Small, but not large, unilamellar liposomes composed of dioleoylphosphatidylethanolamine and oleic acid can be stabilized by human plasma

pH-sensitive niosomes for ATRA delivery: A promising approach to inhibit Pin1 in high-grade serous ovarian cancer

The peptidyl-prolyl cis/trans isomerase Pin1 positively regulates numerous cancer-driving pathways, and it is overexpressed in several malignancies, including high-grade serous ovarian cancer (HGSOC). The findings that all-trans retinoic acid (ATRA) induces Pin1 degradation strongly support that ATRA treatment might be a promising approach for HGSOC targeted therapy. Nevertheless, repurposing ATRA into the clinics for the treatment of solid tumors remains an unmet need mainly due to the insurgence of resistance and its ineffective delivery. In the present study, niosomes have been employed for improving ATRA delivery in HGSOC cell lines. Characterization of niosomes including hydrodynamic diameter, ζ-potential, morphology, entrapment efficiency and stability over time and in culture media was performed. Furthermore, pH-sensitiveness and ATRA release profile were investigated to demonstrate the capability of these vesicles to release ATRA in a stimuli-responsive manner. Obtained results documented a nanometric and monodispersed samples with negative ζ-potential. ATRA was efficiently entrapped, and a substantial release was observed in the presence of acidic pH (pH 5.5). Finally, unloaded niosomes showed good biocompatibility while ATRA-loaded niosomes significantly increased ATRA Pin1 inhibitory activity, which was consistent with cell growth inhibition. Taken together, ATRA-loaded niosomes might represent an appealing therapeutic strategy for HGSOC therapy.

Biomimetic nano-surfactant stabilizes sub-50 nanometer phospholipid particles enabling high paclitaxel payload and deep tumor penetration

Sub-50 nm nanoparticles feature long circulation and deep tumor penetration. However, at high volume fractions needed for intravenous injection, safe, highly biocompatible phospholipids cannot form such nanoparticles due to the fluidity of phospholipid shells. Here we overcome this challenge using a nano-surfactant, a sterilized 18-amino-acid biomimetic of the amphipathic helical motif abundant in HDL-apolipoproteins. As it induces a nanoscale phase (glass) transition in the phospholipid monolayer, the peptide stabilizes 5-7 nm phospholipid micelles that do not fuse at high concentrations but aggregate into stable micellesomes exhibiting size-dependent penetration into tumors. In mice bearing human Her-2-positive breast cancer xenografts, high-payload paclitaxel encapsulated in 25 nm (diameter) micellesomes kills more cancer cells than paclitaxel in standard clinical formulation, as evidenced by the enhanced apparent diffusion coefficient of water determined by in vivo MR imaging. Importantly, the bio-inertness of this biomimetic nano-surfactant spares the nanoparticles from being absorbed by liver hepatocytes, making them more generally available for drug delivery.

Lipid-Based Drug Delivery Systems in Cancer Therapy: What Is Available and What Is Yet to Come

Cancer is a leading cause of death in many countries around the world. However, the efficacy of current standard treatments for a variety of cancers is suboptimal. First, most cancer treatments lack specificity, meaning that these treatments affect both cancer cells and their normal counterparts. Second, many anticancer agents are highly toxic, and thus, limit their use in treatment. Third, a number of cytotoxic chemotherapeutics are highly hydrophobic, which limits their utility in cancer therapy. Finally, many chemotherapeutic agents exhibit short half-lives that curtail their efficacy. As a result of these deficiencies, many current treatments lead to side effects, noncompliance, and patient inconvenience due to difficulties in administration. However, the application of nanotechnology has led to the development of effective nanosized drug delivery systems known commonly as nanoparticles. Among these delivery systems, lipid-based nanoparticles, particularly liposomes, have shown to be quite effective at exhibiting the ability to: 1) improve the selectivity of cancer chemotherapeutic agents; 2) lower the cytotoxicity of anticancer drugs to normal tissues, and thus, reduce their toxic side effects; 3) increase the solubility of hydrophobic drugs; and 4) offer a prolonged and controlled release of agents. This review will discuss the current state of lipid-based nanoparticle research, including the development of liposomes for cancer therapy, different strategies for tumor targeting, liposomal formulation of various anticancer drugs that are commercially available, recent progress in liposome technology for the treatment of cancer, and the next generation of lipid-based nanoparticles.

Controlling subcellular delivery to optimize therapeutic effect

This article focuses on drug targeting to specific cellular organelles for therapeutic purposes. Drugs can be delivered to all major organelles of the cell (cytosol, endosome/lysosome, nucleus, nucleolus, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes and proteasomes) where they exert specific effects in those particular subcellular compartments. Delivery can be achieved by chemical (e.g., polymeric) or biological (e.g., signal sequences) means. Unidirectional targeting to individual organelles has proven to be immensely successful for drug therapy. Newer technologies that accommodate multiple signals (e.g., protein switch and virus-like delivery systems) mimic nature and allow for a more sophisticated approach to drug delivery. Harnessing different methods of targeting multiple organelles in a cell will lead to better drug delivery and improvements in disease therapy.

