Preparation and characterization of liposomes as therapeutic delivery systems: a review

Nanoliposomes and their applications in food nanotechnology

Food nanotechnology involves the utilization of nanocarrier systems to stabilize the bioactive materials against a range of environmental and chemical changes as well as to improve their bioavailability. Nanoliposome technology presents exciting opportunities for food technologists in areas such as encapsulation and controlled release of food materials, as well as the enhanced bioavailability, stability, and shelf-life of sensitive ingredients. Liposomes and nanoliposomes have been used in the food industry to deliver flavors and nutrients and, more recently, have been investigated for their abilityto incorporate antimicrobials that could aid in the protection of food products against microbial contamination. In this paper, the main physicochemical properties of liposomes and nanoliposomes are described and some of the industrially applicable methods for their manufacture are reviewed. A summary of the application of nanoliposomes as carrier vehicles of nutrients, nutraceuticals, enzymes, food additives, and food antimicrobials is also presented.

Macromolecular crowding improves polymer encapsulation within giant lipid vesicles

We report the effect of macromolecular crowding on encapsulation efficiency of fluorescently labeled poly(ethylene glycol) (PEG) and dextran polymers within individual giant lipid vesicles (GVs). Low concentrations of the fluorescently labeled polymers (82 nM to 186 pM) were mixed with varying concentrations of nonfluorescent polymers that served as crowding agents during vesicle formation by gentle hydration. Encapsulation efficiency of the fluorescently labeled polymers in individual GVs (EEind) was determined via confocal fluorescence microscopy. EEind for high molecular weight polymers (e.g., fluorescein isothiocyanate (FITC)-dextran 500 and 2000 kDa) increased substantially in the presence of several weight percent unlabeled PEG or dextran. For example, when 0.24 microM FITC dextran 500 kDa was encapsulated, addition of 3% PEG 8 kDa improved the mean concentration in the GVs from 0.14 microM (+/-50%) to 0.24 microM (+/-12%). Light scattering data indicate reduced hydrodynamic radii for polymers as a function of increasing polymer concentration, suggesting that the improvements in EEind result from polymer condensation due to macromolecular crowding. Polymeric cosolutes did not significantly impact EEind for lower molecular weight polymers (e.g., Alexa Fluor 488-PEG 20 kDa), which already encapsulated efficiently (EEind to approximately 1). However, for both the higher and lower molecular weight labeled polymers, cosolutes led to improved uniformity in EEind for vesicles within a batch. Methods for improving the value and homogeneity of EEind for polymeric solutes in lipid vesicles are important in a variety of applications, including the use of vesicles as microreactors and as vehicles for drug delivery.

Hydrophobic silver nanoparticles trapped in lipid bilayers: Size distribution, bilayer phase behavior, and optical properties

BACKGROUND

Lipid-based dispersion of nanoparticles provides a biologically inspired route to designing therapeutic agents and a means of reducing nanoparticle toxicity. Little is currently known on how the presence of nanoparticles influences lipid vesicle stability and bilayer phase behavior. In this work, the formation of aqueous lipid/nanoparticle assemblies (LNAs) consisting of hydrophobic silver-decanethiol particles (5.7 +/- 1.8 nm) embedded within 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers is demonstrated as a function of the DPPC/Ag nanoparticle (AgNP) ratio. The effect of nanoparticle loading on the size distribution, bilayer phase behavior, and bilayer fluidity is determined. Concomitantly, the effect of bilayer incorporation on the optical properties of the AgNPs is also examined.

RESULTS

The dispersions were stable at 50 degrees C where the bilayers existed in a liquid crystalline state, but phase separated at 25 degrees C where the bilayers were in a gel state, consistent with vesicle aggregation below the lipid melting temperature. Formation of bilayer-embedded nanoparticles was confirmed by differential scanning calorimetry and fluorescence anisotropy, where increasing nanoparticle concentration suppressed the lipid pretransition temperature, reduced the melting temperature, and disrupted gel phase bilayers. The characteristic surface plasmon resonance (SPR) wavelength of the embedded nanoparticles was independent of the bilayer phase; however, the SPR absorbance was dependent on vesicle aggregation.

CONCLUSION

These results suggest that lipid bilayers can distort to accommodate large hydrophobic nanoparticles, relative to the thickness of the bilayer, and may provide insight into nanoparticle/biomembrane interactions and the design of multifunctional liposomal carriers.

Acyclovir liposomes for intranasal systemic delivery: development and pharmacokinetics evaluation

Intranasal route is one of the most attractive routes for distributing drugs to systemic circulation. Liposomes are used as biocompatible carriers to improve delivery properties across nasal mucosa. The objective of the present study was to formulate acyclovir liposomes and partition into poly-N-vinyl-2-pyrrolidone. Entrapment efficiency showed that multilamellar and unilamellar liposomes were 43.2% +/- 0.83 and 21% +/- 1.01, respectively. The bioavailability of acyclovir from nasal mucoadhesive gel was 60.72% compared with intravenous route. The use of liposomes acyclovir and mucoadhesive gel not only promoted the prolonged contact between the drug and the absorptive sites in the nasal cavity, but also facilitated direct absorption through the nasal mucosa.

Alternative measures of lipophilicity: from octanol-water partitioning to IAM retention

This review describes lipophilicity parameters currently used in drug design and QSAR studies. After a short historical overview, the complex nature of lipophilicity as the outcome of polar/nonpolar inter- and intramolecular interactions is analysed and considered as the background for the discussion of the different lipophilicity descriptors. The first part focuses on octanol-water partitioning of neutral and ionisable compounds, evaluates the efficiency of predictions and provides a short description of the experimental methods for the determination of distribution coefficients. A next part is dedicated to reversed-phase chromatographic techniques, HPLC and TLC in lipophilicity assessment. The two methods are evaluated for their efficiency to simulate octanol-water and the progress achieved in the refinement of suitable chromatographic conditions, in particular in the field of HPLC, is outlined. Liposomes as direct models of biological membranes are examined and phospolipophilicity is compared to the traditional lipophilicity concept. Difficulties associated with liposome-water partitioning are discussed. The last part focuses on Immobilised Artificial Membrane (IAM) chromatography as an alternative which combines membrane simulation with rapid measurements. IAM chromatographic retention is compared to octanol-water and liposome-water partitioning as well as to reversed-phase retention and its potential to predict biopartitioning and biological activities is discussed.

