recent experimental and theoretical works focus on the perturbation of lipid properties achieved depending on the nature and strength of binding. Of importance to biomimicry, to tethered model membranes, and drug carriers, the effects achievable include modulation of the lateral diffusivity of lipids, shape distortions, lateral segregations, formation of well-defined nanopores and ultimately the stimuli responsive disruption of the membrane.

Temperature-Responsive Phase Transition of Polymer Vesicles: Real-Time Morphology Observation and Molecular Mechanism

Novel thermosensitive polymer vesicles with controlled temperature-responsive phase transition at the lower critical solution temperature (LCST) varying from 8 to 81 degrees C were prepared via self-assembly of amphiphilic hyperbranched star copolymers having a hydrophobic hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HBPO) core and many hydrophilic polyethylene oxide (PEO) arms. Real-time optical microscopic observation revealed that the polymer vesicles have undergone sequential morphology changes including enrichment, aggregation, fusion, and vesicle-to-membrane transformation near the LCST. Molecular-level investigation indicates that the LCST transition results from the decreasing water solubility of the polymer vesicles with increasing temperature based on the partial dehydration of the PEO vesicle corona. On the basis of these results, a LCST transition mechanism, in view of the molecular configuration, balance of hydrophilic and hydrophobic moieties, and the vesicle morphology transformations, was proposed. As far as we know, the work presented here is the first demonstration of thermosensitive vesicles based on PEO, and the finding may be useful to design the thermosensitive core-shell structures by introducing the PEO segments.

Hyperthermia-Induced Antitumor Activity of Thermosensitive Polymer Modified Temperature-Sensitive Liposomes

Temperature-sensitive liposomes (TS-liposomes) have been studied for chemotherapeutic purposes to enhance the release of anticancer drugs at tumor sites. In this study, we prepared poly(N-isopropylacrylamide-co-acrylamide) (PNIPAM-AAM) and polyethylene glycol (PEG)-modified TS-liposomes (PETS-liposomes). PETS-liposomes significantly increased in vitro drug release in serum compared with PEG-fixed or PNIPAM-AAM-modified liposomes. Furthermore, incorporation of both PNIPAM-AAM and PEG into PETS-liposomes enhanced the stabilities of liposomes in serum by inhibiting protein adsorption. In addition, to investigate the therapeutic efficacy of doxorubicin (DOX)-loaded PETS-liposomes, the in vivo antitumor activity of liposomes in combination with hyperthermia was evaluated in a B16F10 melanoma tumor-bearing mouse model. PETS-liposomes showed much higher levels of tumor growth inhibition than PEG-fixed or PNIPAM-AAM-modified TS-liposomes. Moreover, the antitumor activity of PETS-liposomes was enhanced significantly when they were administered in combination with hyperthermia. PETS-liposomes were found to be highly efficacious carriers for the in vivo delivery of anticancer drugs, and to have potential anticancer applications in combination with hyperthermia.

Study of pH-sensitive copolymer/phospholipid complexes using the langmuir balance technique: effect of anchoring sequence and copolymer molecular weight

The behavior of three copolymers of N-isopropylacrylamide (NIPAM), methacrylic acid (MAA), and hydrophobic moiety was studied at phospholipid monolayer/subphase interfaces. The hydrophobic moieties, N-terminal dioctadecylamine (DODA) and random octadecylacrylate (ODA), were used as anchoring groups. The interactions between a 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC) monolayer and the copolymers were studied using the Langmuir balance technique. The effect of subphase pH, distribution of anchors along the copolymer chain, and copolymer molecular weight on the nature of the interactions between the copolymer chains and the DSPC monolayer were investigated. A first-order kinetics model was used to analyze the copolymers adsorption at the DSPC monolayer/subphase interface and allowed the interaction area between the copolymer chains and the DSPC monolayer, A(x), to be determined. The interaction area appears to depend on the subphase pH and the copolymer molecular weight. On decreasing pH, the interaction area of high molecular weight copolymers increases significantly; this is consistent with the copolymer chain phase transition from an extended coil to a collapsed globule while pH is lowered. In the latter conformation, strong hydrophobic attractive interactions between the copolymer chains and the hydrophobic part of the DSPC monolayer favor the copolymer intercalation, which could eventually provoke the phospholipidic layer destabilization or rupture.

