Distearoylphosphatidylcholine Research Articles

Liposome-induced pulmonary hypertension: properties and mechanism of a complement-mediated pseudoallergic reaction.

Intravenous injection of liposomes can cause significant pulmonary hypertension in pigs, a vasoconstrictive response that provides a sensitive model for the cardiopulmonary distress in humans caused by some liposomal drugs. The reaction was recently shown to be a manifestation of "complement activation-related pseudoallergy" (CARPA; Szebeni J, Fontana JL, Wassef NM, Mongan PD, Morse DS, Dobbins DE, Stahl GL, Bünger R, and Alving CR. Circulation 99: 2302-2309, 1999). In the present study we demonstrate that the composition, size, and administration method of liposomes have significant influence on pulmonary vasoactivity, which varied between instantaneously lethal (following bolus injection of 5 mg lipid) to nondetectable (despite infusion of a 2,000-fold higher dose). Experimental conditions augmenting the pulmonary hypertensive response included the presence of dimyristoyl phosphatidylglycerol, 71 mol% cholesterol, distearoyl phosphatidylcholine, and hemoglobin in liposomes, increased vesicle size and polydispersity, and bolus injection vs. slow infusion. The vasoactivity of large multilamellar liposomes was reproduced with human C3a, C5a, and xenoreactive immunoglobulins, and it correlated with the complement activating and natural antibody binding potential of vesicles. Unilamellar, monodisperse liposomes with 0.19 +/- 0.10 microm mean diameter had no significant vasoactivity. These data indicate that liposome-induced pulmonary hypertension in pigs is multifactorial, it is due to natural antibody-triggered classic pathway complement activation and it can be prevented by appropriate tailoring of the structure and administration method of vesicles.

A new method for determining the phase in the X-ray diffraction structure analysis of phosphatidylcholine/alcohol

A new method based on a sampling theorem is proposed for determining the phase in the X-ray diffraction analysis of the structure of phospholipid systems. The thickness of a lipid layer is changed by changing the length of hydrocarbon chains in order to rebuild the continuous transform from the scattering amplitudes. By employing this method, the phases were accurately determined in a structure analysis of nine phospholipid/alcohol systems at the interdigitated gel phase. The nine systems are dimyristoylphosphatidylcholine(DMPC)/propanol, DPPC/methanol, DPPC/ethanol, DPPC/propanol, DPPC/butanol, distearoylphosphatidylcholine(DSPC)/methanol, DSPC/ethanol, DSPC/propanol and DSPC butanol systems.

Molecular modeling of archaebacterial bipolar tetraether lipid membranes

Membranes composed of glycerol dialkylnonitol tetraether (GDNT) lipids from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius have been studied by molecular modeling. GDNT membranes containing eight cyclopentane rings in the molecule are packed much tighter than those without rings. When containing eight cyclopentane rings, the β- d-galactosyl- d-glucose head-group of GDNT runs almost parallel to the membrane surface. However, when containing no rings, the head-group is oriented perpendicular to the membrane surface. Using molecular dynamics calculations, we have also conducted comparative studies of membrane packing between GDNT and various non-archaebacterial membranes. Compared to gel state dipalmitoylphosphatidylcholine (DPPC) and gel state distearoylphosphatidylcholine (DSPC) bilayers, the GDNT membrane with eight cyclopentane rings has a more negative interaction energy, thus a tighter membrane packing, while the GDNT without rings is less tightly packed than gel state DSPC. Based on the calculated interaction energies, the GDNT membranes (with and without rings) are much more tightly packed than DPhPC (an ester-linked diphytanyl PC) and DPhyPC (an ether-linked diphytanyl PC) bilayers. This suggests that the branched methyl group in the phytanyl chain is not the major contributor of the tight packing of GDNT membranes. The biological implication of this study is that the cyclopentane ring could increase GDNT membrane thermal stability. This explains why the number of cyclopentane rings in archaebacterial lipid increases with increasing growth temperature. Perhaps, through the ring-temperature compensation mechanism, the plasma membrane of thermoacidophilic archaebacteria is able to maintain a tight and rigid structure, consequently, a constant proton gradient between the extracelluar (pH 2.5) and intracelluar compartment (pH 6.5), over a wide range of growth temperatures.

