Dipalmitoylphosphatidylcholine Liposomes Research Articles

Gold-embedded photosensitive liposomes for drug delivery: Triggering mechanism and intracellular release

Liposomes embedded with gold nanoparticles show light-triggered contents release. We investigated the mechanism of the light-induced changes and functionality of the light-induced release in the cells. The real time small angle X-ray scattering (SAXS) analysis revealed time-dependent phase transitions in distearoylphosphatidylcholine (DSPC)/dipalmitoylphosphatidylcholine (DPPC) liposomes upon heating. Similar changes were observed when gold nanoparticle-embedded liposomes were exposed to the UV light: gold nanoparticles absorb light energy and transfer it to heat, thereby causing lipid phase transition from gel phase to rippled phase, and further to fluid phase. Without UV light exposure the gold nanoparticles did not affect the liposomal bilayer periodicity. The light-triggered release of hydrophilic fluorescent probe (calcein) from the gold nanoparticle-loaded liposomes was demonstrated with fluorescence-activated cell sorting after liposome internalization into the ARPE-19 cells. The liposome formulations did not decrease the cell viability in vitro. In conclusion, the light-triggered release from the liposomes is functional in the cells, and the release is triggered by thermal phase changes in the lipid bilayers.

ChemInform Abstract: Structure-Protection Mechanism of Dipalmitoylphosphatidylcholine Liposomes by Adsorption of Sucrose Molecules in the Liposome Surface

Molecular mechanism of structure-protection of dipalmitoylphosphatidylcholine (DPPC) liposomes by sucrose adsorption on/in the liposome surface was studied by performing fluorescence decay, quartz-crystal microbalance (QCM), and infrared reflection-absorption (IRRA) spectroscopic measurements. Thermally hydrated DPPC monolayers were newly introduced in place of the conventional DPPC liposomes, since the hydrated monolayers were expected to have similar surface property to that of liposomes. Studies of the fluidity and permeability of membrane in the monolayers and liposomes suggested that the thermally hydrated DPPC monolayers could be used as a model membrane in place of DPPC liposomes. With the hydrated DPPC monolayers, the adsorption rate of sucrose molecules in the DPPC monolayer was investigated by a combined technique of QCM and IRRA by the use of Langmuir-Blodgett (LB) technique. It was found that the incorporation rate of sucrose into the monolayer in the liquid-expanded state was proportional to the bulk concentration of sucrose in the Langmuir trough. The linear relationship depicts the incorporation mechanism that sucrose molecules were deeply incorporated in the DPPC molecules, and the layer is formed as a single mingled layer. This molecular scheme readily supports the conventional hypotheses based upon water-replacement and vitrification mechanisms. The IRRA spectra were further studied using newly found property of principal component analysis (PCA). PCA was found to have great potential to detect minute bands hidden in dominantly large bands, after theoretical speculation and simulations. The new analytical technique was employed to search for an unknown minute spectrum in the collection of raw spectra. The calculated abstract spectra yielded by PCA strongly suggested that there remained minute quantity of water in the dried LB films. The water molecules were also found to bind to PO2 group in the head group moiety of DPPC. The mass change of adsorbed and incorporated sucrose molecules in the DPPC monolayer were readily distinguished by PCA without any probe molecule, and they were followed semi-quantitatively by calculation of scores.

Interaction of 10-(octyloxy) decyl-2-(trimethylammonium) ethyl phosphate with mimetic membranes and cytotoxic effect on leukemic cells