Study of molecular interactions between a phospholipidic layer and a pH-sensitive polymer using the Langmuir balance technique

Molecular interactions between a terminally alkylated pH-sensitive N-isopropylacrylamide copolymer DODA-poly(NIPAM-co-MAA) and a monolayer of distearoylphosphatidylcholine (DSPC) at the air/water interface are investigated using the Langmuir balance technique. The compression isotherms ofthe copolymer monolayer at the air-water interface confirm that the copolymer undergoes a structural transition with a change in pH ranging from an extended coil state at neutral pH to a collapsed globular state at a pH corresponding to the pH of the polymer phase transition. Adsorption kinetics of DODA-poly(NIPAM-co-MAA) in the DSPC monolayer is analyzed using a first-order kinetics model allowing an effective interaction area Ax between DSPC and DODA-poly(NIPAM-co-MAA) molecules to be evaluated. The results clearly indicate that the interaction area increases with a decrease in pH. The results also suggest that the penetration of the DODA-poly(NIPAM-co-MAA) within the phospholipid monolayer is enhanced by a decrease in pH which causes a change in the copolymer structure and an increase in specific attractive interactions between the copolymer and the phospholipid. Therefore, the copolymer can trigger the destabilization or rupture of the phospholipidic layer through a simple variation in its structure associated with a variation in molecular interactions when coupled or inserted within the membrane. This study greatly supports the prospects of the copolymer-functionalized liposomes as stable and tunable carrier systems for in vivo applications in drug delivery.

Structure of artificial cytoskeleton containing liposomes in aqueous solution studied by static and dynamic light scattering

The structure of three types of liposomes (egg yolk phosphatidylcholine (EPC) without modification and EPC vesicles containing cross-linked N-isopropylacrylamide (NIPAM) networks of low and a high concentration inside the vesicles) were analyzed by static and dynamic light scattering. Upon polymerization the network was assumed to become attached to the membrane by reactive anchoring monomers. For the sample of high poly(NIPAM) content the polymer network was assumed to fill the whole space in the vesicles. The issue of the present study was to examine hard and hollow sphere behavior of the liposomes with networks of high and low poly(NIPAM) content. The theoretical scattering curves differ markedly for uniform hard and uniform hollow spheres by the presence of specific peaks. However, polydispersity washed out the peaks and led to smoothed asymptotes with fractal dimensions of df = 2 for hollow and df = 4 for hard spheres. The experimental data could efficiently be fitted with weakly polydisperse hollow spheres. No clear conclusion could be drawn from the angular dependence alone for the liposome of high poly(NIPAM) content. The two wavelengths from the HeNe and Ar lasers proved to be too long for the studied liposomes of about 100 nm in radius. However, evidence for hollow sphere behavior was found for fractionated liposomes from the ratio rho = Rg/Rh = 1.04 +/- 0.02 (theory rho = 1.00 for hollow spheres). Finally, from the molar mass and the sphere radius, an apparent density was determined. The analysis gave the expected density for the pure EPC lecithin vesicles and a poly(NIPAM) network density of 0.244 g/mL. For the liposome of low poly(NIPAM) content the network appeared to be attached to the inner surface of the lecithin shell to form a layer of about 18 nm thickness.

Mimicking a cytoskeleton by coupling poly(N-isopropylacrylamide) to the inner leaflet of liposomal membranes: effects of photopolymerization on vesicle shape and polymer architecture

Networks of N-isopropylacrylamide (NIPAM) copolymers, coupled to spherical phospholipid bilayers, are suitable as a model for the study of the interaction between the cytoskeleton and cellular membranes, as well as for promising new drug delivery systems with triggerable drug release properties and improved stability. In this article, we describe a simple preparation technique for liposomes from egg phosphatidyl choline (EPC) encapsulating a cross-linked NIPAMminus signTEGDM copolymer skeleton (tetraethylene glycol dimethacrylate, TEGDM) which is coupled only to the inner monolayer by a novel membrane anchor monomer. Polymerization in the lipid vesicles was initiated at the inner membrane surface by the radical initiator 2,2-diethoxy-acetophenone (DEAP) permeating through the membrane from the outside. The effects of photopolymerization and polymer formation on vesicle shape and membrane integrity were studied by transmission electron microscopy (TEM), cryo-TEM, and atomic force microscopy (AFM). Upon UV irradiation, approximately 100% of the vesicles contained a polymer gel and only occasional changes in the spherical shape of the liposomes were observed. The architecture of the polymer network inside the liposomal compartment was determined by the conditions of the photopolymerization. Composite structures of polymer hollow spheres or solid spheres, respectively, tethered to spherical membrane vesicles were produced. The increased stability of the polymer-tethered lipid bilayers against solubilization by sodium cholate, compared to pure EPC vesicles, was determined by radiolabeling the lipid membrane.