Properties, engineering and applications of lipid-based nanoparticle drug-delivery systems: current research and advances

Lipid-based drug-delivery systems have evolved from micro- to nano-scale, enhancing the efficacy and therapeutic applications of these delivery systems. Production of lipid-based pharmaceutical nanoparticles is categorized into top-down (fragmentation of particulate material to reduce its average total dimensions) and bottom-up (amalgamation of molecules through chemical interactions creating particles of greater size) production methods. Selection of the appropriate method depends on the physiochemical properties of individual entities within the nanoparticles. The production method also influences the type of nanoparticle formulations being produced. Liposomal formulations and solid-core micelles are the most widely utilized lipid-based nanoparticles, with surface modifications improving their therapeutic outcomes through the production of long-circulating, tissue-targeted and/or pH-sensitive nanoparticles. More recently, solid lipid nanoparticles have been engineered to reduce toxicity toward mammalian cells, while multifunctional lipid-based nanoparticles (i.e., hybrid lipid nanoparticles) have been formulated to simultaneously perform therapeutic and diagnostic functions. This article will discuss novel lipid-based drug-delivery systems, outlining the properties and applications of lipid-based nanoparticles alongside their methods of production. In addition, a comparison between generations of the lipid-based nano-formulations is examined, providing insight into the current directions of lipid-based nanoparticle drug-delivery systems.

Electroporation optimization to deliver plasmid DNA into dental follicle cells

Electroporation is a simple and versatile approach for DNA transfer but needs to be optimized for specific cells. We conducted square wave electroporation experiments for rat dental follicle cells under various conditions. These experiments indicated that the optimal electroporation electric field strength was 375 V/cm, and that plasmid concentrations greater than 0.18 microg/microL were required to achieve high transfection efficiency. BSA or fetal bovine serum in the pulsing buffer significantly improved cell survival and increased the number of transfected cells. The optimal pulsing duration was in the range of 45-120 ms at 375 V/cm. This electroporation protocol can be used to deliver DNA into dental follicle cells to study the roles of candidate genes in regulating tooth eruption. This is the first report showing the transfection of dental follicle cells using electroporation. The parameters determined in this study are likely to be applied to transfection of other fibroblast cells.

Biodegradable nanoparticles for cytosolic delivery of therapeutics

Many therapeutics require efficient cytosolic delivery either because the receptors for those drugs are located in the cytosol or their site of action is an intracellular organelle that requires transport through the cytosolic compartment. To achieve efficient cytosolic delivery of therapeutics, different nanomaterials have been developed that consider the diverse physicochemical nature of therapeutics (macromolecule to small molecule; water soluble to water insoluble) and various membrane associated and intracellular barriers that these systems need to overcome to efficiently deliver and retain therapeutics in the cytoplasmic compartment. Our interest is in investigating PLGA and PLA-based nanoparticles for intracellular delivery of drugs and genes. The present review discusses the various aspects of our studies and emphasizes the need for understanding of the molecular mechanisms of intracellular trafficking of nanoparticles in order to develop an efficient cytosolic delivery system.

pH-sensitive liposomes--principle and application in cancer therapy

The purpose of this review is to provide an insight into the different aspects of pH-sensitive liposomes. The review consists of 6 parts: the first introduces different types of medications made in liposomal drug delivery to overcome several drawbacks; the second elaborates the development of pH-sensitive liposomes; the third explains diverse mechanisms associated with the endocytosis and the cytosolic delivery of the drugs through pH-sensitive liposomes; the fourth describes the role and importance of pH-sensitive lipid dioleoylphosphatidylethanolamine (DOPE) and research carried on it; the fifth explains successful strategies used so far using the mechanism of pH sensitivity for fusogenic activity; the final part is a compilation of research that has played a significant role in emphasizing the success of pH-sensitive liposomes as an efficient drug delivery system in the treatment of malignant tumours. pH-Sensitive liposomes have been extensively studied in recent years as an amicable alternative to conventional liposomes in effectively targeting and accumulating anti-cancer drugs in tumours. This research suggests that pH-sensitive liposomes are more efficient in delivering anti-cancer drugs than conventional and long-circulating liposomes due to their fusogenic property. Research focused on the clinical and therapeutic side of pH-sensitive liposomes would enable their commercial utility in cancer treatment.

Inhibition of Epstein Barr Virus LMP1 gene expression in B lymphocytes by antisense oligonucleotides: uptake and efficacy of lipid-based and receptor-mediated delivery systems

Epstein Barr Virus (EBV), is associated with an increasing number of lymphoid and epithelial malignancies. Among the genes expressed by EBV during latency, LMP1 plays a key role for growth transformation and immortalization of B lymphocytes. We have previously shown that antisense oligonucleotides (ONs) directed to LMP1 mRNA, effectively suppressed LMP1 gene expression and substantially reduced proliferation of the infected cells. The use of antisense phosphodiester oligonucleotides as therapeutic agents is limited by inefficient cellular uptake and intracellular transport to the target mRNA. We tested the ability of three cationic carriers internalized by different pathways, to increase the delivery of anti-LMP1-ON to their site of action in EBV-infected B lymphocytes. We report here that liposomes, dendrimers or transferrin-polylysine-conjugated ON were internalized by the cells at an extent several fold higher than that of the naked oligomers. However, only the delivery system exploiting the transferrin receptor pathway of internalization, was able to vectorize biologically active antisense LMP1-ON.

Advances in the use of tocols as drug delivery vehicles

There has been increasing interest in recent years in the drug delivery applications of tocols and their derivatives. Their biocompatibility and potential to deliver both poorly soluble and water-soluble drugs make tocols attractive as drug delivery vehicles. This review article will focus primarily on topical, oral, and parenteral drug administration using tocols, although other routes of delivery such as pulmonary and nasal will also be discussed. After an overview of the tocol structures, physicochemical properties with emphasis on their solvent properties, functions, and metabolism, specific case studies will be discussed where tocols have been successfully used in topical, oral, and parenteral drug formulations and marketed drug products. Case studies will be extended to those where tocol-based formulations were administered pulmonarily and nasally. As more clinical data and marketed drug products emerge, the utility and therapeutic value of tocols will certainly increase.