In vitro cytotoxic effect of N-(phosphonacetyl)-L-aspartic acid in liposome against C-26 murine colon carcinoma.

We have investigated the in vitro cytotoxic effect of liposome-encapsulated N-(phosphonacetyl)-L-aspartic acid (PALA) against C-26 murine colon cancer cells, and have compared it in this regard to free PALA. Three different PALA-containing liposomal formulations using distearoylphosphatidylcholine (DSPC), distearoylphosphatidylglycerol (DSPG), and polyethyleneglycol-derivatized distearoylphosphatidylethanolamine (PEG-DSPE) were made and their cytotoxicity was measured. In 72 hr continuous exposure experiment with C-26 cells, the 50% growth inhibitory concentration (IC50) of DSPG-PALA liposome formulation was 0.09 microM, which showed about 65-fold more potent than unencapsulated free PALA (5.1 microM). Similar degree of increase in cytotoxicity was also observed in 1 hr exposure experiment. However, the IC50 of PEG-DSPE-PALA liposome and DSPC-PALA liposome were 10.7 microM and 11.8 microM, respectively, which showed slightly less potent than unencapsulated free PALA. Physical characteristics of PALA-liposomes, such as the size and drug:lipid ratio were also determined. In conclusion, negatively-charged DSPG-PALA liposome showed the highest cytotoxic effect among tested on the C-26 cells in vitro.

The suppression of rat collagen-induced arthritis and inhibition of macrophage derived mediator release by liposomal methotrexate formulations

This study was designed to determine whether liposomes are suitable vehicles for the delivery of methotrexate (MTX-gamma-DMPE) for arthritis therapy. Liposomal formulations containing either egg lecithin (EPC), cholesterol (CHOL) and phosphatidic acid (PA) (MTX-EPC) or distearoylphosphatidylcholine (DSPC), CHOL and distearoylphosphatidylethanolamine conjugated to polyethyleneglycol (PEG) (MTX-PEG) were employed. Rat peritoneal macrophages (rPM phi) were used to test the mechanism of action of these liposomes in vitro, whilst, the rat collagen-induced arthritis (CIA) model was used to evaluate the in vivo efficacy of MTX-EPC and MTX-PEG. In vitro, rPM phi were incubated with liposomal MTX concentrations ranging from 0 to 15 microg/well. In vivo, rats were given 4 daily intravenous injections of liposomal MTX (2.5 mg/Kg). IL-1beta and prostaglandin-E2 (PGE2) release from rPM phi were quantified by immunoradiometric assay. Arthritis progression, in vivo, was measured by serial clinical score and hind paw diameter measurements. MTX-EPC and MTX-PEG respectively (15 microg of MTX and 0.15 mg of lipid) were powerful inhibitors of both IL-1beta (77 +/- 2.3%; 79 +/- 4.0%) and PGE2 (75.5 +/- 4.9%; 68.5 +/- 2.3%) release (mean +/- SEM % inhibition) from lipopolysaccaride stimulated rPM phi. In vivo, only MTX-EPC exerted an anti-inflammatory effect, clinical score (p < 0.001) and paw diameter (p < 0.001) measurements being significantly lower than in control rats, after 2 days treatment. MTX-EPC and MTX-PEG are potent inhibitors of pro-inflammatory mediators in vitro, but liposomes with long circulation times do not appear to have therapeutic potential for treating arthritis in vivo.

Influence of lipids on the mannitol flux during transdermal iontophoresis in vitro

The aim of the present study was to evaluate the influence of the lipids EPC ( l-α-phosphatidylcholine, egg lecithin), DSPC (distearoylphosphatidylcholine), and SA (stearylamine) on the iontophoretic mannitol transport through human skin in vitro. The skin was pretreated with 1 mM lipid suspension with ethanol (32%) for 24 h prior to the iontophoretic experiment with mannitol. In addition, the penetration of fluorescent lipids into the epidermis during the pretreatment was studied by confocal laser scanning microscopy (CLSM). The results of the present study show that pretreatment of the skin with zwitterionic EPC increases the iontophoretic transdermal mannitol flux about three-fold compared to iontophoretic control without pretreatment. However, skin pretreatment with another zwitterionic phospholipid, DSPC, did not influence the iontophoretic flux of mannitol. In contrast, pretreatment of the skin with cationic SA decreased the iontophoretic mannitol flow from the anode. It is concluded that EPC works as a penetration enhancer further increasing the transdermal mannitol flux during iontophoresis. In contrast, the cationic stearylamine changes the charge of the skin, thus leading to decreased electroosmosis and decreased mannitol flux. Hence, the effects of stearylamine are assumed to be mediated by the alterations in the charge of the stratum corneum structures, while EPC is suggested to decrease the permeability barrier of the skin.