10-(Octyloxy) decyl-2-(trimethylammonium) ethyl phosphate (ODPC) is an alkylphospholipid that can interact with cell membranes because of its amphiphilic character. We describe here the interaction of ODPC with liposomes and its toxicity to leukemic cells with an ED-50 of 5.4, 5.6 and 2.9 μM for 72 h of treatment for inhibition of proliferation of NB4, U937 and K562 cell lines, respectively, and lack of toxicity to normal hematopoietic progenitor cells at concentrations up to 25 μM. The ED-50 for the non-malignant HEK-293 and primary human umbilical vein endothelial cells (HUVEC) was 63.4 and 60.7 μM, respectively. The critical micellar concentration (CMC) of ODPC was 200 μM. Dynamic light scattering indicated that dipalmitoylphosphatidylcholine (DPPC) liposome size was affected only above the CMC of ODPC. Differential calorimetric scanning (DCS) of liposomes indicated a critical transition temperature ( T c) of 41.5 °C and an enthalpy (∆H) variation of 7.3 kcal mol − 1. The presence of 25 μM ODPC decreased T c and ∆H to 39.3 °C and 4.7 kcal mol − 1, respectively. ODPC at 250 μM destabilized the liposomes (36.3 °C, 0.46 kcal mol − 1). Kinetics of 5(6)-carboxyfluorescein (CF) leakage from different liposome systems indicated that the rate and extent of CF release depended on liposome composition and ODPC concentration and that above the CMC it was instantaneous. Overall, the data indicate that ODPC acts on in vitro membrane systems and leukemia cell lines at concentrations below its CMC, suggesting that it does not act as a detergent and that this effect is dependent on membrane composition.

In vitro evaluation of inhalable isoniazid‐loaded surfactant liposomes as an adjunct therapy in pulmonary tuberculosis

In this study, exogenous pulmonary surfactant was evaluated as an inhalable drug carrier for antitubercular drug isoniazid (INH). Isoniazid-entrapped liposomes of dipalmitoylphosphatidylcholine (DPPC) (the most abundant lipid of lung surfactant and exogenous surfactant) were developed and evaluated for size, drug entrapment, release, in vitro alveolar deposition, biocompatibility, antimycobacterial activity, and pulmonary surfactant action. Isoniazid-entrapped DPPC liposomes were about 750 nm in diameter and had entrapment efficiency of 36.7% +/- 1.8%. Sustained release of INH from DPPC liposomes was observed over 24 h. In vitro alveolar deposition efficiency using the twin impinger exhibited approximately 25-27% INH deposition in the alveolar chamber upon one minute nebulization using a jet nebulizer. At 37 degrees C, the formulation had better pulmonary surfactant function with quicker reduction of surface tension on adsorption (36.7 +/- 0.4 mN/m) than DPPC liposomes (44.7 +/- 0.6 mN/m) and 87% airway patency was exhibited by the formulation in a capillary surfactometer. The formulation was biocompatible and had antimycobacterial activity. The isoniazid-entrapped DPPC liposomes could fulfill the dual purpose of pulmonary drug delivery and alveolar stabilization due to antiatelectatic effect of the surfactant action which can improve the reach of antitubercular drug INH to the alveoli.

Fluorescence Studies on New Potential Antitumoral Benzothienopyran-1-ones in Solution and in Liposomes

Fluorescence properties of four new potential antitumoral compounds, 3-arylbenzothieno[2,3-c]pyran-1-ones, were studied in solution and in lipid membranes of dipalmitoyl phosphatidylcholine (DPPC), egg yolk phosphatidylcholine (Egg-PC) and dioctadecyldimethylammonium bromide (DODAB). The 3-(4-methoxyphenyl)benzothieno[2,3-c]pyran-1-one (1c) exhibits the higher fluorescence quantum yields in all solvents studied. All compounds present a solvent sensitive emission, with significant red shifts in polar solvents for the methoxylated compounds. The results point to an ICT character of the excited state, more pronounced for compound 1c. Fluorescence (steady-state) anisotropy measurements of the compounds incorporated in liposomes of DPPC, DODAB and Egg-PC indicate that all compounds have two different locations, one due to a deep penetration in the lipid membrane and another corresponding to a more hydrated environment. In general, the methoxylated compounds prefer hydrated environments inside the liposomes. The 3-(4-fluorophenyl)benzothieno[2,3-c]pyran-1-one (1a) clearly prefers a hydrated environment, with some molecules located at the outer part of the liposome interface. On the contrary, the preferential location of 3-(2-fluorophenyl)benzothieno[2,3-c]pyran-1-one (1b) is in the region of lipid hydrophobic tails. Compounds with a planar geometry (1a and 1c) have higher mobility in the lipid membranes when phase transition occurs.