Tuning the MR properties of blood-stable pH-responsive paramagnetic liposomes

Paramagnetic pH-responsive liposomes have recently been suggested as a promising approach for monitoring by magnetic resonance imaging (MRI) pH changes in tumours. In the present study, the effects of variations in bilayer composition on the relaxometric properties of diacylphosphatidylethanolamine (PE)/dipalmitoylglycerosuccinate (DPSG) liposomal GdDTPA-BMA were investigated both in buffer and blood. A factorial experimental design was used with the variables PE chain length and mol% DPSG. All the relaxometric profiles displayed a semi-sigmoidal shape with a minimum plateau at high pH (r1(min)) and a maximum at low pH (r1(max,E)). Relevant sigmoidal curve fit parameters were evaluated by partial least squares regression. Systematic variations in the relaxometric response (r1(max,E)-r1(min)) were shown for the liposomal systems both in buffer and blood. The pH value at which the r1 was 20% of r1(max,E) relative to r1(min), i.e. pH20, decreased significantly both in buffer and blood as a function of the mol% DPSG. This phenomenon could be understood by the increased surface charge density with increasing mol% DPSG and, hence, higher barrier against liposome aggregation with consequent leakage of contrast agent. Furthermore, the pH relaxometric profiles in blood were shifted laterally to higher, and likely more clinically relevant pH values than the corresponding profiles in buffer. The liposome formulations displayed minimal leakage of contrast agent after prolonged incubation in blood at physiological pH and retained their pH sensitivity after pre-incubation in blood.

Innovations in oligonucleotide drug delivery

Oligonucleotides (ONs) are a new class of therapeutic compounds under investigation for the treatment of a variety of disease states, such as cancer and HIV, and for FDA approval of an anti-CMV retinitis antisense molecule (Vitravene trade mark, Isis Pharmaceuticals). However, these molecules are limited not only by poor cellular uptake, but also by a general lack of understanding regarding the mechanism(s) of ON cellular uptake. As a result, various delivery vehicles have been developed that circumvent the proposed mechanism of uptake, endocytosis, while improving target specific delivery and/or drug stability. This review describes various traditional and novel delivery mechanisms that have been employed to improve ON cellular delivery, cost effectiveness, and therapeutic efficacy.

pH-sensitive paramagnetic liposomes for MRI: assessment of stability in blood

The pH-dependent stability of dipalmitoyl phosphatidyl ethanolamine/palmitic acid (DPPE/PA) liposomal GdDTPA-BMA was investigated in human blood and after exposure to selected blood components. Relaxometry, visual observations and cryo-transmission electron microscopy (cryo-TEM) were employed for the assessment of stability. The liposomes were stable in buffer at physiological pH and the T(1)-relaxivity (r(1)) of the system was significantly lowered compared to that of non-liposomal GdDTPA-BMA, which could be explained by an exchange limited relaxation process. Lowering the pH, however, gave a marked increase in r(1), due to liposome aggregation and subsequent leakage of GdDTPA-BMA. After a few minutes incubation in human blood the liposomes were destabilised and leaky at both high and low pH, and blood components likely to cause the instability were studied. Physiological level of Na(+) (150 mM) did not affect the relaxometric behavior of the liposomes at pH 7.4, but shifted the pH-r(1) profile laterally to higher pH-values compared to a level of 50 mM Na(+). Increased screening of the surface charges and, concomitantly, a lowering of the energy-barrier against aggregation is a plausible explanation for this phenomenon. In contrast, both Ca(2+) and Mg(2+) (physiological level, both 2 mM) caused massive aggregation of the liposomes and leakage of contents, and were therefore much more detrimental to the stability of the liposomes than a physiological level of Na(+). This could be due to the higher screening ability of divalent cations, but aggregation could also be induced through an inter-liposomal "bridging" effect. Physiological level of both Na(+) and Ca(2+) caused less leakage than for lower Na(+) concentration (50 mM Na(+) and 2 mM Ca(2+)), probably due to competition for the negative surface charges. Albumin also destabilised the liposomes, and it was shown to be due to an interaction between albumin and PA in the liposomal membrane.

Stable adhesion of phospholipid vesicles to modified gold surfaces

Phospholipid vesicles are well-studied biomembrane mimics that are of increasing interest in drug delivery, immunoassays, and sensor chips. In a number of biosensor applications it is desirable to be able to adhere vesicles to a surface in a manner which does not result in their rupture or fusion. Such behavior should, in principle, be achievable by controlling the vesicle-surface and vesicle-vesicle interactions. We have varied vesicle composition and charge (phosphatidylcholine, phosphatidylcholine-phosphatidic acid 18 mol%) and solution ionic strength, to study the adhesion of fluorescent vesicles to glass, gold, and gold modified with chemisorbed acetyl-cysteine. The extent of chemisorption was characterized with angle-resolved X-ray photoelectron spectroscopy (ARXPS), and vesicle integrity and behavior was studied using entrapped and lipophilic fluorescent markers, together and in separate measurements. Vesicle fusion (by energy transfer), adhesion of intact vesicles (with entrapped calcein) and diffusion coefficients (by photobleaching recovery) were monitored using confocal fluorescence microscopy. Acetyl-cysteine modified gold surfaces were shown to be appropriate substrates for adhesion of intact vesicles. Finally, as a 'proof of principle' for fluorescence amplification, release of a self-quenching entrapped reporter dye (calcein) by the detergent Triton X-100 was followed in real time.