Preparation, Characterization, and Pharmacokinetics of Sterically Stabilized Nimodipine-Containing Liposomes

Nimodipine is a dihydropyridine calcium antagonist used in clinical trials in the treatment of ischemic damage in subarachnoid hemorrhage and commercially available as nimotop intravenous infusion solution and tablets. However, due to its poor solubility in water, intravenous administration depends on the use of the dehydrated alcohol to achieve a clinically relevant concentrated infusion solution while the low bioavailability of the nimotop tablets were far away from content. We have prepared a well-characterized novel lyophilized liposome-based nimodipine formulation that is sterile and easy-to-use. Of the several formulations examined, nimodipine-liposomes composed of ePC/CHOL 20:3 and co-surfactant poloxamer 188/sodium deoxycholate/ePC/3:0.3:5 were chosen for further studies. This composition was found to give more stable liposomes than other formulations. It gave 89.9% entrapment efficiency and particle size of 200 nm after lyophilization. The pharmacokinetic parameters following orally and intravenously administration to New Zealand rabbits were determined and compared with those of commercial nimodipine formulations. Encapsulation of nimodipine in liposomes produced marked differences over those of commercial preparations with an increased C(max), prolonged elimination half-life, and an increased value for AUC. The obtained values for mean residence time (MRT) indicated that nimodipine remains longer for liposomal formulation. Thus an optimum i.v. liposome formulation for nimodipine can be developed for an alternative to the commercial nimodipine preparations.

Effect of buffer composition and preparation protocol on the dispersion stability and interfacial behavior of aqueous DPPC dispersions

The effect of the buffer composition and the preparation protocol on the dynamic surface tension (DST) and vesicle sizes of aqueous dipalmitoylphosphatidylcholine (DPPC) dispersions was studied. Four isotonic buffers were used in preparing DPPC dispersions at physiological conditions for possible biological applications: (1) a standard PBS solution; (2) the above PBS with 1mM CaCl(2); (3) PBS with one tenth the previous standard phosphate salt concentrations and 2.5 mM CaCl(2); and (4) 150 mM NaCl with 2.5 mM CaCl(2) and 10mM HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid). Two protocols, with a new method and an old method (Bangham method), were used in preparing the DPPC dispersions. The DPPC dispersions prepared with the new method contained mostly vesicles and were quite stable at 25 or 37 degrees C. Dynamic light scattering (DLS) and spectroturbidimetry (ST) results showed that the DPPC vesicle sizes in buffer (4) were much smaller than those in the other buffers. When the DPPC dispersions were prepared with the new method, the diameter of the DPPC particles was smaller than those with the old method. The DPPC vesicles with the new method were more stable than those with the other method. The DPPC dispersions of 1000 ppm at 37 degrees C with the new method produced, at pulsating area conditions at 20 cycles per minute, low tension minima (gamma(min)), lower than 10 mN/m, in buffers (1), (2), and (4). With buffer (4) the DSTs were lower and were achieved faster than with the other buffers. A minimum concentration of 1000 or 250 ppm DPPC was needed to produce DSTs lower than 10 mN/m within 10 min or less, with buffer (2) or (4), respectively. IRRAS results suggest that DPPC in buffer (2) or (4) forms a close-packed monolayer at the interface. These results have implications for designing efficient protocols of lipid dispersion preparation and lung surfactant replacement formulations in treating respiratory disease.

Incorporation of peptides in phospholipid aggregates using ultrasound

This study presents the highlights of ultrasonic effects on peptides incorporated on phospholipid aggregates (liposomes). These liposomes or vesicles are known as transport agents in skin drug delivery and for hair treatment. They might be a good model to deliver larger peptides into hair to restore fibre strength after hair coloration, modelling, permanent wave and/or straightening. The preparation of liposomes 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) with peptides (LLLLK LLLLK LLLLK LLLLK; LLLLL LCLCL LLKAK AK) was made by the thin film hydration method. The LUVs (uni-lamellar vesicles) were obtained by sonication, applying different experimental conditions, such as depth (mm) and power intensity (%). Photon-correlation spectroscopy (PCS) and electronic microscopy (EM) results confirmed that the incorporation of these peptides, with different sequence of amino acids, presented differences on the diameter, zeta-potential of membrane surface and shape of liposomes. The liposomes that included peptide LLLLK LLLLK LLLLK LLLLK present an increased in zeta-potential values after using ultrasound and an "amorphous" aspect. Conversely, the liposomes that incorporated the peptide LLLLL LCLCL LLKAK AK presented a define shape (rod shape) and the potential surface of liposome did not change significantly by the use of ultrasound.

Prospects of anionic nanolipoplexes in nanotherapy: Transmission electron microscopy and light scattering studies

Currently nanosystems composed of polynucleotides and lipid vesicles (nanolipoplexes) are considered to be promising tools for gene therapeutics. Successful in vivo application of these vectors depends on their physicochemical, technological and biological characteristics including morphology, size distribution, molecular interactions and stability. Anionic nanoliposomes (DPPC:DCP:CHOL) were prepared by two different techniques, namely the conventional thin-film hydration method followed by extrusion, and the heating method (HM), in which no volatile solvent or detergent is used. A non-viral and non-cationic gene transfer vector was constructed by incorporating plasmid DNA (pcDNA3.1/His B/lacZ) to the HM-nanoliposomes by the electrostatic mediation of Ca(2+) ions. Transfection efficiency of the nanolipoplexes was evaluated using a human bronchial epithelial cell line (16HBE14o-) in the presence of serum. Particle characterisation, stability of the formulations and lipid-DNA interaction studies were performed using transmission electron microscopy (TEM) and light scattering. TEM pictures of nanolipoplexes showed presence of two to four closely packed vesicles with signs of fusion. Efficient delivery of plasmid DNA and subsequent beta-galactosidase expression was achieved using the anionic nanolipoplexes. Transfection efficiency increased with lipid:DNA ratio up to 7:1 (w/w), where transfection efficiency was 12-fold higher than in untreated cells. Further increase in lipid ratio decreased transfection. These nanolipoplexes appear to be safe, stable and efficient in the protection and delivery of DNA to different cells and tissues.