Proteoliposomes Harboring Alkaline Phosphatase and Nucleotide Pyrophosphatase as Matrix Vesicle Biomimetics

We have established a proteoliposome system as an osteoblast-derived matrix vesicle (MV) biomimetic to facilitate the study of the interplay of tissue-nonspecific alkaline phosphatase (TNAP) and NPP1 (nucleotide pyrophosphatase/phosphodiesterase-1) during catalysis of biomineralization substrates. First, we studied the incorporation of TNAP into liposomes of various lipid compositions (i.e. in pure dipalmitoyl phosphatidylcholine (DPPC), DPPC/dipalmitoyl phosphatidylserine (9:1 and 8:2), and DPPC/dioctadecyl-dimethylammonium bromide (9:1 and 8:2) mixtures. TNAP reconstitution proved virtually complete in DPPC liposomes. Next, proteoliposomes containing either recombinant TNAP, recombinant NPP1, or both together were reconstituted in DPPC, and the hydrolysis of ATP, ADP, AMP, pyridoxal-5′-phosphate (PLP), p-nitrophenyl phosphate, p-nitrophenylthymidine 5′-monophosphate, and PPi by these proteoliposomes was studied at physiological pH. p-Nitrophenylthymidine 5′-monophosphate and PLP were exclusively hydrolyzed by NPP1-containing and TNAP-containing proteoliposomes, respectively. In contrast, ATP, ADP, AMP, PLP, p-nitrophenyl phosphate, and PPi were hydrolyzed by TNAP-, NPP1-, and TNAP plus NPP1-containing proteoliposomes. NPP1 plus TNAP additively hydrolyzed ATP, but TNAP appeared more active in AMP formation than NPP1. Hydrolysis of PPi by TNAP-, and TNAP plus NPP1-containing proteoliposomes occurred with catalytic efficiencies and mild cooperativity, effects comparable with those manifested by murine osteoblast-derived MVs. The reconstitution of TNAP and NPP1 into proteoliposome membranes generates a phospholipid microenvironment that allows the kinetic study of phosphosubstrate catabolism in a manner that recapitulates the native MV microenvironment.

Physicochemical and biopharmaceutical characterization of dipalmitoyl phosphatidylcholine liposomes sterically stabilized by copolymers bearing short blocks of lipid-mimetic units

Amphiphilic block copolymers were used to provide steric stabilization of dipalmitoyl phosphatidylcholine (DPPC) liposomes. The copolymers are based on poly(ethylene glycol) (PEG), but unlike the commercial PEG-lipids, bear short blocks of lipid-mimetic units. The liposomes were prepared by the lipid film hydration method and extrusion which yielded liposomes in the range of 140–170 nm in diameter as revealed by dynamic light scattering. The cryogenic transmission electron microscopy study documented unilamellar, spherical, well-separated and predominantly intact liposomes, even at copolymer contents as high as 10 mol%. The leakage of 5(6)-carboxyfluorescein was typically reduced compared to the plain liposomes. Some differences in the leakage profiles and membrane permeability were discussed in terms of shapes of the macromolecules and phase propensity of the copolymers studied. The pharmacokinetic parameters showed that the liposomes modified by a copolymer bearing four lipid-mimetic anchors exhibit superior longevity in vivo and two times lower accumulation in the liver compared to the plain liposomes and liposomes stabilized by commercial PEG-lipid. The cellular interactions were evaluated by means of fluorescence microscopy. Both the plain and the sterically stabilized liposomes were found to adhere to the membrane of MGH-U1 cells thus showing potential of serving as containers for sustained release of cytostatics. By using an ammonium transmembrane gradient method, mitoxantrone was loaded into liposomes with efficiency greater than 96%. The presence of a stabilizing copolymer did not compromise the drug loading efficacy. The liposomal mitoxantrone showed pronounced cytotoxicity towards an HL-60/Dox, which is of clinical importance as far as by-passing of multi-drug resistance is concerned.