Alteration in the Temperature-Dependent Content Release Property of Thermosensitive Liposomes in Plasma

The effect of plasma components on the temperature-dependent content release property of thermosensitive liposomes has been described. Temperature-sensitive liposomes containing mitomycin C (MMC) were prepared from dipalmitoylphosphatidylcholine (DPPC liposomes) and a 7 : 3 mixture of DPPC and dipalmitoylophosphatidylglycerol (DPPC/DPPG liposomes). We defined in this study the difference in the content release between 38 degrees C and 44 degrees C as an index of the temperature-dependent content release efficiency (Delta% release). In the absence of rat plasma, the Delta% release of the DPPC liposomes and the DPPC/DPPG liposomes was 83% and 71%, respectively. However, when the release study was conducted with rat plasma, the Delta% release increased to about 96% for both liposomes. In addition, while the DPPC liposomes were destabilized by rat plasma below the gel-to-liquid crystalline phase transition temperature (T(m)), MMC leakage from the DPPC/DPPG liposomes below T(m) was suppressed by rat plasma. Moreover, the plasma protein binding onto lipid bilayer was concomitant with the gel-to-liquid crystalline phase transition and then enhanced the temperature-dependent release from the DPPC/DPPG liposomes. The possible mechanism of interaction between liposomes and plasma proteins, especially serum albumin, was discussed based on differential scanning calorimetry and protein binding experiments.

Polymer based pH-sensitive carriers as a means to improve the cytoplasmic delivery of drugs

pH-sensitive niosomal and liposomal formulations bearing alkylated N-isopropylacrylamide (NIPAM) copolymers were characterized with regard to vesicle-polymer interaction, pH-responsiveness and stability in human serum. The interactions between the pH-sensitive NIPAM copolymer and the vesicles were studied by spectrofluorimetry, using covalently-attached pyrene as a probe. In contrast to liposomes, where complexation of copolymer to the lipid bilayer is essentially mediated by hydrophobic interactions, the binding between niosomes and PNIPAM was mainly driven by hydrogen bonding. Both formulations were found to rapidly release their contents under mildly acidic conditions. However, the niosomes lost their pH-sensitivity after incubation in serum, whereas liposomes maintained their ability to respond to pH only when complexed with a copolymer containing a high proportion of hydrophobic anchor. The ability of pH-sensitive liposome/polymer complexes to enhance the cytotoxicity of cytosine arabinofuranoside (ara-C) was evaluated in vitro using macrophage-like J774 cells. Ara-C encapsulated in pH-sensitive liposomes exhibited a higher cytotoxicity than the control formulation. This study showed that both niosomes and liposomes can be rendered pH-sensitive by anchoring a randomly-alkylated NIPAM copolymer to their surface. The interactions that take place between the polymer and the vesicles strongly depend on the vesicle nature. pH-sensitive PNIPAM-based liposomes can improve the in vitro efficiency of ara-C.

A novel pH-sensitive liposome formulation containing oleyl alcohol

pH-sensitive liposomes are designed to undergo acid-triggered destabilization. First generation pH-sensitive liposomes, based on the cone-shaped lipid dioleoylphosphatidylethanolamine (DOPE), have been shown to lose fusogenicity in the presence of serum. Here, we report the design and evaluation of novel serum-resistant pH-sensitive liposome formulations that are based on the composition of egg phosphatidylcholine (PC), cholesteryl hemisuccinate (CHEMS), oleyl alcohol (OAlc), and Tween-80 (T-80). When loaded with the fluorescent probe calcein, these liposomes exhibited excellent stability at pH 7.4 and underwent rapid destabilization upon acidification as shown by calcein dequenching and particle size increase. Adjusting the mole percentages of T-80 and OAlc in the formulation could regulate the stability and pH-sensitive properties of these liposomes. Liposomes with a higher T-80 content exhibited greater stability but were less sensitive to acid-induced destabilization. Meanwhile, formulations with a higher OAlc content exhibited greater content release in response to low pH. The pH-triggered liposomal destabilization did not produce membrane fusion according to an octadecylrhodamine B chloride (R(18)) lipid-mixing assay. Compared to DOPE-based pH-sensitive liposomes, the above formulations showed much better retention of their pH-sensitive properties in the presence of 10% serum. These liposomes were then evaluated for intracellular delivery of entrapped cytosine-beta-D-arabinofuranoside (araC) in KB human oral cancer cells, which have elevated folate receptor (FR) expression. The FR, which is amplified in many types of human tumors, has been shown to mediate the internalization of folate-derivatized liposomes into an acidic intracellular compartment. FR-targeted OAlc-based pH-sensitive liposomes, entrapping 200 mM araC, showed approximately 17-times greater FR-dependent cytotoxicity in KB cells compared to araC delivered via FR-targeted non-pH-sensitive liposomes. These data indicated that pH-sensitive liposomes based on OAlc, combined with FR-mediated targeting, are promising delivery vehicles for membrane impermeable therapeutic agents.

Effect of poly(ethylene glycol) grafts on temperature-sensitivity of thermosensitive polymer-modified liposomes

A copolymer of N-isopropylacrylamide and N-acryloylpyrrolidine, which exhibits a lower critical solution temperature (LCST) at 38 degrees C, having a didodecyl group at the chain terminal was synthesized and calcein-loaded liposomes were prepared from a mixture of dioleoylphosphatidylethanolamine and the copolymer. While the contents release from the copolymer-modified liposomes was strongly accelerated around the LCST of the copolymer, some extent of the release was also observed below the LCST, probably because the partly-dehydrated copolymer chains interacted with the liposome membrane. When poly(ethylene glycol) with the number average molecular weight of 550 having a didodecyl group at the chain end was introduced into the copolymer-modified liposomes, the contents release from the copolymer-modified liposomes was effectively suppressed below the LCST but hardly affected above the LCST. As a result, the liposomes exhibited much sharper enhancement of the contents release in a narrow temperature region. The temperature-sensitive contents release property of the copolymer-modified liposomes became less significant in the presence of serum. However, inclusion of the anchor-bearing poly(ethylene glycol) derivative improved the contents release behavior of the copolymer-modified liposomes in the presence of serum.

pH-sensitive paramagnetic liposomes as MRI contrast agents: in vitro feasibility studies