Spectrophotometric measurement of intraliposomal trehalose

Trehalose, a non-reducing glucose disaccharide found at high concentrations in many species of anhydrobiotic organisms, shows significant promise in protecting cellular viability and structural integrity during freezing and desiccation. As mammalian cell membranes are impermeable to trehalose, extensive efforts have been taken to introduce trehalose into mammalian cells. In this study, we report on the characterization of trehalose-containing liposomes, with focus on the entrapment of trehalose inside liposomes, as the first step in establishing liposomes as a delivery system in the biopreservation field. Liposomes were synthesized by hydrating a phospholipid/cholesterol lipid bilayer with 200-400 mM trehalose buffer and repeatedly extruding the lipid suspension to form unilamellar vesicles. The trehalose content of the liposomal lysate was determined spectrophotometrically using a commercial kit Megazyme and confirmed with HPLC measurements. The number of liposomes was calculated from the phosphate content of the liposomal preparation and an estimated number of lipid molecules in a 401+/-8 nm liposome. Based on an intraliposomal trehalose content, the calculated liposomal encapsulation efficiency of 200 mM trehalose liposomes was of 92+/-0.7%. This value was in agreement with the 300 and 400 mM trehalose liposomes (91.1+/-8.2% and 102.1+/-9.4%, respectively). The Megazyme method for trehalose measurement is an inexpensive and sensitive technique that does not require specialized instrumentation or extensive technical expertise. Therefore, it can be used to enhance current efforts in the development of alternative strategies for the cryo- and lyoprotection of mammalian cells.

Importance of divalent cations in nanolipoplex gene delivery

Gene therapy is a promising therapeutic strategy to combat genetic or acquired diseases at their root cause rather than just treating symptoms. It is well recognised that there is an urgent need for non-toxic and efficient gene delivery vectors to fully exploit the current potential of gene therapy in molecular medicine. Cell-specific targeting of bioactive nucleotides is a prerequisite to attain the concentration of nucleic acids required for therapeutic efficacy in the target tissue. Many metal ions such as Mg2+, Mn2+, Ba2+ and, most importantly, Ca2+ have been demonstrated to have significant roles in gene delivery. These inorganic cations show low toxicity, good biocompatibility and promise for controlled delivery properties, thus presenting a new alternative to toxic and immunogenic carriers. Recently, inorganic nanoparticles alone, or in combination with a colloidal particulate system such as nanoliposome, an advanced approach to gene delivery, were found to exert a positive effect on gene transfer. In this report, the role of the divalent cations in nucleic acid delivery, particularly with respect to the potential improvement of transfection efficiency of nanolipoplexes, is reviewed.

Screening and identification of permeable components in a combined prescription of Danggui Buxue decoction using a liposome equilibrium dialysis system followed by HPLC and LC-MS

A new method, i.e., liposome equilibrium dialysis followed by HPLC and LC-MS analysis, has been developed for the screening of permeable components in combined prescriptions of Danggui Buxue decoction (CPDBD). Multiple permeable components were simultaneously predicted by comparison of chromatograms of CPDBD extract before and after interaction with liposome membranes. A diode-array detector (DAD) and an evaporative light scattering detector (ELSD) were used, and the permeable compounds were identified by comparison with the available reference compounds and confirmed by on-line LC-MS. About fifteen compounds in a CPDBD extract were found to interact with liposome membranes. They were identified as calycosin-7-O-beta-D-glucoside (1), senkyunolide I or H (2), ononin (3), (6alphaR,11alphaR)-9,10-dimethoxypterocarpan-3-O-beta-D-glucoside (4), (3R)-2'-hydroxy-3',4'-dimethoxyisoflavan-7-O-beta-D-glucoside (5), calycosin (6), astragaloside IV (7), isoastragaloside II (8), formononetin (9), (6alphaR, 11alphaR),-3-hydroxy-9,10-dimethoxypterocarpan (10), (3R)-7,2'-dihydroxy-3',4'-dimethoxyisoflavan (11), astragaloside I (12), isoastragaloside I (13), E-ligustilide (14), and Z-ligustilide (15), respectively. Among all permeable components, 1, 3, 6, and 9 (flavonoids), 2, 14, and 15 (phthalides), and 7 (saponins) have been considered as major bioactive components in CPDBD. Therefore, this new method appears useful as a first step in the screening of bioactive components in natural products including Traditional Chinese Medicines (TCMs).

Preparation of liposomal gene therapy vectors by a scalable method without using volatile solvents or detergents

A scalable and safe method was developed to prepare liposomal carriers for entrapment and delivery of genetic material. The carrier systems were composed of endogenously occurring dipalmitoylphosphatidylcholine (DPPC), negatively charged dicetylphosphate (DCP), cholesterol (CHOL) and glycerol (3%, v/v). Liposomes were prepared by a modified and improved version of the heating method in which no harmful chemical or procedure is involved. Anionic lipoplexes were formed by incorporating plasmid DNA (pCMV-GFP) to the liposomes by the mediation of calcium ions. Transfection efficiency and toxicity of the lipoplexes were evaluated in CHO-K1 cells using flow cytometry and MTT assay, respectively. Controls included DNA-Ca(2+) complexes (without lipids), anionic liposome-DNA complexes (with no Ca(2+)), and a commercially available cationic liposomal formulation. Results indicated fast and reproducible formation of non-toxic lipoplexes that possess long-term stability, high DNA entrapment capacity (81%) and high transfection efficiency. The lipoplex preparation method has the potential of large-scale manufacture of safe and efficient carriers of nucleic acid drugs.

Lipid vesicles for skin delivery of drugs: reviewing three decades of research

Since liposomes were first shown to be of potential value for topical therapy by Mezei and Gulasekharam in 1980, studies continued towards further investigation and development of lipid vesicles as carriers for skin delivery of drugs. Despite this long history of intensive research, lipid vesicles are still considered as a controversial class of dermal and transdermal carriers. Accordingly, this article provides an overview of the development of lipid vesicles for skin delivery of drugs, with special emphasis on recent advances in this field, including the development of deformable liposomes and ethosomes.

An improved NMR study of liposomes using 1-palmitoyl-2-oleoyl-sn-glycero-3-phospatidylcholine as model

In this paper we report a comparative characterization of Small Unilamellar Vesicles (SUVs), Large Unilamellar Vesicles (LUVs) and Multilamellar Vesicles (MLVs) prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phospatidylcholine (POPC), carried out using two NMR techniques, namely High Resolution NMR in solution and High Resolution-Magic Angle Spinning (HR-MAS). The size and size distributions of these vesicles were investigated using the dynamic light scattering technique. An improved assignment of the (1)H-NMR spectrum of MLVs is also reported.