Localized Photothermal Heating of Temperature Sensitive Liposomes

A drug delivery system consisting of a temperature sensitive liposome coupled to hollow gold nanoshells allows precise spatial and temporal control of drug release. A small fraction of lysolipid in a primarily dipalmitoylphosphatidylcholine (DPPC) liposome lowers the membrane transition temperature to that obtainable by mild hyperthermia, while simultaneously enhancing the membrane permeability at the transition temperature. Hollow gold nanoshells coupled to the liposomes heat the membrane when irradiated by a continuous wave near-infrared laser. The heat generated by the nanoshells can be tuned to control local membrane temperature, and hence the membrane permeability and rate of drug release. This system could be used to deliver anticancer drugs directly to a tumor site. Additionally, the ability to correlate drug release with membrane temperature allows us to empirically determine the local heat generated by the hollow gold nanoshells upon laser irradiation.

Ghrelin–liposome interactions: Characterization of liposomal formulations of an acylated 28‐amino acid peptide using CE

Ghrelin is a pharmacologically interesting peptide hormone due to its effects on appetite and metabolism. The cationic, octanoylated 28 amino acid peptide has a short biological half-life; thus, prolonged release formulations are of interest. Acylated peptides have been suggested to bind to or be incorporated into liposomes. Formulations based on neutral dipalmitoylphosphatidylcholine (DPPC) liposomes and phosphatidylcholine:cholesterol (70:30 mol%) liposomes, and negatively charged dipalmitoylphosphatidylcholine:dipalmitoylphosphatidylserine (DPPC:DPPS) (70:30 mol%) liposomes (2 mM total lipid concentration) were characterized using ACE. Pre-equilibrium CZE and frontal analysis CE methods circumventing capillary wall adsorption of the peptide and the liposomes and suitable for characterizing ghrelin-liposome interactions were developed. The cationic peptide exhibited low affinity (<10% bound) for DPPC and phosphatidylcholine:cholesterol (70:30 mol%) liposomes whereas electrostatic interactions caused a higher affinity for DPPC:DPPS (70:30 mol%) liposomes. Studies on desacyl ghrelin instead of ghrelin demonstrated the significance of the n-octanoyl side chain as an affinity providing moiety towards DPPC:DPPS liposomes (48 and 73% bound peptide, respectively). CE experiments showed that the binding was characterized by rapid dissociation kinetics.

Interactions of Dihydrochloride Fluphenazine with DPPC Liposomes: ATR-IR and 31P NMR Studies

The influence of dihydrochloride fluphenazine (FPh) on the dipalmitoylphosphatidylcholine (DPPC) bilayer structure was investigated using ATR-IR and (31)P NMR methods. The ATR-IR results indicate an increase in conformational disorder in the hydrophobic part compared with pure DPPC liposomes and a decrease in temperature of the chain-melting phase transition in FPh/DPPC liposomes. These effects depended on the concentration of the drug in the DPPC bilayer. The dihydrochloride fluphenazine molecules form H-bonds with the proton-acceptor carbonyl groups of DPPC molecules. At a higher concentration of the drug, the lipid bilayer structure is destroyed, and an isotropic phase is observed using (31)P NMR spectroscopy. The interactions between FPh and the lipid bilayer have a crucial role in MDR (multidrug-resistant) activity of this drug. These results improve one possible strategy of cancer chemoprevention with FPh accompanied by fluidization and destabilization of the model lipid bilayer structure.

Effect of some statin group of drugs on the phase profile of liposomal membrane – a fluorescence anisotropy study

Using the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene, we have investigated the effect of some statin group of drugs, widely used in hyperlipidemia, on the phase transition of model membranes such as dipalmitoyl phosphatidylcholine (DPPC) liposomes. The nature of the changes in the fluorescence anisotropy values suggested that the drugs simvastatin and mevastatin fluidized the membrane both before and after the phase transition temperature (T m), whereas atorvastatin fluidized the membrane below T m but rigidified the same above T m, i.e., introduced an intermediate fluid condition within the lipid matrix. We have calculated the changes in van’t Hoff enthalpy values associated with the phase transitions due to chain melting in all the cases, and observed that the values of enthalpy decreased with increase in drug concentrations. In order to get a better insight, the fraction of motionally restricted lipid molecules was determined.