A novel type of pH-sensitive paramagnetic contrast agent is introduced; a low molecular weight gadolinium (Gd) chelate (GdDTPA-BMA) encapsulated within pH-sensitive liposomes. The in vitro relaxometric properties of the liposomal Gd chelate were shown to be a function of the pH in the liposomal dispersion and the membrane composition. Only a minor pH-dependency of the T1 relaxivity (r1) was observed for liposomal GdDTPA-BMA composed of the unsaturated lipids dioleoyl phosphatidyl ethanolamine (DOPE) and oleic acid (OA). On the other hand, the r1 of GdDTPA-BMA encapsulated within saturated dipalmitoyl phosphatidyl ethanolamine/palmitic acid (DPPE/PA) liposomes demonstrated a strong pH-dependency. At physiological pH and above, the r1 of this system was significantly lowered compared to that of non-liposomal Gd chelate, which was explained by an exchange limited relaxation process. Lowering the pH below physiological value, however, gave a sharp and 6-7 fold increase in r1, due to liposome destabilisation and subsequent leakage of entrapped GdDTPA-BMA. The pH-sensitivity of the DPPE/PA liposome system was confirmed in an in vitro magnetic resonance imaging (MRI) phantom study.

N-isopropylacrylamide copolymers for the preparation of pH-sensitive liposomes and polymeric micelles

Hydrophobically-modified copolymers of N-isopropylacrylamide bearing a pH-sensitive moiety were investigated for the preparation of pH-responsive liposomes and polymeric micelles. The copolymers having the hydrophobic anchor randomly distributed within the polymeric chain were found to more efficiently destabilize egg phosphatidylcholine (EPC)/cholesterol liposomes than the alkyl terminated polymers. Release of both a highly-water soluble fluorescent contents marker, pyranine, and an amphipathic cytotoxic anti-cancer drug, doxorubicin, from copolymer-modified liposomes was shown to be dependent on pH, the concentration of copolymer, the presence of other polymers such as polyethylene glycol, and the method of preparation. Both polymers were able to partially stabilize EPC liposomes in human serum. These polymers were found to self-assemble to form micelles. The critical association concentration was low (9--34 mg/l) and influenced by the position of the alkyl chains. In phosphate buffered saline, the micelles had a bimodal size distribution with the predominant population having a mean diameter of 35 nm. The polymeric micelles were studied as a delivery system for the photosensitizer aluminum chloride phthalocyanine, (AlClPc), currently evaluated in photodynamic therapy. pH-Responsive polymeric micelles loaded with AlClPc were found to exhibit increased cytotoxicity against EMT-6 mouse mammary cells in vitro than the control Cremophor EL formulation.

Optimization of cationic lipid/DNA complexes for systemic gene transfer to tumor lesions

Intravenous (i.v.) administration of cationic lipid N-[( 1-(2-3-dioleyloxy)propyl)]-N-N-N-trimethylammonium chloride (DOTMA)-based transfection complexes in mice with subcutaneous squamous cell tumors yielded plasmid delivery and expression in tumor lesions. The efficiency of gene transfer in tumors was significantly lower than in the lung. This was consistent with low plasmid levels associated with the tumor, suggesting that plasmid delivery to the tumor site was a limiting factor. Lowering the lipid/DNA charge ratio from 5:1 to 0.8:1 (+/-) did not change DNA levels in tumor but significantly reduced DNA levels in lung. However, expression levels were significantly reduced in both tissues at lower lipid/DNA charge ratios. Complexes prepared from small unilamellar liposomes gave significantly lower expression levels in the lungs but similar expression levels in tumors when compared to complexes prepared from larger unilamellar liposomes. The small liposome complexes were better tolerated than large liposome complexes. Varying the cationic lipid to colipid (cholesterol or DOPE) molar ratio from 4: 1 to 1: 1 significantly reduced expression levels in both tumor and lung. Cationic lipid substitution, using a cholesterol cationic lipid, diethyldiamino-carbamyl-cholesterol instead of DOTMA, produced reduced expression in all other tissues except tumor. Incorporation of PEG into preformed transfection complexes reduced DNA delivery to lung, increased circulation half-life, and enhanced DNA delivery to tumor. In a lung metastatic mouse tumor model, where the accessibility of the i.v. administered transfection complexes to tumor lesions should be less challenging, DOTMA: CHOL complexes (4: 1 lipid to colipid molar ratio, 3: 1 +/- lipid to plasmid charge ratio) were preferentially localized in tumor lesions. These data demonstrate that systemic gene transfer to distal tumor sites by lipid/ DNA complexes may be limited by low plasmid delivery. Modifying the chemical surface properties of transfection complexes enhanced both DNA delivery and expression in tumor and is one approach that may overcome limitations.

Involvement of membrane surface charge in thermal stability of the rat ovarian LH/hCG receptor

Analysis of fluorescence of membrane-bound 8-anilino-1-naphthalene sulfonate and monodansylcadaverine probes revealed that a negative membrane surface charge derived from free fatty acids (FFA) resulted in destabilization of structure-functional properties of the rat ovarian LH/hCG receptor. Removal of FFA from rat luteal and porcine ovarian granulosa cells by BSA increased gonadotropin responsiveness of cells in cAMP formation.

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.