Enhanced ion transfer rate due to the presence of zwitterionic phospholipid monolayers at the ITIES

The transfer of cations across phospholipid monolayers at ITIES is studied both experimentally and theoretically. Further evidence of the enhanced rate for cation transfer due to the presence of the monolayer is presented, and a theoretical model that can explain these observations is worked out. The system considered experimentally is Li+ ion transfer across a hemispherical water ∣ 1,2-dichloroethane interface covered by distearoyl phosphatidylcholine. The theoretical description is based on the electrical double layer correction to the Butler–Volmer equation, coupled with a solution of the Poisson–Boltzmann equation across the interfacial region. The phospholipid monolayer is modelled as two parallel planes of charge, with the negative charge located at the ITIES and the positive charge located at a distance d into the aqueous phase. The enhancement factors predicted are found to be in agreement with our experimental results as well as with those obtained by other authors.

Probing adsorption reactions at the liquid∣liquid interface by area-step experiments

The results of novel area step experiments at the interface between two immiscible electrolyte solutions (ITIES) in the presence of a supporting electrolyte only and with distearoylphosphatidylcholine (DSPC) in the bulk organic phase are presented. The interface was formed by extrusion of an aqueous drop into 1,2-dichloroethane from the tip of a micropipette. An apparatus is described which allows the dimensions of the interface to be stepped with a rise time of milliseconds, and to be controlled within 5% of a mean area for many hours. In the absence of DSPC, the relaxation of the interfacial electric current after steps in the interfacial area was shown to occur in two distinct stages, typified by an initial rapid current decay on the 50 ms time scale, followed by a much slower decay over approximately 5 s. The first part was due to the relaxation of the charge stored in the interfacial capacitance with its integrated charge going through zero at the potential of zero charge. The theory is given. In addition there was a current due to ion transfer across the interface, which was distinguishable from the charging current, and relaxation of which gave the second part of the transient. The interfacial adsorption of DSPC from the bulk organic phase and the formation of elastic lipid layers were observed visually over a period of hours and via the creation of new current transients following area modulation. The presence of DSPC increased the rate of interfacial ion transfer at all potentials studied and the effect was shown to depend on surface concentration. The presence of DSPC altered the potential of zero charge negatively as shown by a shift in the potential at which the polarity of the initial transient inverted.

Biodistribution of Liposomes of Terbutaline Sulfate in Guinea Pigs

A series of liposomes was prepared with various lipid (egg phosphatidyl choline [egg PC], phosphatidyl glycerol [PG], dipalmitoyl phosphatidylcholine [DPPC], distearoyl phosphatidyl choline [DSPC], dipalmitoyl phosphatidyl glycerol [DPPG], phosphatidyl ethanolamine [PE], cholesterol [CH], and stearylamine [SA]) compositions, such as egg PC:PG:CH (55:5:40), DPPC:PG:CH (55:5:40), DSPC:DSPG:CH (55:5:40) egg PC:SA:CH (55:5:40), DSPE:DSPG:CH (55:5:40) in molar ratio. Liposomal formulations were administered to guinea pigs intravenously; 3 hr after the treatment, serum samples and various organs (e.g., liver, spleen, lung) were removed and analyzed for drug concentration by a high-performance liquid chromatographic (HPLC) method. Based on the above study, a liposomal preparation with better lung specificity was selected, and the time profile of these liposomes was determined in guinea pigs. Three hours postadministration, a significant difference in blood levels was observed between free terbutaline sulfate and the various liposomal formulations. Localization of the drug in the lungs increased considerably when encapsulated drug was used, and the highest percentage localization was observed with DSPC:DSPG:CH (55:5:40) liposomes. The percentage recovery of the drug in the lungs with egg PC:CH:SA (55:40:5) liposomes did not change significantly when compared with egg PC:CH:PG (55:40:5) liposomes. To establish the time course of disposition of the liposomes, DSPC:SPG:CH (55:5:40) liposomes were selected. Terminal half-life t1/2 of the drug in blood with free drug solution was about 12 hr, whereas with liposomes, a twofold increase in t1/2 was observed. The disposition data indicated that the clearance of the drug was delayed by 1.5 times when incorporated into liposomes.