Liposomal nanocapsules in food science and agriculture

Liposomes, spherical bilayer vesicles from dispersion of polar lipids in aqueous solvents, have been widely studied for their ability to act as drug delivery vehicles by shielding reactive or sensitive compounds prior to release. Liposome entrapment has been shown to stabilize encapsulated, bioactive materials against a range of environmental and chemical changes, including enzymatic and chemical modification, as well as buffering against extreme pH, temperature, and ionic strength changes. Liposomes have been especially useful to researchers in studies of various physiological processes as models of biological membranes in both eukaryotes and prokaryotes. Industrial applications include encapsulation of pharmaceuticals and therapeutics, cosmetics, anti-cancer and gene therapy drugs. In the food industry, liposomes have been used to deliver food flavors and nutrients and more recently have been investigated for their ability to incorporate food antimicrobials that could aid in the protection of food products against growth of spoilage and pathogenic microorganisms. In this review we briefly introduce key physicochemical properties of liposomes and review competing methods for liposome production. A survey of non-agricultural and food applications of liposomes are given. Finally, a detailed up-to-date summary of the emerging usage of liposomes in the food industry as delivery vehicles of nutrients, nutraceuticals, food additives, and food antimicrobials is provided.

New self-assembled nanogels based on host–guest interactions: Characterization and drug loading

We show here, for the first time, that two neutral polymers may completely associate together in water to spontaneously form supramolecular nanoassemblies (nanogels) of spherical shape. The cohesion of these stable structures of about 200 nm is based upon a "lock and key" mechanism: inclusion complexes are formed between the hydrophobic alkyl chains grafted on a polysaccharide (dextran) and the molecular cavities contained in a poly-cyclodextrin polymer. Production yields reached 95%. It was established that all the alkyl chains were included within the cyclodextrins' cavities in these nanoassemblies. The multivalent character of the interactions between the two polymers ensures the stability of the nanoassemblies. Moreover, empty cyclodextrin units remained accessible for the inclusion of compounds of interest such as benzophenon or tamoxifen.

Preparation of Double Liposomes and Their Efficiency as an Oral Vaccine Carrier

The usefulness of double liposomes (DL), liposomes containing liposomes inside, as an oral vaccine carrier was examined. Ovalbumin (OVA) encapsulating liposomes sized to 230 nm (small liposomes, SL) were prepared by the glass-beads (GB) method and sequential sonication and extrusion. For the purpose of stabilizing the model antigen, DL containing SL were prepared by the GB method and the reverse-phase evaporation (REV) method. They were named GB-DL and REV-DL, respectively. The morphological structure of DL was confirmed using confocal laser scanning microscopy and scanning electron microscopy by the freeze-fracture method. DL showed suppressed release of OVA and stabilized OVA in pepsin solution as compared with SL. BALB/c mice were immunized with OVA solution, SL and DL suspension by oral administration. Significantly higher levels of IgA in feces were observed in mice immunized with SL and REV-DL as compared with OVA solution, and REV-DL tended to show the higher level of IgA than SL. REV-DL elicited significantly higher anti-OVA IgG responses as compared with OVA solution. Furthermore, GB-DL tended to raise the IgG level as compared with SL. The results suggest that DL have the potential to be an effective carrier for oral immunization.

Preparation and characterization of dehydration–rehydration vesicles loaded with aminoglycoside and macrolide antibiotics

Enhanced activity of liposomes-encapsulated antibiotics against clinical isolates of Pseudomonas aeruginosa has been documented with liposomes of low encapsulation efficiency. We sought to construct liposomes with high yield entrapment of aminoglycoside and macrolide antibiotics as well as favorable stability in storage and physiological conditions. Liposome-entrapped aminoglycosides (amikacin, gentamicin, tobramycin) and a macrolide (erythromycin) were prepared by a modified dehydration-rehydration vesicles (DRVs) method, and their particle size and entrapment efficiency were determined. We studied in vitro stability of these vesicles over a 48 h period at 4 and 37 degrees C in phosphate-buffered saline (PBS) and in plasma at 37 degrees C. The mean particle size of DRVs loaded with antibiotics varied from 163.37+/-38.44 to 259.83+/-11.80 nm with no significant difference in regard with the type of the antibiotics encapsulated. Encapsulation efficiency of DRVs loaded with amikacin, gentamicin, tobramycin, and erythromycin were 29.27+/-1.17, 33+/-0.76, 22.33+/-1.48 and 32.06+/-0.82% of initial amount of the drug, respectively. These vesicles were stable regardless of the experimental temperature. Indeed, the liposomes retained more than 75% of the initially encapsulated drugs for the study period of 48 h. DRVs incubated in plasma however, released more antibiotics than those incubated in PBS. In conclusion, using this modified DRV method, we obtained small sized vesicles with high yield entrapment for aminoglycoside and macrolide antibiotics. The technique may be utilized to overcome the low encapsulation efficiency associated with aminoglycoside and macrolide antibiotics.

Increasing bioavailability of silymarin using a buccal liposomal delivery system: preparation and experimental design investigation

Silymarin is a natural lipotropic agent of low bioavailability from oral products. The aim of our study is to prepare buccal liposomal delivery system of silymarin with higher bioavailability. The effect of lecithin:cholesterol molar ratio on the percentage drug encapsulated was investigated. The influence of fluctuating the amount of added drug was also determined. The effect of additives such as positive charge inducer, negative charge inducer and surfactants was studied using two different 2(3) full factorial designs. Furthermore, additives used to optimize liposomal product were also investigated for their optimal concentrations, release properties and in vitro permeation and absorption through chicken cheek pouch. Optimal liposomal encapsulation efficiency was found at 7:4 lecithin to cholesterol molar ratio. A decrease in entrapment efficiency with increasing cholesterol content was observed. Tween 20 or Tween 80 beyond 0.5 molar ratio decreased the entrapment efficiency. Positively charged liposomes showed superior entrapment efficiency over neutral and negatively charged liposomes. Release studies as well as permeation and absorption studies showed that hybrid liposomes prepared according to formula 3 containing lecithin, cholesterol, stearyl amine and Tween 20 in 9:1:1:0.5 molar ratio, respectively, gave the best drug absorption and permeation. It showed steady state permeation through chicken cheek pouch for 6h. This is expected to improve the bioavailability of silymarin in the developed liposomal buccal delivery system, as the results show an increase in drug penetration compared to free drug powder.