Biophysical studies and intracellular destabilization of pH-sensitive liposomes

We examined changes in membrane properties upon acidification of dioleoylphosphatidylethanolamine/cholesterylhemisuccinate liposomes and evaluated their potential to deliver entrapped tracers in cultured macrophages. Membrane permeability was determined by the release of entrapped calcein or hydroxypyrene-1,3,6-trisulfonic acid (HPTS)-p-xylene-bis-pyridinium bromide (DPX); membrane fusion, by measuring the change in size of the liposomes and the dequenching of octadecylrhodamine-B fluorescence; and change in lipid organization, by 31P nuclear magnetic resonance spectroscopy. Measurement of cell-associated fluorescence and confocal microscopy examination were made on cells incubated with liposomes loaded with HPTS or HPTS-DPX. The biophysical studies showed (i) a lipid reorganization from bilayer to hexagonal phase progressing from pH 8.0 to 5.0, (ii) a membrane permeabilization for pH <6.5, (iii) an increase in the mean diameter of liposomes for pH <6.0, and (iv) a mixing of liposome membranes for pH <5.7. The cellular studies showed (i) an uptake of the liposomes that were brought from pH 7.5-7.0 to 6.5-6.0 and (ii) a release of approximately 15% of the endocytosed marker associated with its partial release from the vesicles (diffuse localization). We conclude that the permeabilization and fusion of pH-sensitive liposomes occur as a consequence of a progressive lipid reorganization upon acidification. These changes may develop intracellularly after phagocytosis and allow for the release of the liposome content in endosomes associated with a redistribution in the cytosol.

Structure-stabilizing effect of albumin on rat ovarian LH/hCG receptors

The stabilizing effect of albumin on structure-functional alteration of LH/hCG receptors was analyzed by thermal perturbation technique. On exposing the membranes to bovine serum albumin (BSA) the heat inactivation profile of hCG-binding sites was shifted to a temperature higher by about 5 degreesC (T50 values). The receptor destabilizing action of arachidonic and oleic acids incorporated into ovarian membranes and reversal of this effect when BSA was used as fatty acid scavenger, may indicate that free fatty acids are responsible for the thermal instability of hCG-binding sites. This presumption was corroborated by digestion of membranes with phospholipase A2 (PLA2). This enzyme exerted effects on the thermal stability of the receptor protein resembling those observed upon insertion of fatty acids. The membrane fluidization induced by arachidonic acid can be reversed by BSA. However, alterations of lipid fluidity in membranes were not found to be a necessary prerequisite for stabilization of the LH/hCG receptor structure. Fluorescence quenching studies indicated that incorporation of oleic acid or digestion of membrane phospholipids with PLA2 elevated the accessibility of fluorophores for acrylamide. BSA scavenging of free fatty acids approached the quenching rate of control membranes. Analysis of fluorescence of membranes bound to monodansylcadaverine probe revealed that the negative surface charge derived from free fatty acids resulted in destabilization of the receptor protein. The effects of free fatty acids on membranes suggest that altered lipid-protein interactions may directly affect the stability of the LH/hCG receptor structure.

pH sensitivity and plasma stability of liposomes containing N-stearoylcysteamine

In this study, we investigated the pH sensitivity of different liposomal formulations containing 10 mol% N-stearoylcysteamine, as pH sensitive molecule. Liposome stability was monitored by determining the release of different entrapped water soluble molecules, 5,6-carboxyfluorescein (CF) being the marker of leakage mainly used. Small unilamellar vesicles composed of egg phosphatidylcholine (EPC) and N-stearoylcysteamine (9:1 molar ratio) incubated at 20 degrees C in citrate phosphate buffer released, at pH 6.8, 2.5 fold the amount of CF released at pH 7.4. The addition of plasma to the incubation medium and an increase of temperature to 37 degrees C led to significantly increased the CF release from EPC/N-stearoylcysteamine SUV, both at pH 7.4 and 6.8. The addition of cholesterol had a stabilizing effect on liposomal vesicles with respect to both temperature and plasma, without affecting pH sensitivity. In fact, at 37 degrees C and in 25% plasma the ternary mixture showed the highest CF release, as a consequence of the moderate acidification of the medium from 7.4 to 6.8. Thus, these liposome formulations are potentially a useful tool for specific drug delivery to pathological tissues such as tumours, inflammation sites and ischemic areas where it is known that a lowering of the pH can occur.

Sterically stabilized pH-sensitive liposomes. Intracellular delivery of aqueous contents and prolonged circulation in vivo

Liposomes that destabilize at mildly acidic pH are efficient tools for delivering water-soluble drugs into the cell cytoplasm. However, their use in vivo is limited because of their rapid uptake from circulation by the reticuloendothelial system. Lipid-anchored polyethylene glycol (PEG-PE) prolongs the circulation time of liposomes by steric stabilization. We have found that addition of PEG-PE to the membrane of pH-sensitive liposomes composed of cholesteryl hemisuccinate (CHEMS) and dioleoylphosphatidylethanolamine (DOPE) confers steric stability to these vesicles. This modification significantly decreases the pH-dependent release of a charged water-soluble fluorophore, calcein, from liposomes suspended in buffer or cell culture medium. However, the ability of such liposomes to release calcein intracellularly, measured by a novel flow cytometry technique involving dual fluorescence labeling, remains unaltered. As expected, the release of calcein from liposomes endocytosed by cells is inhibited upon pretreatment of the cells with NH4Cl, an inhibitor of endosome acidification. The unique properties of these liposomes were also demonstrated in vivo. The distribution kinetics of 111In-containing CHEMS/DOPE/PEG-PE liposomes injected intravenously into rats has pharmacokinetic parameters similar to control, non-pH-sensitive, sterically stabilized CHEMS/distearoylphosphatidylcholine/PEG-PE liposomes. In contrast, regular pH-sensitive liposomes lacking the PEG-PE component are cleared rapidly. Sterically stabilized pH-sensitive liposomes may therefore be useful for the intracellular delivery in vivo of highly negatively charged molecules such as genes, antisense oligonucleotides, and ribozymes for the treatment of various diseases.