Process characteristics of liposomal encapsulation of proteins

Encapsulation of functional proteins inside hydrophilic core space of vesicular liposomes is an important issue in such applications of targeted delivery of therapeutic proteins, immobilized reactions of industrial enzymes [1], and bionano sensing of diagnostic proteins. The encapsulation yield is generally quite low and needs to be improved for the applications to be industrially viable. Several parameters are known to affect the yield [2]; liposomal lipidstypes, liposome size, buffer type, pH and ionic strength, protein size (or, molecular weight). Enzymes were selected as an encapsulant because it can replace protein drug due to physical and chemical similarity and enable to measure bioactivity. In this study, by fixing the liposomal lipid as DPPC (dipalmitoylphosphatidylcholine) and using various sized proteins as modelproteins (i.e., trypsin, 15 kDa; horseradish peroxidase, 44 kDa; hyaluronidase, 80 kDa; enterokinase, 250 kDa), we evaluated the effects of these parameters on encapsulation yield. After encapsulation process, size exclusion chromatography (Sepharose 4B) was applied to separate liposomes and free proteins [3]. Encapsulation yield was calculated in two units; volumetric encapsulation (mole of protein per unit volume) and surface encapsulation (mole of protein per unit surface area). Effects of buffer pH and salt concentration on proteinlipidinteraction were measured by surface plasmon resonance. 5 mM of DPPC liposome dissolved 6 ml of 1mg/ml of enzyme solution represented over 50% of encapsulation yield. These encapsulation properties demonstrated optimal condition to achieve high encapsulation yield with more than 80% of initial bioactivity.

Interaction of N,N,N-trialkylammonioundecahydro-closo-dodecaborates with dipalmitoyl phosphatidylcholine liposomes

N,N,N-Trialkylammonioundecahydrododecaborates (1-), a novel class of compounds of interest for use as anions in ionic liquids, interact with DPPC liposomes. Increasing compound concentration causes an increasing negative zeta potential. Dissociation constants demonstrate that the binding capacity increases strongly with longer chain length. N,N,N-Trialkylammonioundecahydrododecaborates with longer alkyl chains show a detergent-like behavior: the compounds incorporate into the liposome membrane and differential scanning calorimetric experiment show already low concentrations cause a complete disappearance of the peak representing the gel-to-liquid crystalline phase transition. In contrast, compounds with shorter alkyl chains only interact with the headgroups of the lipids. Investigations by means of cryo-TEM reveal that all derivatives induce significant morphological changes of the liposomes. N,N,N-Trialkylammonioundecahydrododecaborates with short alkyl chains produce large bilayer sheets, whereas those with longer alkyl chains tend to induce the formation of open or multi-layered liposomes. We propose that the binding of N,N,N-trialkylammonioundecahydrododecaborates is mainly due to electrostatic interactions between the doubly negatively charged cluster unit and the positively charged choline headgroup; the positively charged ammonium group might be in contact with the deeper-lying negatively charged phosphate. For N,N,N-trialkylammonioundecahydrododecaborates with longer alkyl chains hydrophobic interactions with the non-polar hydrocarbon part of the membrane constitute an additional important driving force for the association of the compounds to the lipid bilayer.

Induced surface activity of supramolecular cyclodextrin–statin complexes: Relevance in drug delivery

Complexation of active therapeutic agents with cyclodextrins (CDs) offers potential uses in pharmaceutical and biomedical applications for controlling drug delivery and targeting. This paper reports on possible enhancement of the aqueous solubility and bioavailability of sparingly soluble statins (simvastatin and lovastatin) by inclusion complexation with native β-cyclodextrin and a chemically modified β-cyclodextrin, respectively. Complexation-induced surface activity of the supramolecular associates and the effect of the pure CDs and the amphiphilic CD–statin complexes on the physical stability of colloidal liposomes of dipalmitoyl phosphatidyl choline (DPPC) are discussed. It was shown that complexation with either cyclodextrin may lead to considerable improvement of the aqueous solubilities of both statins. Randomly methylated β-cyclodextrin (RAMEB) showed particularly outstanding solubilizing effects. The cyclodextrin molecules dissolved in the medium of liposome dispersions strongly reduced the physical stability of the phospholipid membranes. Complexation of the hydrophobic DPPC chains with cyclodextrins may ultimately lead to disintegration of the vesicles. In ternary systems, where due to the complexation of the pharmacon with the cyclodextrin amphiphilic CD–statin associates could develop, an enhanced and prolonged physical stability of the vesicles could be ensured.