Targeted Delivery of Prostaglandin E1 to Hepatocytes Using Galactosylated Liposomes

Prostaglandin E1 (PGE1) was incorporated in galactosylated liposomes containing cholesten-5-yloxy-N-(4-((1-imino-2-β-D-thiogalactosylethyl)amino)butyl)formamide (Gal-C4-Chol) intended for hepatocyte-selective delivery. Liposomes composed of distearoylphosphatidylcholine (DSPC)/cholesterol (Chol)/Gal-C4-Chol (60:35:5) were prepared and compared with DSPC/Chol (60:40) liposomes. After intravenous injection of [3H]-labeled PGE1 or cholesteryl hexadecyl ether (CHE) with the liposomal formulation, mice were sacrificed at a series of times, and the radioactivity in tissues was determined. Up to about 80% of [3H]CHE in galactosylated liposomes had accumulated in the liver 10 min after intravenous injection and the liver accumulation of the incorporated [3H]PGE1 was significantly higher than that in control liposomes during the entire test period. The pharmacological activity was examined in mice with fulminant hepatitis induced by peritoneal injection of carbon tetrachloride. Intravenous injection of PGE1 incorporated in DSPC/Chol/Gal-C4-Chol (60:35:5) liposomes significantly suppressed the GPT increase, whereas PGE1 (dissolved in saline) and PGE1 incorporated in DSPC/Chol (60:40) liposomes had little effect.

Temperature-dependent phase behaviour of dihydroxy octadecanoic acid methyl esters: Influence of stereochemistry and position of the second polar moiety

Racemic mixtures and enantiomers of amphiphiles based on methyl octadecanoate with two vicinal hydroxy groups as a second polar moiety have been investigated at the air/water interface on ultrapure water. In contrast to methyl octadecanoate these bipolar substances show a distinct two-phase coexistence region. Earlier studies showed the dependence of the surface pressure–area (π–A) isotherms and the structure and domain shapes of the condensed phases on the position of the vicinal hydroxy groups within the hydrophobic alkyl chain of methyl octadecanoate. To obtain more information about the influence of the insertion of a second polar moiety within an amphiphilic structure on film properties and film stability the temperature-dependent phase behaviour is studied by measuring π–A isotherms within a temperature range of 275–313 K. Applying a suitable form of the Clausius–Clapeyron equation to the π–A isotherms, the enthalpy ΔH was calculated for the liquid-expanded (LE) to liquid-condensed (LC) phase transition. From these calculations we conclude that the hydroxy groups at different positions along the alkyl chain act as a disturbing moiety between the molecules of the condensed film and influence the hydrophobic interaction between the alkyl chains for threo-dihydroxyoctadecanoate (DHO) with the hydroxy groups in the middle of the molecules. As a consequence the condensed monolayers are metastable and the more stable multilayered order is preferred. Films of the bolaamphiphile methyl rac-17,18-DHO as well as films of methyl rac-threo-2,3-DHO show smaller effects on film properties. The observed effects are discussed with respect to distearoylphosphatidylcholine (DSPC), which serves as a reference. In contrast, racemic erythro-DHO form stable condensed monolayers. The hydroxy groups act not as a disturbing moiety but as a spanning moiety between the particular amphiphiles by formation of hydrogen bonds. The enantio-enriched monolayers of all DHO investigated show a bent surface pressure–temperature course due to two different condensed phases which are in equilibrium with the expanded phase. Therefore, the DHO show chiral discrimination with respect to thermodynamic investigations.

Interactions of cyclodextrins with dipalmitoyl, distearoyl, and dimyristoyl phosphatidyl choline liposomes. A study by leakage of carboxyfluorescein in inner aqueous phase of unilamellar liposomes.