Rose Bengal located within liposome do not affect the activity of inside-out oriented Na,K-ATPase

DPPC:DPPE-proteoliposomes (in which the enzyme is inside-out oriented) and DLOPC:DLOPE-proteoliposomes (in which the enzyme is only 40% inside-out oriented) is an excellent model for studying the selective effect of the reactive oxygen species, produced by the photo-activation of Rose Bengal. Both proteoliposomes used, when submitted to photo-irradiation with laser using 1200 mJ/cm2 energy dose, in the absence of the Rose Bengal, did not shown any effect in the ATPase activity and in the integrity of its systems. Also, no effect was observed using 50 microM of Rose Bengal encapsulated in the interior of the DPPC:DPPE-proteoliposome system. But, when we use 50 microM of Rose Bengal, present only in the extravesicular environment, and photo-irradiation with a laser dose of 200 mJ/cm2, it results in the loss of 40-50% of the ATPase activity, with damage of the DPPC:DPPE-proteoliposome integrity. Using a dose of 400 mJ/cm2 the ATPase activity was totality lost. Consequently, these effects could be correlated with direct damage in the peptide structure. The photo-irradiation of the system constituted by DLOPC:DLOPE-proteoliposome in the presence of Rose Bengal, encapsulated only in the interior compartment or in the extra-liposomal environments, revealed a gradual decrease of the ATPase activity, maintaining it at 30% after a dose of 1200 mJ/cm2 and losing total ATPase activity at 800 mJ/cm2, respectively, with the loss of integrity of this vesicular system in both conditions studied. The generated singlet oxygen could attack the double linkage present in the fatty acid structure of the lipid instead of the amino acid in the protein structure and, in a second step, result in an indirect inactivation of the enzyme activity. In summary, these results indicated that singlet oxygen species produced by photo-oxidation of Rose Bengal using laser light could act in protein and lipid structure depending on its proportion or distribution.

Poly(ethylene oxide)-modified poly(epsilon-caprolactone) nanoparticles for targeted delivery of tamoxifen in breast cancer

This study was carried out to evaluate and compare the biodistribution profile of tamoxifen when administered intravenously (i.v.) as a simple solution or when encapsulated in polymeric nanoparticulate formulations, with or without surface-stabilizing agents. Tamoxifen-loaded, poly(ethylene oxide)-modified poly(epsilon-caprolactone) (PEO-PCL) nanoparticles were prepared by solvent displacement process that allowed in situ surface modification via physical adsorption of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock polymeric stabilizer (Pluronic). The nanoparticles were characterized for particle size and surface charge. Presence of PEO chains on nanoparticle surface was ascertained by electron spectroscopy for chemical analysis (ESCA). In vivo biodistribution studies were carried out in Nu/Nu athymic mice bearing a human breast carcinoma xenograft, MDA-MB-231 using tritiated [(3)H]-tamoxifen as radio-marker for quantification. PEO-PCL nanoparticles with an average diameter of 150-250 nm, having a smooth spherical shape, and a positive surface charge were obtained with the formulation procedure. About 90% drug encapsulation efficiency was achieved when tamoxifen was loaded at 10% by weight of the polymer. Aqueous wettability, suspendability, and ESCA results showed surface hydrophilization of the PCL nanoparticles by the Pluronics. The primary site of accumulation for the drug-loaded nanoparticles after i.v. administration was the liver, though up to 26% of the total activity could be recovered in tumor at 6h post-injection for PEO-modified nanoparticles. PEO-PCL nanoparticles exhibited significantly increased level of accumulation of the drug within tumor with time as well as extended their presence in the systemic circulation than the controls (unmodified nanoparticles or the solution form). Pluronic surfactants (F-68 and F-108) presented simple means for efficient surface modification and stabilization of PCL nanoparticles to achieve preferential tumor-targeting and a circulating drug reservoir for tamoxifen.

Unique technology for solubilization and delivery of highly lipophilic bioactive molecules

We have produced a family of novel carriers enabling water solubilization of highly lipophilic molecules. The compound carriers were synthesized by conjugating polyethylene glycol to alpha-tocopherol, tocotrienols, beta-sitosterol or cholesterol via an alkanedioyl linker. These PEG- conjugates were amphiphilic and formed stable non-covalent complexes (nanomicelles) with a wide range of molecules including vitamins, carotenoids, ubiquinones, poly-unsaturated fatty acids and polyene macrolide antibiotics. The resulting formulations were water-soluble, non-toxic and had excellent stability. This solubilization method represents a major advance in the delivery of lipophilic molecules and could be used to reformulate drugs with near term patent expiry or those that have failed clinical trials due to low solubility. Furthermore, the technology could also be applied for delivery of active ingredients for dietary supplement, functional food, cosmetic and animal health industries.

Topical liposomal gel of idoxuridine for the treatment of herpes simplex: pharmaceutical and clinical implications