Membrane binding requirements for the cytolytic activity of Leishmania amazonensis leishporin

To lyse cells, some pore-forming proteins need to bind to receptors on their targets. Studying the binding requirements of Leishmania amazonensis leishporin, we have shown that protease-treated erythrocytes are as sensitive to leishporin-mediated lysis as untreated cells, indicating that protein receptors are dispensable. Similarly, carbohydrate receptors do not seem to be needed, since several sugars do not inhibit leishporin-mediated hemolysis. Conversely, dipalmitoylphosphatidylcholine (DPPC), but not cholesterol, completely inhibits leishporin-mediated lysis. DPPC liposomes, with or without cholesterol, are lysed by leishporin and remove its lytic activity. Our results demonstrate that leishporin is a cholesterol-independent cytolysin that binds directly to phospholipids.

Biophysical studies on chitosan-coated liposomes

Liposomes have been used as delivery vehicles for stabilizing drugs, overcoming barriers to cellular and tissue uptake, and for directing their contents toward specific sites in vivo. Chitosan is a biological macromolecule derived from crustacean shells and has several emerging applications in drug development, obesity control, and tissue engineering. In the present work, the interaction between chitosan and dipalmitoyl phosphatidylcholine (DPPC) liposomes was studied by transmission electron microscopy (TEM), zeta potential, solubilization using the nonionic detergent octylglucoside (OG), as well as Fourier transform infrared (FTIR) spectroscopy and viscosity measurements. The coating of DPPC liposomes by a chitosan layer was confirmed by electron microscope images and the zeta potential of liposomes. Coating of liposome by chitosan resulted in an increase in liposomal size by addition of a layer of 92 +/- 27.1 nm. The liposomal zeta potential became increasingly positive as chitosan concentration increased from 0.1 to 0.3% w/v, then it held at a relatively constant value. The amount of detergent needed to completely solubilize the liposomal membrane was increased after coating of liposomes with chitosan, indicating an increased membrane resistance to the detergent and hence a change in the natural membrane permeation properties. In the analysis of FTIR spectra of DPPC, the symmetric and antisymmetric CH(2) (at 2,800-3,000 cm(-1)) bands and the C=O (at 1,740 cm(-1)) stretching band were investigated in the absence and presence of the chitosan. It was concluded that appropriate combining of the liposomal and chitosan characteristics might be utilized for the improvement of the therapeutic efficacy of liposomes as a drug delivery system.

Interaction of arginine-based cationic surfactants with membranes. An experimental and molecular simulation study

The effect of two arginine-based cationic surfactants, arginine N-lauroyl amide dihydrochloride and arginine O-lauroyl ester dihydrochloride, upon dipalmitoylphosphatidylcholine liposomes, is addressed in this work. Some aspects concerning the synthesis of these surfactants are also presented in some detail. Differential scanning calorimetry, as well as fluorescence anisotropy using several probes, is used to assess the relative effects on the lipid model membranes. In spite of the structural similarity among these molecules, which differ only by the group that connects the polar head to the tail, experimental results suggest differences in their behavior. Molecular dynamics simulation provides significant insight into the molecular mechanism that governs this interaction. A rationale is provided in terms of lipid-surfactant hydrogen bonding and the consequent positioning of the surfactant molecule within the bilayer.

Glucose-sensitivity of liposomes incorporating conjugates of glucose oxidase and poly( N-isopropylacrylamide-co-methacrylic acid-co-octadecylacrylate)

Liposomes, which release their contents in response to the concentration of glucose, were prepared by modifying the liposomal surface with the conjugate of poly( N-isopropylacrylamide-co-methacrylic acid-co-octadecylacrylate) (P(NIPAM-co-MAA-co-ODA)) and glucose oxidase (GOD). The maximum enzymatic activity of copolymer conjugated GOD (Polym-GOD) was observed around pH 5.0 and the value was about 40% of that of native GOD. Nine lysine residues per GOD molecule, on average, were found to be covalently attached to the copolymers. Egg phosphatidylcholine liposomes bearing Polym-GOD released their contents in response to the concentration of glucose and the sensitivity was higher than dipalmitoylphosphatidylcholine liposomes.