The interaction of cyclodextrins (CDs) with L-alpha-dipalmitoyl phopsatidyl choline (DPPC), L-alpha-distearoyl phosphatidyl choline (DSPC), and L-alpha-dimyristoyl phosphatidyl choline (DMPC) unilamellar liposomes was investigated by the leakage of carboxylfluorescein (CF) entrapped in the inner aqueous phase of liposomes, at 25 degrees C (DPPC and DSPC liposomes) and at 5 degrees C (DMPC liposomes). The efficiency of CDs for CF leakage was remarkable in the order of heptakis (2,6-di-O-methyl)-beta-CD (DOM-beta-CD) > alpha-CD > heptakis (2,3,6-tri-O-methy)-beta-CD (TOM-beta-CD) from DPPC liposomes, in the order of DOM-beta-CD > TOM-beta-CD > alpha-CD from DSPC liposomes and in the order of alpha-CD > DOM-beta-CD > TOM-beta-CD from DMPC liposomes. The other CDs used in the present studies, beta-CD, 2-hydroxylpropyl beta-CD, and gamma-CD scarcely induced the CF leakage from above the three liposomes. From the profiles of % CF leakage, together with measurements of differential scanning calorimetry, it was found that hydrophobic DOM-beta-CD penetrates the matrix of the liposomes to interact with them as well as TOM-beta-CD, and that less hydrophobic alpha-CD exists at the surface of the membrane to interact with the liposomes. Further, it was found that the interaction of CDs with liposomes changes depending not only on the length of fatty acid chain of phospholipid (condensation force and hydrophobicity) but also the hydrophobicity and the cavity size of CD.

Dilinoleoylphosphatidylcholine selectively modulates lipopolysaccharide-induced Kupffer cell activation

Polyenylphosphatidylcholine (PPC), a mixture of polyunsaturated phosphatidylcholines extracted from soybeans, protects against alcoholic and non-alcoholic liver injury. Because Kupffer cells mediate liver injury, we hypothesized that PPC may modulate their activation. The activation of Kupffer cells by lipopolysaccharide (LPS) leads to an enhanced production of cytokines. Among these, tumor necrosis factor-α (TNF-α) exerts mainly a hepatotoxic effect, whereas interleukin-1β (IL-1β) appears to be hepatoprotective. The present study evaluated whether dilinoleoylphosphatidylcholine (DLPC), the main component of PPC (40% to 52%), affects LPS-induced Kupffer cell activation in vitro. For comparison, palmitoyl-linoleoylphosphatidylcholine (PLPC), the other major component of PPC (23% to 24%), and distearoylphosphatidylcholine (DSPC), the saturated counterpart of DLPC, were also tested. Rat Kupffer cells were cultured in serum-free RPMI-1640 medium containing 10 μmol/L of either DLPC, PLPC, or DSPC in the presence or absence of LPS (1 μg/mL). After 20 hours in culture, the media were collected for cytokine measurements by enzyme-linked immunosorbent assays. LPS significantly stimulated TNF-α and IL-1β production by 62% and 328%, respectively. Treatment of Kupffer cells with LPS plus DLPC decreased the production of TNF-α by 23% (12.17 ± 1.83 pg/ng DNA vs 15.72 ± 2.74 pg/ng DNA, P < .05, n = 6) and increased that of IL-1β by 17% (1.80 ± 0.16 pg/ng DNA vs 1.54 ± 0.08 pg/ng DNA, P < .05, n = 6). No effect of PLPC or DSPC on LPS-induced TNF-α or IL-1β generation was observed, thereby illustrating the selective effect of DLPC in this process. Thus DLPC selectively modulates the LPS-induced activation of Kupffer cells by decreasing the production of the cytotoxic TNF-α while increasing that of the protective IL-1β. This dual action of DLPC on cytokines may provide a mechanism for the protective effect against liver injury, but its significance still needs to be determined by in vivo studies.