The optimization of the method of preparation of idoxuridine (IDU) liposomes by the reverse phase evaporation (REV) method was carried out by three variables at three levels (3(3)) factorial design. The three independent variables selected were volume of organic phase (x1), volume of aqueous phase (x2), and drug/phosphatidylcholine/cholesterol in molar ratio (x3). Twenty-seven batches of IDU liposomes were prepared by the REV method and subjected to evaluation for percentage drug entrapment (PDE), size, and size distribution. A reduced polynomial equation was derived by multiple regression of the data of PDE and the transformed values of the three independent variables. Three contour plots at fixed level of-- 1 (low), 0 (medium), and 1 (high) of major contributing variable (x3) were plotted between x1 and x2 at predetermined PDE to understand the physical meaning of independent variables. Liposomal gels were prepared by dispersing optimized IDU liposomes in 2%w/w and 5%w/w (HPMC) K4M gel bases so as to contain 1%w/w IDU (LIG-1 and LIG-2, respectively). The percentage of drug retention (PDR) studies of optimized batch 14 (Lipo-14) and LIG-1 and LIG-2 were carried out at three different storage conditions (2-8 degrees C, 25 +/- 2 degrees C, and 37 degrees C). A comparative diffusion study of LIG-1 and LIG-2 with PIG-1 and PIG-2 (1%w/w IDU with components of liposome dispersed in 2%w/w and 5%w/w HPMC K4M gel bases, respectively), respectively, through human cadaver skin was conducted. A comparative double blind clinical pilot study of optimized LIG-2 gel was carried out for eight weeks and compared with PIG-2 on 20 Herpes simplex patients (10 patients each for HSV-1 and HSV-2, divided into two groups each of 5 patients). Batch 14 (Lipo-14) was found to have maximum PDE of 74.4%. The PDR study showed maximum drug retention at 2-8 degrees C. A significant increase in PDR (p<0.05) was observed in LIG-1 and LIG-2 when compared with Lipo-14 at all the three temperatures. In the diffusion studies, a significant (p<0.05) flux reduction; 3.5 times in LIG-1 when compared with PIG-1 and 2.3 times in LIG-2 when compared with PIG-2 was observed. Approximately 2.2- and 2.5-fold increase in skin drug retention in LIG-1 and LIG-2, respectively, was determined. A double blind clinical study demonstrated an approximately 2.0- and 1.6-fold increase in average percentage improvement in healing of the lesions in patients suffering from HSV-1 and HSV-2 diseases, respectively, when treated with LIG-2 compared with PIG-2. However, complete removal of lesions was not observed. Local side effects such as itching, burning, inflammation in HSV-1 and HSV-2, and burning micturation in HSV-2 associated with the use of PIG-2 were reduced considerably with the use of LIG-2. The findings of this investigation establish the role of the derived equation and plotted contour plots in predicting the values of independent variables for preparation of IDU liposomes by the REV method. The study also demonstrated that IDU liposomal gels retain more drug when compared with plain liposomes at all temperatures for the period of three months, while maximum PDR was found at refrigeration temperature. The skin retention of IDU was enhanced due to its entrapment in the liposomal vesicles. The clinical study suggested the improvement of therapeutic efficacy of IDU entrapped in liposomes in treatment of HSV-1 and HSV-2 patients.

Liposome fluidization and melting point depression by pressurized CO2 determined by fluorescence anisotropy

The influence of CO2 on the bilayer fluidity of liposomes, which are representative of model cellular membranes, was examined for the first time at the elevated pressures (up to 13.9 MPa) associated with CO2-based processing of liposomes and microbial sterilization. Fluidization and melting point depression of aqueous dipalmitoylphosphatidylcholine (DPPC) liposomes by pressurized CO2 (present as an excess phase) were studied by steady-state fluorescence anisotropy using the membrane probe 1,6-diphenyl-1,3,5-hexatriene (DPH). Isothermal experiments revealed reversible, pressure-dependent fluidization of DPPC bilayers at temperatures corresponding to near-gel (295 K) and fluid (333 K) phases at atmospheric pressure, where the gel-to-fluid phase transition (Tm) occurs at approximately 315 K. Isobaric measurements (PCO2 =1.8, 7.0, and 13.9 MPa) of DPH anisotropy demonstrate substantial melting point depression (DeltaTm = -4.8 to -18.5 K) and a large broadening of the gel-fluid phase transition region, which were interpreted using conventional theories of melting point depression. Liposome fluidity is influenced by CO2 accumulation in the hydrocarbon core and polar headgroup region, as well as the formation of carbonic acid and/or the presence of buffering species under elevated CO2 pressure.

Studies on the encapsulation of diclofenac in small unilamellar liposomes of soya phosphatidylcholine

The encapsulation of acid (AD) and sodium diclofenac (SD) in small unilamellar liposomes (SUV) as well as the interactions of the drug with the bilayer was studied. SUV was prepared by sonication from multilamellar liposomes containing soya phosphatidylcholine and diclofenac at various proportions. The size distribution obtained from dynamic light scattering showed that the incorporation of SD decreases significantly the size of the liposomes suggesting that the drug interacts with the bilayer of the liposomes. This size decrease is related with the phase transition of liposomes to mixed micelar solution. The encapsulation of the hydrophilic dye indocyanine green in the aqueous compartment of liposomes showed that the rate of captured dye decreases with SD concentration suggesting the transition of liposomes to mixed micelles. The (31)P NMR analysis indicates that SD interacts with the phosphate of phosphatidylcholine head groups. A schematic model for interaction of SD with phosphatidylcholine of the liposomes in which the diclofenac anion interacts with the ammonium group of the phospholipid and the dichlorophenyl ring occupies a more internal site of bilayer near phosphate group was proposed.

Liposome/water lipophilicity: methods, information content, and pharmaceutical applications

This review discusses liposome/water lipophilicity in terms of the structure of liposomes, experimental methods, and information content. In a first part, the structural properties of the hydrophobic core and polar surface of liposomes are examined in the light of potential interactions with solute molecules. Particular emphasis is placed on the physicochemical properties of polar headgroups of lipids in liposomes. A second part is dedicated to three useful methods to study liposome/water partitioning, namely potentiometry, equilibrium dialysis, and (1)H-NMR relaxation rates. In each case, the principle and limitations of the method are discussed. The next part presents the structural information encoded in liposome/water lipophilicity, in other words the solutes' structural and physicochemical properties that determine their behavior and hence their partitioning in such systems. This presentation is based on a comparison between isotropic (i.e., solvent/water) and anisotropic (e.g., liposome/water) systems. An important factor to be considered is whether the anisotropic lipid phase is ionized or not. Three examples taken from the authors' laboratories are discussed to illustrate the factors or combinations thereof that govern liposome/water lipophilicity, namely (a) hydrophobic interactions alone, (b) hydrophobic and polar interactions, and (c) conformational effects plus hydrophobic and ionic interactions. The next part presents two studies taken from the field of QSAR to exemplify the use of liposome/water lipophilicity in structure-disposition and structure-activity relationships. In the conclusion, we summarize the interests and limitations of this technology and point to promising developments.