Thermosensitive polymer-modified liposomes

Temperature-sensitive liposomes are considered to be a promising tool to achieve site-specific delivery of drugs. These liposomes have been prepared using lipids whose membranes undergo a gel-to-liquid crystalline phase transition a few degrees above physiological temperature. However, recently, temperature-sensitization of liposomes has been attempted using thermosensitive polymers. So far, functional liposomes whose contents release behavior, surface properties, and affinity to cell surface can be controlled in a temperature-dependent manner, have been developed according to this strategy. The design and function of these thermosensitive polymer-modified liposomes have been outlined in this review.

In vitro characterization of a novel polymeric-based pH-sensitive liposome system

This study demonstrates rapid and pH-sensitive release of a highly water-soluble fluorescent aqueous content marker, pyranine, from egg phosphatidylcholine liposomes following incorporation of N-isopropylacrylamide (NIPA) copolymers in liposomal membranes. The pH-sensitivity of this system correlates with the precipitation of the copolymers at acidic pH. In vitro release can be significantly improved by increasing the percentage of anchor in the copolymer and thus favoring its binding to the liposomal bilayer. In the case of liposomes containing a poly(ethylene glycol)-phospholipid conjugate, the insertion of the pH-sensitive copolymer in the liposomal membrane appears to be sterically inhibited. Dye release from these formulations at acidic pH can still be achieved by varying the anchor molar ratio and/or molecular mass of the polymers or by including the latter during the liposome preparation procedure. Removal of unbound polymer results in decreased leakage only when the copolymer is inserted by incubation with preformed liposomes, but can be overcome by preparing liposomes in the presence of polymer. Aqueous content and lipid mixing assays suggest contents release can occur without membrane fusion. The results of this study indicate that the addition of pH-sensitive copolymers of NIPA represents promising strategy for improving liposomal drug delivery.

Temperature-controlled interaction of thermosensitive polymer-modified cationic liposomes with negatively charged phospholipid membranes

To obtain cationic liposomes of which affinity to negatively charged membranes can be controlled by temperature, cationic liposomes consisting of 3beta-[N-(N', N'-dimethylaminoethane)carbamoyl]cholesterol and dioleoylphosphatidylethanolamine were modified with poly(N-acryloylpyrrolidine), which is a thermosensitive polymer exhibiting a lower critical solution temperature (LCST) at ca. 52 degrees C. The unmodified cationic liposomes did not change its zeta potential between 20-60 degrees C. The polymer-modified cationic liposomes revealed much lower zeta potential values below the LCST of the polymer than the unmodified cationic liposomes. However, their zeta potential increased significantly above this temperature. The unmodified cationic liposomes formed aggregates and fused intensively with anionic liposomes consisting of egg yolk phosphatidylcholine and phosphatidic acid in the region of 20-60 degrees C, due to the electrostatic interaction. In contrast, aggregation and fusion of the polymer-modified cationic liposomes with the anionic liposomes were strongly suppressed below the LCST. However, these interactions were enhanced remarkably above the LCST. In addition, the polymer-modified cationic liposomes did not cause leakage of calcein from the anionic liposomes below the LCST, but promoted the leakage above this temperature as the unmodified cationic liposomes did. Temperature-induced conformational change of the polymer chains from a hydrated coil to a dehydrated globule might affect the affinity of the polymer-modified cationic liposomes to the anionic liposomes.