Interaction of pulmonary surfactant protein A with phospholipid liposomes: a kinetic study on head group and fatty acid specificity

Recent work on surfactant protein A (SP-A) has shown that Ca 2+ induces an active conformation, SP-Â, which binds rapidly to liposomes and mediates their aggregation. Employing sensitive real time assays, we have now studied the lipid binding characteristics of the SP-Â liposome interaction. From the final equilibrium level of the resonant mirror binding signal, an apparent dissociation constant of ca. K d=5 μM is obtained for the complex between SP-A and dipalmitoylphosphatidylcholine (DPPC) liposomes. At nanomolar SP-A concentrations, this complex is formed with a subsecond (0.3 s) reaction time, as measured by light-scattering signals evoked by photolysis of caged Ca 2+. With palmitoyloleoylphosphatidylcholine (POPC), the complex formation proceeds at half the rate, compared to DPPC, leading to a lower final equilibrium level of SP-A lipid interaction. Distearoylphosphatidylcholine (DSPC) shows a stronger interaction than DPPC. Regarding the phospholipid headgroups, phosphatidylinositol (PI) and sphingomyelin (SM) interact comparable to DPPC, while less interaction is seen with phosphatidylethanolamine (PE) or with phosphatidylglycerol (PG). Thus both headgroup and fatty acid composition determine SP-A phospholipid interaction. However, the protein does not exhibit high specificity for either the polar or the apolar moiety of phospholipids.

Size-dependent extravasation and interstitial localization of polyethyleneglycol liposomes in solid tumor-bearing mice

We have examined the size dependence of extravasation and interstitial localization of polyethyleneglycol-coated liposomes (PEG-liposomes) in the solid tumor tissue by means of electron microscopic observation. Liposomes composed of distearoyl phosphatidylcholine, cholesterol and distearoylphosphatidylethanolamine derivative of polyethyleneglycol (PEG) were prepared in various size ranges. PEG-liposomes with an average diameter of 100–200 nm showed the most prolonged circulation time and the greatest tumor accumulation in all the solid tumors employed in this experiment. Although large PEG-liposomes with a diameter of 400 nm showed a short circulation time in normal mice, the results in splenectomized mice indicated that they do have an intrinsic prolonged circulation character in vivo. However, large PEG-liposomes could not extravasate into solid tumor tissue. These results indicate that the size of liposomes is critical for extravasation. The electron microscopic observations revealed the almost exclusive engulfment of extravasated liposomes by tumor-associated macrophages; very few were taken up by tumor cells.

Stabilization of aerosolized IFN-γ by liposomes

Aerosolized IFN-γ is very unstable. We have improved the stability of IFN-γ in the jet nebulizer by adding small liposomes. Aerosolized IFN-γ was recovered in PBS solution by bubbling and its concentration was determined. After nebulization for 30 min, aerosolized IFN-γ was detected only 0.4±0.2% of the initial amount in the PBS solution and 3.1±0.7% in the reservoir. On the other hand, the addition of small liposomes (HSPC/DSPG=10/1 (molar ratio), 45±24 nm) in the nebulizer increased the stability of IFN-γ, 27.2±4.7% of the initial amount in the PBS solution and 25.7±12.6% in the reservoir. The present study also examined the effects of composition and concentration of liposomes on the stabilization of aerosolized IFN-γ. Liposome prepared from distearoyl phosphatidylcholine (DSPC) or hydrogenated soy phosphatidylcholine (HSPC) was very effective for stabilization of aerosolized IFN-γ (DSPC/DPPG=10/1, HSPC/DSPG=10/1). HSPC/DSPG liposome was efficient at the concentration higher than 12.5 μmols/ml for the stabilization of 5×10 5 JRU/ml of IFN-γ. In considering the mechanism of this stabilization, the results of gel filtration chromatography suggest that IFN-γ is inactivated by polymerization or aggregation in nebulization, while the inactivation is suppressed by liposomes due to their adsorption to IFN-γ.

Association of Blood Clotting Factors I and VII with Phospholipid Monolayers at the Air–Water Interface

Phospholipid monolayers adsorbed at the air–water interface are useful model membranes and have been employed to study the interactions between phospholipids and blood clotting factors I and VII. Factor I is a non-membrane-binding protein and was found to penetrate both distearoylphosphatidylcholine (DSPC) and dipalmitoylphosphatidylcholine (DPPC) monolayers at low lipid pressures. At high lipid pressures, the protein was crowded out of the interface. Factor I penetration of phospholipid monolayers was independent of hydrocarbon chain length, while penetration was maximized with electrolytes in the subphase. Factor VII is a membrane binding protein and was found to penetrate a DPPC monolayer only when electrolytes were added to the subphase. Factor VII penetrated DSPC monolayers regardless of electrolyte addition in the subphase, and its interactions with DSPC films are attributed to protein–lipid hydrophobic interactions.