Lipid composition-dependent incorporation of multiple membrane proteins into liposomes

Membrane proteins from bacteria Pasteurella multocida were used as a model for studying its incorporation into liposomes. An important step to achieve efficient high yield protein incorporation in proteoliposomes is the study of the more suitable lipid composition. To this end, we compared the amount of total protein, reconstituted by co-solubilization methods, into liposomes of phospholipids with different polar head groups and acyl chain lengths. The liposomes and proteoliposomes were characterised by isopycnic centrifugation in sucrose gradient and by dynamic light scattering. Experimental and theoretical results were compared considering the effects exerted through the hydrocarbon chain length, volume, and optimal cross-sectional area of the phospholipid (combined in the geometrical critical packing parameter, lipid-protein matching), critical spontaneous radius of curvature of the bilayer vesicle, phase transition temperature of the lipid and ratio of lipid-protein molecules present in the vesicles. The highest incorporation of multiple proteins was found with dipalmitoylphosphatidylcholine (DPPC), reaching a yield of 93% compared to the lower relative amounts incorporated in proteoliposomes of the other lipids. The incorporation of multiple proteins induces a proportional enhancement of vesicular dimension, since DPPC-proteoliposomes have an average diameter of 1850A, compared to the 1430A for pure DPPC vesicles.

Sterically stabilized etoposide liposomes: evaluation of antimetastatic activity and its potentiation by combination with sterically stabilized pentoxifylline liposomes in mice

Cytotoxic activity of chemotherapeutic agents can be enhanced by site-specific delivery or by combination with other less toxic agents. In the present study, enhancement in the antimetastatic activity of etoposide (ETP) by encapsulation in sterically stabilized liposomes was evaluated in the murine experimental B16F10 melanoma model. Further, potentiation of its antimetastatic activity by combination with pentoxifylline (PTX) solution or sterically stabilized PTX liposomes was evaluated in the same animal model. Upon intravenous administration, ETP solution and ETP liposomes inhibited pulmonary tumor nodule formation in a dose-dependent manner. Encapsulation of ETP in liposomes resulted in significant enhancement in its antimetastatic activity at doses of 0.5 mg/kg and 0.75 mg/kg as compared to ETP solution at similar doses. In combination therapy, the effect of sequence of administration of the drugs, ETP and PTX, was evaluated. Enhancement of antimetastatic activity of ETP solution when used in combination with PTX solution was effected by the sequence in which the solutions were administered. However, a combination of ETP liposomes and PTX liposomes led to potentiation of antimetastatic activity in a sequence-independent manner. The results indicate that antimetastatic activity of ETP is significantly enhanced by encapsulation in liposomes. Administration of ETP liposomes with PTX liposomes further potentiated the activity, suggesting the usefulness of this combination in clinical practice for reducing the dose-limiting toxic effects of ETP.

Multilamellar liposomes formed by phosphatidyl nucleosides: an NMR-HR-MAS characterization

We present an NMR investigation of multilamellar vesicles (MLVs) obtained from phosphatidyl nucleosides, 5'-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)cytidine (1), 5'-(1-palmitoyl-2-oleoyl-sn-glycero(3)phospho)inosine (2), and their mixtures. Because of the lower stability of liposomes obtained from 2, studies have been preferentially performed in this case with mixed liposomes 2/POPC (4:1). The investigation is conducted mostly via the HR-MAS technique and the general observation is that the resolution achieved in this way is superior to that obtained in the past with small unilamellar vesicles (SUVs). A full assignment is now possible, which includes the spectral region of the ribose ring and part of the glycerol moiety. Also in the case of MLVs, both for 1 and 2, a stacking between the aromatic bases of the same liposome layer seems to be ruled out, although in both cases the nucleobases appear to be exposed to the aqueous phase. The splitting of both aromatic H-5cyt and H-6cyt is ascribed to the presence of two aggregate populations that may correspond to the two syn and anti conformations observed for cytidine monophosphate in aqueous solution. On the basis of NOESY cross-peaks, it is not always possible to discriminate between inter- and intramolecular interactions; however, the distances found for 1 appear to be compatible with the intramolecular contacts in the anti conformation of the cytidine and also with intermolecular interactions between neighboring molecules of 1. We also find that the glycerol moiety does not seem to interact with the cytidine; however, part of the ribose ring seems to be close to the glycerol moiety. More generally, the interaction of one base with the sugar moiety of a neighboring base, previously observed for SUVs, still appears to be true for MLVs. Studies have been performed also for mixed liposomes obtained from the mixture of 1 and 2, where it is observed that the HR-MAS spectra of the corresponding MLVs are not simply the sum of the spectra of the two isolated components. In particular, there is the presence of a NOESY cross-peak between the aromatic protons H-6cyt and H-2ino, and this permits us to rule out large patchwork domains containing only one nucleoside components in the mixed liposomes. Finally, a study is performed on the time evolution of the system obtained by mixing the previously prepared liposomes of 1 and 2. No interaction is obtained in this case, i.e., the spectra are constitutive, which is consistent with the general picture of liposomes as kinetic traps that are not fusing with each other.

HPLC determination of encapsulated 5-fluorouracil in lyoiphilized liposomes

5-fluorouracil is used as an antineoplastic agent in the treatment of several malignant neoplasm, alone or in a combination with other antineoplastic drugs. Due to the difficulties in delivery of antineoplastic drugs on the target place, serious adverse events and hazardous effects, recent advanced researches are focused on the topical colloidal systems as drug carriers for the chemical substance. Colloidal drug carriers, as liposomes, can improve the therapeutic effects of the chemical substances by modifying their distribution and increasing their efficacy. The aim of this study was to develop HPLC method for quantitative determination of encapsulated 5-fluorouracil in lyophilized liposomes. HPLC method was developed using the following chromatographic conditions: column- LiChrospher 60, RP Selected B, 125 x 4 mm, 5 µm; mobile phase - 0.02 M phosphate buffer pH 4.7; flow rate of 1.0 ml/min; column temperature 20 ºC; UV - detection at 266 nm; injection volume - 20 µl. Linear relationship was obtained in the concentration range of 10-100 µg/ml, r=0.9999. Accuracy of the method was confirmed by standard addition method. Determination of encapsulated 5-fluorouracil in lyophilized liposomes was carried out with RSD of 0.71 %. The proposed method could be used for rapid and precise quality control of lyophilized liposomes with 5-fluorouracil, used as drug delivery system, prepared for further incorporation in pharmaceutical dosage form-gel.