Stability, liposome interaction, and in vivo pharmacology of ghrelin in liposomal suspensions

Ghrelin is an appetite-stimulating peptide hormone. It is a pharmacologically interesting peptide because of its involvement in e.g. appetite and metabolism, but it has a very short half-life in the body. Ghrelin carries a Ser-3-octanoyl group, and it has previously been suggested that acylated peptides can bind to or be incorporated into liposomes. Therefore, neutral dipalmitoylphosphatidylcholine (DPPC) liposomes and phosphatidylcholine:cholesterol (PC:Chol) (70:30) liposomes as well as negatively charged dipalmitoylphosphatidylcholine:dipalmitoylphosphatidylserine (DPPC:DPPS) liposomes (70:30) were prepared, and ghrelin was added. The chemical and physical stability of ghrelin was examined. Affinity capillary electrophoresis (ACE) revealed an interaction between ghrelin and the negatively charged (DPPC:DPPS) liposomes, whereas only very small affinities were discerned in the other liposomal formulations of ghrelin. Differential scanning calorimetry showed no changes in phase transitions ( T m). In vivo pharmacokinetics following subcutaneous administration of ghrelin in buffer and in the liposomal formulations was examined in rats. The PC:Chol formulation had a longer-lasting effect as compared to the ghrelin buffer solution and the other two liposomal formulations. The prolonged effect of the PC:Chol formulation is suggested not to be caused by association between ghrelin and the liposome.

The effect of acids on dipalmitoyl phosphatidylcholine (DPPC) monolayers and liposomes

Dipalmitoyl phosphatidylcholine (DPPC) is the most abundant phospholipid in lung surfactant, primarily responsible for the reduction of surface tension to near 0 mN/m during expiration. In addition, DPPC liposomes are also used for drug delivery. In this study, the effects of hydrochloric acid, nitric acid and sulphuric acid on various properties of DPPC was investigated. The surface pressure versus area isotherms of DPPC monolayers at different concentrations of acid were obtained using a Langmuir–Blodgett trough. Distinct changes in the shapes of DPPC isotherms were observed in presence of acid. The lift off area for DPPC control at pH 7.4 was 58 Å 2. The lift off area for DPPC was 105, 100 and 95 Å 2 at concentration of 15.5 × 10 −2 mol/dm 3 of nitric acid, sulphuric acid and hydrochloric acid respectively. Collapse plateaus were longest in presence of nitric acid compared to hydrochloric acid and sulphuric acid. The nitric acid treated liposomes showed improved adsorption as evidenced by lower surface tension values achieved within 1 s at concentrations ranging from 3.1 × 10 −6 to 15.5 × 10 −4 mol/dm 3 having pH 6.2–2.9. The inhibitory concentrations of nitric acid which significantly increased the average surface tension values achieved within 1 s were 3.1 × 10 −3 to 15.5 × 10 −2 mol/dm 3 (pH 2.6–1.0). The sulphuric acid treated liposomes showed improved adsorption at concentrations of 3.1 × 10 −6 to 3.1 × 10 −2 mol/dm 3 (pH 6.1–1.7). The adsorption was significantly inhibited at pH 1.4–0.4 achieved with sulphuric acid. The HCl-treated liposomes showed an improved adsorption at concentrations ranging from 3.1 × 10 −6 to 15.5 × 10 −2 mol/dm 3 (pH 6.4–1.3). The average surface pressure value achieved within 1 s at inhibitory concentration of 3.1 × 10 −1 mol/dm 3 of HCl at a pH of 0.83 was 10.1 ± 2.2 mN/m. Acids did not affect the compressibility of DPPC. Increased leakage of calcein from DPPC liposomes at decreasing pH suggested increased permeability of DPPC liposomes under acidic conditions. The results of this study may have implications for pH dependent drug delivery in acidic conditions as well as for lung surfactant dysfunction in acid lung injury.