Ion transfer across a phospholipid monolayer adsorbed at the water∣1,2-dichloroethane interface under surface pressure control

The influence of a monolayer of a lipid, distearoyl phosphatidylcholine, formed at a water ∣ dichloroethane interface in a special Langmuir trough, which allows simultaneous control of the state of the monolayer and the potential across the interface, on the transfer of ions across this interface has been studied using two probe ions: the cationic drug propranolol and anionic picrate. From measurement of the isotherms of the lipid monolayer and of the cyclic voltammograms of propranolol and picrate at controlled surface pressure it was found that picrate stabilises the monolayer and that its transfer across the interface requires more energy in the presence than in the absence of lipid. Propranolol, however, has a destabilising influence on the monolayer and accelerates lipid desorption. The kinetics of transfer of both ions was too fast to be followed by cyclic voltammetry.

Uptake of liposome-encapsulated 99Tcm-MIBI by sensitive and multidrug-resistant tumour cell lines.

It is well established that accumulation of 99Tcm-sestamibi (99Tcm-MIBI) is much higher in sensitive than multidrug-resistant tumour cells expressing the permeability glycoprotein 170 (Pgp 170) as well as a multidrug-resistance related protein (MRP). Thus 99Tcm-MIBI is a good candidate for diagnosing the multidrug-resistance phenotype by in vivo imaging. However, the blood clearance of 99Tcm-MIBI is too rapid to achieve optimal accumulation in tumours and uptake in the liver, spleen, heart and muscle is too high for it to be an excellent in vivo tumour tracer. One way of prolonging the bioavailability of 99Tcm-MIBI is to use liposomes which do not affect its accumulation in tumour cells. We explored this possibility in vitro using two sensitive and five resistant cell lines, two of them expressing Pgp 170 and three others over-expressing MRP. 99Tcm-MIBI was incorporated into liposomes prepared by thin film hydration with phosphate-buffered saline using distearoyl phosphatidyl choline, distearoyl phosphatidyl ethanolamine and cholesterol in a ratio of 1.85:0.15:1.00. Liposome diameter was 97.9 +/- 4.5 nm as determined by dynamic light scattering. 99Tcm-MIBI uptake was quantified by measuring radioactivity retained in the cells incubated at 37 degrees C with liposome-encapsulated 99Tcm-MIBI or with free radiotracer in the presence of empty liposomes. In both experimental cases, 99Tcm-MIBI accumulation was similar to that obtained in the presence of free 99Tcm-MIBI only: it was much higher in sensitive than in resistant Pgp 170-positive and MRP-positive cells. Encapsulation in liposomes does not alter the potency of 99Tcm-MIBI to distinguish the sensitive and resistant tumour cells. Our results suggest that future studies should assess the usefulness of the encapsulated form of 99Tcm-MIBI for in vivo imaging of tumours.

Monte Carlo Simulation of Two-Component Bilayers: DMPC/DSPC Mixtures

In this paper, we describe a relatively simple lattice model of a two-component, two-state phospholipid bilayer. Application of Monte Carlo methods to this model permits simulation of the observed excess heat capacity versus temperature curves of dimyristoylphosphatidylcholine (DMPC)/distearoylphosphatidylcholine (DSPC) mixtures as well as the lateral distributions of the components and properties related to these distributions. The analysis of the bilayer energy distribution functions reveals that the gel–fluid transition is a continuous transition for DMPC, DSPC, and all DMPC/DSPC mixtures. A comparison of the thermodynamic properties of DMPC/DSPC mixtures with the configurational properties shows that the temperatures characteristics of the configurational properties correlate well with the maxima in the excess heat capacity curves rather than with the onset and completion temperatures of the gel–fluid transition. In the gel–fluid coexistence region, we also found excellent agreement between the threshold temperatures at different system compositions detected in fluorescence recovery after photobleaching experiments and the temperatures at which the percolation probability of the gel clusters is 0.36. At every composition, the calculated mole fraction of gel state molecules at the fluorescence recovery after photobleaching threshold is 0.34 and, at the percolation threshold of gel clusters, it is 0.24. The percolation threshold mole fraction of gel or fluid lipid depends on the packing geometry of the molecules and the interchain interactions. However, it is independent of temperature, system composition, and state of the percolating cluster.