Temperature-dependent interaction of thermo-sensitive polymer-modified liposomes with CV1 cells

Egg yolk phosphatidylcholine liposomes modified with a copolymer of N-acryloylpyrrolidine and N-isopropylacrylamide having a lower critical solution temperature at ca. 40 degrees C were prepared and an effect of temperature on their interaction with CV1 cells was investigated. The unmodified liposomes were taken up by the cells approximately to the same extent after 3 h incubation at 37 and 42 degrees C. In contrast, uptake of the polymer-modified liposomes by CV1 cells decreased slightly at 37 degrees C but increased greatly at 42 degrees C, compared to the unmodified liposomes. Proliferation of the cells was partly prohibited by the incubation with the unmodified liposomes encapsulating methotrexate at 37 and 42 degrees C. The treatment with the polymer-modified liposomes containing methotrexate at 37 degrees C hardly effected the cell growth. However, the treatment at 42 degrees C inhibited the cell growth completely. It is considered that the highly hydrated polymer chains attached to the liposome surface suppressed the liposome-cell interaction below the lower critical solution temperature of the polymer but the dehydrated polymer chains enhanced the interaction above this temperature. Because interaction of the polymer-modified liposomes with cells can be controlled by the ambient temperature, these liposomes may have potential usefulness as efficient site-specific drug delivery systems.

Improvement of temperature-sensitivity of poly(N-isopropylacrylamide)-modified liposomes

As a novel temperature-sensitive drug delivery system, we have designed liposomes coated with poly(N-isopropylacrylamide), which exhibits a coil-globule transition at 32-35 degrees C. In a previous study [ K. Kono, H. Hayashi, T. Takagishi, J. Control. Release 30 (1994) 69-75], it was shown that the release of contents from the polymer-coated liposomes is enhanced above the transition temperature of the polymer but the release is not controlled completely by the conformational change of the polymer. In this study, to improve temperature-sensitivity of the poly(N-isopropylacrylamide)-modified liposomes, the influence of lipid composition on the temperature-sensitivity of the liposomes was investigated. A copolymer of N-isopropylacrylamide and N, N-didodecylacrylamide was synthesized by free radical copolymerization. While the copolymer was insoluble in water, it exhibited a coil-globule transition around 28 degrees C when incorporated into an egg yolk phosphatidylcholine membrane. The copolymer-modified egg yolk phosphatidylcholine/dioleoylphosphatidylethanolamine liposomes were prepared by sonication or reverse phase evaporation. Release of calcein from the copolymer-modified liposomes was very slow below the transition temperature of the copolymer, whereas the release was enhanced above the transition temperature. Incorporation of dioleoylphosphatidylethanolamine into liposome membranes enhanced release greatly above the transition temperature of the copolymer. Fluorometric measurements using 8-anilino-1-naphthalenesulfonate and 1,6-diphenyl-1,3,5-hexatriene suggested that the copolymer decreases membrane fluidity of the liposomes near and above the transition temperature of the copolymer and that interaction between the copolymer and the liposome membranes is enhanced by inclusion of dioleoylphosphatidylethanolamine.

Thermosensitive polymer-modified liposomes that release contents around physiological temperature

To obtain temperature-sensitive liposomes which release their contents around the physiological temperature, we designed dioleoylphosphatidylethanolamine liposomes modified with copolymers of N-isopropylacrylamide and acryloylpyrrolidine. Copolymers of acryloylpyrrolidine and N-isopropylacrylamide, which exhibit a lower critical solution temperature around the physiological temperature, were prepared by free radical copolymerization using azobis(isobutyronitrile) as the initiator. The copolymers with anchors to the liposome membrane were obtained by using N, N-didodecylacrylamide as an additional comonomer. The copolymer having the anchor group at the terminal of the polymer chain was also synthesized by copolymerization of these monomers in the presence of 2-aminoethanethiol and subsequent conjugation of N, N-didodecyl succinamic acid to the terminal amino group of the copolymer. Calcein-loaded dioleoylphosphatidylethanolamine liposomes modified with these copolymers were prepared and release of the contents from these liposomes was investigated. It was found that the release from these copolymer-modified liposomes was promoted around and above the lower critical temperature of the copolymer. Also, the liposomes modified with the terminal anchor-type copolymer released the contents more drastically responding to a small temperature change than the liposomes modified with random copolymers containing N,N-didodecylacrylamide units as the anchor.