Stability and pH sensitivity of sulfatide-containing phosphatidylethanolamine small unilamellar vesicles

The bilayer stabilization effect of sulfatide and the pH sensitivity of sulfatide-containing 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) small unilamellar vesicles were examined by light scattering and the release of entrapped calcein. At 30 mol% sulfatide, stable DOPE/sulfatide vesicles were formed at the physiological pH and their stability was preserved in the presence of human plasma. These vesicles were found to be pH-sensitive and became leaky at pH 6.0 or when there was a pH-gradient across the membrane bilayer. Under such conditions, the amount of calcein released after 24 h incubation at 37 degrees C was increased by one-fold compared to that found at pH 7.4. Our results suggest that the hydration and partial dehydration of the headgroup of sulfatide upon changing pH play an essential role in determining the pH sensitivity of DOPE/sulfatide vesicles, while the importance of the condensing effect of the glycolipid on membrane bilayer is less significant.

Tumor targeting in vivo by means of thermolabile fusogenic liposomes

Thermolabile fusogenic liposomes were devised based on the stoichiometric 1/2 mixtures of dipalmitoylphosphatidylcholine (DPPC) and elaidic acid (ELA) and from the similar stoichiometric mixtures of DPPC, dipalmitoylphosphatidylglycerol (DPPG) and elaidoyl alcohol (EL-OH) or palmitelaidoyl alcohol (PEL-OH). The resulting vesicle suspensions are fusogenic in the region of hyperthermia (> or = 42 degrees C) and can be targeted selectively to the heated tumor tissue. Incorporation of DPPG or fatty alcohols into the vesicle membranes also leads to a non-specific, temporary vesicle material accumulation in the lung, however, probably due to platelet activation. Vesicle material accumulation in A-431 tumors, xenotransplanted in nude mice, after 30 min of local hyperthermia (42 degrees C) is 4-fold higher for the DPPC/ELA (1/2), 2.8-fold higher for the DPPC/DPPG/EL-OH (0.8/0.2/2) and 3.7-fold higher for the DPPC/ELA/EL-OH (1/1/1) mixtures than for similar vesicles used at the physiological temperature. Extension of hyperthermia to 60 min induces a 7.8-fold relative material accumulation in the tumor tissue when the thermolabile, fusogenic DPPC/ELA/EL-OH (1/1/1) vesicles are used. Simple DPPC vesicles only reach concentrations in the heated tumor or muscle tissue that are 1.85-fold and 1.38-fold higher than in the normothermic control, respectively. This is probably a consequence of simple vasodilatation. In vitro experiments revealed that the adsorption of serum proteins to the vesicle membrane decreases the chain-melting phase transition temperature and the transition enthalpy of vesicle suspension. Adsorption is most prominent at the chain-melting phase transition temperature of the mixed lipid bilayers, which is also the critical temperature for the induction of liposome fusion. This hampers the practical use of the resulting vesicle suspension in vivo. The serum-induced decrease of the chain-melting phase transition temperature, which is likely to change as a function of time in vivo, depends on the lipid composition and on the local surface charge density of vesicles. Incorporation of ELA and DPPG concentrations above 15 mol-%, for example, reduce the extent of protein adsorption onto vesicles. This has to be borne in mind when devising vesicles for practical applications.

Thermosensitive sterically stabilized liposomes: formulation and in vitro studies on mechanism of doxorubicin release by bovine serum and human plasma

PURPOSE

To formulate thermosensitive sterically stabilized liposomes and to study the effects of plasma and serum components in vitro.

METHODS

The rate of release of encapsulated doxorubicin (Dox) from liposomes of various compositions was followed by fluorometric assay at 37 degrees, 42 degrees and 45 degrees C, in buffer and also in both calf serum and human plasma up to 50% by volume.

RESULTS

The optimal composition for the maximal differential release of doxorubicin between 37 degrees C and 42 degrees C in human plasma was a mixture of dipalmitoylphosphatidylcholine/hydrogenated soy phosphatidylcholine/cholesterol and distearoylphosphatidylethanolamine derivatized with polyethylene glycol at a molar ratio of 100:50:30:6. In experiments designed to study the mechanism causing increased permeability of liposomes in bovine serum, we found two different distinct release patterns: a slow linear rise of rate of Dox release for fluid liposomes and fast exponential rise reaching plateau within 5 minutes for solid phase (rigid) liposomes. This release of Dox from rigid but not fluid liposomes was inhibited by pre-heating serum at 55 degrees C for 30 minutes or by addition of EDTA (but not EGTA) or antiserum to the C3 component of complement.

CONCLUSIONS

A formulation of sterically stabilized liposomes with the proper thermal sensitivity in human plasma has been obtained. In addition, the results suggest that complement may play an important role in the interaction of rigid but not fluid liposomes with bovine serum. Human plasma did not show this effect.

N-palmitoylphosphatidylethanolamine stabilizes liposomes in the presence of human serum: effect of lipidic composition and system characterization

Liposomes containing negatively-charged phospholipid, N-palmitoylphosphatidylethanolamine (NPPE) were examined for stability in the presence of human serum, using the release of the entrapped 5,6-carboxyfluorescein as an aqueous marker. Either small unilamellar vesicles (SUV) or large unilamellar vesicles (LUV) were used. Incorporation of NPPE into PC SUV decreases leakage in the presence of serum or phosphate-buffered saline, no strictly related to size increase observed and to the surface negative charge present. The stabilizing effect of NPPE and Chol were synergistic. Inhibition of destabilization induced by serum of PC/Chol liposomes was observed when NPPE concentrations were above 12 mol%. Change in the membrane fluidity or incorporation of a monosialoganglioside into liposomes do not significantly change the half-life of liposomes in the presence of a high NPPE concentration. Incorporation of NPPE into PC/Chol liposomes increases membrane rigidity which does not change after serum incubation. The presence of NPPE in liposomes decreases lipid transfer/exchange between liposomes and lipoproteins although the same amount of serum proteins were incorporated as in PC/Chol liposomes. As expected, these proteins are accessible to trypsin digestion. In accordance with these results, the liposome agglutination assay shows no steric barrier activity. As a whole, the results obtained in this paper suggest a complex mechanism for stabilization of NPPE containing liposomes in human serum.