Modeling of matrix vesicle biomineralization using large unilamellar vesicles

Stable, large unilamellar vesicles (LUV) have been constructed that model matrix vesicles (MV) in inducing de novo mineral formation when incubated in synthetic cartilage lymph (SCL). Using a dialysis method for incorporation of predetermined pure lipid, electrolyte and protein constituents, the detergent n-octyl beta-D-glucopyranoside enabled formation of stable, impermeable LUV with a diameter ( approximately 300 nm), lipid composition (phosphatidylcholine-phosphatidylserine-cholesterol, 7:2:2, molar ratio) and enclosed inorganic phosphate level (25-100 mM) similar to that of native MV. Mineral formation by these LUVs was measured by 45Ca(2+) uptake and FTIR analysis following incubation in SCL. Addition of the ionophore A23187 to SCL enabled 45Ca(2+) uptake comparable to that of native MV. FTIR analysis revealed that crystalline mineral formed in the LUV during incubation in SCL, but not in the absence of ionophore. This mineral had an IR absorption spectrum like that of the acid-phosphate-rich, octacalcium phosphate-like mineral formed by native MV. Perturbing the LUV membrane with either detergents or phospholipase A(2) following prior incubation in SCL enabled egress of mineral crystallites from the vesicle lumen, stimulating further mineral formation. Annexin V, a major protein in native MV with known Ca(2+) channel activity, incorporated into the LUV lumen or added to the external medium, induced only limited 45Ca(2+) uptake. This indicates that additional factors are required for annexin V to form Ca(2+) channels. Nevertheless for the first time, stable LUVs have been constructed with MV-like lipid, electrolyte, and protein composition and size that induce formation of mineral like that formed by native MV.

Preparation and comparative clinical evaluation of liposomal gel of benzoyl peroxide for acne

A novel topical benzovl peroxide (BP) gelformulation containing liposomal BP was shown to significantly reduce local irritation relative to its nonliposomal BP gel (plain BP gel) preparation and also to improve clinical efficacy (almost twofold) in the treatment of acne. BP liposomes were prepared, optimized, and formulated into a carbopol 934gel base. Drug release evaluated using dialysis membrane has repeatedly shown that a new topical gel formulation containing liposomal BP (liposomal BP gel) significantly reduced BP penetration. Clinical evaluation data were also compared with those obtained with liposomal tretinoin (TRE) gel in an earlier investigation of ours. The overall improvement in terms of percentage reduction in total number of skin lesions demonstrated almost similar results for both BP and TRE. However, variation was observed in the treatment of separate types of lesions in which liposomal TRE gel was found to be more effective in treating comedones and liposomal BP gel in treating papules and pustules. Also, the liposomal gel formulation of both the drugs significantly reduced the local adverse effects, thereby improving patient compliance.

Effect of liposomes and niosomes on skin permeation of enoxacin

The skin permeation and partitioning of a fluorinated quinolone antibacterial agent, enoxacin, in liposomes and niosomes, after topical application, were elucidated in the present study. In vitro percutaneous absorption experiments were performed on nude mouse skin with Franz diffusion cells. The influence of vesicles on the physicochemical property and stability of the formulations were measured. The enhanced delivery across the skin of liposome and niosome encapsulated enoxacin had been observed after selecting the appropriate formulations. The optimized formulations could also reserve a large amount of enoxacin in the skin. A significant relationship between skin permeation and the cumulative amount of enoxacin in the skin was observed. Both permeation enhancer effect and direct vesicle fusion with stratum corneum may contribute to the permeation of enoxacin across skin. Formulation with niosomes demonstrated a higher stability after 48 h incubation compared to liposomes. The inclusion of cholesterol improved the stability of enoxacin liposomes according to the results from encapsulation and turbidity. However, adding negative charges reduced the stability of niosomes. The ability of liposomes and niosomes to modulate drug delivery without significant toxicity makes the two vesicles useful to formulate topical enoxacin.

Topical liposomal gel of tretinoin for the treatment of acne: research and clinical implications

An attempt was made to pharmaceutically develop a topical liposomal tretinoin (TRE) gel and clinically evaluate the developed formulation for the treatment of acne in patients. Liposomes of TRE were prepared using the lipid film hydration technique and the entrapment efficiency of TRE in liposomes was optimized to 79.96%. The drug retention in liposomes and in liposomal TRE gel (Carbopol 934 gel base) studied at three storage conditions indicated maximum drug retention at refrigeration temperature. For liposomal TRE gel, reduced drug leakage was observed as compared to that of liposomes at all three storage conditions. Diffusion studies of plain TRE gel and liposomal TRE gel suggested prolongation (3.4 times reduction in flux value) of drug diffusion and almost two-fold increase in skin drug retention after liposomal encapsulation of drug. A comparative double-blind clinical study of the developed liposomal TRE gel, carried out on 30 acne patients over a period of 3 months, demonstrated significant enhancement (about 1.5-fold) in drug efficacy. More remarkable improvement was observed in the treatment of comedones, where the mean percent reduction in lesions increased from 62.36% for plain TRE gel to 94.17% for liposomal TRE gel. Erythema and irritation associated with the use of plain TRE gel was reduced considerably with the use of liposomal TRE gel. The findings of this investigation therefore underscore potential utility of commercialization of liposomal TRE gel in the treatment of acne.

Enhancement of anti-metastatic activity of pentoxifylline by encapsulation in conventional liposomes and sterically stabilized liposomes in murine experimental B16F10 melanoma model

Pentoxifylline has been shown to exhibit anti-metastatic activity by inhibiting homing of B16F10 melanoma cells in the murine experimental metastasis model. In this study, the effect of encapsulation of pentoxifylline in conventional and sterically stabilized liposomes on its anti-metastatic activity in the murine experimental metastasis model was investigated. After a single intravenous dose (10, 20 or 40 mg kg(-1)), pentoxifylline solution, as well as conventional pentoxifylline liposomes, significantly reduced the number of pulmonary nodules compared with the untreated control group. Conventional pentoxifylline liposomes showed significantly higher inhibition (69%) of pulmonary tumour nodule formation at a dose of 20mg kg(-1) as compared with pentoxifylline solution (49%) at the same dose. Encapsulation of pentoxifylline in sterically stabilized liposomes prepared by incorporation of monomethoxypolyethyleneglycol (5000)-cholesteryl ester further enhanced the inhibition of pulmonary nodule formation (77%) at a dose of 20 mg kg(-1) as compared with conventional pentoxifylline liposomes. Overall, the results suggest that encapsulation of pentoxifylline in conventional liposomes enhanced its anti-metastatic activity. Steric stabilization of pentoxifylline liposomes also resulted in a two-fold increase in anti-metastatic activity (at dose of 10 mg kg(-1)) as compared with conventional liposomes.