Novel pH-sensitive liposomes: liposomes bearing a poly(ethylene glycol) derivative with carboxyl groups

Three kinds of succinylated poly(glycidol)s were synthesized as poly(ethylene glycol) derivatives having carboxyl groups by the reaction of poly(glycidol) with varying amounts of succinic anhydride in dimethylformamide. These polymers promoted fusion of egg-yolk phosphatidylcholine liposome more intensively with decreasing content of carboxyl groups at pH 7.4, although the extent of fusion was limited. However, the polymer with 56% of succinylated residues induced fusion of the liposome much more strongly at pH 4.0. Egg-yolk phosphatidylcholine liposomes bearing the succinylated poly(glycidol) which was combined with long alkyl chains as anchors to the liposomal membrane were prepared. The leakage of calcein entrapped in the inner aqueous phase of the liposomes was slight at pH 7.4. However, the leakage increased with decreasing pH. The turbidity measurement and the fusion assay indicate that the liposomes bearing the polymer fuse more intensively with decreasing pH and with increasing amount of the polymer bound to the liposomes.

Plasma dependent pH sensitivity of liposomes containing sulfatide

In this study we investigated the possibility to define relatively plasma-stable liposomal preparations in which the sensitivity to moderate drops of pH (i.e., from 7.4 to 6.8) would be induced by the presence of plasma itself. The liposome stability was monitored by determining the release of entrapped 5,6-carboxyfluorescein (CF). Using small unilamellar vesicles composed of egg phosphatidylcholine (EPC) and bovine brain sulfatide (CS) (4:1, molar ratio), the amount of CF released at pH 6.8 in the presence of 50% plasma was 3-fold that at pH 7.4, whereas no significant differences in the amount of CF released were observed when the same liposomes were incubated in buffer at pH 7.4 and 6.8, respectively. The increase in plasma induced leakage as a consequence of a drop in the pH medium, seems to specifically depend on the presence of sulfatide molecule in the bilayer since neither the acidic cholesterol 3-sulfate nor galactocerebroside, are able to induce pH sensitivity in EPC liposomes. Of all the plasma components considered (VLDL, LDL, HDL, protein fraction), VLDL seemed preferentially involved in the pH sensitivity induced by CS since they promoted an almost complete release of CF from EPC/CS small unilamellar vesicles. Thus, these liposomes are potentially a useful tool for a specific drug delivery to those pathological tissues such as tumors, inflammation sites and ischemic areas in which it is known that a lowering of the pH can occur.

In vivo potentiation of ricin toxicity by monensin delivered through liposomes

Monensin, a carboxylic ionophore, which is known to raise intravesicular pH, was intercalated in liposomes and its effect on the toxicity of ricin in mice was studied. The toxicity of ricin in vivo was found to be significantly enhanced by the administration of monensin intercalated in liposomes (liposomal monensin). The observed enhancement of the toxicity of ricin by monensin was highly dose-dependent and was maximal when ricin was injected within 60 min of monensin injection. The survival time was found to be reduced in the range of 8-20 h, depending on the dose of ricin used, by liposomal monensin. Stability of liposomes containing monensin as inferred from the release of entrapped calcein or FITC-dextran under both in vivo and in vitro conditions was comparable to that observed for liposomes without monensin. Liposomal monensin remains in circulation for 2 h and was cleared from the blood stream after 4 h. In contrast, 15 min was required for the clearance of monensin when administered in free form. Studies on the distribution of liposomal monensin and 125I-ricin in various tissues have revealed that monensin is mainly localized in the liver and spleen which are also the major sites for ricin accumulation. Our observation on the substantial enhancement of ricin toxicity in vivo by liposomal monensin strongly supports the potential usefulness of the latter as a potentiating agent in the enhancement of the toxicity of immunotoxin or hormonotoxin for selective elimination of cancer cells.

Trypsin-induced lysis of lipid vesicles: effect of surface charge and lipid composition

We have made a curious observation that the proteolytic enzyme, trypsin, induced a rapid and complete release of the contents of vesicles composed of dioleoylphosphatidylethanolamine (DOPE) and oleic acid (OA). Content release at 37 degrees C, monitored by the release of an entrapped fluorescence marker (calcein), was accompanied by an extensive vesicle aggregation. The lytic activity of trypsin on the vesicles depended on pH and liposome composition. The optimal pH for vesicle lysis was below pH 7.4, which was different from the optimal pH for catalytic activity of trypsin. The lytic activity of trypsin was specific for vesicles composed of DOPE and fatty acids such as OA and palmitoleic acid; vesicles composed of dioleoylphosphatidylcholine, N-methyl-DOPE, and OA, or DOPE combined with other negatively charged lipids such as phosphatidylserine and phosphatidic acid were not sensitive to trypsin. Inhibition of enzyme activity by trypsin inhibitors did not abolish the lytic activity, suggesting that the lytic activity of trypsin is not related to the catalytic activity. However, the lytic activity of trypsin on vesicles composed of DOPE and OA was inhibited in the presence of excess vesicles containing negative charges, or by a pretreatment of trypsin with acylating reagent to reduce the positive-charge content of trypsin. These data demonstrate that vesicle aggregation and lysis are the results of electrostatic interactions of positive charges on trypsin and negative charges on the vesicles. Phase separation and transition to nonbilayer phases of the vesicle lipids are likely involved.